Impact of Petroleum Refinery Wastewater on Activated Sludge

Transcription

1 Impact of Petroleum Refinery Wastewater on Activated Sludge M. A. R. M. M. Amin, S. R. M. Kutty, H. A. Gasim, M. H. Isa Civil Engineering Department, Universiti Teknologi PETRONAS Bandar Seri Iskandar, Tronoh, Perak D.R., MALAYSIA Abstract: - The conventional removal of dissolved organic compounds requires biological treatment such as an activated sludge system. In this study, petroleum refinery wastewater was co-treated with municipal wastewater in continuous flow bench scale reactors. Two bench scale reactors, A and B each of liquid volume of 21 L treated municipal wastewater and a mixture of municipal and refinery wastewater, respectively. The organic load of the refinery wastewater in reactor B was increased from 20% to 40%, 60%, 80% and 100% throughout the study period. The flow rate, Q for both reactors was maintained at 10.5 L/day with the hydraulic retention time (HRT) and sludge retention time (SRT), set at 1 day and 40 days, respectively. Food to microorganism (F/M) ratio, concentration of effluent total chemical oxygen demand (COD), mixed liquor suspended solids (MLSS), and mixed liquor volatile suspended solids (MLVSS) were monitored throughout the study period. The result of five months study period indicated high removal efficiencies of organic matter of approximately 90% as COD with 40% loading of the refinery wastewater. Key-Words: Petroleum refinery wastewater, Organic loading, Extended aeration activated sludge. 1 Introduction As Petroleum refining is a complex combination of interdependent industrial processes that generates wastewater effluent containing oil, ammonia, sulfides, chlorides, mercaptans, phenols and other hydrocarbons [1]. The most important pollutants are organics, oils, suspended solids and other toxic materials referred to as priority pollutants [2]. The presence of toxic organic compounds in receiving waters and water supplies has modified the emphasis of wastewater treatment during the past several decades. Stringent effluent concentration limits are required especially for the organic priority pollutants. Thus, the complete range of available treatment technologies has been applied in this field, very often with disregard to high treatment costs [3]. However, recent studies indicated that conventional biological treatment is also capable of removing many organic compounds, and is effective in removing most of the compounds. Modifications and alternatives to conventional biological treatment methods has been suggested and evaluated in bench, pilot and full-scale systems [4-6]. Among such modifications has been the extended aeration to the aeration basin of activated sludge facilities [7-9]. The sludge retention time (SRT) is the time where the activated sludge remains in the reactor before wasted. Long SRT (28 d) resulted in high chemical oxygen demand (COD) and biochemical oxygen demand (BOD) removal [9]. More studies [9-11] have concluded that extending the SRT generally improved removal efficiency of biological degradation. The studies showed that extended aeration has the highest percent removal efficiency for biological degradation (>90%). Zhao-Bo et al., [12] found that during the pilot experiment, the SRT varied from 1200 to 2400 h and the average removal rate of COD remained at 98%. It proved that the removal rate of organic pollutants was high and stable with 2400 h SRT. Petroleum refinery wastewater treatment using biological methods is widely investigated. Petroleum refinery wastewater was found to be ultimately biodegradable in batch treatment over 24 hours of reaction time [13]. Anaerobic treatment of petroleum refinery wastewater under different loads was successfully used with 83% removal, but the final effluent was not up to discharge limit and aerobic polishing stage was required [14]. High COD removal (88%) was achieved when petroleum refinery was co treated with domestic wastewater (50%) in sequencing batch reactor in 24 hours cycle [15] or less [16]. In this study, the extended aeration (SRTs = 40 d), continuous flow bench scale activated sludge bioreactors were investigated on the impact of organic load of petroleum refinery wastewater in municipal wastewater treatment. The objectives of this study were to determine the impact of increased organic load of refinery wastewater on an activated sludge operated at long sludge age of 40 days. The impact of increased organic load of the refinery wastewater was evaluated by increasing the loading of refinery wastewater at 10%, 20%, 40%, 60%, 80% and 100% in the influent feed. The COD in influent and effluent tanks was continuously monitored throughout the study period. ISBN:

2 2 Methodology Two continuous flow bench scale reactors were constructed. The height and width were 20 cm and 30 cm, respectively. The length of the aeration chamber and the settling chamber were 31 cm and 16 cm, respectively. Total liquid volume in the tank was 21 L. The experimental set-up consisted of a raw refinery wastewater influent tank, a stirrer with stand, an influent line, a peristaltic pump, an aeration chamber, air supply, air diffuser stones connected to air supply, settling chamber, overflow line for effluent and effluent tank. Raw refinery wastewater was obtained from an oil refinery facility and stored at 4 C prior to treatment. Wastewater in the influent tank was continuously stirred to keep solids in suspension. The wastewater was fed into two parallel reactors A and B. Reactor A act as a control and treat only municipal wastewater while reactor B treat a mixture of municipal wastewater and refinery wastewater. The percentage of refinery wastewater in the influent to reactor B was increased in stages from 10% to 20%, 40%, 60%, 80% and 100%. Initially, both reactors were acclimatized with municipal wastewater for 13 days prior to feeding of the refinery wastewater to reactor B. Both reactors were seeded with 1.1 L biomass taken from a sewage treatment plant. Aeration to the reactors was supplied through stone diffusers at mg/l of oxygen. The study was conducted at room temperature (26 C) and and ph 7.0. The flow rate, Q for both reactors was maintained at 21 L/d with 1 day hydraulic retention time (HRT) and 40 days SRT. The sludge age was maintained at 40 days by daily wasting 0.45 L of biomass from the clarifier. Sampling of the feed and effluent wastewater were collected daily and analyzed in triplicates according to the Standard Methods [17 and 18]. Table 1 shows the characteristics of the refinery wastewater and municipal wastewater used in this study. Table 1: Petroleum refinery & Municipal Wastewater Characteristic Wastewater Parameter Unit Refinery Municipal COD mg/l Ammonia mg/l Nitrate mg/l Phosphorus mg/l ph Results and discussion Influent COD Loading Figure 1 shows influent COD loading vs sampling days for both reactors throughout the study period. COD loading of both reactors was observed to be unsteady during the acclimatization period. It should be noted that influent to reactor A was taken from the domestic wastewater and was observed that the COD concentration was inconsistent due to the fluctuation of domestic wastewater. However, the COD load for both reactors A and B stabilized after 30 th sampling day and averaged at approximately 0.32 kg COD/m 3 d. During the acclimatization period, average influent COD concentration was approximately 300 mg/l for both reactors. The loading was observed to be relatively constant for reactor A throughout phase 2 with slight decrease on 42 nd sampling day to 0.30 kg COD/m 3 d and increased to 0.34 kg COD/m 3 d on 62 nd sampling day. However, influent COD for reactor A was steady at an average concentration of 336 mg/l from 44 th sampling day until the end of phase 2. Organic Loading (kg COD / m 3 ) % - 90% - A 20% - 80 % -A 0.0 Samping days Reactor A 40% - 60% -A 60% - 40% -A Reactor B 80% - 20% -A Figure 1: Organic Loading vs sampling days 100%- 0% -A COD loading and influent COD concentration for reactor B were simultaneously increased as per increased of refinery wastewater added from 10% to 100%. When 10% of the refinery wastewater was added into the influent tank of reactor B, the COD loading increased to approximately 0.5 kg COD/m 3 d from 2 nd sampling day to 60 th sampling day with influent COD concentration increased to an average of approximately 570 mg/l. When 20% of the refinery wastewater was added on 61 st sampling day, COD loading and influent COD concentration increased to an average of approximately, 0.84 kg COD/m 3 d and 820 mg/l, respectively. On 89 th sampling day, the addition of refinery wastewater was doubled from 20% to 40% caused COD loading increased to approximately 1.23 kg COD/m 3.d with influent COD concentration increased to 1200 mg/l from 89 th sampling day to 111 th sampling day. With 60% addition of the refinery wastewater on 114 th sampling day, COD loading and influent COD concentration increased to an average of 1.82 kg COD/m 3 d and 1800 mg/l, respectively and they were constant until end of the loading period. The loading significantly increased to approximately 3.07 kg COD/m 3 d when 80% of refinery wastewater was added and consequently ISBN:

3 increased the influent concentration to 3000 mg/l from 138 th sampling day to 160 th sampling day. When 100% of raw refinery wastewater was added on 161 st sampling day, COD loading and influent concentration increased to 3.48 kg COD/m 3 d and 3400 mg/l, respectively until the end of the period. The variations of COD loading for the study and average values for influent COD concentration in reactor B were summarized in Table 2 and Table 3, respectively. During the study, the SRT and HRT were maintained at 40 days and 1 day, respectively. Table 2: Average variations of COD loading Sampling days W (%) Reactor B (kg/m 3 d) Reactor A (kg /m 3 d) W (kg/m 3 d) The influent COD sample was filtered using a microfibre filter paper to obtain soluble COD concentration (scod). Figure 1 shows the influent scod concentration vs sampling days. Influent scod concentration was observed to be fluctuating during the acclimatization period. On the 36 th sampling day, influent scod concentration for reactor A was observed to have increased to an average concentration of 146 mg/l. From 89 th sampling day to 113 th sampling day, scod averaged at 124 mg/l after which it stabilized to an average concentration of 150 mg/l. Table 3 shows the summary of influent scod concentrations. Table 3: Average influent scod concentration of reactor A and B W scod scod Ratio Ratio Sampling added (B) (A) COD:sCOD COD:sCOD days (%) (mg/l) (mg/l) (B) (A) Influent scod concentration for reactor B was observed to be approximately 110 mg/l during acclimatization period. When 10% refinery wastewater was added, average influent scod increased to 210 mg/l. With an addition of 20% of refinery wastewater, influent scod concentration was further increased to an average of 250 mg/l. When 40% of refinery wastewater was added into the influent tank of reactor B, average influent scod increased significantly to 680 mg/l. Further increment in the refinery wastewater addition to 60% and 80%, influent scod concentration increased to 1280 and 1500 mg/l, respectively. When 100% raw refinery wastewater was added, average influent scod was 2170 mg/l. From Table 3, ratio of COD:sCOD for reactor A was observed to be relatively consistent throughout the phase 2 study, with a ratio COD:sCOD of 2.82 at beginning until 43 rd sampling day. However, it decreased to 2.29 from 44 th sampling day to 62 nd sampling day. On 63 rd sampling day, the COD:sCOD ratio increased again to 2.72 and constant until 88 th sampling day. However, the COD:sCOD ratio decreases to the lowest value of 2.22 from 89 th sampling day until 113 th sampling day. On 114 th sampling day, the ratio COD:sCOD increase slightly to 2.35 and stabilized until the end of the study. For reactor B, ratio of COD:sCOD was observed to be inconsistent throughout the study. Initial ratio of COD:sCOD calculated was 2.61 until 43 rd sampling day. The COD:sCOD ratio increased to 2.78 from 44 th sampling day until 62 nd sampling day. Then COD:sCOD ratio was further increased to 3.30 from 63 rd sampling day until 88 th sampling day. However, on 89 th sampling day, COD:sCOD ratio decreased to 1.78 and was further decreased to 1.40 from 114 th sampling day until 137 th sampling day. However, COD:sCOD ratio increased to 2.00 on 138 th sampling day until 161 st sampling day. COD:sCOD ratio decreased again 1.58 on 162 nd sampling day and constant until the end of the study period. Effects of Loading on MLSS & MLVSS Figure 2 shows the MLSS and MLVSS vs sampling days, respectively, for both reactors, A and B throughout the study period. Conc. (mg/l) MLSS and MLVSS vs Sampling days 10% - 90% -A 20% - 80 % -A 40% - 60% -A 60% - 40% -A 80% - 20% -A Sampling Days MLSS Reactor A MLSS Reactor B MLVSS Reactor A MLVSS Reactor B Figure 2: MLSS & MLVSS in reactor A and B 100%- 0% -A The MLVSS and MLSS were observed to be unstable at the start of the acclimatization period for both reactors. This was due to the low population of microorganisms in the beginning as well as the adaptation of microorganisms to the new environment of refinery wastewater [19]. During the acclimatization period, MLSS for both reactors A and B stabilized at the average concentration of approximately 6000 mg/l from 31 st sampling day. ISBN:

4 However, MLVSS for reactor A and B, stabilized at an average of approximately 3500 and 3000 mg/l, respectively from 26 th sampling day. MLSS and MLVSS for reactor A were generally higher than reactor B throughout the study period. Reactor B had experienced major decrease towards the end of the phase 2 while reactor A had only slight decrease. This was corresponding to the changes of COD loading for respective reactors. The decreased in both MLSS and MLVSS was due to the endogenous respiration over the time as well as may be due to insufficient amount of nutrients supplied to the growing population of microorganisms [19]. Throughout the study period, the MLSS and MLVSS for reactor A, averaged approximately at 5900 and 3500 mg/l, respectively giving a ratio of MLVSS/MLSS of only For reactor B, when COD loading was increased from 0.32 kg COD/m 3 d to 0.50 kg COD/m 3 d, an immediate impact was observed on MLSS which decreased to an average concentration of approximately 5000 mg/l throughout this loading period with a corresponding MLVSS that averaged approximately 2900 mg/l. Toxic impact from the refinery wastewater on the biomass may be the cause of the drop in the concentrations. When COD loading was increased to 0.84 kg COD/m 3 d on 63 rd sampling day, no observed changes in MLSS and MLVSS was observed as the biomass may have acclimatized to the refinery wastewater. However, when the COD loading was increased to 1.23 kg COD/m 3 d on 89 th sampling day, a slight decrease was observed on MLSS and MLVSS resulting with an average concentration of approximately 4700 mg/l and 2300 mg/l, respectively. When COD loading was further increased to 1.82 kg COD/m 3 d, further decrease of MLSS and MLVSS was observed to an average concentration of approximately 3800 mg/l and 1700 mg/l, respectively, for reactor B throughout the loading period. However it was observed that, when COD loading was further increased to 3.07 kg COD/m 3 d, both MLSS and MLVSS increased to an average concentration of approximately 4000 mg/l and 2100 mg/l, respectively, from 138 th sampling day until 160 th sampling day. This may be due to new stock of refinery influent was fed into the reactors. The MLSS and MLVSS stabilized when COD loading was increased to 3.48 kg COD/m 3 d, with an average of approximately 4000 and 2000 mg/l respectively and stabilized until the end of the study. Removal of COD Figure 3 shows effluent COD concentration vs sampling days for reactors A and B. COD (mg/l) % - 90% -A 20% - 80% -A Effluent A 40% - 60% -A Sampling days Effluent B 60% - 40% -A 80% - 20% -A 100%- 0% -A Figure 3: Effluent COD concentration vs. sampling days From Figure 3, it can be observed that effluent COD concentration for reactor A was lower than reactor B during the acclimatization period even though the influent COD concentration was same for both reactors, A and B. The average concentration for reactor A and B during acclimatization period were approximately 10 mg/l and 25 mg/l, respectively. Effluent COD concentration for reactor A was observed to be stable from 24 th sampling day onwards until the end of the study period, with an average COD concentration of approximately 17 mg/l. This was still below the standard effluent discharge of standard A, 50 mg/l. Reactor A only received domestic wastewater throughout the study. For reactor B, effluent COD concentration was observed to be immediately impacted with the addition of 10% refinery wastewater (0.50 kg COD/m 3 d) in the influent. This resulted in effluent COD concentration to increase from 25 mg/l to 56 mg/l on 43 rd sampling day and stabilized at an average of approximately 55 mg/l at the end of the loading period. When 20% of refinery wastewater was added (0.84 kg COD/m 3 d) in the influent to reactor B, effluent COD concentration was observed to further increase and stabilized at average concentration of approximately 67 mg/l from 63 rd sampling day until 88 th sampling day. The increased influent COD loading may have impacted the biomass. However, effluent COD concentration was observed to have little decrement on 89 th sampling day with an average value of approximately 57 mg/l when 40% refinery wastewater was added (1.23 kg COD/m 3 d) in influent to reactor B. This still meets the limit of effluent discharge for refinery wastewater sample, of 100 mg/l. However, effluent COD concentration was over the limit when addition of 60% of refinery wastewater was added (1.82 kg COD/m 3 d) in influent to reactor B. The effluent COD concentration was significantly increased to an average value of approximately 177 mg/l from 114 th sampling day until 137 th sampling day. Effluent COD concentration was further increased when 80% of refinery wastewater was added (3.07 kg COD/m 3 d) in influent to reactor ISBN:

5 B with an average of approximately 279 mg/l. However, when 100% of refinery wastewater was added, (3.48 kg COD/m 3 d) in influent to reactor B, no changes in effluent COD concentration was observed since it has stabilized with an average value of approximately 279 mg/l until the end of the study phase. From Figure 3, it can be observed that throughout the study period, the percent COD removal for reactor A was relatively completed and it remain constant averaged approximately at 97% with averaged effluent COD approximately at 17 mg/l. This meets the standard limits for discharge 50 mg/l. Reactor A was operated at sludge age of 40 days with the MLSS averaging approximately at 5900 mg/l [20]. In Figure 3, prior to addition of 10% refinery wastewater, effluent COD averaged approximately at 25 mg/l with an average COD removal of approximately 88%. However, when the organic load was increased with an addition of 10% refinery wastewater, effluent COD concentration of reactor B was immediately impacted resulting in increased effluent COD to approximately 54 mg/l. When the organic load was increased to 20% by addition of refinery wastewater, a slight increase in effluent COD was observed throughout this addition period with the final effluent COD and effluent COD removal averaged approximately at 67 mg/l and of 91%, respectively. However, the reactor acclimatized and stabilized to a lower effluent COD approximately at 57 mg/l at the 89 th sampling day with corresponding COD removal of approximately 97% at the end of the period. However, effluent COD averaged at 57 mg/l throughout this loading period still meet the limit for effluent discharge. However, when the refinery wastewater loading was increased to 60%, significant increased was observed on effluent COD concentration with effluent COD averaged approximately at 177 mg/l throughout this loading period with corresponding percent COD removal decreased to 89%. On 138 th sampling day, when 80% load of refinery wastewater was increased, effluent COD concentration observed to further increased to an average approximately 279 mg/l. However percent COD removal observed to remain constant at 90%. It was also observed that as the reactor acclimatized, the average percentage removal of COD increased from 90% to 91% at 161 st sampling day when 100% of refinery wastewater was loaded corresponding with a constant effluent COD concentration average at approximately at 279 mg/l. Hence, it can be observed that the discharge limits was met up until 40% loading of the refinery wastewater [20] only, which below 100 mg/l. Even though percent COD removal at 60% loading of refinery wastewater onwards was relatively high (90%), it does not mean the effluent COD concentration still met the discharge limit. Impact of Organic Load on F/M ratio Figure 4 shows the F/M ratio vs. sampling days for reactors A and B throughout the study period. F/M (kg COD / kg MLVSS) % 90% A 20% 80% A F/M Ratio Sampling days reactor A 40% 60% A 60% 40% A reactor B 80% 20% A Figure 4: F/M ratio vs. sampling days 100% 0% A It can be observed that during the acclimatization period, the F/M ratio for both reactors was approximately 0.10 kg COD/kg MLVSS d which was within the range of EAAS system [20]. The F/M ratio for reactor A stabilized at approximately 0.10 kg COD/kg MLVSS d throughout the study period as it only treats municipal wastewater. The F/M ratio was observed to remain stable towards the end of the study period due to a relatively consistency of the municipal wastewater loading. Even though the population of the microorganisms will increase over the time [21], the constant F/M ratio was also expected since there might be endogenous respiration occurs within the reactor. For Reactor B, when the loading of the refinery wastewater was increased to 10%, it can be observed that the F/M ratio in reactor B was increased and stabilized to an average of approximately 0.20 kg COD/kg MLVSS d from 44 th sampling day to 62 nd sampling day. This F/M ratio now exceeds the range of an extended aeration system of kg COD/kg MLVSS d [20]. When the refinery loading in reactor B was increased to 20%, the F/M ratio also increased to an averaged at approximately 0.27 kg COD/kg MLVSS d until the end of the 20% refinery loading period. Thus, the F/M ratio was already exceeding the range of EAAS system [20]. When the refinery loading was increased to 40% at the 89 th sampling day, the F/M ratio in reactor B was further increased to 0.55 kg COD/kg MLVSS d until 113 th sampling day. However, when fresh batch samples were added on the 114 th sampling day, the F/M ratio significantly increased to an average of approximately 1.10 kg COD/kg MLVSS d. This was due to the increased of the influent COD ISBN:

6 concentration with the average MLVSS relatively constant. F/M ratio was further increased when 80% of refinery wastewater loading was added on 138 th sampling day with an average of approximately 1.41 kg COD/kg MLVSS d. Thus, an increased in the food supply will increase the F/M ratio when the microorganisms remain constant. It then stabilized to an average of 1.71 kg COD/kg MLVSS d from the 160 th sampling day until the end of the study when 100% loading of refinery wastewater added. It can be concluded that increased organic load to the system will increase the F/M ratio as more substrate is provided and hence promotes higher growth of biomass [21]. 4 Conclusions The co-treatment of petroleum refinery wastewater with municipal wastewater up to 40% showed minimal impact on effluent COD while still meeting the Malaysian standard effluent discharge limits. Approximately 90% removal of scod was achieved with 40% loading of the refinery wastewater. References [1] Jou, C.-J.G., & Huang, G.-C., A pilot study for oil refinery wastewater treatment using a fixed-film bioreactor. Advance in Env. Res. 7, pp , [2] Yamada, Y., & Kawase, Y., Aerobic composing of waste activated sludge: kinetic analysis for microbial reaction and oxygen consumption. Waste Manage. 26, pp , [3] Chih-Ju, G., Jou, & Guo-Chiang, H., A pilot study for oil refinery wastewater treatment using a fixedfilm bioreactor. Advance in Environ. Res. 7, pp , [4] Nemukula, A., Bacterial treatment of precious metals refinery wastewater. Honours thesis, Dept. of Bioch. Microbiol. and Biotechnol., Rhodes University, Grahams town, South Africa, [5] Tanya, A.B., Jeanne, M.T., & Sheldon, J.B.D., Effect of HRT, SRT and temperature on the performance of activated sludge reactors treating bleached kraft mill effluent. Wat. Res, 30, pp , [6] Malakahmad, A., Hasani, A., Eisakhani, M., Isa, M. H. Sequencing Batch Reactor (SBR) for the removal of Hg 2+ and Cd 2+ from synthetic petrochemical factory wastewater. J. Hazard. Mater. 191(1 3), , [7] Mohamed, A., Z., Walid, E., Characterization and assessment of Al Ruwais refinery wastewater. J. Hazard. Mater. 136, pp , [8] Izanloo, H., Mesdaghinia, A., Nabizadeh, R., Naddafi, K., Nasseri, S., Mahvi, A. H., & Nazmara, S., The treatment of wastewater containing oil with aerated submerged fixed-film reactor. Pak. J. Bio. Sci. 10(17), pp , [9] Arceivala, S.J., Wastewater treatment and disposal, Marcel Dekker New York, pp. 892, [10] Qasim, S.R., Wastewater treatment plants. Planning, design, and operation, Holt, Rinehart & Winston: USA, pp. 726, [11] Von Sperling, M., Comparison among the most frequently used systems for wastewater treatment in developing countries. Wat. Sci. Tech, 33(3), pp , [12] Zhao-Bo, C., Dong-Xue, H., Nan-Qi, R., Yu, T., Zhen-Peng, Z., Biological COD reduction and inorganic suspended solids accumulation in a pilotscale membrane bioreactor for traditional Chinese medicine wastewater treatment. Chem. Eng. J. 155, pp , [13] H. A. Gasim, S. R. M. Kutty and M. H. Isa, Biodegradability of petroleum refinery wastewater in batch reactor, Proceeding of ICSBI 10, Kuala Lumpur Malaysia [14] H. A. Gasim, S. R. M. Kutty, M. H. Isa and M. P. M. Isa, Treatment of petroleum refinery wastewater by using UASB reactors, Int. J. Chem. Bio. Eng. 6(1), pp , [15] S. R. M. Kutty, H. A. Gasim, P. F. Khamaruddin and A. Malakahmad, Biological treatability study for refinery wastewater using bench scale sequencing batch reactor systems, in Water Resources Management VI, 2011, pp [16] Gasim H. A., S. R. M. Kutty and M. H. Isa, Petroleum refinery effluent biodegradation in sequencing batch reactor, Int. J. App. Sci. Technol. 1(6), pp , [17] APHA. Standard methods for the examination of water and wastewater, 15th ed. New York, [18] HACH Company, Water Analysis Handbook, HACH Company, 4 th edition: Colorado, USA, 2002 [19] Aoyi Ochieng, John O. Odiyo, and Mukayi Mutsago, Biological treatment of mixed industrial wastewaters in a fluidised bed reactor. J. Hazard. Mater. B96, 79 90, [20] Wastewater Engineering Textbook, Metcalf & Eddy, 21 st edition, 2010, pp 747. [21] E. Vaiopoulou, P. Melidis, and A. Aivasidis, (2007). An activated sludge treatment plant for integrated removal of carbon, nitrogen and phosphorus. Desalination. 211, ISBN: