Evaluation of feasibility, sustenance and microbial ecology of enhanced biological phosphorus removal in aerated-anoxic activated sludge systems

Activated sludge systems are commonly used secondary processes for wastewater treatment. Phosphorus removal in activated sludge systems is achieved through enhanced biological phosphorus removal (EBPR), which is executed by a group of bacteria known as Polyphosphate Accumulating Organisms (PAOs). These organisms, when exposed to cyclic anaerobic and aerobic conditions are capable of accumulating phosphorus. Although several bacterial species have been identified as PAOs, only Candidatus Accumulibacter Phosphatis is dominantly found in laboratory scale acetate fed and full scale EBPR systems. Owing to the complex biochemical mechanisms involved in EBPR and to the lack of solid microbiological basis, the development of actual EBPR processes is largely dependent on good observations of full scale and pilot scale plants. For this reason, most EBPR processes emphasize the need of separate anaerobic and aerobic stages However, there are certain biological processes which do not strictly maintain an anaerobic phase within their process, but can still achieve efficient phosphorus removal, such as the aerated-anoxic oxidation ditches.;It was concluded that though aerated-anoxic process configurations do not follow conventional biochemical EBPR paradigm, they have sufficient potential for efficient biological phosphorus removal. The sustainability of such a system was checked using a bench scale reactor operated for more than 400 days. An average of 86% phosphorus and above 98% COD removal was recorded throughout the operation. The PAOs identified in the full-scale systems and those enriched in the bench-scale system was found to be phylogenetically distant from Candidatus Accumulibacter Phosphatis, and more close related to genus Dechloromonas. This was proved through a full-cycle rRNA analysis and by testing the ecophysiology of the PAOs.;With a proposed hypothesis that a strict anaerobic environment is not required for EBPR and the presence of small concentrations of dissolved oxygen in the stage of treatment where PHA is synthesized during EBPR (typically anaerobic stage) exerts a selective pressure against Candidatus Accumulibacter phosphatis and favors growth of other PAOs, this study evaluated full scale aerated-anoxic systems for EBPR and checked the sustainability of the process through a bench scale reactor. The organisms actively participating in EBPR under such conditions were also identified.