Phosphorus takes 1-2% of microorganisms’ dry mass in general. It is an essential element of DNA (Deoxyribonucleic acid) that stores genetic information of organisms. It is also an important constituent of ATP (Adenosine-5′-triphosphate) that is an essential component of energy metabolism. Phosphorus exists in wastewater as orthophosphate (PO43-), polyphosphate, organically bound phosphorus, etc. Around 6 mg of P is removed per g COD in raw wastewater by assimilation mechanisms in typical conventional activated sludge (CAS) process treating municipal wastewater, which corresponds to 25-50 % of total incoming P (Haandel, 2007).
Additional phosphorus can be removed by enriching phosphorus-accumulating organisms (PAO) in microbial population using enhanced biological phosphorus removal (EBPR) process. PAO is a group of bacteria that can accumulate phosphorus in cell mass at the level much higher than 1-2 %, i.e. up to 38% (Haandel, 2007). Acinetobacter has been identified as one of the primary PAO and is a Gram-negative genus belonging to the Gammaproteobacteria.
The metabolism of PAO is depicted in Fig. 1. In aerobic condition, PAO uptakes excess phosphorus and stores it as polyphosphate in the cell mass using the energy from the heterotrophic oxidation of organic materials (BOD/COD). If PAO is exposed to anaerobic condition, where little molecular and combined oxygen molecules are available, it obtains energy from the hydrolysis of the accumulated polyphosphate to uptake volatile fatty acids (VFA) as poly-hydroxyalkanoates (PHA) and poly-hydroxybutyrates (PHB).
Fig. 2. Molecular structure of polyphosphate and poly-hydroxyalkanoate (PHA) (Mino, 2004)
In EBPR process, mixed liquor is periodically circulated through aerobic, anoxic, and anaerobic conditions. In such cyclic environment, PAO has survival advantages relative to other heterotrophes since PAO can sustain its metabolism and grow while most other heterotrophes cannot. The soluble phosphate level is highest in anaerobic tank and lowest in aerobic tank in EBPR process due to the phosphate release and uptake in each tank. The ORP required in anaerobic tank to enrich PAO is -200 mV to -50mV as can be seen in Fig. 3, but the actual ORP required can vary significantly by 50-100 mV depending on pH and water chemistry. The preferred range might be -300 mV to -200 mV.
Fig. 4 summarizes the principle and the mechanisms of biological phosphorus removal process.
© Seong Hoon Yoon