The effect of glucose on the enhanced biological phosphorus removal in a sequencing batch reactor

Abstract

Bu çalışmada evsel atıksularda yaygın olarak bulunan glikozun biyolojik fosfor giderimi üzerine etkisi incelenmiştir. Laboratuvar ortamında glikozla beslenen anaerobik-aerobik Ardışık Kesikli Reaktörler (AKR) işletilmiş; çalışma süresince proses performansında ve mikrobiyal türlerde meydana gelen değişimler izlenmiştir. AKR’nin ilk döneminde Biyolojik Aşırı Fosfor Giderimi (BAFG) aktif olarak görülmüştür. Ancak reaktörün ikinci döneminde BAFG aktivitesi bozulmaya başlamış, son döneminde ise kabarma problemi ile karşılaşılmış ve koşullar iyileştirilemediği için işletilmesine son verilmiştir. AKR’nin başlangıç döneminde laktik asit oluşması ve oluşan laktik asitin anaerobik süreçte tüketilmesi sonucu fermentasyon bakterileri ile fosfor depolayan organizmaların (PAO) biyolojik fosfor giderimini birlikte gerçekleştirdiği belirlenmiştir. Laktik asit bakterilerinin glikozu laktik asite fermente ettiği ve PAO’ların anaerobik fosfor salınımından enerji sağlayarak oluşan laktik asidi polihidroksialkonata (PHA) dönüştürdüğü düşünülmüştür. Bu dönemdeki mikroskobik gözlemlerde poli-P depolayan kokların yoğun olarak görülmesi ve filogenetik analiz sonucunda Firmicutes filumuna ait Lactococcus türlerinin mikrobiyal topluluğun önemli bir bölümü olarak tespit edilmesi bu varsayımı desteklemektedir. Ayrıca oluşan PHA’nın %77’sinin 3-hidroksivalerat (3HV) olması ve anaerobik süreçte laktik asitin tüketilmesi laktik asitin PAO’larca kullanılan esas karbon kaynağı olduğunu kanıtlamaktadır. AKR’nin 29’uncu gününde  glikojen depolayan organizmalar (GAO) olarak tanımlanan Candidatus Competibacter Phosphatis türü filogenetik analizde yoğun olarak gözlemlenmiş ve BAFG aktivitesinin bozulmasının bu türün baskı hale gelmesinden kaynaklandığı belirlenmiştir. 29’uncu günle birlikte glikojen tüketiminin önemli miktarda artması, glikoz tüketimine karşı salınan fosfor değerinin (0.07 mol P/ mol C) düşmesi sistemde GAO’ların aktif olduğunu göstermektedir. Anahtar Kelimeler: Biyolojik aşırı fosfor giderimi, fosfat depolayan organizmalar, glikojen depolayan organizmalar, glikoz.Since short chain fatty acid (SCFA) is believed to be the favorable substrates for biological phosphorus removal, the majority of the studies on enhanced biological phosphorus removal (EBPR), focus on the metabolism of acetate. However EBPR process can also occur successfully with organic substrates other than acetate. A wide range of organic substances like carboxylic acids, sugars and amino acids can be taken up anaerobically by phosphate accumulating organisms (PAO) enriched sludges but the metabolism of these organic substrates is not clear yet. Hence, the effect of carbon sources other than acetate on EBPR has to be considered deeply. The composition of the organic substrates in domestic wastewater varies remarkably among countries and/or wastewater treatment plants and glucose is a significant simple sugar found widely in wastewaters with an important role in biochemical pathways. EBPR mechanism with glucose found wide interest but results of the reported studies in the related literature are not consistent with each other indicating many different mechanisms of anaerobic uptake and storage of glucose can act in favor of, or against EBPR. The objective of this study was to investigate the effect of glucose feeding on the performance of the enhanced biological phosphorus removal process. The effect of glucose on process performance and microbial community was studied by operating laboratory-scale alternating anaerobic-aerobic sequencing batch reactors (SBRs). The SBR fed with glucose as sole carbon source achieved biological phosphorus removal but deteriorated gradually along the operation of the reactor. During the good EBPR period (day 9), 63% of the glucose fed to the reactor was metabolized within 10 minutes of the anaerobic period and a rapid increase in glycogen concentration observed which showed the conversion of external glucose into glycogen. Lactate and acetate were detected in the supernatant and pH was dropped upon the glucose addition which indicated that part of the glucose was fermented to mainly lactate and to small amount of acetate. The results of the molecular analysis performed during this period showed the presence of many diverse fermentative bacteria proving clearly the glucose fermentation. It is assumed that lactate was the major substrate converted to polyhydroxyalkaonates (PHA) by PAOs due to the significant amount of 3-hydroxyvalerate (3HV) formation and low level of glycogen consumption under anaerobic conditions. But if lactate was the only substrate uptaken by PAOs to be converted to PHA, lactate should be metabolized to acetyl-CoA and propionyl-CoA equally to maintain the redox balance which will result in formation of 3HV and 3-hydroxy-2-methylbutyrate (3H2MB) only. However 12% of the PHA was consisted of 3-hydroxy-2-methylvalerate (3H2MV) and small amount of glycogen was also consumed. Hence glycogen consumption together with lactate changed the ratios of acetyl-CoA and propionyl-CoA metabolized which could explain the formation of 3H2MV. Thus PHA was thought to be derived not only from the lactate but also from glycogen and from small amount of acetate fermented from glucose. The EBPR activity was remarkably deteriorated on the 29th day of the SBR operation. The occurrence and predominance of Candidatus Competibacter Phosphatis detected on day 29 sludge was significant. 17% of the g-Proteobacteria were closely related to the Candidatus Competibacter Phosphatis. They were postulated as putative glycogen accumulating organisms (GAO) as they compete with PAOs. The dominance of GAOs detrimentally affects phosphorus removal by out-competing the PAOs since glycogen can be used as the energy source and reducing power for PHA accumulation reducing the dependency on polyphosphate degradation for energy supply. The significant increase in glycogen consumption was in accordance with the predominance of GAOs on day 29. The decrease in the total phosphorus content of the sludge (4.3% of the MLVSS) and phosphate release/carbon uptake ratio (0.07 mol P/mol C) indicated clearly the abundance of GAOs over PAOs. SBR was ended due to the bulking problem at the 54th day of operation. In the beginning of the operation of the SBR, the fermentation products were depleted at the end of the anaerobic period but in the latter phase of the operaion significant amounts of fermentation products were detected at the end of the anaerobic period and these fermentation products probably stimulated the growth of filamentous bacteria. The lactate accumulation at the end of the anaerobic phase was related to the abundance of GAOs over PAOs. Keywords: Enhanced biological phosphorus removal, phosphate accumulating organisms, glycogen accumulating organisms, glucose

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