Investigation of enhanced biological phosphorus removal characteristics in İzmir Wastewater Treatment Plant

Phosphorus (P) is an essential nütrient for all life forms. It is also one of the limited and non-renewable natural resources. Furthermore, treated wastewater containing high level of P could cause serious problems associated with eutrophication in the receiving water bodies (Janssen et al., 2002). Removal of nutrients by biological methods is cost effective and environmentally sound alternative to the chemical treatment of wastewater (Osee et al., 1997). İzmir bay is one of the great natural bays of the Aegean Sea. Total surface area of the bay is 500 km2 and total water volume is 11.5 billion m3 (Kucuksezgin et al., 2005). To prevent discharge of untreated wastewaters into the bay, İzmir WWTP was taken into operation in early 2000. The plant was designed to treat both domestic and pre treated industrial wastewater collecting from the İzmir metropolitan area. Since previous scientific investigations indicated that both Nitrogen (N) and P concentrations of the sea were critical level with respect to eutrophication problem, the plant design was performed for the combined removal of Carbon (C), N, P in the activated sludge process following adequate physical treatment including fine screens, aerated grit removal chambers and circular primary sedimentation tanks. The average design capacity of the plant is approximately 605.800 m3/d.  Mechanism of EBPR is based on selection of P accumulating microorganisms (PAOs) in the activated sludge culture by exposing the microorganisms into anaerobic, anoxic and aerobic environments. Preferential selection of PAOs in the system is attributed to energy conversion ability of these microorganisms from storage of simple carbon forms (mainly in the form of volatile fatty acids). In these assimilative reactions energy is derived from hydrolysis of intracellular poly-P reserves (Comeau et al, 1986; Mino et al., 1998). Generated energy from the P release in the anaerobic zone is used for transportation of volatile fatty acids (VFAs) from bulk liquid interface to the cell inventory of PAOs. These substrate forms are stored in the intracellular environment as polyhydroxy­alkanoates (PHA). In the following aerobic and anoxic zones of the EBPR process, stored PHA is utilized to generate required energy for reproduction of new cells, maintenance and restoring depleted poly-P reserves using electron acceptors either in the form of dissolved oxygen (DO) or nitrate (USEPA, 1987; Kuba et al., 1996; Lee et al., 2003;Panswad et al., 2007 ). Availability of readily biodegradable COD (rbCOD) in the anaerobic zone is among the essential considerations. It was reported that at least 20 mg as acetic acid (Janssen et al., 2002; Abu-ghararah, 1991), 50 mg as COD (Ekama and Marais, 1984) are required to remove 1 mg of P. According to the previous scientific investigations, EBPR process could be considered as COD limited when the COD/TP ratio is low (<20:1 for settled domestic sewage), whereas it is P limited when the COD/TP ratio is high (Randall et al., 1992). While low COD/TP ratios could cause EBPR failures, very low effluent P concentrations achievable at sufficient COD/TP ratios. Although EBPR systems are well established, there are several unclear issues in identification of microbial dispersal. It was also reported that although several methods developed to analyze the microbial structure of the EBPR process; several critical issues are still unclear (Panswad et al., 2007). Moreover, many scientific investigations in this field were based on laboratory scaled studies. This study aims to investigate fundamental EBPR characteristics and identify microbial responses to variable organic loading rates in the large scale EBPR process. Investigations were conducted in İzmir WWTP, serving 3.5 million population equivalent, between 2006 and 2007. In order to evaluate the EBPR process accurately, influent and effluent wastewater were characterized for various forms of nutrients. Mass balances were performed around anaerobic, anoxic and aerobic zones considering all main and side streams. All required data for accurate evaluation of the EBPR process including environmental and operational variables including pH, temperature, mixed liquor volatile suspended solids concentrations, hydraulic retention times in biological treatment units, sludge age, inflow rate, return sludge and internal recirculation rates were determined during the monitoring period. Batch scale tests were also performed parallel to the full scale investigations to identify microbial responses. All of the experimental results were statistically analyzed and evaluated considering previously obtained theoretical background in the field. Keywords: Enhanced biological phosphorus removal, mass balance, phosphorus accumulating microorganisms, denitrification.  Atıksu arıtımında biyolojik aşırı fosfor giderimi (BAFG) ülkemizde ve Avrupa Birliği ülkelerinde oldukça tercih edilen bir süreç haline gelmiştir. Bu çalışmada değişen organik yükleme hızlarına ve çevresel faktörlere bağlı olarak tam ölçekli bir BAFG sürecinde oluşan biyokimyasal ve mikrobiyolojik değişimler incelenmiştir. BAFG süreçlerinin temel filozofisi, aktif çamurun sırası ile anaerobik, anoksik ve aerobik ortamlarda tutularak fosfor depolama yeteneğine sahip bakteri türlerinin (FDB) süreç içerisinde baskın hale getirilmelerine dayanmaktadır. Bu çalışma kapsamında, büyük ölçekli bir BAFG sürecinin anaerobik, anoksik ve aerobik ortamlarında kütle dengeleri oluşturularak, kinetik bağıntılar yardımı ile sistemde oluşan bakteri türleri ve nütrient giderim hızları tespit edilmeye çalışılmıştır. Yapılan saha çalışmaları laboratuar ölçekli deneylerle desteklenmiştir. İncelemeler neticesinde FDB türlerinin sistemdeki kütlesel oranının %9 ile %34 arasında değiştiği tespit edilmiştir. Daha önceki bilimsel çalışma sonuçlarını doğrular nitelikte, FDB’lerin hücre ağırlığının %32'si oranında, aşırı miktarda fosfor (P) depolayabilecekleri belirlenmiştir. Buna ilave olarak, aktif çamur kültüründeki FDB kütlesel oranının artması ile birlikte sadece P giderim verimliliği değil aynı zamanda karbon (C) ve azot (N) giderim hızlarının da önemli ölçüde artabileceği saptanmıştır. Bu bakteri türlerinin (FDB) sistemdeki oranlarının ise atıksu içerisindeki basit karbon formları ile yakından ilişkili olduğu saptanmış olup, bu konuda yapılmış olan bilimsel çalışmaları doğrular nitelikte sonuçlara ulaşılmıştır. Sistemdeki FDB kütlesel oranının %30'un üzerine çıktığı durumlarda, anoksik ve aerobik P giderim hızlarının 0.1 mg P (g UAKM(Uçucu askıda katı madde)-1 dk-1, denitrifikasyon hızının 0.04 mg NO3-N (g UAKM)-1 dk-1 ve uçucu yağ asidi (UYA) giderim hızının ise 0.5 mg UYA (g UAKM)-1 dk-1 değerlerine ulaşabileceği belirlenmiştir.  Anahtar Kelimeler: Biyolojik aşırı fosfor giderimi, kütle dengesi, fosfor depolayan bakteri, denitrifikasyon.&nbsp