Critical Role of Aerobic Uptake in Biological Phosphorus Removal

The use of biological phosphorus removal (BPR) to remove phosphorus (P) from municipal wastewaters is well-established, and especially in recent years, there has been a significant increase in the number of utilities practicing BPR. The basic BPR mechanism includes two distinct steps: anaerobic P release, and aerobic P uptake. However, BPR design and optimization efforts have typically focused on the anaerobic release step of the process, with a lot of attention being paid to maximizing anaerobic P-release through augmentation with volatile fatty acids (VFAs) and by minimizing interferences from nitrate and dissolved oxygen. In contrast, there is little published work, especially from full-scale wastewater plants, on optimization of the aerobic uptake step of the BPR process. This paper presents new information on the significance of the aerobic uptake step in the BPR process, based on full-scale experimentation conducted at the Durham Advanced Wastewater Treatment Facility (Durham AWTF), run by Clean Water Services. Comprehensive side-by-side sampling and profiling was conducted on four different reactor configurations in 2005 as part of a BPR optimization study. A major outcome of this study was an improved understanding of the significance of aerobic uptake in BPR. Aerobic uptake was found to be the critical step in the process, with the P uptake in the initial aerobic zones having a particularly significant impact on the ability to achieve very low P levels reliably.

The basic BPR process utilizes an alternating anaerobic-aerobic sequence in the bioreactor to promote the growth of phosphorus accumulating organisms (PAOs) that can accumulate P in excess of normal growth requirements. Under anaerobic conditions, the PAOs take up volatile fatty acids (VFAs) into the cell and store them as polyhydroxyalkanoates (PHA). The energy for this transport comes from the hydrolysis of the intracellular polyphosphate to orthophosphate (OP), which is released from the cell into the liquid. The energy required for the conversion of the VFA to PHA is supplied partially by the polyphosphate hydrolysis reaction and partially by the degradation of glycogen, an intracellular storage polymer. The glycogen degradation reaction also supplies the reducing equivalents required for the formation of the PHA. Under aerobic conditions, the cycle is reversed, and the PAOs use the stored PHA to take in P from the liquid, thereby “recharging” their polyphosphate batteries. The PAOs also use the PHA-derived energy for growth and replenishment of their glycogen reserves. This complementary relationship between the anaerobic and aerobic phases - the anaerobically-produced PHA is consumed in the aerobic phase to replenish the polyphosphate and glycogen consumed in the anaerobic phase - is a key characteristic of the BPR process. Overall P removal occurs when the PAOs with high P content are removed from the system in the waste activated sludge (WAS).

Even though phosphorus is effectively removed from the liquid only in the aerobic phase, BPR optimization efforts have typically focused on the anaerobic step. The primary emphasis has usually been on maximizing the anaerobic P release, either by increasing the VFA feed to the anaerobic zones through pre-fermenters, etc., or through reactor configurations designed to minimize interferences from dissolved oxygen (DO) and nitrate (VIP, UCT configurations)). In contrast, the aerobic uptake step has received much less attention, the underlying assumption being that the anaerobic P release is the limiting step in the BPR process. The Durham BPR optimization study has generated some interesting findings that challenge this assumption and suggest that aerobic uptake plays a very significant role in the BPR process.

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