Comparison on the Treatment Performance of Full-scale Sewage Treatment Plants using Conventional and Modified Activated Sludge Processes FATIHAH SUJA 1, RAKMI ABDUL RAHMAN 2 AND OTHMAN JAAFAR 3 1,3 Department of Civil and Structural Engineering, 2 Department of Chemical and Process Engineering, and 1 Research Centre for Sustainable Technology (CESPRO) Universiti Kebangsaan Malaysia 43600 Bangi Selangor MALAYSIA fati@eng.ukm.my Abstract: - The objective of the study is to identify and carry out studies on the selected five (5) existing sewage treatment plants (STPs) in terms of their treatment performance. The five selected sewage treatment plants have different treatment processes, a) Sequencing Batch Reactor (SBR) b) Modified Oxidation Ditch (MOD) c) Advanced Oxidation Ditch (AOD) d) Advanced Conventional Activated Sludge System (ACAS) and e) Extended Aeration (EA) types. Assessment on the removal efficiency and sludge quality of the major pollutant parameters namely BOD, COD, TSS, O&G NH 3 -N, PO 4 of the effluent and heavy metals content of sludge were carried out based on the sampling done by an accredited laboratory. The health of the aeration tanks was also investigated. On average COD recorded highest removal for all STPs at 91% followed by TSS (88%), O&G (86%), BOD (84%) and PO 4 (71%). The removal of NH 3 -N was low, at only 72%; with a big range of removal rate between the highest of 97% for SBR type and only 17% removal for the EA type. Comparison with other sludge approved for use in agriculture as well as compost standards showed that the heavy metals contents of raw sludges comply with those approved for sewage sludges for agriculture, as can be found in the USA, Australia and Spain. Comparison with levels limited by the UK Composting Association Quality (CAQ) and European EC Compost Ecolabel (ECCE) shows that the raw sludge levels exceed most of the ECCE s levels. Key-Words: activated sludge system, sequencing batch reactor, oxidation ditch, extended aeration 1 Introduction The sewage treatment plants in Malaysia generally employ the activated sludge treatment process. The basic principle behind all activated sludge processes is that as microorganisms grow, they form particles that clump together. These particles (floc) are allowed to settle to the bottom of the tank, leaving a relatively clear liquid free of organic material and suspended solids. By analysing the different characteristics of the activated sludge or the sludge quality, plant operators are able to monitor how effective the treatment plant s process is. Efficient operation is ensured by keeping accurate, up-to-date records; routinely evaluating operating and laboratory data; and troubleshooting, to solve problems before they become serious. A wide range of laboratory and visual and physical test methods are recommended. Principally, these include floc and settleability performance using a jar test, microscopic identification of the predominant types of bacteria, and analysis of various chemical parameters. The treatment environment directly affects microorganisms. Changes in food, dissolved oxygen, temperature, ph, total dissolved solids, sludge age, presence of toxins, and other factors create a dynamic environment for the treatment organisms. The operator can change the environment (the process) to encourage or discourage the growth of specific microorganisms. 2 Sewage Treatment Plant Facilities Five (5) existing sewage treatment plants (STPs) in Malaysia indicated by A, B, C, D and E were investigated. The designs of the five selected STP is based on average sewage flows of 0.225 m 3 /d per PE, with organic loads of 55 g BOD and 68 g SS per PE per day. Treated effluent is required to comply to the Environmental Quality Act (EQA), 1974, which specifies two standards ISBN: 978-1-61804-301-6 179
for effluent discharge: Standard A for discharge upstream of any raw water intake and Standard B for discharge downstream of any raw water intake [1]. 2.1 Sewage Treatment Plant To achieve specific effluent goals of meeting the discharge standards, most of the wastewater operators in Malaysia use the different adaptations and modifications of basic activated sludge design. 2.1.1 Sequencing Batch Reactor Sewage Treatment Plant A is based on the Sequencing Batch Reactor (SBR) treatment process with a current inflow of 71,200 PE. The existing treatment plant consists of two lift stations, grit and grease chambers, two equalisation tanks, six numbers of SBR tanks over two aeration tanks and sludge treatment facilities which include sludge thickener, sludge dewatering and sludge cake storage. Secondary treatment is carried out in the SBR which is operated with prescribed programmable cycles of fill, react, settle, draw and idle according to flow conditions and effluent requirements. Air is supplied by roots blower located in the blower house and channeled to the submersible aerators installed at the bottom of the aeration tanks. The treated effluent is discharged during lowering of the decanter. The treated effluent is passed through the measuring tank which can be used as a disinfection tank when necessary. The treated effluent is discharged to the adjacent stream. Waste sludge pumps, located in sludge pump houses, remove the excess sludge for storage in the waste sludge holding tank. The thickened sludge is then transferred by pump to the sludge dewatering unit. Sludge dewatering is carried out by screw press. Polymer is added as conditioner during thickening and dewatering. The dewatered sludge is conveyed to the hoppers for offsite disposal. 2.1.2 Modified Oxidation Ditch Sewage Treatment Plant B is a mechanical plant using the Modified Oxidation Ditch (MOD) system with a current sewage flow of 58,125 PE. Treatment process includes primary and secondary screening using automatically raked mechanical screens. The screen openings for primary screens shall be less than 25mm and less than 12 mm for secondary screens. The sewage then flows to the grit and grease removal chamber for removal of grit, oil and grease. The preliminary treated sewage is then passed through the flow measurement channel before to the three concentric Orbal tanks. Aeration in the Orbal tank is via aeration discs with triangular nodules on the surface of the discs to provide mixing and aeration. The mixed liquor in the final channel is transferred to the clarifier for settling and separation of sludge. Part of the sludge is pumped back to the aeration tank as returned activated sludge and the remaining sludge is wasted to the sludge holding tank. The final effluent is discharged to the nearby river. The waste sludge produced at the plant is currently transported to other STP for treatment. 2.3 Advanced Oxidation Ditch Sewage Treatment C involved the use of Advanced Oxidation Ditch (AOD) treatment process and the current flows is only about 70,000 PE. The treatment process used in the oxidation ditches is based on the advanced oxidation ditch process. The term is used to describe the new generation of deeper (up to 5 m) extended aeration ditches using fine diffusers together with propeller mixers and designed to achieve nitrogen removal. The deeper ditches also allow for better oxygen transfer thus providing better energy efficiency. Flows to the existing sewage treatment plant is discharged to the grit and grease chamber which include an automatically raked mechanical fine screen with opening of 12mm spacing. The preliminary treated sewage is then transferred to the oxidation ditches for secondary treatment. Aeration is provided via propeller mixers and fine diffusers. The mixers are continuously operated while the blowers are designed to operate intermittently to enable anoxic condition within the reactors for nitrification and denitrification. The reactors are designed for a hydraulic retention time of 24 hours and the mixed liquor overflows at the outlet end to the ISBN: 978-1-61804-301-6 180
clarifiers for settling and separation of sludge. Part of the sludge is pumped back to the aeration tank as returned activated sludge and the remaining sludge is wasted to the sludge holding tank. The sludge at the holding tank is then pumped to the mechanical thickener for thickening prior to mechanical dewatering. Sludge dewatering is carried out by screw press. Polymer is added as conditioner during thickening and dewatering. The dewatered sludge is conveyed to the hoppers for off site disposal. 2.1.4 Advanced Conventional Activated Sludge Sewage Treatment Plant D uses Advanced Conventional Activated Sludge (ACAS) treatment system and the current sewage flow to the overall STP is about 161,341 PE. The term ACAS is used to imply a conventional activated sludge process with the provision of anoxic zones to encourage denitrification. The reaction tank is designed for denitrification to occur through the adoption of a four compartment tank comprising anoxic, aerobic, anoxic and aerobic zones. The design incorporated a recycle from the second to the first anoxic zone to aid denitrification. The main advantages of the process include high quality effluent, reduced reactor volume and high solids concentration of sludge from primary clarifier which allows easier dewatering of sludge. Main disadvantages include larger quantities of sludge generated and the use of primary clarifiers may give rise to odour. Primary sludge collected at the primary clarifier is pumped to the gravity sludge thickener for further treatment. The primary clarified sewage overflows to the aeration tank for secondary treatment. Air is supplied by roots blower located in the blower house and channelled to the submersible aerators installed at the bottom of the aeration tanks. Submersible propeller mixers are provided at the anoxic zones to prevent settlement of suspended solids. The blowers and propeller mixers are designed for continuous operation. The reactors are designed for a hydraulic retention time of 8 hours and the mixed liquor overflow at the outlet end to the rectangular secondary clarifiers for settling and separation of sludge. Part of the sludge is pumped back to the aeration tank as returned activated sludge and the remaining sludge is wasted to the sludge holding tank. 2.5 Extended Aeration Sewage Treatment Plant E is a mechanical plant using the Extended Aeration (EA) activated sludge system. The current flow to the STP is more than 30,000 PE. The existing treatment plant consists of an internal sewage pump sump, secondary screen chamber, aerated grit and grease chamber, distribution chambers, anoxic tank, aeration tank, two circular clarifiers and sludge treatment facilities which include sludge holding tank, sludge dewatering, standby sand drying beds and dried sludge cake storage. The treatment process is a modified version of the conventional activated sludge process. The secondary biological treatment operates at extended aeration and long sludge age to produce relatively stabilised sludge. The advantage of having longer hydraulic retention times for the process allows the STP to operate effectively over widely varying flows and waste loadings. Other advantages include reduced odorous gases emission due to direct feed to aeration tank and easier to operate and maintain. Main disadvantages include large footprint and generated sludge are difficult to dewater. 3 Data Collection and Analysis Data collected for the study cover the broad scope on the current status of the five existing sewage treatment plants on aspects such as plant layout, flows, influent and effluent characteristics, sludge production, energy consumption, process designs, capital and operating expenditures. Sampling and laboratory analysis programme were carried out by an Accredited Laboratory to provide essential data on the plant operations, for the period March 2013 to 10 May 2013. Some data were also obtaine. The review in this paper has analysed both the data collected from the plant operators and the results from the Accredited Laboratory. 4 Performance Comparison 4.1 Effluent Quality ISBN: 978-1-61804-301-6 181
Effluent quality at EA type STP shows the followings: Low nitrification, NH 3 -N reduced from about 20 mg/l in raw sewage to about 18 mg/l after clarifier; Data from the plant operator also shows similar results, with NH 3 N ranging from 7 to 30 mg/l despite the long aeration period; Very low MLSS with s range from 364 mg/l to 572 mg/l; High SVI ranging from 111 to 370 ml/g, with most s above 200 ml/g. The combination of low MLSS and high SVI, indicating possible sludge bulking, results in washout. As nitrifiers are slow growing, under such conditions there will not be enough of them to effectively lower NH 3 -N levels in the effluent. The low MLSS content is expected since the STP only receives approximately 45,000 PE inflows, much lower when compared with the original design of 100,000 PE. Very low level of BOD in the inflow is one of the main factors MLSS is not achieved since the concentration of MLSS is difficult to increase without adequate food. In addition, maintenance of sewer pipes by the developer is still in progress which may have contributed to the mixing of sewage with sludge which in turn affects the whole plant process. It was informed that there was no significant increase in MLSS eventhough some plant breeding activities have been carried out. At ACAS type STP, observations made include: Reasonable removal of NH 3 -N, being reduced from about 32 mg/l in raw sewage down to about 9 mg/l after clarifier, although there are spikes of up to about 22 mg/l. Data from the plant operator also show similar results with effluent NH 3 N ranging from 1 to 25 mg/l and concentrations of <10 mg/l were obtained in later times of the sampling periods. A one whole month sampling (March 2013) by the plant operator reported higher s of MLSS in the aerobic tank ranging from 2108 to 4333 mg/l, and an average of 3574 mg/l. High SVI ranging from about 200 to 550 ml/g and these high s could be attributable to high MLSS of fluffy sludge. Overall, the process looks stable, with good MLSS levels and reasonable SVI s. For SBR type STP, data from sample analysis show reasonable removal of NH 3 -N, being reduced from about 20-34 mg/l in raw sewage down to about 7-10 mg/l after clarifier with data from the plant operator showed much lower levels of NH 3 -N in the final effluent, at s of 1 5 mg/l. High MLSS was obtained at all stages: at about 2,200-3600 mg/l at 1 st 30 minutes aeration stage, about 1500 2900 mg/l at 1 st 30 minutes nonaeration stage, about 2200-5200 mg/l at 2 nd 30 minutes aeration stage, and about 1500 2700 mg/l at 2 nd 30 minutes non-aeration stage. Moderate SVI ranges from about 180-200 ml/g, showing good settling sludge. Data from sample analysis at MOD type STP showed the followings: Reasonable removal of NH 3 -N, being reduced from 135-139 mg/l in the influent to about 9.1-13.3 mg/l at the effluent, however data from the plant operator showed much higher levels, at about 20 35 mg/l; MLSS data for the inner and outer orbitals obtained by the plant operator from 6 th April 2013 till 27 th June 2013 show constant readings at 3000 mg/l since the meter can only read up to a maximum of 3000 mg/l; Very high SVI ranging from 345 to >900 ml/g, showing bulking sludge. The extremely high levels of ammonia discharged by some industries into the STP influent resulted in high NH 3 -N levels shown in the plant operator s data. At MOD type STP, the followings were observed: Reasonable removal of NH 3 -N, being reduced from about 31 mg/l in the influent to about 1.0 mg/l at the effluent, data obtained by the plant operator in 2012 are also comparable; High MLSS at all stages: at about 2100-2800 mg/l at 1 st stage with aeration, about 2500 2700 mg/l at 1 st stage without aeration, about 1800-2600 mg/l at 2 nd stage with aeration, and about 1700 2500 mg/l at 2 nd stage without aeration; and reasonable SVI ranging from 128 to 358 ml/g. Table 1.1 shows the average concentration of the various parameters in the influent and effluent of the STPs. The reduction of the various parameters after being treated at the respective STPs were determined and shown in Table 1.2 and illustrated in Figure 1.1. It is apparent that the percentage reductions of the various parameters were different between the STPs. On average COD recorded highest removal for all STPs at 91% followed by TSS (88%), O&G (86%), BOD (84%) and PO 4 (71%). The removal of NH 3 -N was low, at only 72%; with a big range of removal rate ISBN: 978-1-61804-301-6 182
between the highest of 97% at AOD type STP and only 17% removal at EA. Table 1.1: Average concentration of various parameters in the influent and effluent of the STPs Parameters Unit SBR MOD AOD ACAS EA Inf. Eff. Inf. Eff. Inf. Eff. Inf. Eff. Inf. Eff. Biochemical mg/l 108 18 71 22.5 37 4.5 106 2.7 75.5 11.5 Oxygen Demand @ 20 0 C, 5 days Chemical Oxygen mg/l 259 23.5 642 90 558 29.5 289 20 219 26.5 Demand Total Suspended mg/l 82 25 394 27 183 10 114 5 170 22 Solids Oil & Grease mg/l 12 2 64 1 46 4.5 10.5 2 3.5 1 Ammoniacal Nitrogen (NH 3 N) mg/l 27 8.4 137 11.2 31.4 0.98 32 4.3 22.0 18.3 Total Phosphorus mg/l 12 2.22 17.8 11.6 10.8 2.4 7.7 1.0 4.9 1.3 Table 1.2: Removal efficiency of various water quality parameters by respective STPs STPs BOD COD TSS O&G NH 3 -N PO 4 SBR 83% 91% 70% 83% 69% 82% MOD 68% 86% 93% 98% 92% 35% AOD 88% 95% 95% 90% 97% 78% ACAS 97% 93% 96% 81% 86% 87% EA 85% 88% 87% 71% 17% 74% Average 84% 91% 88% 86% 72% 71% ISBN: 978-1-61804-301-6 183
120% 100% Removal Efficiency 80% 60% 40% 20% BTR SBR Sg MOD Besi Damansara AOD Puchong ACAS Cyberjaya EA Average Average 0% BOD COD TSS O&G NH3-N PO4 WQ Parameter Figure 1.1: Removal efficiency of various water quality (WQ) parameters by respective STPs 4.2 Sludge Characteristics Of foremost concern on the sludge quality is its heavy metal content. Table 1.3 shows record of heavy metal contents of sludge from the various STP types. In order to gauge suitability of Malaysian STP sludges for agricultural application, the concentrations of heavy metals from the tables above are compared to the limits being adopted by; a) Limits in sewage sludge for agricultural application (available for Spain) [2]; b) Limits in UK Composting Association Quality (CAQ)Label [3]; and c) Limits in European Compost Ecolable (ECCE) (http://ec.europa.eu/environment/ecolabel/d ocuments/soil_improvers.pdf) The highest heavy metal concentration from the sampling of sludge carried out by the Accredited Laboratory is shown in Table 1.3. Comparison with other sludge approved for use in agriculture as well as compost standards showed that: a) The heavy metals contents of raw sludges comply with those approved for sewage sludges for agriculture, as can be found in Spain. b) Comparison with levels limited by the UK Composting Association Quality (CAQ) Label shows that the raw sewage levels exceed most of the CAQ s levels. c) Comparison with levels limited by the European EC Compost Ecolabel (ECCE) shows that the raw sludge levels exceed most of the ECCE s levels. The (b) and (c) exceedences above do not at all mean that the sewage sludges cannot be turned into compost. Sewage sludges can always be treated in anaerobic digesters and the digester sludges characteristics would be rather different from those of raw sludges. In the worst situation, feed mixing for composting, with wastes low in metals, such as sludges from agricultural or food industry treatment plants can be employed to adjust final compost heavy metals contents. ISBN: 978-1-61804-301-6 184
Table 1.3: Heavy metal concentration of dried sludge from STPs record and sampled by the Accredited Laboratory mg/kg DM STP Types Cd Cr total Cr VI Cu Hg Ni Pb Zn As Data from STP operators ACAS 3.5 15.2 Na 305 3.9 45 26 1063 na AOD 1.07 10.8 Na 123 2.8 19 28 842 na Data from the Accredited Laboratory ACAS <0.002 84 38 183 2.48 35 <0.01 373 53.3 SBR 5.79 82 18 81 <0.00 104 98 454 19.5 1 MOD 12.88 307 36 119 14.86 322 46 1490 56.2 AOD <0.002 102 14 215 <0.00 28 45 441 22.7 1 EA No sludge 4.3 Aeration Basin Environment Table 1.4 summarizes the findings on plant operation. In the aeration basin environment, the F/M ratio is of utmost important control parameter. Four out of the five plants, ACAS, SBR, MOD and AOD fulfill the desired ratios for effective treatment for the designed type of activated sludge process. ACAS STP plant operates at low F/M levels (0.075) or low BOD. The effluent quality can be very good but the plant can actually cater for more sewage. Whilst EA STP is overloaded in high of F/M 0.22, as the desired F/M ranges from 0.05 to 0.15 for such type of extended aeration systems. Almost all STPs have sufficient aeration to maintain a dissolved oxygen of > 1.0 mg/l in the aerobic tanks. EA type STP, ACAS type STP, MOD type STP and AOD type STP successfully maintain DO concentrations above the minimum 1.0 mg/l DO. Even though the DO levels in SBR type fall slightly below 1 mg/l (0.72-0.80 mg/l DO), the SBR system yet operates well because it is still operating within an acceptable and comparatively low F/M range. Fine bubble aeration known for a high oxygen transfer efficiency has provided excessive aeration (3.50-4.78 mg/l DO) in the aeration tank at EA type STP for such a low MLSS range. In the aerobic tanks at ACAS type STP, SBR type STP, MOD type STP and AOD type STP, the MLSS concentrations are within the desired range for efficient operating parameters. EA type STP is operating with very low MLSS concentrations at an average of 459 mg/l. Therefore, the aerobic process in the STP is not operating efficiently and is wasting energy. The low MLSS levels at EA type STP is caused by low BOD influent. Control of Solids Retention Time (SRT) in the activated sludge process is critical for ensuring effective wastewater treatment. The SRT of all the treatment plants except for MOD type STP meet the minimum retention time required to achieve a satisfactory stabilisation of the solids. A conventional activated sludge system generally works with a hydraulic retention time in a range of 5 to14 hours and extended aeration types 20 to 40 hours. The retention time is especially important for nitrification activities and population dynamics of an activated sludge system. All of the treatment plants are operated within proper efficient HRTs. The study also reveals that only SBR type STP generates sludge with very good settling properties (182-212 ml/g). At ACAS type ISBN: 978-1-61804-301-6 185
STP, the SVI of the sludge becomes high (SVI 321-558 ml/g) due the low F/M. High SVI in low F/M ratio is possibly caused by the present of filamentous microorganism. The relatively high DO (mixing) in the extended aeration tank at EA type STP creates turbulence condition which causes break-up of flocs and give rise to turbid effluent (SVI 208-370 ml/g). High HRT condition (46.57 hours) result in pin-point floc which also contributes to high SVI s. Astoundingly high SVI of 345 to 962 ml/g were observed at MOD type STP. The system which operates under a high F/M and low SRT will typically Table 1.4: Summary of Findings on Plant Operation Specific Process at STP Aeration Basin Environment F/M 0.25-0.50 DO [mg/l] ACAS EA SBR MOD AOD >1.0 0.79-5.484 0.075 0.05-0.15 >1.0 3.50-4.78 0.22 0.05-0.30 >1.0 0.72-0.80 Value 0.12 0.05-0.15 >1.0 1.83-1.86 Value 0.052 0.05-0.10 0.017 >1.0 0.20-1.98 MLSS [mg/l] SRT [day] HRT [hour] 1500-3000 2108-4333 (3574) 2500-5000 364-472 (459) 3000-6000 2190-3650 (2888) 2500-5000 3000 2500-5000 1770-2760 (2440) 5-10 9.71 15-25 19.29 10-30 33.93 20-30 5.27 20-30 34.56 5-14 11.87 20-35 22.19 15-40 20.42 15-35 13.66 20-35 31.54 Clarifier Conditions SVI [ml/g] 80-200 321-558 80-200 208-373 80-200 182-212 80-200 345-962 80-200 128-358 Sludge Generation Dewatered sludge [tonne/day] Production rate [kg/m 3 treated influent] 20 No generation 6.07 8.18 6.55 0.932 0.339 0.931 0.551 ISBN: 978-1-61804-301-6 186
settle and compact more slowly and leave large straggler type floc and a slightly cloudy supernatant. At this elevated SVI, the sludge settles very slowly and compacts poorly in the settleability test. The SVI s at AOD type STP may sometimes increase substantially to 358 ml/g. It appears that low F/M and low aeration basin dissolved oxygen (DO) concentration for the applied organic loading (F/M) leads to filamentous bulking in the clarifier. It can be seen the extended aeration plants at SBR type STP and AOD type STP produces less sludge than a conventional activated sludge. The secondary sludge at EA type STP is basically inert so no sludge generation. The low MLSS content in the aeration tank causes less sludge generation. The amount of sludge generated depends highly on the SRT of the system. The comparatively high sludge production at ACAS type STP (0.932 kg/m 3 ) is due to the low F/M of the system. 4 Conclusion The activated sludge wastewater treatment process is capable of producing an excellent effluent quality when properly designed, constructed and operated. The five selected treatment plants for sewage treatment include conventional activated sludge system and its modifications. The various pollutant parameters of sewage can be effectively removed via the biological systems provided that the operating conditions are optimized and monitored. There are three areas of major concern for the operator of an activated sludge plant. Acknowledgements The authors wishes to extend sincere thanks to the many individuals and organisations for their kind support in the study, in particular, the staff of Jabatan Perkhidmatan Pembetungan and Indah Water Konsortium Sdn. Bhd. We wishes to acknowledge their invaluable assistance and highly appreciated cooperation provided to complete the study. References [1] Environmental Quality Act 1974. Laws of Malaysia, Regulations, Rules and Orders. Environmental Quality (Industrial Effluent) Regulations, 2009, pp. 421-423. [2] Mosquera-Losada M.R., Muñoz-Ferreiro N, Rigueiro-Rodríguez A. Agronomic characterisation of different types of sewage sludge: policy implications. Waste Management Vol.30, 2010, pp.492-503. [3] EU (European Union) 1986. DOCE nº L 181 04/07/1986.Council Directive 86/278/EEC of 12 June 1986 on the protection of the environment and, in particular of the soil, when sewage sludge is used in agriculture. (a) The characteristics of the influent that is going to the aeration basin. (b) The environment in the aeration basin that must be maintained to ensure good treatment. (c) The operating conditions within the secondary clarifier, which affect how well solids separation will occur. All three of these areas are closely related and influence each other. ISBN: 978-1-61804-301-6 187