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Wastewater solids fermentation for volatile acid production and enhanced biological phosphorus removal

Fermentation of wastewater solids for volatile acid (VA) production was studied as part of a full-scale demonstration of the Orange County Water and Sewer Authority (OWASA) process for enhanced biological phosphorus removal (EBPR). The VA yields in bench-scale fermentation tests ranged between approximately 6% and 26% of feed volatile solids (VS) under a range of test conditions that included solids retention times (SRTs) between 2 and 6 days, fermenter solids concentrations between 0.43% and 2.6%, and temperatures between 14 o C and 23 o C. Higher VA yields were observed at higher SRTs, lower fermenter solids concentrations, and higher temperatures. Settleability tests indicated that gravity separation of fermented sludge may be limited to an underflow concentration of 3%. After blending a phosphorus-rich waste activated sludge (WAS) with fermented primary solids, up to 42% of the WAS phosphorus was released within 5 hours. Two full-scale demonstration fermentation designs were investigated during the study that included a 2-month simultaneous testing period. Demonstration Fermenter No. 1 used rectangular dissolved air flotation thickeners that were retrofitted with submersible mixers, liquid-solids separation zones, and covers for odor control. This unit proved difficult to operate and maintain because of the design of the unit and the demanding duty. During the 2-month side-by-side test period, the VA yield averaged 0.05 mg/mg VS fed to the unit. The average VA composition of the fermenter product stream consisted of 41 no acetic acid, 44% propionic acid, 9% butyric acid, and 5% valeric acid. Suspended solid, organic, and nutrient loads in the fermenter product stream averaged 3.1 mg solids/mg VA, 1.7 mg carbonaceous biochemical oxygen demand (CBOD 5 )/mg VA, 0.04 mg ammonia/mg VA, and 0.05 mg phosphorus/mg VA during the 8-month testing period. The second fermenter used an existing sludge-holding tank mixed intermittently with coarse bubble aeration and dewatering centrifuges for liquid-solids separation. Demonstration Fermenter No. 2 proved to be quite reliable, although it provided less discretionary control over the fermentation process compared with Fermenter No. 1. During the side-by-side test, the VA yield averaged 0.05 mg/mg VS fed to the unit. The VA composition of the fermenter product stream averaged 38% acetic acid, 36% propionic acid, 16% butyric acid, and 10% valeric acid. During the 10-month testing period, the solid, organic, and nutrient loads averaged 1.0 mg solids/mg VA, 2.4 mg CBOD 5 /mg VA, 0.15 mg ammonia/mg VA, and 0.11 mg phosphorus/mg VA in the fermenter product stream. Both demonstration fermenters produced an acceptable quality VA source for use in a 22 000 M 3 /day (6 mgd) full-scale demonstration of the OWASA EBPR process. Performance of bench-scale and full-scale demonstration fermenters was highly variable (0.05 to 0.2 mg VA/mg VS feed). Commercial acetic acid was also used as the sole volatile acid source and as a supplement to the VA stream produced from the demonstration fermenters to stabilize EBPR performance