Abstract
Enhanced biological phosphorus removal (EBPR) is an engineered process that can be employed to sustainably recover phosphorus from dilute wastewater solutions. While EBPR is operated quite successfully, process upsets occur. In considering potential sources of EBPR instability, the effects of the requisite anaerobic zone are at the top of the list. Exposing mixed liquor to anaerobic conditions initiates critical EBPR metabolic responses, including storing/cycling internally stored carbon (ISC – polyhydroxyalkanoates (PHA) and glycogen). However, engineering design guidance for the anaerobic zone remains generic at best with most being rules of thumb that do not explicitly incorporate influent wastewater
characteristics such as volatile fatty acids (VFAs) concentration or the carbon fraction/form [i.e., guidance principally centers on minimum hydraulic or solids residence time]. VFAs play a pivotal role in EBPR where anaerobically phosphorus accumulating organisms uptake and store VFAs as PHA, which kickstarts the EBPR process. Lacking the requisite VFAs can lead to process instability and supplementation is required either through side stream fermentation or chemical addition. Instead, longer EBPR anaerobic hydraulic retention times (AN HRT) could be used to achieve in-line fermentation of the slowly biodegradable carbon fraction. This could allow more PHA to be accumulated, which could help stabilize EPBR. Beyond EBPR, the ISC could be used in an anoxic environment to denitrify the wastewater. Having the culture accumulate carbon anaerobically that can be used anoxically would increase efficiency of BNR and reduce or eliminate extracellular carbon addition. Batch testing was conducted using sludge from multiple EBPR systems to elucidate how carbon substrate, AN HRT effect ISC synthesis as well as how does ISC and its relative fraction impact EBPR function. Experimental factors included anaerobic HRT, sludge source, and carbon substrate (raw wastewater, rbCOD rich, and sbCOD rich). Results from this research showed that the relative quantity of ISC does not effect AN ISC synthesis. Results also showed that a more rbCOD rich waste stream paired with longer AN HRTs led to higher ISC synthesis and a more sbCOD rich waste stream had higher ISC yields suggesting possible fermentation. This enhanced ISC synthesis led to enhanced phosphorus removal and enhanced denitrification that were primarily driven by total PHA consumption.