IMPROVING THE DESIGN OF A FULL SCALE WASTEWATER TREATMENT PLANT WITH THE USE OF THE COMPLEX ACTIVATED SLUDGE MODEL

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1 POLITECNICO DI MILANO Como Campus M. Sc. in Environmental and Geomatic Engineering IMPROVING THE DESIGN OF A FULL SCALE WASTEWATER TREATMENT PLANT WITH THE USE OF THE COMPLEX ACTIVATED SLUDGE MODEL Supervisor: prof. Giorgio Guariso Master Thesis by: Vladimir Katić Student ID Academic Year 2015/2016

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3 Table of Contents Abstract... 5 LIST of abbreviations... 6 LIST of figures... 7 LIST of tables... 8 CHAPTER 1 - INTRODUCTION Basics of biological wastewater treatment Activated Sludge Process (ASP) General information Activated Sludge Process variables Membrane Bioreactor technology (MBR) General information Efficiency and comparison with the conventional ASP Porec project background Framework of the project Description of the project Scope of the Work CHAPTER 2 - MODELLING OF ACTIVATED SLUDGE PROCESSES Overview Activated sludge models Activated sludge model development Activated sludge models assumptions and limitations Simulation environments Model applications WWTP model simulation for learning WWTP model simulation for design WWTP model simulation for process optimization CHAPTER 3 - APPLICATION OF THE BIOWIN MODELLING TOOL About BioWin Implemented Biological/Chemical models Activated sludge processes Other important physical phenomena implemented

4 3.3 Model simulation for WWTP Porec South Project design parameters Plant configuration Performed simulations and results Input data Dynamic simulation, summer (variable inflow) Dynamic simulation, winter (variable inflow) Yearly average SRT of the WWTP Comparison with ATV-DVWK rules and standards Required Sludge Age - winter Required Sludge Age - summer Determination of the proportion of the reactor volume for denitrification Phosphorous removal Sludge production Volume of the biological reactor Summary of obtained results Simulation of an extreme event Assumed scenario Performed simulation and results CHAPTER 4 - CONCLUSIONS BIBLIOGRAPHY ANNEX - BIOWIN PARAMETERS (ASP)

5 Abstract This thesis discusses the application of a complex activated sludge modelling tool which was implemented to assure the predictability and improve the design and effectiveness of a biological wastewater treatment for the full-scale Wastewater Treatment Plant (WWTP) Porec South, situated in the touristic coastal town of Porec (Parenzo) - Republic of Croatia. The above mentioned plant represents a part of a larger project consisting of four wastewater treatment plants, all designed with the Membrane Bioreactor Technology (MBR). This project has been awarded to a consortium of companies SUEZ-STRABAG in August 2015, for which the Design & Build process is still ongoing during the development of this work. The selected tool used for the simulation is BioWin 5.0 (EnviroSim Associates Ltd.). Since the plant is characterized by a large seasonal difference in terms of its hydraulic and mass loads, the model was applied for the period with the heaviest load (summer) and then the simulation was repeated for the period with the lowest load (winter). The obtained study and simulations describe and confirm the chosen configuration of the WWTP Porec South and its design using a modelling tool which is widely used and universally recognized in the scientific community. The results will permit to check and validate the design and to confirm process tank volumes (anaerobic, anoxic and aerated volumes), sludge concentration, excess sludge extraction and sludge age, recirculation rates and most importantly, the compliance to the discharge limits and the requirements of the project. Furthermore, an additional simulation is performed to demonstrate the effect of an extreme peak flow event (5-day storm event) on the plant and how it might affect the performance of the wastewater treatment process. This would give further performance indicators of the project, thus assuring the safety and operability of the plant. 5

6 LIST of abbreviations ASDM Activated Sludge/Digestion Model ASM Activated Sludge Model ASP Activated Sludge process BOD Biological Oxygen Demand BIO-P Biological Phosphorus Removal COD Chemical Oxygen Demand DO Dissolved Oxygen F/M Food to Microorganism ratio HRT Hydraulic Retention Time IAWPRC International Association on Water Pollution Research and Control IWA International Water Association MBR Membrane Bioreactor MBBR Moving Bed Biofilm Reactor MLSS Mixed Liquor Suspended Solids PAO Phosphorus Accumulating Organisms PE Population Equivalent RBC Rotating Biological Reactor SBR Sequencing Batch Reactor SRT Solids Residence Time SS Suspended solids TSS Total Suspended Solids TUDP Metabolic model developed at the Delft University of Technology VFA Volatile Fatty Acids VSS Volatile Suspended Solids WWTP Wastewater Treatment Plant 6

7 LIST of figures Figure 1: Generalized, schematic diagram of an activated sludge process (complete mixing) Figure 2: Example of a WWTP with a conventional ASP technology (WWTP Milano San Rocco: PE) Figure 3: Submerged MBR with internal vacuum-driven membrane filtration Figure 4: Side-stream MBR with external pressure driven membrane filtration Figure 5: Overview of the filtration processes Figure 6: Scope of the MBR process compared to the conventional activated sludge process with extensions Figure 7: Example of MBR technology with submerged membranes (WWTP Rimini: PE) Figure 8: Project location Figure 9: Substrate flows for autotrophic and heterotrophic biomass in ASM1 and ASM3 models Figure 10: Substrate flows for storage and growth of PAOs in the ASM2 model Figure 11: Substrate flows for storage and aerobic growth of PAOs in the TUDP model Figure 12: Example of a plant configuration in BioWin (WWTP Porec South) Figure 13: Simplified scheme of WWTP Porec South biological section Figure 14: BioWin main simulator window for WWTP Porec South (summer period) Figure 15: BioWin main simulator window for WWTP Porec South (winter period) Figure 16: Pattern of flow distribution employed for winter and summer simulations, only the applied coefficients - that were not modified in the three simulations - are reported (and not the flows) Figure 17: Effluent Nitrogen fractions in summer Figure 18: Total excess sludge production from dewatering in summer Figure 19: Weekly operation of the centrifuge, where the 1 represent an hour of duty of the equipment and the 0 an hour of standby Figure 20: Total waste material produced from pretreatment in summer Figure 21: Biomass concentrations Figure 22: Effluent nitrogen fractions in winter Figure 23: Total excess sludge production from dewatering in winter Figure 24: Total waste material produced from pretreatment in winter Figure 25: Effluent Nitrogen fractions (summer - extreme event) Figure 26: Total waste material produced from pre-treatment (summer - extreme event) Figure 27: Total excess sludge production from dewatering (summer - extreme event) Figure 28: Air flow rate need to perform the aeration of biomass (summer - extreme event) 7

8 LIST of tables Table 1: Comparison of the performance: membrane bioreactor process and conventional activated sludge process (with and without extensions for disinfection) Table 2: Estimated pollution parameters for the project Table 3: Estimated pollution and influent loads for the project Table 4: Overview of activated sludge models Table 5: Typical flow values of influent wastewater for WWTP Porec South Table 6: Inhabitant-specific loads in g/(i d) Table 7: Design daily loads of influent wastewater for WWTP Porec South Table 8: Limit values considered for water discharge from the WWTP Porec South Table 9: Influent characterization - Input values for BioWin simulation for summer and winter period Table 10: Summary of the biological tank volumes for WWTP Porec South Table 11: Calculations to determine the yearly average SRT Table 12: Dimensioning sludge age in days dependent on the treatment target and the temperature as well as the plant size (intermediate values are to be estimated) Table 13: Standard values for the dimensioning of denitrification for dry weather at temperatures from 10 to 12 C and common conditions (kg nitrate nitrogen to be denitrified per kg influent BOD5) Table 14: Values used in the calculation of the anaerobic HRT Table 15: Parameters and calculations employed for the comparison on sludge production Table 16: Sludge production as a function of the different SRT obtained or stated in the ATV-DVWK-A 131E Table 17: Parameters for biological reactor volume determination Table 18: Summary of the compliance of the WWTP design with ATV-DVWK- A 131E/Wastewater Engineering: Treatment and Reuse 8

9 CHAPTER 1 - INTRODUCTION The progressive deterioration of water resources globally and the large amount of polluted water generated in industrialized societies gives wastewater treatment processes a fundamental importance in the water loss prevention. New guidelines and regulations (i.e. Directive 91/271/CEE) enforce the adoption of specific quality indexes for the treated wastewater. Taking into account current environmental problems, it is not unrealistic to believe that this trend will continue. At the same time loads on existing plants are expected to increase due to growth of urban areas. This situation demands more efficient treatment procedures for wastewater. Effluents from wastewater treatment plants has been reported as the main cause of eutrophication in surface waters. Small amounts of nutrients can lead to eutrophication and stimulate excessive production of chemical oxygen demand (COD) in the form of algae, loss of oxygen resources, changes in aquatic population and subsequent deterioration of water quality. In the field of domestic wastewater treatment, there is an increasing requirement to improve effluent quality for the benefit of receiving surface waters. Additionally, it is required to minimise energy consumption and reduce the use of chemicals in the treatment process. Inside a biological wastewater treatment plant, the Activated Sludge Process (ASP) is the most commonly used technology to remove organic pollutant from wastewater, even if the process was developed in the early 20 th century. This is because it is the most cost-effective, it is very flexible (it can be adapted to any kind of wastewater), it is reliable and has the capacity of producing high quality effluent (Mulas, 2006). Further technological developments in recent years have led to the application of a membrane bioreactor (MBR) technology for full-scale municipal wastewater treatment. The MBR is a suspended growth-activated sludge system that utilizes microporous membranes for solid/liquid separation instead of secondary clarifiers that are used in a conventional ASP. It represents a decisive step forward concerning effluent quality by delivering a hygienically pure effluent and by exhibiting a very high operational reliability. Advanced MBR wastewater treatment technology is being successfully applied at an ever-increasing number of locations around the world. The design and operation of biological wastewater treatment plants that implement the above mentioned technologies can be simplified through the use of mathematical models. The activated sludge models elaborated in the last two decades have resulted in several mathematical models comprehensively describing biological wastewater treatment processes, especially with regard to activated sludge systems. A fundamental meaning in this area had the formulation of the Activated Sludge Model no. 1 (ASM1) by the International Association on Water Pollution Research and Control (IAWPRC, formerly known as IAWQ and IWA). Although neither biological nor chemical phosphorus removal was incorporated into ASM1, this model provided the matrix notation system and the nomenclature used in further models (inter alia ASM2 and ASM2d). The development of these initial models has allowed the prediction of the 9

10 effluent composition including the content of carbon, nitrogen and phosphorus compounds. They have proven to be very helpful in the optimization studies for the existing wastewater treatment plants (WWTPs) and the design and development of control strategies for the existing or new WWTPs (Liwarska-Bizukojc, 2013). 1.1 Basics of biological wastewater treatment Biological treatment is an important and integral part of any wastewater treatment plant that treats wastewater from either municipality or industry having soluble organic impurities or a mix of the two types of wastewater sources. The obvious economic advantage, both in terms of capital investment and operating costs, of biological treatment over other treatment processes like chemical oxidation; thermal oxidation etc. has cemented its place in any integrated wastewater treatment plant. Biological treatment using aerobic activated sludge process has been in practice for well over a century. Increasing pressure to meet more stringent discharge standards or not being allowed to discharge treated effluent has led to implementation of a variety of advanced biological treatment processes in recent years (e.g. Wastewater Engineering: Treatment and Reuse, Metcalf & Eddy, 2002). In principle, the biological wastewater treatment is based on metabolism of natural microorganisms to eliminate pollution caused by dissolved substances and to achieve the prescribed parameters for secure release into the environment. These microorganisms eliminate dissolved contamination by assimilating it for the needs of their own growth and reproduction, leading to an increase of biological sludge (biomass) that has to be separated from treated water. Development of microorganisms may be organized in the form of suspended growth or as attached growth: a) Suspended growth In suspended growth systems, such as activated sludge (also aerated lagoons and aerobic digestion) waste and microorganisms are combined while oxygen diffuse and penetrate into the cell. The microorganisms develop freely in a liquid environment and they naturally group in floccules. The settled flocs are retained in a clarifier while part of the sludge is recycled to the aeration tank. The ratio of recycled sludge influences the performance of biological treatment. In relation to activated sludge concentration, time and volume needed for purification (in relation to natural purification in rivers) are significantly reduced. Excess sludge is regularly extracted and conveyed to the sludge treatment section. b) Attached growth Contrary to suspended solid systems, microorganisms can also develop on submerged or fixed media, watered by water that needs treatment: it is an attached growth procedure. Microorganisms as a biofilm are maintained and grown on the media and they get in 10

11 contact with fresh wastewater. Trickling filters and rotating biological contactors (RBCs) are two popular attached growth processes which are commonly used in industrial wastewater treatment. The trickling filter consists of a fixed bed media of rocks, plastic material, or textile media. In this process wastewater flows downward and passes and creates a biofilm on the media, that becomes thick and falls off when the thickness of biofilm increase considerably. This phenomenon is known as sloughing. Also, RBCs consist of a series of circular disks rotating through the wastewater flow, partially submerged. These rotating disks are usually plastic. Microorganisms as biofilm are developed on exterior surface of the disks and eventually sloughs off if the film gets thick. Advantages of the attached growth are its compactness and reactivity. On the other hand, it requires complex pre-treatment (primary straining or settling, depending on the case) and the remaining sludge is very fermentable. The suspended growth technique is more extensive than the attached growth technique, but on the other hand it is characterized by greater culture stability. Combined systems also exist: organisms from the fixed culture on mobile carriers are added and mixed in the activated sludge mixture (e.g. Moving Bed Biofilm Reactors). In the following chapters 1.2 and 1.3, we shall briefly discuss the fundamentals and the differences between the conventional activated sludge process and the membrane bioreactor process, given that the former represents the base for the implemented modelling tool, while the latter represents the implemented technology of the project we will examine (Case study). In chapter 1.4 are represented all the basic information and parameters that make up the considered application. 1.2 Activated Sludge Process (ASP) General information The most common suspended growth process used for municipal wastewater treatment is the activated sludge process as shown in figure: Figure 1: Generalized, schematic diagram of an activated sludge process (complete mixing) 11

12 In activated sludge process wastewater containing organic matter is aerated in an aeration basin which promotes microorganisms to metabolize the suspended and soluble organic matter. Part of organic matter is synthesized into new cells and part is oxidized to CO2 and water to derive energy. In activated sludge systems the new cells formed in the reaction are removed from the liquid stream in the form of a flocculent sludge in settling tanks. A part of this settled biomass, described as activated sludge is returned to the aeration tank and the remaining forms waste or excess sludge. Activated sludge plant involves: wastewater aeration in the presence of a microbial suspension; solid-liquid separation following aeration; discharge of clarified effluent; wasting of excess biomass; return of remaining biomass to the aeration tank. Activated sludge is today the most common procedure for municipal wastewater biological treatment, mainly because it has proved to be the most flexible and cost effective. It enables treatment of primary municipal pollutants (carbon, nitrogen, phosphorus, suspended solids) with production of relatively stable sludge. Figure 2: Example of a WWTP with a conventional ASP technology (WWTP Milano San Rocco: PE) 12

13 1.2.2 Activated Sludge Process variables The main variables of activated sludge process are the mixing regime, loading rate, and the flow scheme. Mixing Regime Generally two types of mixing regimes are of major interest in activated sludge process: plug flow and complete mixing. In the first, the regime is characterized by orderly flow of mixed liquor through the aeration tank with no element of mixed liquor overtaking or mixing with any other element. There may be lateral mixing of mixed liquor but there must be no mixing along the path of flow. In complete mixing, the contents of aeration tank are well stirred and uniform throughout. Thus, at steady state, the effluent from the aeration tank has the same composition as the aeration tank contents. The type of mixing regime is very important as it affects (1) oxygen transfer requirements in the aeration tank, (2) susceptibility of biomass to shock loads, (3) local environmental conditions in the aeration tank, and (4) the kinetics governing the treatment process. Loading Rate A loading parameter that has been developed over the years is the hydraulic retention time (HRT) θ defined as: θ = V Q where V= volume of aeration tank, m 3, and Q = sewage inflow, m 3 /d Another empirical loading parameter is the volumetric organic loading which is defined as the BOD applied per unit volume of aeration tank, per day. A rational loading parameter which has found wider acceptance and is often preferred is the specific substrate utilization rate, q, per day. q = Q ( S o S e ) V X where So and Se are influent and effluent organic matter concentration respectively, measured as BOD5 (g/m 3 ), A similar loading parameter is the mean cell residence time or sludge retention time (SRT) θc : 13

14 θ c = V X Q w X r + (Q Q w X e ) where X, Xe and Xr are MLSS concentration in aeration tank, effluent and return sludge respectively, and Qw = waste activated sludge rate. Under steady state operation, the mass of waste activated sludge is given by: Q w X r = YQ (S o S e ) k d XV where Y= maximum yield coefficient (microbial mass synthesized / mass of substrate utilized) and kd = endogenous decay rate (d -1 ). From the above equation it is seen that 1/θc = Yq - kd If the value of Se is small as compared So, q may also be expressed as Food to Microorganism ratio, F/M F/M = Q(S o S e ) XV = QS o /XV The θc value adopted for design controls the effluent quality, and settleability and drainability of biomass, oxygen requirement and quantity of waste activated sludge. Flow Scheme The flow scheme involves: the pattern of sewage addition; the pattern of sludge return to the aeration tank and; the pattern of aeration. Sewage addition may be at a single point at the inlet end or it may be at several points along the aeration tank. The sludge return may be directly from the settling tank to the aeration tank or through a sludge reaeration tank. Aeration may be at a uniform rate or it may be varied from the head of the aeration tank to its end. Conventional System and its Modifications The conventional system maintains a plug flow hydraulic regime. Over the years, several modifications to the conventional system have been developed to meet specific treatment objectives. In step aeration settled sewage is introduced at several points along the tank length which produces more uniform oxygen demand throughout. Tapered aeration attempts to supply air to match oxygen demand along the length of the tank. Contact stabilization provides for reaeration of return activated sludge from 14

15 the final clarifier, which allows a smaller aeration or contact tank. Completely mixed process aims at instantaneous mixing of the influent waste and return sludge with the entire contents of the aeration tank. Extended aeration process operates at a low organic load producing lesser quantity of well stabilized sludge. 1.3 Membrane Bioreactor technology (MBR) General information The combination of an activated sludge tank with a membrane filtration for the separation of the activated sludge is called the membrane bioreactor process. The membrane filtration takes over the separation of the activated sludge in place of the conventional final clarification. While in secondary settling tanks only the part of the activated sludge that is settleable is separated, i.e. forms settleable flocks. During membrane filtration all parts of the activated sludge are separated which are larger than the molecular separation size of the membrane. Thus, the separation of the activated sludge from the treated waste water becomes independent of the settling characteristics of the activated sludge and depends only on the membrane applied. In addition, a higher solids content can be maintained in the bioreactor than in the conventional activated sludge process so that less reactor space is needed (pg. 14, Merkblatt DWA-M 227 manual). The two main MBR configurations for WWTPs are described below. a) Internal/submerged membranes The filtration element is installed in either the main bioreactor vessel or in a separate tank. The membranes can be flat sheet or tubular or a combination of both, and can incorporate an online backwash system which reduces membrane surface fouling by pumping membrane permeate back through the membrane. In systems where the membranes are in a separate tank to the bioreactor, individual trains of membranes can be isolated to undertake cleaning regimes, however the biomass must be continuously pumped back to the main reactor to limit TSS concentration increase. Additional aeration is also required to provide air scouring to reduce fouling. Where the membranes are installed in the main reactor, membrane modules are removed from the vessel and transferred to an offline cleaning tank. 15

16 Figure 3: Submerged MBR with internal vacuum-driven membrane filtration (Image from b) External/sidestream The filtration elements are installed externally to the reactor, often in a plant room. The biomass is either pumped directly through a number of membrane modules in series and back to the bioreactor, or the biomass is pumped to a group of modules, from which a second pump circulates the biomass through the modules in series. Cleaning and soaking of the membranes can be undertaken in place with use of an installed cleaning tank, pump and pipeline. Figure 4: Side-stream MBR with external pressure driven membrane filtration (Image from 16

17 In the following figure an overview of the filtration process is given. To separate the activated sludge with its microorganisms and particles from the treated waste water, microfiltration membranes with a molecular separation size of maximally 0.5 µm are used for the membrane bioreactor process. Figure 5: Overview of the filtration processes Efficiency and comparison with the conventional ASP The membrane bioreactor process reaches performance values that are better than those of a conventional activated sludge process with the same size (pg. 18, Merkblatt DWA- M 227 manual). To achieve comparable values, a conventional activated sludge plant should show process steps as shown in the following figure: 17

18 Figure 6: Scope of the MBR process compared to the conventional activated sludge process with extensions The advantages of the membrane bioreactor process result from the possible higher MLSS contents in the activated sludge tank and complete separation of all solid matter by the membranes. Therefore, nitrogen, phosphorus and carbon in the effluent of membrane bioreactors are reduced by the fraction which in conventional plants results from solid matter in the effluent. Table 1 shows the achievable performance values that can be expected under conventional municipal supply conditions (pg. 18, Merkblatt DWA-M 227 manual). Parameter Membrane bioreactor plant Conventional activated sludge Without extensions With extensions Solids mg/l CCSB mg/l < Microbiological quality Bathing water quality 1) - Bathing water quality 1) REMARK: 1) With regard to EC directive 76/160/ECC Table 1: Comparison of the performance: membrane bioreactor process and conventional activated sludge process (with and without extensions for disinfection) 18

19 All the main advantages of MBR system over conventional activated sludge systems are listed below (Arun Mittal, 2011): Membrane filtration provides a positive barrier to suspended bio-solids that they cannot escape the system. This contrasts gravity settling in activated sludge process, where the bio-solids continuously escape the system along with clarified effluent and sometimes a total loss of solids is also encountered due to process upsets causing sludge-bulking in the clarifier. As a result, the bio-solids concentration measured as MLSS/MLVSS can be maintained 3 to 4 times larger in an MBR process (~ mg/l) in comparison to the activated sludge process (~2500 mg/l). Due to the above aspect of MBR, aeration tank size in the MBR system can be one-third to one-fourth the size of the aeration tank in an activated sludge system. Further, instead of gravity settling based clarifier, a much more compact tank is needed to house the membrane cassettes in case of submerged MBR and skid mounted membrane modules in case of non-submerged, external MBR system. Thus, MBR system requires only 40-60% of the space required for activated sludge system, therefore significantly reducing the concrete work and overall foot-print. Due to membrane filtration (micro/ultrafiltration), the treated effluent quality in case of MBR system is far superior compared to conventional activated sludge, so the treated effluent can be directly reused as cooling tower make-up or for gardening etc. Typical treated water quality from MBR system is: o BOD5 < 5 mg/l o Turbidity < 0.2 NTU Figure 7: Example of MBR technology with submerged membranes (WWTP Rimini: PE) 19

20 In summary, membrane bioreactor technology has become more popular, abundant, and accepted in recent years for the treatment of many types of wastewaters, whereas the conventional ASP process cannot cope with either composition of wastewater or fluctuations of wastewater flow rate. MBR technology is also used in cases where demand on the quality of effluent exceeds the capability of conventional ASP. Although MBR capital and operational costs exceed the costs of conventional process, it seems that the upgrade of conventional process occurs even in cases when conventional treatment works well. Microorganisms are retained by membranes in a very high degree. Studies have shown that the limit values and guide values for all microorganisms (total number of bacteria coliforms, faecal coliforms and streptococci) in accordance with the EC Directive on the quality of bathing water (76/160/EEC 1976) are independent of the weather conditions (dry weather, storm, continuous rain) and were met in all cases. Even viruses, the smallest pathogenic organisms which theoretically may pass through the membrane pores, are retained by the membrane bioreactor process. The viruses typically accumulate with particles and microorganisms so that they are removed from the wastewater by the elimination of larger particles. During the studies mentioned above (source: pg. 19, Merkblatt DWA-M 227 manual)., it was possible to significantly reduce the concentrations of intestinal viruses. Filtration units available in municipal wastewater treatment with membranes with a nominal pore size below 0.5 µm do not differ with respect to the efficiency of particle removal from each other. For a long-term high performance it should be guaranteed that no short-circuits between treated and non-treated wastewater exist and that membranes and connections are always secure and a contamination of the permeate side is minimized. 1.4 Porec project background Framework of the project During the accession period towards the European Union (2013), the Republic of Croatia had been obliged to transpose the Urban Waste Water Treatment Directive (91/271/EEC) into the Croatian legal system. The adoption of the aforementioned Directive demanded the implementation of extensive and financially heavy investments for construction of integrated wastewater collection and treatment systems. However, co-financing of such investments was made possible via means of the Cohesion Fund and European Fund for Regional Development. The extensiveness of investments, rules for EU co-financing and professional rules have sought a detailed and careful preparation of the project documentation and attentive evaluation of submitted tenders during the public procurement procedure of such projects. 20

21 One such project, which was awarded to a consortium of companies STRABAG - SUEZ, during August 2015, is the design and construction of four wastewater treatment plants with Odvodnja LLC as the local public water authority for the town of Porec. Figure 8: Project location Located in Istria, a region on the Adriatic coast that is popular with tourists, the town of Porec increases its population from out of season to residents during the holiday season. The coastal waters in the area have been declared as a sensitive area. With the backing of the European Union, the local authorities have launched a vast programme to optimise the town's wastewater treatment infrastructures. The project covers the design and construction of four wastewater treatment plants, equipped with membrane bioreactors and with a total capacity of population equivalent (PE), as foreseen with the estimated horizon by the end of year The treated water will be reused for agricultural irrigation and the sludge from the treatment of the wastewater will be recovered by solar drying or by composting Description of the project The sewage systems in the coastal areas are reasonably well developed. The sewage currently only receives rudimentary treatment (mainly screening and, in some cases, grit collection) and is disposed through submarine outfalls at various points along the coast line. The sewage systems in settlements to the inland are not developed and wastewater is disposed in septic tanks and cesspits. According to the Urban Wastewater Treatment Directive the Porec area has a high priority for compliance. Each of the existing four agglomerations generates more than PE and discharges into a sensitive marine recipient and hence requires appropriate treatment. Tourist activity and growth rates The annual tourist activity, measured in overnights and residing on campsites, in hotels and apartment complexes is estimated to grow from the current 6,6 million annual 21

22 overnights to 7,5 million during a 30 year period. The peak is in July and August with approximately 2 million overnights in each of these months and represents a tourist inflow of between and people plus another people in weekend houses and in private rooms. Demand analysis and seasonality The water consumption and related waste water generation have an explicit seasonal pattern. The minimum water consumption and waste water generation takes place in the four winter months (November, December, January, February). The maximum water consumption and hence wastewater generation takes place in the period from mid June to mid - September. Wastewater is generated by resident population, weekend house users, tourists (camps, self-catering and hotels) and the ancillary tourist infrastructure. The total generated load in the summer months is estimated to be approx PE for the predicted start of operation (2017) and will increase to approx PE at the end of design horizon. Climate and weather The project area is in the border zone between sub-mediterranean and mild continental climate, but under strong maritime influence. The climate is mildly Mediterranean, with dry and warm summers, frequent and intensive autumn and spring rains-showers, and comparatively mild winters, mainly without snow. According to Köppen classification, the climate of the City of Porec is moderately warm and rainy sub-humid. The mean temperature in the coldest month of the year is higher than 5 C, and lower than 22 C. There is no distinctive dry period and minimum rainfall in summer, with mean temperature in the warmest month of the year higher than 25 C, and with at least four consecutive months with mean temperature above 10 C, rainy period in autumn, and in addition to the main rainfall minimum in winter, with a minor dry period in summer. The average annual rainfall ranges between 780 and mm. The precipitation occurs mainly in the form of rain, very rarely as hail or snow. The maximum average monthly rainfall occurs in September, October and November and is over 100 mm per month. The driest period is winter, in particular February and March with monthly average rainfall not exceeding 40 mm per month. Influent data The flows and pollution loads on the WWTP Porec South have been estimated for the years 2011 and 2045 taking into consideration the existing resident population, nonhousehold consumption and the tourist overnights. A slight growth is expected in flows/loads up to the estimated horizon. The estimated flows and the loads for the year 2045 are summarized in the following table: 22

23 WWTP Porec South Estimated values Flows/Loads M.U. Winter Summer Pollution load PE = 60 g BOD BOD load kg/d 516 2,880 Average daily flow m 3 /day 958 7,800 Peak factor = 16/24 Maximum dry weather flow m 3 /hour Inflow/infiltration m 3 /day Peak factor = 4/24 Maximum inflow/infiltration m 3 /hour Peak daily flow m 3 /day Peak hourly flow m 3 /hour Table 2: Estimated pollution parameters for the project For the estimation of the influent flows and pollution loads the following data were used: Influent and pollution loads Months Population Tourist Average water consumption Overnights (m 3 /month) 2011 Average Households Non-Households Estimated growth of tourist overnights from 2011 to % Permanent population for year Table 3: Estimated pollution and influent loads for the project 23

24 1.5 Scope of the Work In the previous chapters, we have been briefly introduced to the importance of water conservation and wastewater treatment considering the phenomena of climate change and increase of urban populations globally. Also, we have been introduced to the basics of biological wastewater treatment and all the information, which represents a foundation for the development of this study. In the scope of the project, pollution loads are calculated for each month according to Table 3 and ATV specific pollution load per inhabitant (pg. 19, table 1, ATV-DVWK-A 131E manual). However, for the design of the plant, it was considered the month of August, given that it is a month with the highest pollution loads (worst case scenario to assure the proper design of the plant). Additionally, the month of January is a period of highest load with respect to other winter months. With this in mind and given that for the assumed winter temperature (12 C) the biomass growth rate is much slower, this month was selected for the simulation of the winter period. It is also important to note that in the design of the WWTP Porec South, it was taken into consideration the fact that due to changes in holiday periods and weather conditions, the number of tourists can vary significantly and hence the flows and loads in the transition months between low season (winter) and high season (summer) can substantially deviate from the above mentioned estimated values. With this in mind, this study is essentially split into the following parts: Chapter 2 which discusses the theory behind the activated sludge process modelling and simulation, while the Chapter 3 deals with actual implementation of the BioWin modelling tool for the expected summer and winter loads of the WWTP Porec South, along with an additional simulation for an extreme peak inflow event as a further step of assuring the behaviour of the plant during unusual weather circumstances and inflow patterns. The goal is not only to have a plant designed according to the specifications set out by the project and the applied regulations with the respect to the set limits (ATV standards), but also to go above the needs and to perform additional simulation(s) that could give further performance indicators of the plant. 24

25 CHAPTER 2 - MODELLING OF ACTIVATED SLUDGE PROCESSES 2.1 Overview The purpose of Chapter 2 is to demonstrate how the model selection, the data collection and the WWTP model calibration all relate to the modelling purpose. Note that there is an essential difference between an activated sludge model and a WWTP model. A WWTP usually consists of a set of activated sludge tanks, combined with a sedimentation tank, with a range of electron acceptor conditions occurring in the tanks. Depending on the concentrations of dissolved oxygen (DO) and nitrate present in the tanks, aerobic (oxygen present), anoxic (nitrate present, no oxygen) or anaerobic (no oxygen, no nitrate) tanks can be distinguished. The term WWTP model is used to indicate the ensemble of activated sludge model, hydraulic model, oxygen transfer model and sedimentation tank model needed to describe an actual WWTP. The term activated sludge model is used to indicate a set of differential equations that represent the biological (and chemical) reactions taking place in one activated sludge tank. Activated sludge model will thus refer exclusively to white-box models, i.e. models based on first engineering principles. The hydraulic model describes tank volumes, hydraulic tank behaviour (e.g. perfectly mixed versus plug flow behaviour, constant versus variable volume, etc.) and the liquid flow rates in between tanks, such as return sludge flow rate and internal recycle flow rate. The sedimentation tank models are available in varying degrees of complexity. Dedicated WWTP simulators allow construction of WWTP models based on libraries of activated sludge models, sedimentation tank models, etc. (Gernaey V.K. et al., 2004). A number of factors are to be considered with regard to activated sludge modelling and model applications, and a step-wise approach is needed to evolve from the model purpose definition to the point where a WWTP model is available for simulations. The following main steps can be distinguished in this process: Definition of the WWTP model purpose or the objectives of the model application (control, design, simulation); 25

26 Table 4: Overview of activated sludge models Model selection: choice of the models needed to describe the different WWTP units to be considered in the simulation, i.e. selection of the activated sludge model, the sedimentation model, etc.; Hydraulics, i.e. determination of the hydraulic models for the WWTP or WWTP tanks; Wastewater and biomass characterisation, including biomass sedimentation characteristics; Data reconciliation to a steady-state model; Calibration of the activated sludge model parameters; Model unfalsification. In this task it is determined whether or not the model is sufficiently accurate for its intended purpose. If this is the case, the model is said to be unfalsified with respect to the available data. If this is not the case, a number of the preceding steps needs to be repeated until the model is unfalsified; Scenario evaluations. The following paragraph will provide a number of key references as guidance through some of the abovementioned steps. The activated sludge models that are most frequently used today will be summarized. Available WWTP simulators will be described just briefly. 2.2 Activated sludge models The most frequently used activated sludge models will be considered in an attempt to support the modeller in the model selection phase Activated sludge model development The focus will be on the recent developments of activated sludge models, mainly the family of activated sludge models developed by the International Water Association (IWA) and the metabolic model developed at the Delft University of Technology (TUDP model). Table 4 summarises essential features of these and several other activated sludge models. The Activated Sludge Model No. 1 (ASM1; Henze et al., 1987) can be considered as the reference model, since this model triggered the general acceptance of WWTP 26

27 modelling, first in the research community and later on also in industry. This evolution was undoubtedly supported by the availability of more powerful computers. Many of the basic concepts of ASM1 were adapted from the activated sludge model defined by Dold et al. (1980). Even today, the ASM1 model is in many cases still the state of the art for modelling activated sludge systems. ASM1 has become a reference for many scientific and practical projects (Roeleveld and van Loosdrecht, 2002), and has been implemented (in some cases with modifications) in most of the commercial software available for modelling and simulation of WWTPs for N removal (Copp, 2002). ASM1 was primarily developed for municipal activated sludge WWTPs to describe the removal of organic carbon compounds and N, with simultaneous consumption of oxygen and nitrate as electron acceptors. The model furthermore aims at yielding a good description of the sludge production. Chemical oxygen demand (COD) was adopted as the measure of the concentration of organic matter. In the model, the wide variety of organic carbon compounds and nitrogenous compounds are subdivided into a limited number of fractions based on biodegradability and solubility considerations. Figure 9: Substrate flows for autotrophic and heterotrophic biomass in ASM1 and ASM3 models The ASM3 model (Gujer et al. 1999) was also developed for biological N removal WWTPs, with basically the same goals as ASM1. The ASM3 model is intended to become the new standard model, correcting for a number of defects that have appeared during the usage of the ASM1 model (Gujer et al. 1999). The major difference between 27

28 the ASM1 and ASM3 models is that the latter recognises the importance of storage polymers in the heterotrophic activated sludge conversions. In the ASM3 model, it is assumed that all readily biodegradable substrate (SS) is first taken up and stored into an internal cell component (X STO) prior to growth (see Figure 9). The heterotrophic biomass is thus modelled with an internal cell structure, similar to the phosphorus accumulating organisms (PAOs) in the biological phosphorus removal (Bio-P) models. The internal component X STO is subsequently used for biomass growth in the ASM3 model. Biomass growth directly on external substrate as described in ASM1 is not considered in ASM3. A second difference between ASM1 and ASM3 is that the ASM3 model should be easier to calibrate than the ASM1 model. This is mainly achieved by converting the circular growth decay growth model, often called death regeneration concept, into a growth-endogenous respiration model (Figure 9). Whereas in ASM1 effectively all state variables are directly influenced by a change in a parameter value, in ASM3 the direct influence is considerably lower thus ensuring a better parameter identifiability. Koch et al. (2000)concluded that ASM1 and ASM3 are both capable of describing the dynamic behaviour in common municipal WWTPs, whereas ASM3 performs better in situations where the storage of readily biodegradable substrate is significant (industrial wastewater) or for WWTPs with substantial non-aerated zones. The ASM3 model can be extended with a Bio-P removal module (Ky et al., 2001; Rieger et al., 2001). Figure 10: Substrate flows for storage and growth of PAOs in the ASM2 model The overview of models including Bio-P will start with the ASM2 model (Henze et al. 1995), which extends the capabilities of ASM1 to the description of Bio-P. Chemical P removal via precipitation was also included. The ASM2 publication mentions explicitly that this model allows the description of bio-p processes, but does not yet include all observed phenomena. For example, the ASM2d model (Henze et al., 1999) builds on the ASM2 model, adding the denitrifying activity of PAOs which should allow a better description of the dynamics of phosphate and nitrate. Bio-P modelling in ASM2 is illustrated in Figure 10: the PAOs are modelled with cell internal structure, where all 28

29 organic storage products are lumped into one model component (X PHA). PAOs can only grow on cell internal organic storage material; storage is not depending on the electron acceptor conditions, but is only possible when fermentation products such as acetate are available. In practice, it means that storage will usually only be observed in the anaerobic activated sludge tanks. The TUDP model (van Veldhuizen et al. 1999; Brdjanovic et al., 2000) combines the metabolic model for denitrifying and non-denitrifying Bio-P of Murnleitner et al. (1997) with the ASM1 model (autotrophic and heterotrophic reactions). Contrary to ASM2/ASM2d, the TUDP model fully considers the metabolism of PAOs, modelling all organic storage components explicitly (X PHA and X GLY), as shown in Figure 11. The TUDP model was validated in enriched Bio-P sequencing batch reactor (SBR) laboratory systems over a range of sludge retention time (SRT) values (Smolders et al., 1995) for different anaerobic and aerobic phase lengths and for oxygen and nitrate as electron acceptor (Murnleitner et al., 1997). Figure 11: Substrate flows for storage and aerobic growth of PAOs in the TUDP model In some cases, such as high ph (>7.5) and high Ca 2+ concentrations, it can be necessary to add biologically induced P precipitation to the Bio-P model (Maurer et al., 1999; Maurer and Boller, 1999). Indeed, under certain conditions the Bio-P reactions coincide with a natural precipitation that can account for an important P removal effect that is not related to the Bio-P reactions included in the models described thus far. The formation of these precipitates, mostly consisting of calcium phosphates, is promoted by the high P concentration and increased ionic strength during the anaerobic P release of the PAOs Activated sludge models assumptions and limitations Influence of environmental effects Temperature: Kinetic model parameters are temperature dependent, and consequently one has either to estimate the model parameters when calibrating the model for a specific temperature, or to develop appropriate temperature correction factors to include 29

30 the temperature dependency of the reaction kinetics in the simulations. Henze et al. (1987) provide two sets of typical parameters for 10 and 20 C, respectively. Later models, such as ASM2 and the TUDP model, use an Arrhenius type temperature dependence. Different reactions have different temperature dependencies, where nitrification is generally most sensitive. Hellinga et al. (1999) provide a detailed explanation of the influence of temperature on nitrification kinetics. Finally, Henze et al. (1995) warn that the ASM2 temperature coefficients are only valid between 10 and 25 C. ph: In ASM1, it is assumed that the ph is constant and near neutrality. Including alkalinity as one of the state variables in the model allows detection of possible ph problems. For some reactions, specific functions can be added to the model to describe inhibitory ph effects, as illustrated by Helinga et al. (1999) for the nitrification reaction. Toxic components: Nitrification is especially sensitive to inhibition by toxic components. In ASM1, the nitrification parameters are assumed to be constant. This means that any inhibitory effect of the wastewater on the nitrification kinetics is assumed to be included in the calibrated nitrification parameters. It is thus only possible to represent an average inhibitory effect of the wastewater. Alternatively, the nitrification rate equation can be extended to represent sudden acute inhibition by specific chemicals (Nowak et al., 1995). It is then up to the modeller to select the best inhibition kinetics model for the actual inhibition problem. Wastewater composition: The models in Table 4 were developed for simulation of municipal WWTPs. Model modifications are typically needed for WWTP systems where industrial contributions dominate the wastewater characteristics. Acute nitrification inhibition by toxic components related to industrial activity is one of the model modifications that are often necessary. Ky et al. (2001) combined the ASM3 model with the bio-p reactions of the TUDP model. In their modelling study, the simulation of a SBR treating the wastewater of a cheese industry, Mg 2+ Monod switching functions were added to specific Bio-P model reactions to account for Mg 2+ limited kinetics. Coen et al. (1998) proposed a modified ASM1 model extended to three different soluble biodegradable organic substrates to describe a WWTP in the pharmaceutical industry. Biodegradation kinetics Cell growth limitations due to low nutrient concentrations (e.g. N and P) are not considered in ASM1. Later models have included these limitations, e.g. the ASM3 model includes N and alkalinity limitations (Gujer et al., 1999). The Bio-P models usually include P limitations too. Biomass decay in ASM1 is modelled according to the death regeneration concept (Dold et al., 1980). In the ASM3 model this was replaced by the endogenous respiration or maintenance concept (see Table 4). As a result, the conversion reactions of both autotrophs and heterotrophs are clearly separated in ASM3, whereas the decay product regeneration cycles of the autotrophs and heterotrophs are strongly interrelated in ASM1 (see Figure 9). Moreover, the use of the endogenous respiration concept in the ASM3 model should allow easier comparisons between the results of kinetic parameters derived from respirometric 30

31 batch experiments with activated sludge of the plant to be modelled (Vanrolleghem et al. 1999) and the activated sludge model used to describe the phenomena in the full-scale plant. Note that the TUDP model uses the death regeneration concept for the autotrophic and heterotrophic (non-pao) reactions, whereas the maintenance concept is used for the PAOs. Effectively you want to describe maintenance, viruses, decay, protozoa, rotifers, nematodes, etc., in the model, since all these processes lead to a decreased sludge production or oxygen consumption in the absence of external substrate in the full-scale WWTP (van Loosdrecht and Henze, 1999). It has been shown that all these processes can conveniently be lumped in one activated sludge model reaction. The hydrolysis of organic matter and organic nitrogen are coupled and occur simultaneously with equal rates. In the Bio-P models this was extended to include also organic phosphate. ASM1 cannot deal with elevated nitrite concentrations, i.e. nitrification is modelled as a one-step process thereby ignoring the possible appearance of nitrite, a nitrification intermediate, in full-scale WWTPs. Typically, the assumption of onestep nitrification is acceptable. However, when modelling a WWTP where considerable nitrite concentrations occur, or where the temperature is above 20 C, a two-step nitrification model with nitrite as intermediate might be useful. Nitrogen gas, a denitrification product, is not included in the ASM1 model. As a consequence, the model does not allow checking the N balances. Most of the later models included nitrogen gas as a model component (Henze et al., 1995, 1999; Gujer et al., 1999; Brdjanovic et al., 2000). Clearly, the modeller can easily add nitrogen gas to the model as an extra component. The P-balances in the Bio-P models are always closed. In ASM1, the type of electron acceptor present does not affect the biomass decay rate. In contrast, ASM3 allows differentiation between aerobic and anoxic heterotrophic biomass, storage product (X STO) and autotrophic biomass decay rates. According to the experimental result reported in Siegrist et al. (1999), this differentiation between aerobic, anoxic, and, if necessary, anaerobic autotrophic biomass decay rates seems to be justified. In ASM1, the type of electron acceptor does not affect the heterotrophic biomass yield coefficient, whereas the ASM3 model (Gujer et al., 1999) and the model of Barker and Dold (1997) allow inclusion of different aerobic and anoxic heterotrophic biomass yield coefficients in the model. It has been theoretically proven, based on metabolic process energetics, that anoxic yields are consistently lower than aerobic ones (Orhon et al., 1996). Indeed similar differences between aerobic and anoxic yield were obtained experimentally with activated sludge (McClintock et al., 1988; Sperandio et al., 1999). A metabolic model takes this explicitly into account because a different energetic efficiency for the different electron acceptors is included. In the ASM1 model, hydrolysis reaction rates depend on the electron acceptor present (aerobic or anoxic conditions). In the ASM3 model, hydrolysis is independent of the available electron acceptor (Gujer et al., 1999). ASM2 31

32 acknowledges that hydrolysis reaction rates may depend on the available electron acceptor, also under anaerobic conditions (Henze et al., 1995). The Bio-P models cannot handle two extreme situations (van Veldhuizen et al., 1999): (1) full depletion of the organic storage product pool X PHA in the PAOs; (2) simultaneous presence of volatile fatty acids (= substrate for storage reactions) and electron acceptors. Model extensions are needed to handle these two situations. Storage of substrate by non-paos is not accounted for in ASM2/ASM2d and TUDP. The models are not able to describe filamentous biomass growth and sludge bulking. 2.3 Simulation environments A WWTP simulation environment can be described as software that allows the modeller to simulate a WWTP configuration. General-purpose simulation environments can be distinguished from specific WWTP simulators. General-purpose simulation environments normally have a high flexibility, but the modeller has to supply the models that are to be used to model a specific WWTP configuration. The latter task can be very time consuming. However, it is better to spend time on the model implementation and debugging, to avoid running lots of simulations with a model that afterwards turns out to be erroneous for the specific application task. As a consequence, general-purpose simulation environments require a skilled user that fully understands the implications of each line of code in the models. A popular example of a generalpurpose simulator environment is MATLAB/Simulink ( Specific WWTP simulation environments usually contain an extended library of predefined process unit models, for example a perfectly mixed ASM1 or ASM2d bioreactor, and a one-dimensional 10-layer settler model. The process configuration to be simulated can easily be constructed by connecting process unit blocks. Pop-up windows allow modifying the model parameters. Examples of specific commercial WWTP simulator environments are (in alphabetic order): AQUASIM ( BioWin ( EFOR ( GPS-X ( SIMBA ( STOAT ( and WEST ( 2.4 Model applications A model may be applied in the following roles (Russel et al., 2002): (1) a service role, where the model, when solved, provides the needed numerical values for further analysis; (2) an advice role, where the model provides insights that help to understand and solve related sub-problems contributing to the solution of an overall problem; (3) an analysis role, where simulations with the model indicate how to use models to solve a specific task. The purpose for WWTP model studies can be (Hulsbeek et al., 2002; 32

33 Petersen et al., 2002): (1) learning, i.e. use of simulations to increase process understanding, and to develop people s conception of the system; (2) design, i.e. evaluate several design alternatives for new WWTP installations via simulation; (3) process optimisation and control, i.e. evaluate several scenarios that might lead to improved operation of existing WWTPs. The latter two are applications of the model in a service role. An application of the model in an analysis role can for example be a study where the suitability to describe a particular process is evaluated for several modelling concepts enclosed in different activated sludge models WWTP model simulation for learning Simulations with WWTP models can be applied in different ways to increase the process understanding of the user. For the WWTP operator, simulations might for example be useful to indicate the consequences of process operation modifications on the activated sludge composition and the WWTP effluent quality. Similarly, simulations with e.g. the ASM1 benchmark plant (Coop, 2002) for different weather disturbance scenarios are very informative to get an idea of the behaviour of a WWTP under variable weather conditions. From a research perspective, Brdjanovic et al. (2000) used the TUDP model to increase the understanding of a full-scale Bio-P process. Siegrist et al. (1999) noticed in the experimental work that the decay rate of autotrophic bacteria is lower under anaerobic and anoxic conditions, compared to aerobic conditions. Simulations with a WWTP model incorporating this hypothesis showed that avoiding excess aeration in the activated sludge tanks, for example via intermittent aeration, not only saves aeration energy but also improves the nitrification capacity of the plant WWTP model simulation for design During the design phase, process alternatives can be evaluated via simulation. Such a model study was presented e.g. by Salem et al. (2002), where different alternatives for the upgrade of a biological N removal plant were evaluated with focus on appropriate treatment of sludge reject water. The WWTP model simulations provided the knowledge basis that was needed to decide on full-scale implementation of one of the proposed alternatives. In this context, modelling can substantially reduce the scale-up time, because different options can be evaluated before a pilot plant is built. The model thus contributes significantly in bridging the gap between lab and full-scale application (Hellinga et al, 1999). A WWTP model thus transforms data obtained from lab scale experiments into quantitative knowledge, which helps in decision-making processes. Yuan et al. (1998) evaluated a sludge storage concept via ASM1 simulations, based on the reduced decay of autotrophic bacteria under anaerobic conditions. The concept provides spare nitrification capacity for nitrogen shock load situations by storing the waste activated sludge temporarily in an anaerobic tank with a retention time of a few days, whereas the SRT in the activated sludge plant is reduced considerably. The concept thus results in a WWTP with less sludge but a similar nitrification capacity compared to traditional reactor design, and was successfully evaluated in pilot plant 33

34 studies (Yuan et al., 2000). Savings on reactor volume were evaluated to be around 20%, but increased sludge production could be a problem with respect to operational costs WWTP model simulation for process optimization Process optimization can be used in different contexts. Off-line process optimisation refers to applications where off-line simulations with the calibrated model are used to determine how to optimally run the process, whereas the result is later implemented and tested on the full-scale plant. In on-line process optimization, simulations with the calibrated model are applied in an on-line optimization scheme. Off-line process optimization is often needed because new stricter demands are imposed to existing WWTPs, or considerable changes in the plant load have occurred, or deficiencies have appeared during WWTP operation such that the initially required effluent quality cannot any longer be obtained. In this context, simulations are often used to evaluate whether the pollutant removal efficiencies can be improved within the existing plant lay-out, e.g. via improved process control. The ASM1- based benchmark WWTP (Copp, 2002) was specifically developed for simulation-based objective evaluation of different control strategies on a N removal WWTP, and includes several criteria to evaluate the WWTP performance. Scenario evaluations with ASM1/ASM3 usually aim at upgrading a WWTP for biological N removal (Coen et al., 1996), evaluating the possibilities for improved biological N removal within an existing WWTP configuration (Ladiges et al., 1999), or predicting the effect of a change in load on the WWTP performance. During scenario evaluations with Bio-P models, evaluation of different process alternatives often results in a trade-off between Bio-P capacity and nitrification, where increased DO concentrations will promote nitrification but negatively influence the Bio-P process due to increased aerobic decay of PAO storage products (Cinar et al., 1998). Gernaey et al. (2002) illustrate the implementation of chemical P precipitation on an existing N removal WWTP. 34

35 CHAPTER 3 - APPLICATION OF THE BIOWIN MODELLING TOOL 3.1 About BioWin 5.0 Biowin is a computer simulation package that is able to dynamically simulate the wastewater treatment process. It combines various published models, such as the activated sludge models, anaerobic digestion model, and solids settling model, into a single simulation platform (EnviroSim Associates Ltd). It is a flexible platform that allows investigations into changes in configurations or operations without having to alter conditions in the actual treatment plant. The user can define and analyse behaviour of complex treatment plant configurations with single or multiple wastewater inputs. An example of a plant configuration window is shown (where in the background is the graphical representation of WWTP Porec South): Figure 12: Example of a plant configuration in BioWin (WWTP Porec South) Most types of wastewater treatment systems can be configured in BioWin using the many process modules. These include: A range of activated sludge bioreactor modules suspended growth reactors (diffused air or surface aeration), various SBRs, media reactors for IFAS and MBBR systems, variable volume reactors; Anaerobic and aerobic digesters; Various settling tank modules primary, ideal and 1-D model settlers; 35

36 Different input elements wastewater influent (COD- or BOD-based), userdefined (state variable concentrations), metal addition for chemical phosphorus precipitation (ferric or alum), methanol for denitrification; Other process modules holding tanks, equalization tanks, dewatering units, flow splitters and combiners. A crucial component of BioWin is the biological process model. The BioWin model is unique in that it merges both activated sludge and anaerobic biological processes. Additionally, the model integrates ph and chemical phosphorus precipitation processes. The BioWin simulator suite presently includes two modules: A steady state module for analysing systems based on constant influent loading and/or flow weighted averages of time-varying inputs. This unit is also very useful for mass balancing over complex plant configurations. An interactive dynamic simulator where the user can operate and manipulate the treatment system "on the fly". This module is ideal for training and for analysing system response when subjected to time-varying inputs or changes in operating strategy. BioWin is a very powerful analysis tool. The program has been evaluated against an extensive data set and has been demonstrated to provide accurate simulation results for a range of systems. Nevertheless, it is still merely a tool. BioWin incorporates a number of models. These necessarily are a simplification of reality and have limited ranges of applicability. It is the responsibility of the user to carefully assess results generated by the program. 3.2 Implemented Biological/Chemical models BioWin uses a general Activated Sludge/Anaerobic Digestion (ASDM) model which is referred to as the BioWin ASDM. The BioWin ASDM has more than fifty state variables and over eighty process expressions. These expressions are used to describe the biological processes typically occurring in wastewater treatment plants. This complete model approach frees the user from having to map one model s output to another model s input which significantly reduces the complexity of building full plant models, particularly those incorporating many different process units. In the following chapter an overview of the parameters included in the model (kinetic, stoichiometric and chemical constants) with a brief description of the model process. In the performed simulations, only the default BioWin ASDM was used. 36

37 3.2.1 Activated sludge processes The process employed in the simulation were: a. Growth and Decay of Ordinary Heterotrophic Organisms; b. Hydrolysis, Adsorption, Ammonification and Assimilative denitrification; c. Growth and Decay of Ammonia Oxidizing Biomass; d. Growth and Decay of Nitrite Oxidizing Biomass; e. Growth and Decay of Phosphorus Accumulating Organisms. A brief description of each is provided in the following text. (a) Growth and Decay of Ordinary Heterotrophic Organisms (OHO) Number of sub-processes: 24 Engineering objective: BOD removal, denitrification Implementation: Always active in the BioWin model Module description: This group of processes describes the growth and decay of ordinary heterotrophic organisms under all conditions. The activated sludge model allows for direct ordinary heterotrophic aerobic growth on acetate, propionate, readily biodegradable complex substrate and methanol. The base rate expression for each of the growth processes is the product of a maximum specific growth rate, the heterotrophic biomass concentration and a Monod expression for the substrates. This base rate is modified to account for environmental conditions (dissolved oxygen, nitrate and nitrite), nutrient limitations (ammonia, phosphate, other cations and anions) and ph inhibition. BioWin uses ammonia as a nitrogen source for cell synthesis with all of the substrates under aerobic, anoxic and anaerobic conditions. At low ammonia concentrations BioWin allows for assimilative ammonia production from either nitrate or nitrite in order to satisfy synthesis demands. Although the maximum specific growth rate under aerobic and anoxic conditions is the same, under anoxic conditions the base rate is also multiplied by an anoxic growth factor. This allows for anoxic growth at a different rate or for only a fraction of the OHOs being able to perform any kind of denitrification (or both of these). Of the OHOs that can perform denitrification, a fraction can use either nitrate or nitrite (with nitrogen gas as an end product), and the remainder of the denitrifying OHOs can only use nitrate (with nitrite as an end product). There are two pathways for ordinary heterotrophic growth through fermentation of readily biodegradable (complex) substrate to acetate, propionate, carbon dioxide and hydrogen. The dominant pathway is governed by the dissolved hydrogen concentration. In activated sludge vessels there is also an anaerobic growth factor applied to all growth through fermentation. 37

38 There are decay processes appropriate for each environment (aerobic, anoxic and anaerobic). (b) Hydrolysis, Adsorption, Ammonification and Assimilative denitrification Number of sub-processes: 10 Engineering objective: Conversion of organic, nitrogen and phosphorous fractions Implementation: Always active in the BioWin model Module description: These processes are discussed here separately for each different organism groupings because they involve more than one organism type (in general both the ordinary heterotrophic organisms and the phosphate accumulating organisms). Hydrolysis of biodegradable particulate organic substrate to readily biodegradable complex substrate: The base rate is the product of the hydrolysis rate constant, the sum of the ordinary heterotrophs and the phosphate accumulating organisms, and a Monod expression for the ratio of particulate substrate to organism COD. There is an efficiency factor applied for anoxic conditions and another for anaerobic conditions. Hydrolysis of biodegradable particulate organic nitrogen and phosphorus: The hydrolysis of biodegradable particulate nitrogen (phosphorus) is assumed to proceed at the same rate as the biodegradable particulate organics but is adjusted by the ratio of biodegradable particulate organic nitrogen (phosphorus) to biodegradable particulate organic. Adsorption or flocculation of colloidal organic material to particulate organic material (occurring spontaneously as opposed to chemically facilitated flocculation with metal (ferric or alum) addition: The rate is the product of the adsorption rate constant, the colloidal substrate concentration and the sum of the ordinary heterotrophs and the phosphate accumulating organism concentrations. The rate is decreased as the ratio of particulate substrate to organism COD approaches the maximum adsorption ratio constant. Ammonification of soluble organic nitrogen to ammonia: The rate is the product of the ammonification rate constant, the soluble organic nitrogen concentration and the sum of the ordinary heterotrophic and the phosphate accumulating organisms concentrations. Assimilative denitrification of nitrate or nitrite to ammonia for synthesis: BioWin allows for the production of ammonia for synthesis by OHOs, PAOs and methylotrophs under low ammonia conditions (as ammonia becomes limiting for growth). The assimilative process will use nitrite if it is available otherwise it will use nitrate. The base rate is the product of the assimilation rate constant and the organism COD. This base rate is modified to account for environmental conditions (off with ammonia, and selecting between nitrate and nitrite). Slow decay of endogenous products to particulate substrate: BioWin allows for the conversion of endogenous decay products to particulate substrate. The rate is the product of the specified rate constant and the endogenous products concentration. 38

39 (c) Growth and Decay of Ammonia Oxidizing Biomass (AOB) Number of sub-processes: 4 Engineering objective: Nitrification Implementation: Always active in the BioWin model Module description: This biomass grows by oxidizing ammonia to nitrite and using the energy to synthesize organic material from inorganic carbon (fixing CO2). Nitrogen source for cell synthesis is ammonia. The base rate expression for the growth process is the product of the maximum specific growth rate, the ammonia oxidizing biomass concentration and a Monod expression for ammonia. This base rate is modified to account for environmental conditions (off at low dissolved oxygen), nutrient limitations (phosphate, inorganic carbon, other cations and anions) and ph inhibition. The decay rate varies between an aerobic value and an anoxic/anaerobic value depending on the dissolved oxygen concentration. (d) Growth and Decay of Nitrite Oxidizing Biomass (NOB) Number of sub-processes: 2 Engineering objective: Conversion of organic, nitrogen and phosphorous fractions Implementation: Always active in the BioWin model Module description: This biomass grows by oxidizing nitrite to nitrate and using the energy to synthesize organic material from inorganic carbon (fixing CO2). Nitrogen source for cell synthesis is ammonia. The base rate expression for the growth process is the product of the maximum specific growth rate, the nitrite oxidizing biomass concentration and a Monod expression for nitrite. This base rate is modified to account for environmental conditions (off at low dissolved oxygen and inhibited by ammonia), nutrient limitations (ammonia, phosphate, inorganic carbon, other cations and anions) and ph inhibition. The decay rate varies between an aerobic value and an anoxic/anaerobic value depending on the dissolved oxygen concentration. (e) Growth and Decay of Phosphorus Accumulating Organisms (PAOs) Number of sub-processes: 17 Engineering objective: Biological phosphorous removal Implementation: Always active in the BioWin model Module description: 39

40 This group of processes describes the growth and decay of polyphosphate accumulating organisms (PAOs) under all conditions. This includes descriptions of aerobic and anoxic growth, volatile fatty acid (VFA) sequestration and polyphosphate lysis. There are two maximum specific growth rates for PAOs under aerobic conditions. The lower growth rate constant is used under P limited conditions and has a different stoichiometry (no polyphosphate storage). There are also two anoxic growth processes, one uses nitrate and the other nitrite. Growth processes under phosphate rich conditions result in uptake of phosphate, as well as balancing calcium ions magnesium ions and other cations. A lack of these ions will stop the growth processes by appropriate Monod switches. For all of these growth processes, the base growth rate is the product of the maximum specific rate constant, the PAO concentration and a Monod switch on the ratio PHA to PAO. This base rate is modified to account for environmental conditions (dissolved oxygen, nitrate and nitrite), nutrient limitations (ammonia, anions, cations, for polyphosphate storage magnesium, and calcium are also required) and ph inhibition. BioWin uses ammonia as a nitrogen source for cell synthesis under aerobic, anoxic and anaerobic conditions. At low ammonia concentrations BioWin allows for assimilative ammonia production from either nitrate or nitrite in order to satisfy synthesis demands. Although the maximum specific growth rate under aerobic and anoxic conditions is the same, under anoxic conditions the base rate is also multiplied by an anoxic growth factor. This allows for anoxic growth at a different rate or for only a fraction of the PAOs being able to perform any kind of denitrification (or both of these). Of the PAOs that can perform denitrification, a fraction can use either nitrate or nitrite (with nitrogen gas as an end product), and the remainder of the denitrifying PAOs can only use nitrate (with nitrite as an end product). The PAOs use polyphosphate as an energy source to sequester VFAs under anaerobic conditions. The sequestered VFAs are stored internally as polyhydroxy alkanoates (PHA). In the BioWin model the PAOs can use both acetate and propionate for this process. The base sequestration rate is the product of the sequestration rate constant, the PAO concentration and a Monod switch on the appropriate substrate (acetate or propionate). The rate is also dependent on the availability of the stored polyphosphate (poly-p). There are two decay processes (aerobic/anoxic and anaerobic). Associated with each decay process is a lysis process for PHA, low and high molecular weight polyphosphate. The lysis rates are directly proportional to the decay rate itself. There is a polyphosphate cleavage process for anaerobic maintenance that releases phosphate if no oxygen is present (default off). There is also an aerobic/anoxic maintenance process that releases organism COD as well as synthesis nitrogen and phosphorus but no polyphosphate or PHA (default off). 40

41 ph Other important physical phenomena implemented It has been recognized from the early stages of wastewater process modelling that ph is an important factor in simulating the performance of biological wastewater treatment processes. The ph impacts the species distribution of the weak acid systems (carbonate, ammonia, phosphate, acetate, propionate, etc.) present in the process. This in turn dictates the rate of many of the biological and physico-chemical phenomena occurring in these systems. For example, biological activity, that can be severely limited outside an optimal ph range. It is difficult to model ph because the underlying components and reactions are so fast and complex. BioWin uses a mixed kinetic/equilibrium based approach to minimize the negative impact on simulations speed. This approach is applicable across a wide range of biological treatment process models (i.e. activated sludge and anaerobic digestion, etc.). Alkalinity The model determines alkalinity by noting that at the H2CO3 equivalence point [H+] = [HCO3- ]. This additional equation can then be used to solve the carbonate equilibrium explicitly to determine the [HCO3- ] concentration at the equivalence point (and consequently the ph). Gas Transfer and Aeration Models There are seven gas-liquid mass transfer processes implemented in BioWin to allow interphase transfer of oxygen, carbon-dioxide, methane, nitrogen, ammonia, hydrogen and nitrous oxide. The main parameters related to mass transfer are the Liquid phase mass transfer and the Henry s law constants coefficient for the above mentioned compounds. Other important parameters are the ones for aeration (for example, % in the off-gas of the compounds) and for diffuser system. Supply of oxygen constitutes a major operating cost for biological wastewater treatment systems. Emphasis on energy conservation has highlighted the need to develop effective methods for design and operation of aeration systems. Oxygen demand in activated sludge reactors varies with time, necessitating a varying oxygen supply rate to maintain the desired dissolved oxygen (DO) concentration. In diffused air systems bubbles are distributed from diffusers at the base of the reactor. Mass transfer occurs between the rising bubbles and the mixed liquor. The transfer of oxygen from the gas to the liquid is required to supply the oxygen requirements for the biological process. A number of equipment and operational parameters interact to influence the efficiency and rate of transfer of oxygen; inter alia, diffuser pore size and density, and air flow rate. These parameters determine factors such as bubble size, the rate of bubble rise, the bubble residence time in the reactor, the fractional gas hold-up, the interfacial surface area available for mass transfer, the change in oxygen partial pressure in the rising bubbles and the degree of turbulence. Conditions in the mixed liquor also impact on the transfer; for example, temperature, ionic strength, presence of 41

42 surface-active compounds, and solids concentration. Quantifying the impact of all these factors on the overall mass transfer behaviour is very difficult. 3.3 Model simulation for WWTP Porec South Project design parameters Design flows and loads As it was already mentioned, the modelling simulation has been applied to WWTP Porec South for summer and winter conditions. The plant will receive wastewater originating from different locations. Both municipal and industrial wastewaters (suitable for biological treatment) will be delivered to the plant by various pumping stations and networks. The sewage collection network is a separate system except from a small part of the old city of Porec. Certain number of connections of roofs drains and the drainage of storm water from fixed surfaces are existing and this results in some stormwater inflow into the drainage system. The flows and pollution loads on WWTP Porec South have been estimated for the years 2011 and 2045 taking into consideration the existing resident population, non-household consumption and the tourist overnights. Hydraulic load Design flow values of influent wastewater are summarized in Table 5. The values refer to typical conditions considering both summer and winter seasons. Parameter M.U. WWTP Porec South Winter Summer Average daily flow m³/day Maximum dry weather flow Peak factor 16/24 m³/h Infiltration m³/day Infiltration Peak factor 4/24 m³/h Peak daily flow m³/day Peak hourly flow m³/h Table 5: Typical flow values of influent wastewater for WWTP Porec South 42

43 Pollution load Design daily loads values are derived from ATV-DVWK-A 131E (pg. 19, ATV- DVWK-A 131E manual), and are shown in Table 6. The considered values for the influent wastewater are summarized in Table 7. Parameter Raw wastewater BOD 5 60 COD 120 SS 70 TKN 11 P 1,8 Table 6: Inhabitant-specific loads in g/(i d) Parameter M.U. WWTP Porec South Winter Summer Capacity PE BOD 1 5 kg/day COD kg/day TSS kg/day VSS/TSS % N-NTK kg/day 94,6 528,0 P t kg/day Table 7: Design daily loads of influent wastewater for WWTP Porec South The results obtained in the following simulation are strictly dependent on the characteristics of the influent, for example the fraction of volatile suspended solids over the total suspended solids in the influent that affects the amount of volatile matter in the wasted sludge. Wastewater design temperature Design wastewater temperature has been considered equal to 12 C for winter conditions and 20 C for summer ones. 1 The Inlet BOD5 is an important parameter for ATV that will be used later in the calculations at pg

44 Effluent quality requirement and guarantees Given the sensitivity of the recipient, effluent limits are aligned with the requirements for discharge from the treatment of urban wastewater into sensitive areas which are subject to eutrophication, as defined in Annex IIA of the Directive of the European Council 91/271/EEC concerning urban wastewater purification and the amendment 98/15/EEC for WWTP of a capacity of PE. The effluent requirements set out by the Republic of Croatia are defined by the ordinance on the limitation of emissions of wastewater (Official Gazette 80/13). The limit values taken into account for the discharge of water from the plant into the Adriatic sea are shown in Table 8. Parameter Limit value Suspended Solids < 5 mg/l BOD 5 (20 C) < 10 mgo 2/l COD Cr < 125 mgo 2/l Total Phosphorus Total Nitrogen * (organic N + NH 4-N + NO 2-N + NO 3-N) Turbidity Coliforms Coliforms of faecal origin Streptococcus of faecal origin Escherichia Coli Intestinal enterococci < 2 mgp/l < 15 mgn/l < 1 NTU < CFU/100 ml < 500 CFU/100 ml < 200 CFU/100 ml < 10 CFU/100 ml < 200 CFU/100 ml ph 6-9 * Limit value for the total nitrogen is applied when the wastewater temperature at the effluent of the aeration tank is equal or greater than 12 C. Table 8: Limit values considered for water discharge from the WWTP Porec South Plant configuration In the modelling of WWTP Porec South, the whole mechanical pretreatment section of the plant (fine screen, aerated grit-grease removal and microscreen) is represented as a single unit, whose output is grit and screened material. After the pretreatment section, the plant is designed with four identical biological treatment lines in order to guarantee a high flexibility in the operation according to the variable influent flowrate and load (seasonal variations). Each biological line is designed in three different zones (environments) where different processes happen: an anaerobic zone, and anoxic zone, an aerated zone (with a final 44

45 volume that can be switched on and off to guarantee a complete nitrification/denitrification, according to process needs). To reproduce the real behaviour of the plant, the aerated zone has been represented with two different reactors: one aerated and one switch zone (aerobic/anoxic) that can be dynamically controlled in order to mimic optimized SUEZ s aeration system (Greenbass 2 ). Figure 13: Simplified scheme of WWTP Porec South biological section Three recirculation flows have been implemented, according to the WWTP design: from anoxic volume to anaerobic volume, from aerobic volume to anoxic volume and from membrane trains to aerobic volume. After the biological section, there is the membrane compartment for solid-liquid separation. This section is foreseen with three treatment lines. However, when modelling the WWTP, just one reactor is used to represent the behaviour of the process because the simulation does not cover the membrane train operational modes. As shown below, the MLSS form dewatering is also taken into account. Figure 12 shows the BioWin simulator applied to WWTP Porec South. Inlet Sidestream Mixer20 Anaerobic_1 Anox_1 General Mixer22 Ox_1.1 Ox_1.2 General Mixer28 Membrane Tank Effluent Anaerobic_2 Anox_2 Ox_2.1 Ox_2.2 grit and screened material Anaerobic_3 Anox_3 Ox_3.1 Ox_3.2 Anaerobic_4 Anox_4 Ox_4.1 Ox_4.2 Sludge Figure 14: BioWin main simulator window for WWTP Porec South (summer period)

46 To simulate the plant in the winter period, due to the lower load and flowrate, only two of the four lines are considered in operation (see Figure 13 below). in Sidestream Mixer20 Anaerobic_1 Anox_1 General Mixer22 Ox_1.1 Ox_1.2 General Mixer28 Membrane Tank out Anaerobic_2 Anox_2 Ox_2.1 Ox_2.2 grit and screened material Sludge Figure 15: BioWin main simulator window for WWTP Porec South (winter period) 3.4 Performed simulations and results Two simulations are carried out: Dynamic simulation with variable inflow, summer conditions; Dynamic simulation with variable inflow, winter conditions. The objective of a dynamic simulation with variable inflow is to mimic the behaviour of the system in real conditions, thus meaning that the inputs of the system are timevarying, as in real operation. This type of simulation really allows to check and verify the design of the WWTP and its performance. The dynamic simulation is carried out with a variable inflow, daily trend of the flow is presented in Figure 16. It is used the same distribution of the flow for summer and winter conditions, supposing that the pattern of water consumption is constant during the year. The average and the peak values are the ones required by the project while assumed distribution is based on SUEZ s experience and know-how 3. Two peaks of three and four hours are considered

47 1,8 1,6 1,4 1,2 1 0,8 0,6 0,4 0, Figure 16: Pattern of flow distribution employed for winter and summer simulations, only the applied coefficients that were not modified in the three simulations - are reported (and not the flows) Input data Starting from the influent values as estimated by the project, a list of chemical parameters has been defined as input of the BioWin model. The characterization of the COD, Nitrogen, suspended solid and phosphorous components, that is also necessary as in input for BioWin model, is detailed according to the typical fractionation of municipal wastewaters. Parameter M.U. WWTP Porec South Winter Summer Capacity PE BOD 5 kg/day COD kg/day TSS kg/day VSS/TSS % N-NTK kg/day 94,6 528,0 P t kg/day Table 9: Influent characterization - Input values for BioWin simulation for summer and winter period 47

48 Other input data that are necessary to set the model are the volume of the tanks as detailed below in Table 10. Please note that the considered tank volumes in BioWin simulations are conservative because they are calculated with the minimum water height. Biological treatment step Volume per tank Total volume Anaerobic 200 m m 3 Anoxic 175 m m 3 Aerobic 360 m m 3 Aerated membrane 75 m m 3 Table 10: Summary of the biological tank volumes for WWTP Porec South The results obtained by the simulation of the plant of both summer and winter period are shown in the following text Dynamic simulation, summer (variable inflow) For the summer period, the equilibrium sludge retention time is 6.48 days. The obtained value is compliant with ATV-DVWK-A 131E, and the effluent quality respects the project requirements. It should be noted that all the values provided in the following paragraphs are expected values, while guaranteed values are higher and equal to the one presented in Table 8. As to say, COD concentration < 125 mgo2/l and TN concentration < 15 mgn/l. Expected TN outlet value, presented in Figure 17 is less than 15 mgn/l according to the limit and a complete nitrification is achieved (N-NH4 and N-NO2 below 1 mgn/l); the Figure 17 also shows the guaranteed TN value (orange line). The annual average of the samples for each parameter shall conform to the relevant parametric values, according to the Directive 91/271/EEC. Aeration system is controlled and optimized in order to achieve complete nitrification. Controls implemented in BioWin mimic the behaviour of SUEZ s patented Greenbass that adjusts the air supply to completely oxidize inflow ammonium to the aerated tank. The equilibrium concentrations of pollutants in the effluent that are shown in Figure 17 (and the following for winter simulations) are a consequence of the implemented controller that allows for keeping outlet concentrations constant. 48

49 Figure 17: Effluent Nitrogen fractions in summer The concentration in the membrane tanks is around 10 gtss/l with a volatile fraction close to 60%. The production of excess sludge is estimated to be around 2440 kgtss/d 4 from the sludge line to the centrifuge that were assumed with a solid retention efficiency of the 95% 5, corresponding to a production of sludge to disposal of about 2320 kgtss/d (see Figure 19). Please note that sludge dewaterability depends on the ratio of VSS over TSS headed to the centrifuge. 4 The results from the dynamic simulation are, by definition, not constant in time. Therefore a slight variation from values is a part of the intrinsic nature of the simulation. 5 The considered efficiency is conservative because this equipment, already installed in many WWTPs can achieve higher performances 49