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Polyphosphate Accumulating Organisms and Nitrifying Population Ecology in an Activated Sludge Process Aimed to Achieve Nutrient Removal and Sludge Reduction Simultaneously

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Activated sludge systems are among the most commonly used secondary processes for wastewater treatment in developed countries worldwide. The process can be optimized for biological nitrogen and phosphorus removal using different reactor configurations, in addition to the effective removal of organic matters and suspended solids. In all cases, the stable operation of an activated sludge process relies on routinely removing large amounts of excess sludge from the system to maintain a desired solids retention time and a targeted volatile suspended solids concentration. The treatment and disposal of the wasted excess sludge requires much energy and labor, and often face social and environmental challenges.

Though alternative operating processes that result in sludge reduction, such as the Cannibal™ and Oxic-Settling Anoxic (OSA) activated sludge processes have evolved in recent times, they are not postulated to achieve nutrient removal. Coupling sludge minimization with biological nitrogen and phosphorus removal could be challenging and complex.

In this study, simultaneous nutrient removal and sludge reduction was demonstrated using biomass feasting and fasting approach. Two sequencing batch reactors (SBRs) were operated alongside for duration of 370 days. One SBR was operated to achieve nutrient removal (control- SBR) at 10 day solids retention time (SRT), while the other (modified-SBR) was operated to achieve nutrient removal along with sludge reduction. Sludge reduction in the modified-SBR was accomplished by subjecting the returned biomass to fasting and feasting at infinite SRT. More than 99% ammonia nitrogen removal, greater than 80% phosphorus removal and consistent denitrification could be sustained in the reactor, along with 63% net sludge reduction.

Microbial ecology of the polyphosphate accumulating organisms (PAOs) and the nitrifying bacteria in the control and modified-SBR were investigated and the findings were interesting. The microbial population responsible for ammonia oxidation was investigated through Terminal Restriction Fragment Length Polymorphism (T-RFLP), fluorescent in situ hybridization (FISH) and comparative sequence analyses of the amoA gene and, the nitrite oxidizers were identified through FISH and comparative sequence analyses of the 16S rRNA gene. The polyphosphate-accumulating organisms (PAOs) closely related to Rhodocyclus were investigated using FISH and comparative sequence analyses of the 16S rRNA gene. Attempts are also being made to identify the presence of denitrifying PAOs (DNPAOs) through stable isotope probing and a full-cycle rRNA analysis and the results obtained will be presented at the conference.

The amoA gene fragments of the DNA samples obtained from the reactors were cloned and sequenced to identify the ammonia oxidizing bacteria (AOB). The phylogenetic comparison of the amoA sequences with other published sequences showed most of the clones from both the reactors to have close affinity with the N. oligotropha lineage, showing the presence of N. oligotropha genus as the dominant AOB.

To target Candidatus Accumulibacter Phosphatis (CAP), a known PAO in EBPR systems, RHC439f was used as forward primer for cloning and sequencing of 16S rDNA fragments. Most of the clones were closely associated with genus Propionivibrio and Denitratisoma, with none representing CAP. The reactors were operated in anaerobic-aerobic-anoxic mode. It was observed that all the COD was consumed during the anaerobic phase of every cycle in both the SBRs, with no COD left for the last anoxic phase. The anoxic phase was provided to achieve denitrification, and it is known that a stable carbon source is required by the denitrifying bacteria to achieve denitrification. Despite of the absence of carbon source, differences in nitrate concentrations at the beginning and at the end of the anoxic phase was consistently recorded in both the reactors. This emphasized the possible presence of denitrifying PAO population in the reactors. Identification of genus Denitratisoma, a denitrifying bacterium proves the existence of possible DNPAOs.

Additional work is being done to distinguish the DNPAOs through stable isotope probing, whereby microbial groups enriched with 13C-carbon source, can be resolved from 12C-DNA by density-gradient centrifugation. The DNA isolated from the target group will then be characterized taxonomically by sequence analysis. Further analysis on nitrite oxidizing population will also be performed and presented at the conference.
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Document Type: Research Article

Publication date: 2008-01-01

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