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SCELSE researchers reveal mystery behind enhanced biological phosphate removal in tropical wastewater treatment plants
26 April 2017

The treatment of wastewater in tropical regions may be a step closer to sustainability with the discovery that biological phosphorus removal is achievable in tropical conditions, and not just relevant to temperate climates as previously understood, according to a study by a multidisciplinary, international team from the Singapore Centre for Environmental Life Sciences Engineering (SCELSE).

The results of the study overturn long-held conventional thinking and have significant implications for the design and operation of tropical wastewater treatment plants.

The SCELSE researchers demonstrated that enhanced biological phosphorus removal (EBPR) is achievable in warmer conditions, thus eliminating the need for chemical precipitation.

In much of the developed world, EBPR is used as a sustainable, efficient way of removing phosphorus from wastewater and is an important step in preventing eutrophication of surface waters. It also ensures higher quality reclaimed water, enhancing sustainability of the precious resource.

Knowledge of EBPR is primarily derived from studies conducted in temperate climates, and there have been few investigations of the microbes involved in such processes in equatorial regions.

When conventional (temperate climate) operation of EBPR is conducted in warmer conditions, the microbes that mop up phosphorus (polyphosphate accumulating organisms; PAOs) are outcompeted by their neighbours (glycogen accumulating organisms; GAOs) present in the sludge, rendering the process unviable.

However, the SCELSE team found that specific PAOs, as resolved on a molecular scale, could maintain their needed function and withstand GAO competition. This result was achieved only after a rigorous investigation involving thorough analyses of the microbes involved, what they were doing and the physicochemical outcomes of their activity.

“Microbial communities in activated sludge are extremely complex and highly interactive,” said project leader Prof. Stefan Wuertz.

The researchers undertook a systematic investigation of a wastewater treatment plant in Singapore, using multiple lines of evidence to determine that a functioning PAO community could co-exist and outcompete GAOs, thus rendering EBPR viable.

They first determined that EBPR was occurring in the tropical activated sludge and that PAO and GAO bacteria were present.

Used water treatment involves a process of cycling through different anaerobic/aerobic stages to optimise water reclamation. For this study, oxygenation levels in the treatment plant’s aerobic tank were also varied during the cycling, enabling the researchers to discern interactions involving physicochemical state, community composition and EBPR performance.  

The process was greatly aided by the ability to adjust aeration levels in a full scale plant, having first established the outcomes in laboratory studies.

“By manipulating competition dynamics under different operational parameters we could observe the microbes adjust and perform the functions required,” said Dr Yingyu Law, the research fellow and first author of the research paper who performed much of the analysis.

Standard physicochemical surveys were used to demonstrate that EBPR was, in fact, occurring in the manipulated system at the warmer temperatures.

In one of the first such studies on wastewater treatment plants worldwide, “the microbial community in the activated sludge was sequenced to reveal the genomes of the organisms involved (metagenomics), along with their level of activity (metatranscriptomics)” according to SCELSE computational biologist Rohan Williams who led this component of the study .

Biochemical activity was characterised by modeling metabolic profiles, with metagenomics and metatranscriptomics providing further insights into the microbial community behaviour.

“Discerning the biological mechanisms behind the waste water treatment is crucial to plant design and operation both in terms of performance and minimization of energy input,” said Wuertz.

“This finding highlights the power of delving deeper to understanding a complex microbial system at the community level, to achieve a more sustainable outcome.”