Management of phosphorus removal in municipal wastewater treatment plants

Abstract

Bu çalışmanın amacı, evsel atıksu arıtım tesislerinde, değişen atıksu muhtevasına bağlı olarak fosfor giderim veriminin optimizasyonudur. Bu tez çalışmasında büyük ölçekli bir atıksu arıtma tesisinde GBFG mekanizması hem saha ölçekli hem de laboratuar ölçekli çalışmalarla incelenmiştir. İncelemeler 5-kademeli Modifiye Bardenpho prosesine oldukça benzerlik gösteren bir proses yapısında gerçekleştirilmiştir. Giriş ve çıkış atıksuyu, nütrientler ve bunların çeşitli formları için karakterize edilmiş ve biyolojik arıtma ünitelerini kapsayan kütle dengeleri oluşturulmuştur. Atıksu karakterizasyonuna ilave olarak aktif çamur fosfor içeriği, fosfor depolayan bakterilerdeki (FDB) hücre içi fosfor içeriği ve bu bakterilerin aktif çamurdaki oranı, değişken nütrient yükleri ve işletme parametreleri altında irdelenmiştir. Saha ölçekli araştırmaların yanı sıra, laboratuar ölçekli yöntemlerden de faydalanılarak, aktif çamurun oksijensiz ortamda fosforu salım hızı ve çözünmüş formdaki besin maddelerini tüketim hızı, anoksik ortamda fosforun bakteri bünyesine geri alım hızı ve eş zamanlı olarak meydana gelen nitrat asimilasyonu, oksijenli ortamda fosforun bakteri bünyesine alım hızı tespit edilmiştir. Partikül formda bulunan besin formlarının BFG süreci açısından önemi hem saha hem de laboratuar ölçekli kesikli deneyler ile irdelenmiştir. As a result of uncontrolled discharge of wastewaters containing high levels of phosphorus to the receiving bodies could create adverse effects on water quality. Therefore phosphorus should be eliminated prior to discharging into aquatic environment together with carbon and nitrogen to reduce eutrophication risk. Control of effluent phosphorus level by enhanced biological methods became a standard wastewater treatment application due to its high wastewater purification efficiency and its enhancing effect on overall biological treatment stages. In enhanced biological phosphorus removal (EBPR) processes, carbon, nitrogen and phosphorus could be removed simultaneously with higher efficiencies as compared to the conventional biological treatment methods. Another common method applied in wastewater treatment plants (WWTPs) for removal of phosphorus is chemical precipitation method in which aluminum and ferrous salts, lime were used as coagulants. Although these methods are not able to remove phosphorus completely from the theoretical aspect, significant phosphorus removal efficiencies could also be achieved. However chemical phosphorus removal methods could increase sludge production rate significantly in WWTPs. Investigations indicated that characteristics of this chemical sludge are very different from typical activated sludge. It was also reported that sludge treatment applications including thickening - dewatering, anaerobic digestion, thermal drying and incineration could be more complex and costly as compared to the non-chemical sludge. In this study, fundamental characteristics of EBPR were investigated in a large wastewater treatment plant by full-scale and laboratory scale methods. The process configuration in which investigations were carried out was very similar to 5-Stage Modified Bardenpho process. To obtain accurate results mass balances were established around biological treatment units performing detailed influent and effluent characterization. In addition to the wastewater characterization, phosphorus content of both activated sludge and phosphorus accumulating organisms (PAOs) were examined under variable nutrient loading ratios and variable operational conditions. Anaerobic phosphorus release rate, soluble substrate utilization rate, anoxic phosphorus uptake rate, simultaneous denitrification rate and aerobic phosphorus uptake rate were also determined by laboratory-scaled batch tests in addition to the full-scale investigations. Importance of particulate substrate forms in EBPR was also studied using both full-scale and batch-scale methods. Results of the investigations proposed that EBPR method could be the main phosphorus management method in WWTPs. However it was also observed that the stability of the EBPR processes could not be maintained continuously since the process influences by several wastewater and operational parameters. Therefore EBPR processes could be supported by chemical phosphorus removal methods to achieve target effluent phosphorus level. It was also demonstrated that COD/TP ratio of the influent is very important for the optimization of the EBPR processes. Obtained results indicated that COD/TP ratio of 65 could be required for the domination of phosphorus accumulating microorganisms (PAOs) that are perquisites EBPR processes in the scale of İzmir WWTP. Serious adverse effects of electron acceptors, present in both influent and return sludge line, on EBPR performance were also determined. In the scope of this thesis, several operational strategies for the optimization of the EBPR plant were also developed.