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Investigation of the methanol biological phosphorus removal phenomenon Fugère, Raphaël

Abstract

The removal of nutrients from a wastewater stream is a well-known process. The typical nutrients in wastewaters are carbon, nitrogen and phosphorus. Since phosphorus is usually the limiting agent for growth, it usually exhibits the most stringent discharge guidelines. In order to meet these discharge guidelines, municipalities and the industry have installed sophisticated treatment processes. Two main types of nutrient removal processes can be used; chemical nutrient removal and biological nutrient removal, with the latter gaining more popularity. While carrying out denitrification in anoxic tanks with the addition of methanol, some researchers have reported "additional" phosphorus removal. This study was aimed at replicating the operating conditions that might yield this "additional phosphorus removal", performing a mass balance and investigating thoroughly the possible physical or chemical mechanisms responsible. Using a laboratory-scale bioreactor fed with synthetic sewage in which the only exogenous source of carbon was methanol, enhanced phosphorus removal was successfully demonstrated. Significant phosphorus removal was observed with this methanol addition, even without any addition of VFAs, often considered to be essential to trigger the phosphorus uptake and release phenomenon. Low oxic conditions seemed to favor good phosphorus removal in these experiments. It was also found that pH and alkalinity had very little impact on the phosphorus removal phenomenon. However, the addition of ethanol to a biomass, already acclimated to methanol, seemed to boost, significantly, the phosphorus release mechanisms, while having little effect on the phosphorus uptake phenomenon. The replacement of methanol by acetate on .a carbon-based stoichiometric ratio was unsuccessful. While the phosphorus release seemed enhanced, acetate totally destabilized the biomass and resulted in a "reactor crash". A possible metabolic pathway was identified, by which the methanol could be converted to acetate and PHB, thus explaining this methanol-induced biological phosphorus removal phenomenon. However, additional research will have to be undertaken to confirm this metabolic pathway, as well as identifying the various organisms involved in the enhanced phosphorus removal process. If the methanol biological phosphorus removal process could be controlled operationally, it could provide a relatively cheap and efficient way of removing both nitrogen and phosphorus within the same biotreatment sequence.

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