Municipal Wastewater Treatment Improvement Using Computer Simulating

Transcription

1 Municipal Wastewater Treatment Improvement Using Computer Simulating STEFANIA IORDACHE, PETRESCU NICOLAE, CEZARINA NECULA, GABRIELA BUSUIOC Faculty of Environmental Engineering and Biotechnology Valahia University of Targoviste Address: Bd. Carol I, Nr. 2, , Targoviste, Dambovita, ROMANIA Abstract - Modern wastewater treatment is a complex process, which involves several treatment steps in order to obtain quality indicators with a high performance, established by environmental regulations for treated effluent. Optimal wastewater treatment plant management is concerned with trying to obtain good wastewater treatment efficiency in condition of maintaining process stability. In order to meet increasing environmental regulation, the paper presents a new wastewater treatment schema for municipal wastewater treatment plant of Targoviste city, from Romania, which must adapt the actual treatment process and must modernizing it. The aim of this work was to evaluate the possibility to improve nutrients removal efficiency according with the new, stricter discharge limits, imposed by the European Union Directives. The simulation was realized using BioWIN 3.0, a simulator software package in which the user can define and analyze behavior of complex treatment plant configurations with single or multiple wastewater inputs. The simulation allowed to obtain some data regarding parameters of treatment process in order to increase the efficiency in operation. Keywords - wastewater treatment, nutrient removal, optimization, computer simulation, BioWin. 1 Introduction Many of municipal wastewater treatment plants in Romania were built in a period of very difficult socioeconomic conditions. After few years of exploitation, it can be clearly seen that pollution load is much higher than expected. In the same time, in the last years, the amount of municipal sewage to be processed in many wastewater treatment plants is at the level of 50-60% of their hydraulic capacity. On the other hand, unitary load of contamination that can be found in wastewater is so high that many objects are beyond their processing capacity. Increase of nitrogen compounds in raw sewage was shown to occur in many plants. Optimising nitrogen and phosphorus removal in existing wastewater treatment plants (WWTP) is important in order to meet increasing environmental regulations[1]. The European Union Directive 91/271 raised the need to adapt most existing WWTP to new, stricter discharge limits. The directive s requirements must be obeyed by every EU centre with an equivalent population of at least 2000 discharging into continental waters and, in general, by all centres with an equivalent population of or over. European regulations have also established a calendar and specific purification levels as a function of the sensitivity of the receiving environment. Thus, organic matter and excess nutrients (particularly nitrogen and phosphorus) in wastewater must be removed prior to discharging into especially sensitive media. These new requirements can be reached in various ways that involve changes in the operating procedures (optimising current processes or developing new treatments), alterations of existing treatment works (extensions, purchases of new equipment) or the use of control systems to optimize processes. In the frame of adapting the European Union environmental regulation, the nutrient pollution, especially from nitrogen and phosphorus is a main upto-date issue for all municipalities from Romania. In Romania water management activity is realized as a unified, rational and integrated approach. It is organized and conducted on river basins as geographical entities indivisible management quantity and quality of water resources, for sustainable development. Dambovita territory is included in two river basins: Buzau - Ialomita and Arges Vedea. In general, ISSN: ISBN:

2 Dambovita county may be mentioned two categories of impact on quality of waterways: produced by municipal wastewater that are insufficiently treated, discharged in the river courses (e.g.ialomita river - receiving waste water from settlements Fieni, Pucioasa, Targoviste,) impacts on water bodies that cross the area of activity of oil field. In this paper will be analized only the first category of impact presented above. requirements for direct discharge in surface water[5]. Table 1. Parameter Units Maximum value ph - 6,5 8,5 BOD5 mg/l 25 COD mg/l 125 TSS mg/l 35 Ammonium NH4 + mg/l 2 Total N mg/l 10 Total Phosphorus mg/l 1 2 Problem formulation Targoviste city has an important impact on surface water quality of Dambovita county because all municipal wastewater are discharged in Ialomita river by 2 points: Targoviste South and Targoviste-North. In this paper there are presented some simulation experiments that were made to find the best solution for treatment process for the wastewater treatment plant of Targoviste city, which is a centre with an equivalent population of or over. Currently, Targoviste city has a centralized water supply network and sewerage. The existing sewerage system accomplishes collection, transport, cleaning and unloading of water in emissary, Ialomita river. Municipality of Targoviste city has buit 2 wastewater treatment plants: Targoviste-South and Targoviste- North. In this paper it were analized only the treatment processes of wastewater for Targoviste-South plant According with European regulation, treated wastewater effluents can be discharged to a surface water, if the values of specific indicators are less than that are presented in table 1, which indicates effluents Description of the Targoviste- South WWTP The considered WWTP use in the present a single technological line, whose treatment schema is a very usual one and is presented in figure 1[3]. Like any other typical wastewater treatment plant, it includes a primary treatment and a secondary treatment to remove organic matter and suspended solids from wastewater. Primary treatment is designed to physically remove solid material from the incoming wastewater. Coarse particles are removed by screens or reduced in size by grinding devices. Inorganic solids are removed in grit catchers and many of the organic suspended solids are removed by sedimentation. The primary treatment removes almost one-half of the suspended solids in the raw wastewater. Secondary treatment consists of a biological conversion of dissolved and colloidal organic compounds into stabilized, low-energy compounds and new biomass cells, caused by a diversified group of microorganisms, in the presence of oxygen. This mixture of microorganisms, together with inorganic as well as 6 Effluent Return Activated Sludge 7 9 Waste Activated Sludge 8 Valorization Figure 1 - Municipal Wastewater Treatment Plant 1-Bar Screen; 2-Grit chamber; 3-Primary Settler; 4-Anaerobic Reactor; 5-Aerobic Basin; 6-Secondarily settler; 7-Clorination; 8-Thickening Clarifier; 9-Dewatering Unit. ISSN: ISBN:

3 organic particles contained in the suspended solids constitutes the activated sludge. This mixture is kept moving in wastewater by stirring done by aerators, turbines or rotators, which simultaneously supply the required oxygen for the biological reactions. A biological reactor followed by a secondary settler or clarifier constitutes the activated sludge process, which is the most well known process of secondary treatment because it is also the most widely used. Date that were monitored by Environmental Protection Agency in the last 3 years shows important overtaking of emission limitations Table 2 presents the values monitored for COD, Ammonium and Psosphorus. Year Table 2 Indicator Values Maximum Average admissible Measured [mg/l] [mg/l] COD Ammonium Phosphorus COD Ammonium Phosphorus COD Ammonium Phosphorus With the aim to evaluate a new treatment scheme for WWTP of Targoviste-South, there were analized characteristics of the wastewater obtained in The values of data collected in Targoviste-Soth WWTP in 2009, presented in table 3, represents the average values of the most important monitored data. Table 3 Parameter Unit WWTP WWTP Effluent ph TSS [mg/l] TotalBOD [mg/l] Total COD [mg/l] Ammonia [mg/l] Total N [mg/l] Total Kjeldhal [mg/l] Nitrate [mg/l] Total P [mg/l] In the context of international rigors it can be observed that the sewage treatment process used is not sufficiently developed. It is necessary the modernizing of treatment scheme in order to increase the efficiency of purification process. 3 Problem solution 3.1. Model configuration The simulations were realized using BioWin3 software from Environsim. BioWin is a Microsoft Windows-based simulator used world-wide in the analysis and design of wastewater treatment systems. The model was built using a treatment schema for R0 Anaerobic Anoxic Aerobic1 Aerobic2 Aerobic3 Aerobic4 Sec_Anox Aerobic Effluent Anaerobic Digester Effluent 2 WAS Sludge Figure 2 ISSN: ISBN:

4 biological phosphorus removal with simultaneous nitrification-denitrification in activated sludge systems.the BioWin model of the WWTP investigated in this study is shown on Figure 2. As usually, primary treatment is composed of screens and grit chamber followed by primary clarifier and primary sludge digester and thickener. Secondary treatment is designed to enhanced biological nutrient removal and it is comprising: anaerobic reactor, V Anaerob = 2400 m 3, anoxic reactor, V Anoxic = 3000 m 3, four aerobic reactors in series, having a total volume V Aerob1-4 = 7200m 3, secondary anoxic reactor V Sec - Anoxic = 800 m 3, secondary aerobic reactor V Aerob = 1600 m 3. With the aim to improve biological phosphorus removal process by eliminating nitrate nitrogen in return activated sludge, there was included an additional small anoxic reactor, R0, placed in return activated sludge line. Total volume of reactors series is m 3. The technological line ends with a secondary clarifier Results and discussion The computer simulation was realized by specifying some characteristics of the influent. Operating data used for computer simulation are presented in table 4. Table 4 Element name Flow [m3/d] Total COD [mgcod/l] Total Kjeldahl Total P [mgp/l] 8.77 Nitrate N 8.00 ph 7.50 Alkalinity [mmol/l] 6.00 Inorganic S.S. [mgtss/l] For characterizing the composition of the raw wastewater, it were taken into consideration the following COD fraction values as BioWin software variables: readily biodegradable (fbs = 22%); acetate readily biodegradable (acetate) (fac=21%); unbiodegradable particulate (fup = 21%); unbiodegradable soluble (fus = 6%); Fraction of slowly degradable COD which is particulate (fxsp = 72%). There were realized several simulations until the prediction values for effluent quality were satisfactory. Technological parameters used in simulation for which it we obtained the best results for effluent quality are presented in table 5. Table 5 Parameter Unit Value flow rate m 3 /d Wastewater temperature C 20 Return sludge flow rate m 3 /d % of COD 100 influent Internal recycle rate m 3 /d % of COD 190 influent Primary sludge flow rate m 3 /d 5075 Waste sludge flow rate m 3 /d 1800 Hidraulic retention time in bioreactors h 7.9 Results of computer simulation for the variant taken into consideration are presented in Table 6. As result of proposed schema it were obtained the following results: The total Kjeldahl nitrogen (TKN) decreased from 54 mg/l to 3.02 mg/l (reducing of 94.40%) ; Total decreased from 62 mg/l to mg/l(reducing of 83.61%) ; Total Phosphorus decreased from 8.77 mg/l to 0.67 mg/l (reducing of 92.36%) ; Total BOD decreased from 238,8.69 mg/l to 2.14 mg/l (reducing of 99.1%) ; Figure 3 and figure 4 presents Plant Profile respectively variation of soluble PO4-P. The results demonstrated that wastewater analyzed treated using the proposed scheme is efficient from the point of view of reducing of nutrients concentration level. It can be observed a significant decreasing of effluent BOD 5 concentration too. ISSN: ISBN:

5 Ammonia N Nitrate N Plant Profile CONC (mg/l) R0 Anaerobic Anoxic Aerobic1 Aerobic2 Aerobic3 Aerobic4 Sec_Anox Aerobic Ideal clarifier23 Effluent Figure 3 Soluble PO4-P Plant Phosphorus Profile CONC (mg/l) R0 Anaerobic Anoxic Aerobic1 Aerobic2 Aerobic3 Aerobic4 Sec_Anox Aerobic Ideal clarifier23 Effluent Figure 4 Elements Total BOD [mg/l] Total COD [mg/l] Ammoni a N Total N Table 6 TKN VSS [mgvss/ L] TSS [mgtss /L] Soluble PO4-P [mgp/l] Total P [mgp/l] R Anaerobic Anoxic Aerobic Aerobic Aerobic Aerobic Second clarifier Effluent ph ISSN: ISBN:

6 Table 7 presents efficiency of proposed treatment schema having in view the most important characteristics of effluent. It can be observed a significant reducing of pollutants level in the WWTP effluent. Parameters Table 7 Conc. Effluen t Conc. Pollutant Reduction (mg/l) (mg/l) [%] Volatile suspended solids Total suspended solids Particulate COD Filtered COD Total COD Filtered Carbonaceous BOD Total Carbonaceous BOD Soluble PO4-P Total P Filtered TKN Particulate TKN Total Kjeldahl Total N Total inorganic N Total inorganic suspended solids Ammonia N Nitrate N Conclusions Because of its high levels of nutrients concentration in effluent, discharging the wastewater from the WWTP analized into the surface water could cause a severe pollution problem. The paper presents computer simulation scheme of a modified activated sludge ways which can be used for removing nutrients and for increasing efficiency. The simulation was realized using BioWIN 3.0, a simulator software package in which the user can define and analyze behavior of complex treatment plant configurations with single or multiple wastewater inputs. The main aim of simulation was to find a new solution for treatment process so that the effluent quality be in the range imposed by European regulation. In the same time, the study can offer some data regarding technological parameters of treatment process in order to increase the efficiency in operation. Computer simulation demonstrated that the nitrogen concentration of the analyzed wastewater can be reduced by an activated sludge treatment, without using an external source of carbon. In wastewater treatment particularly for removal it was obtained a reduction by 94,4% and for P removal it was obtained a reduction by 92.36%. It can be observed that all requirements imposed to the quality indicators of effluents were in the allowed levels, with a single exception: concentration of Total N in the treated effluent. The results were obtained by adjusting of operating factors like internal recycle rate, SRT and RAS so that The final results showed that the proposed treatment schema is suitable for a significant improving the effluent quality. REFERENCES [1] J.A. Baeza, D. Gabriel, J. Lafuente, Effect of internal recycle on the nitrogen removal efficiency of an anaerobic/anoxic/oxic (A2/O) wastewater treatment plant (WWTP),Process Biochemistry 39 (2004) [2] Henryk Melcer Methods for wastewater characterization in activated sludge modeling, [3] St. Iordache, D. Dumitru, C. Ianache, Developing an integrated automatic control system for wastewater treatment improvement, Annals. Food Science and Technology, 2009, vol10, issue2, pp [4] Z. Kazmierczak, L. Plonka, A. Sochacki, J. Surmacz-Gorska Use Of Hydrolyzed Primary Sludge As Carbon Source For Denitrification. A Case Study. [5] Council Directive 91/271/EEC of 21 May 1991 concerning urban waste-water treatment. ISSN: ISBN: