Conserving Lakes and Water Bodies Through State of-the-art Sewage Treatment Technologies

Transcription

1 Sengupta, M. and Dalwani, R. (Editors) Proceedings of Taal2007: The 12 th World Lake Conference: Conserving Lakes and Water Bodies Through State of-the-art Sewage Treatment Technologies Ajay Popat and Partha Chakravarty Ion Exchange Waterleau Ltd., Ion House, Plot No. 2, Sector 18, Vashi, Navi Mumbai ABSTRACT Conservation of lakes and inland water bodies from eutrophication requires proper treatment of incoming wastewaters. Domestic sewage is one of the major contributors to this process as it contains organics along with nutrients like nitrogen and phosphorus compounds. Conventional two stage activated sludge plants often lack sufficient nutrient removal performances due to substrate limitation for denitrification in the second stage. Ion Exchange Waterleau Ltd. proposes two State-of-the-Art Sewage treatment processes eg. LUCAS and INDION MBR (Membrane Bio-Reactor), which consistently, meet stringent outlet qualities of treated sewage discharged to lakes and inland water bodies. LUCAS, Leuven University Cyclic Activated Sludge System LUCAS is the family name of a complete range of waste-water treatment processes developed by Ion Exchange Waterleau Ltd. Lucas stands for Leuven University Cyclic operating activated sludge system with a compact and modular design. LUCAS also stands for Low-cost Unobtrusive Compact Advanced Sustainable, summarizing the advantages of this technology. LUCAS combines the advantages of the conventional technology and of the fill-and-draw/sbr technology. As in the conventional system, the reactor volume and the level in the tanks are always constant. It is continuous system for both influents influent feed and effluent discharge. As in an SBR (Sequential Batch Reactor) system, the reactor operates according to a time controlled process cycle that allows for the alternation of all essential processes (accumulation, regeneration, sedimentation) in a single compartment. Over the years the LUCAS systems have been extended to meet stringent biological nutrient removal criteria for discharge of treated effluents to surface water sources. INDION MBR is a high-end submerged membrane technology wherein membranes are directly emerged in biological aeration tank. This system produces highest quality of treated sewage with maximum organics and nutrients removal, and also removes pathogens up to six log values. This system is a single tank unit with minimum footprint, modular and flexible. This paper discusses advantages of these State-of-the-Art technologies along with case studies and represents their applicability in conserving lakes and water bodies in Indian context. Keywords: Eutrophication, Sewage, Nutrients, Activated Sludge, LUCAS, SBR, MBR, nitrification, denitrification, ultrafiltration INTRODUCTION Aquatic ecosystems in lakes and inland water bodies have been subjected to various levels of stresses in India, due to unplanned developmental activities from the last century leading to serious environmental degradation. Anthropogenic activities involving changes in land use ultimately affects the receiving water in that drainage. Activities include agriculture inorganic fertilizer, pesticides and herbicides applied to crops, silt washed away because of vegetation removal, or even atmospheric deposition, or disposal of solid and liquid wastes. In India different lakes receive a heavy influx of sewage, industrial effluents, domestic and agricultural waste which consists of varying hazardous chemical and causing deleterious effects on fish and other aquatic organisms. Among these wastewaters, domestic sewage is the major contributor of pollution considering total bulk pollution load. The concerns of those involved in designing sewage treatment systems have changed over the years. Originally, the biochemical oxygen demand (BOD) and total suspended solids (TSS) received most of the attention. This was primarily because excessive BOD and TSS levels could cause severe and readily apparent problems, such as oxygen deficits that led to odors and fish kills and sludge deposits that suffocated benthic organisms. By removing BOD and TSS, other contaminants were also removed and other benefits

2 were realized; so even today, in India, discharge norms for sewage contain mainly limits for BOD and TSS. The importance of nutrient removal and its ill effect causing eutrophication is well known long time among academics; but it is still yet to spread within the common mass in India. In addition to this, control of pathogens during sewage treatment is not considered at all. But recently many progressive municipalities and urban development authorities in India have come up with a positive attitude to address these problems and have taken lake cleaning activities, mostly when the urban sewage discharges are limited to large lakes within the city. Among the first contaminants to be added to the requirements for discharge permits in developed countries, other than BOD and TSS, were nutrients. The most commonly regulated nutrients are phosphorus and nitrogen. Originally removing phosphorus and nitrogen could only be done through expensive, advanced chemical methods. But scientists have recently discovered ways to accomplish enhanced removals of nutrients in conventional biological treatment plants with relatively minor operational and structural adjustments. Till then, in developed countries, biological nutrient removal has been practiced along with organic biodegradation. The oxidation ditch process was most popular in European countries and there are various modifications done to this basic treatment process, like the AO/ A2O/ BIODENPHO etc. But the conventional oxidation ditch process and all its varieties are basically space-specific processes. They can handle unlimited quantity of sewage inflow and are less sophisticated, but require large area, several civil tanks and moving equipments. In recent period, time-specific sequential batch reactors [SBR] became popular because it requires quite less space and provide better outlet characteristics than conventional processes. But necessarily most of the cyclic SBR processes require sophisticated and highly complicated decanter mechanisms, controlled and effective water level change monitoring instrumentation. Also, there are some limitations in flow quantity beyond which typical SBRs cannot handle. Ion Exchange Waterleau Ltd, has successfully introduced two State-of-the-Art Sewage treatment processes eg. LUCAS and INDION MBR, in India and South East Asian market, those are more efficient and consistent in output quality of the treated sewage discharged to lakes and inland water bodies. Ion Exchange Waterleau Ltd., a joint venture of Ion Exchange (India) Ltd. and the Waterleau Group, Belgium offers a complete portfolio of advanced environmental solutions and services for industrial, infrastructure and municipal applications. Technology solutions encompass water, liquid & gaseous effluents, solid waste & bio-solids and renewable energy, while services include consultancy, turnkey contracting, O&M and BO(O)T projects. Leveraging the strengths of both organizations, Ion Exchange Waterleau Ltd., provides state-of-the-art turnkey environmental solutions and services to the growing Asian market. The Lucas Process LUCAS is an advanced aerobic technology that combines advantages of both conventional and sequencing batch reactor systems [Figure 1]. Treated effluent following LUCAS process, is consistently of highest quality, allowing for safe effluent discharge and stable process performance. The system does not required external clarifiers, sludge rakers, recycle pumps/screws/piping, sophisticated and heavy moving equipment like decanters. LUCAS system, operates in Cyclic Activated Sludge Process mode. The entire system is of single tank with three to four chambers; compact with small foot-print, modular, flexible with maximum redundancy. The dynamics is optimized by substrate gradient and integrated selector effect Control in time enables flexibility by adapting times for nitrification, denitrification, biological phosphorus removal, sedimentation, depending on influent characteristics. The LUCAS system may be designed using three/ four cells in series [Figure 2]. In case there are multiple cells in use, they are all hydraulically interconnected. They are all equipped with an inlet and outlet, an aeration device, an agitator, an excess sludge pump and an overflow launder. Each reactor can be completely isolated from the others by closing the openings in the common walls with a pen stock valve; this is the best proof that the LUCAS system is completely redundant. The cyclic phases of LUCAS system are as follows Figure 3]: 924

3 Figure 1. LUCAS is a hybrid system, combining advantages LUCAS is a cyclic activated sludge system LUCAS is a multiple unit tank Figure 2. LUCAS Hydraulic Schemes - The System can be described as a multiple unit tank - Units within the tank are hydraulically connected with 1 or 2 adjacent units - Thus the hydraulic connections combine the individual units into a single tank 925

4 Figure 3. LUCAS Cyclic operation Phase 1: Feed and react During this phase raw sewage is added to the reactor. Aerated fill results in the start of the aerobic reactions. During the feed phase, organic matter is adsorbed and partially biodegraded, meaning removal of organic matter, oxidation of nitrogen and surplus phosphorous uptake. This is called accumulation. Phase 2: React During this phase no sewage is fed to the reactor. When the reactor is aerated, biodegradation of the organic matter is completed. This phase is also called the regeneration phase. The alternation of an accumulation phase by a regeneration phase promotes the growth of floc forming bacteria which are necessary for a good separation of the final sewage from the bacteria in the next phase. Phase 3: Settle During the settling phase, the bio-solids are separated from the sewage. The draw phase is being prepared. Phase 4: Draw The final sewage is flowing out of the reactor. In contrast with the conventional SBR technology the water level in the reactor remains constant. During this phase, excess bio-solids will be extracted from the reactor. In LUCAS a continuous flow system is maintained where it is required at least three similar reactors and their cycles are dislocated in time so that only one tank is fed at a time. Like conventional SBR systems, no sludge recycle systems are necessary between tanks and the different tanks can be built in a compact manner using common walls. To activate biological nutrient removal in wastewater treatment plants, specific conditions should be applied to the activated sludge microorganisms [Figure 4]. Similar to any activated sludge process, in LUCAS system soluble organics are degraded aerobically by microorganisms. In order to ensure required population of bacteria Mixed Liquor Suspended Solids (MLSS) and Food to Micro-organisms ratio (F/M) is maintained. In the reactor, the bacterial culture carries out the conversion in general accordance with the stoichiometry as under: 926

5 Figure 4. LUCAS Cyclic Operation for Advanced nutrient removal Reaction: COHNS +O 2 + NUTRIENTS --bacteria- CO 2 +NH 3 + C 5 H 7 NO 2 + other end (organic matter) products In the aeration phase, biological activity works in the endogenous phase of the bacterial growth curve. The system is designed to maintain high MLSS and low F/M ratio while the extra oxygen/ Kg BOD requirement is provided. The endogenous respiration phase is represented by following equation: bacteria C 5 H 7 NO O CO 2 + 2H 2 O + NH 3 + energy Nutrients such as nitrogen and phosphorous requirement of micro-organisms are satisfied by mixing part of sewage into it. In addition to the nutrients requirements the aerobic micro organisms requires oxygen to sustain their microbial activities. This oxygen is supplied by aeration systems. Enhanced biological nutrient removal in LUCAS systems The functional cycle can also be used for the introduction of enhanced biological nutrient removal. Next to organic matter sewage contains usually also nitrogen and phosphorus compounds. These compounds exist under different forms like particulate or solubilized organic bound nitrogen and phosphorus and solubilized ammonia, nitrates, nitrites and orthophosphates. A distinction can be made between assimilative and dissimilative nutrient removal. Hydrolysis of organic bound nitrogen or phosphorus to solubilized ammonia and orthophosphates is a process that is important to make the nitrogen and phosphorus available to the micro organisms. During the assimilation process the micro organisms incorporate nitrogen and phosphorus in the new cell material according to a specific ratio based on the ideal cell material composition. This ratio is estimated to be approximately 100: 5: 1 as BOD: N: P In general the assimilative nutrient removal is depending on the sludge production and thus on the organic loading of the plant. Since sludge production is normally kept as low as possible the assimilative removal capacity is rather limited (nitrogen typically between 2-10 g/100g sludge, phosphorus between g / 100 g sludge). The specific conditions for dissimilative enhanced biological nitrogen and phosphorus removal processes are different. Therefore these processes are described separately, nevertheless their interaction is important and described later. Biological nitrogen removal The dissimilative nitrogen removal is a process that is carried out by a specific group of bacteria in the activated sludge during different environmental conditions or states. The first step is called nitrification or ammonia oxidation, carried out by autotrophic nitrifying organism (e.g. Nitrosomonas sp. and Nitrobacter sp.) during aerobic conditions. The nitrifying microorganisms in the activated sludge oxidize ammonia till nitrite (Nitrosomonas sp.) and then nitrate (Nitrobacter sp.), while using alkalinity. This process requires oxygen (4.57 kg O 2 /kg NH 4 -N). 927

6 Nitrification : NH 4 -N + 2O 2 NO 3 -N + H 2 O + 2H+ + energy The second step is called denitrification or nitrite/nitrate dissimilation carried out by facultative anaerobic heterotrophic bacteria (e.g. Pseudomonas sp.). During denitrification the nitrate is first reduced to nitrite and in a second step nitrite is reduced via several intermediates like nitric oxide (NO) and nitrous oxide (N 2 O) to harmless nitrogen-gas that is evacuated to the atmosphere. In this process nitrate serves as electron acceptor in the oxidation-reduction reactions of the carbon substrate to provide energy for cell growth. Denitrification is achieved under conditions of low (<0.5 mg/l) or zero dissolved oxygen concentrations within ORP values of +50 to -50 mv, referred to as anoxic conditions. As carbon source for denitrification raw wastewater is normally used. The oxidation capacity of nitrate oxygen equals kg O 2 per kg NO 3 -N. This means the recuperation of most of the oxygen used during nitrification. Alkalinity lost during nitrification is partly recovered during denitrification. DENITRIFICATION : NO 3 --N + 5E - + 6H N H 2 O + ENERGY CH 2 O + H 2 O CO 2 + 4e- + 4H+ Important control parameters are the aerobic and anoxic retention time of the bacteria, the availability of rapidly biodegradable organic substrates (volatile fatty acids), ph and temperature and especially the overall sludge retention time in the system (typically > 10 days at 15 C). Biological phosphorus removal Enhanced biological phosphorus removal is based on the capacity of certain strictly aerobic bacteria (e.g. Acinetobacter sp.) to store phosphates in the cell in the form of polymers (polyp). The biop removal is achieved in a sequence of anaerobiosis and aerobiosis. The strictly aerobic organisms can survive a certain time under anaerobic conditions. They survive on short chain fatty acids like acetic, propionic and butyric acid, produced by other facultative anaerobic bacteria. These acids are taken up using energy derived from the polyp. The short chain fatty acids are transformed into hydroxy-butyric acid and accumulated as Poly-β-OHbutyric acid to avoid endogenous acidification of the bacterial cells. As a final result polyp is degraded and ortho-phosphates are secreted in the medium. When the sludge comes back in aerobic circumstances oxygen is again available as electron acceptor and used rapidly since Poly-β-OH-butyric acid is already present in the cells as endogenous carbon source. During the oxygen reduction ATP is produced from ADP and P and the P that is taken up is stored into PolyP again. To obtain a net P-removal the uptake should exceed the previous release. Important control parameters are the aerobic and anaerobic retention time of the bacteria, the availability of rapidly biodegradable organic substrates (volatile fatty acids), the nitrate concentration (as low as possible), ph and temperature and especially the overall sludge retention time in the system. Major control parameters with respect to the nutrient removal in LUCAS are the influent feeding position, phase duration time and alternation of the different biological states (anoxic, anaerobic and aerobic states). Summarising main advantages of LUCAS system Process advantages The main advantages of the LUCAS wastewater treatment process are the complete control flexibility and the optimal circumstances for all essential biological and physical processes including: symmetric cyclic operation (equal sludge distribution, equal reactor capacity, equal oxygen demand and supply) no short circuiting of influent to clarifier integrated selector properties (accumulationregeneration) maximal dynamics in substrate gradients in all reactors Civil construction advantages Easy, quick and modular construction Complete redundancy Easy to cover Easy to extend Common wall structure with one flat bottom. Advantages of continuous flow and constant level No headloss (volume is always used for 100%) Continuous influent and effluent flow means lower maximal flow rate and therefore smaller piping diameter and less installed capacity of pumps, aerators, weirs No moving mechanical parts 928

7 No varying pressure on walls simplifies use of fine bubble aeration, no suppressor flow rate control based on water level is necessary Main advantages compared to conventional systems No need for external clarifiers, sludge scrapers, recycle pumps/screws/piping No extra disturbances in clarifier caused by sludge bulking/ recirculation Optimal dynamics in substrate gradient and integrated selector effect, responsible for the formation of well settable sludge flocks Control in time enables flexibility by adapting times for nitrification, denitrification, biological phosphorus removal, sedimentation, depending on influent characteristics Easy and very compact construction (no separate clarifier) Maximal redundancy Easy to cover Easy to extend, modular construction Common walls with same flat bottom. Main advantages compared to Sequencing Batch Reactors No raw wastewater enters a compartment in the phase preceding the sedimentation sequence. This guarantees optimal BOD, COD, N, P removal efficiencies, no shortcircuiting. No headloss (volume is always used for 100%) Continuous influent and effluent flow means lower maximal flow rate and therefore smaller piping diameter and less installed capacity of pumps, aerators, weirs. No moving mechanical parts No varying pressure on walls Enables use of surface aerators Simplifies use of fine bubble aeration, no suppressor flow rate control based on water level is necessary Name of the project/ employer : Aquafin Belgium Project details in brief : municipal sewage treatment Process adopted: brief : LUCAS Raw sewage characteristics (COD/BOD/SS/N tot /P tot ): 600/300/250/45/10 Design effluent standards (COD/BOD/SS/N tot /P tot ): 125/25/35/15/2 Effluent standards achieved in the plant : as per design or better Power consumption is approximately 20% less than conventional system, considering the power required for maintaining high recirculation ratio in later system. Area required for main LUCAS reactor : 80 m x 32 m This area is approx. 2/3 of a conventional oxidation ditch with clarifiers. Period of successful operation and maintenance : till present Contract period at award : 2003 Start date : 2003 STP at CIE, Coloane, Macau - Capacity of the plant is approx. 48 MLD [Figure 6] FEW CASE STUDIES ON LUCAS based STP with NUTRIENT REMOVAL a) STP for Aquafin Antwerp, Belgium. Capacity of the plant is approx. 20 MLD [Figure 5] This was a turnkey installation by Waterleau NV, Belgium. 929

8 Name of the project/ employer : Macau government Project details in brief : municipal WWTP Process adopted: brief : LUCAS Raw sewage characteristics (COD/BOD/SS/Ntot/Ptot): 450/250/200/38/8 Design effluent standards (COD/BOD/SS/Ntot/Ptot): 125/25/35/10/1 Effluent standards achieved in the plant : as per design or better Power consumption is approximately 20% less than conventional system, considering the power required for maintaining high recirculation ratio in later system. Area required for main LUCAS reactor is 160 m x 100m which is approximately 78% of a conventional oxidation ditch with clarifiers. Period of successful operation and maintenance : 2003-present Contract period at award : 2003 Start date : 2003 STP at Nanjing, China, - Capacity of the plant 300 MLD [Figure 7] Name of the project/ employer : Nanjing city of China Project details in brief : municipal sewage treatment Process adopted: brief : LUCAS Raw sewage characteristics (COD/BOD/SS/N tot /P tot ): 300/120/300/30/8 Design effluent standards (COD/BOD/SS/N tot /P tot ): 125/10/35/10/1 Effluent standards achieved in the plant : as per design or better Power consumption is approximately. 22.5% less than conventional system, considering the power required for maintaining high recirculation ratio in later system and mixing requirements. Area required for main LUCAS reactor is approximately 80% of a conventional oxidation ditch with clarifiers. Period of successful operation and maintenance : 2003-present Contract period at award : 2003 Indion MBR Process Ion Exchange Waterleau Ltd. has left the limited endof-pipe thinking approach and has switched to processintegrated approach. The INDION Membrane Bio- Reactor ideally fits in this philosophy. It produces an effluent that is extremely pure and appropriate for immediate reuse: no wonder that it is considered worldwide as the most promising technology for wastewater treatment. Micro-and ultrafiltration membranes can be used for the separation of sludge and biologically purified wastewater. By replacing the sedimentation step by a membrane separation it is possible to operate the bioreactor at much higher sludge concentrations (typically 12 to 15 g). Hence, the volume of the bioreactor can be reduced drastically (by a factor of 3). An additional advantage of this system is the fact that the filtrate is virtually free of suspended solids and bacteria since these cannot pass the membranes. The filtrate will thus be ready for reuse or for further treatment by reverse osmosis. There are two types of membrane bioreactors. The membranes can either be placed externally, that is in an external loop to the bioreactor (X-Tractor MBR) or they can be submerged in the activated sludge, that is in the bioreactor itself (INDION MBR). Both membrane bioreactor concepts have there own advantages and they have their own range of application. This paper highlights the INDION MBR process for treating domestic and municipal sewage. In this process membrane-bioreactor reinforced hollow fibre membranes or flat sheet membranes are submerged in the activated sludge, either directly or in a separate extraction compartment in a submerged MBR, the membranes are kept clean by means of an efficient coarse bubble aeration which ensures a good sludge management in the membrane module and guarantees turbulent in the membrane surface. A submerged MBR is hence characterized by a very low energy consumption and is able to treat wastewater in a very compact installation. The INDION MBR submerged solution is generally used for treating larger flows or wastewater not requiring harsh and frequent cleanings of the membranes. The INDION MBR is equipped with durable reinforced hollow fibre/flat sheet membranes allowing 930

9 easy cleaning and efficient backpulse on one hand and ensuring long membrane lifetime on the other. Advantages of Indion MBR IONDION MBR (Membrane Bio Reactor) is the latest technology in wastewater treatment with many advantages as listed below: 1. It requires much less space when compared to conventional activated sludge process. It does not require clarifier tank where as conventional activated sludge process requires clarifier, which further adds to the area requirement and cost. 2. Biological reaction in INDION MBR can be carried out under the condition of higher sludge concentrations (MLSS g/l) compared to conventional activated sludge process (MLSS 2-4 g/l). It means the biological reaction is faster and more organic components included in the feed wastewater can be decomposed in shorter time or in smaller space. This also gives good treatment efficiency and high stability of sludge, which is easy to dispose off. 3. The quality of treated water in case of INDION MBR is much superior than conventional biological systems. As the membrane acts as a physical barrier, it does not allow any sludge particles and to great extent bacteria and viruses to pass through it. Microorganisms like coliform or cryptosporidium can be easily removed in MBR. This increases the reliability of the system multifold. Table 1 gives typical values of treated sewage. 4. Conventional biological systems require further costlier tertiary treatment to match the performance of the INDION MBR system. This may include units for biological phosphorous and Nitrogen removal, coagulation, filtration, chlorination, adsorption, UV treatment etc. 5. As conventional treatment systems require disinfections with chlorine, it needs to be removed completely before reuse for gardening or of green belt development. Otherwise, high amount of residual chlorine may damage green plants. In the INDION MBR process (Fig. 8&9), membrane act as physical barrier and removes bacteria and viruses up to a degree of 4-6 log removal (10 4 to 10 6 times reduction), independent of type or life form of organism. It also helps in complete retention of biomass in the aeration basin. Table 1. Indion MBR Performance with Sewage. Parameter Mbr Removal Effluent Efficiency BOD < 2 mg/l > 99% TSS < 1 mg/l > 99% TKN < 2 mg/l > 96% NH 3 < 0.3 mg/l > 97% Total P < 0.1 mg/l -- Turbidity < 1 NTU > 99% Total Coliforms < 100 cfu/100 ml > 6 log ( %) Feacal Coliforms < 10 cfu/100 ml > 6 log ( %) NOTE: Values for Municipal Sewage with an organic loading of 2.5 kg/m 3 per day of BOD, and a Hydraulic retention time of less than 2 hrs. 6. As chlorination is not required, INDION MBR offered does not produce disinfection by-products or bad odors The energy consumption of INDION MBR is extremely low (0.30 kwh/m 3 ) and the treated domestic effluents has a four to six log (99.99%) removal of total coliform. The MBR system does not require the use of chemicals, thereby making it extremely safe during operation and re-use. Table 2. Extended Aeration Activated Sludge v/s INDION MBR Conventional INDION MBR Process Extended Aeration Final Clarifier Yes No Tertiary Filter Yes No MLSS (mg/l) < Sludge Retention Time (days) 5-10 (N/DN) Footprint Large 3-5 times smaller Process Stability Sensitive to Sludge Bulking Not Sensitive to upsets COD load (Kg/m3 of Tank Volume)

10 CONCLUSION: At the present day scenario, to prevent eutrophication and for protecting the natural aquatic eco system, due care is required before discharging domestic sewage in the inland water bodies and lakes. With the increasing population and rapid economic growth in urban areas, highly efficient yet compact treatment systems are required to cope up with land limitations. Both the advance treatment technologies, LUCAS and INDION MBR, can meet up the expectation and provide high quality treated sewage. 932