Application of dissolved oxygen (DO) level control for polyhydroxyalkanoate (PHA) accumulation with concurrent nitrification in surplus municipal activated sludge

Abstract

Mixed microbial cultures are a viable means for polyhydroxyalkanoate (PHA) production, which can produce polymers of commercial quality with high yields. Various PHA co-polymer blends can be produced by surplus full-scale municipal activated sludge fed with fermented waste feedstocks. In biological nutrient removal, ammonia is converted to nitrate by ammonia and nitrite oxidizing bacteria (AOBs and NOBs) through nitrification and removed as nitrogen gas through denitrification. Activated sludge can be enriched with significant PHA storage potential alongside nitrogen removal by denitrifying heterotrophic and nitrifying autotrophic bacteria. The latter adds complexity and aeration demand during the aerobic side-stream PHA accumulation stage since fermented organic residuals often contain significant amounts of ammonia. In the present work, the influence of dissolved oxygen (DO) levels on both PHA accumulation and nitrification rates for a municipal activated sludge were evaluated. The objective was to identify potential for a DO control strategy for PHA accumulation, which would mitigate the unnecessary nitrification activity during PHA production. A much higher apparent Michaelis–Menten DO affinity for volatile fatty acid (VFA) consumption (KDO _VFA 0.1 ± 0.06 mg/L) was found as compared to nitrification (KDO _NH4 2.87 ± 1.31 mg/L). Consequently, with lower DO levels, PHA production was not limited by oxygen supply, while nitrogen was removed by simultaneous nitrification and denitrification processes. This study suggests a method for PHA accumulation using nitrifying activated sludge, while feeding ammonia-containing organic feedstocks by means of DO level control where: (1) NOB activity and growth are both mitigated, (2) nitrogen removal is facilitated, (3) alkalinity is controlled through simultaneous denitrification, and (4) energy demand for aeration is reduced.