Sewage Sludge as a Substrate for Microbial Poly-β-hydroxyalkanoate Production

Polydroxyalkanoates (PHAs) as biodegradable plastic material could be produced from sewage sludge in aerobic bioreactors. The sludge is hydrolysed and acidified to volatile fatty acids (VFAs) in bioreactors. Microorganisms can then convert VFAs to PHAs, which accumulate intracellularly in the aerobic fermenter. The aim of this research is to develop such bioreactors to produce VFAs and PHAs from wastewater sludge. VFAs production in bioreactors with high volatile solids (VS) and high fixed solids (FS) was investigated at pH values of 9, 7, 5 and without pH adjustment. The results showed that the highest VFAs accumulation was achieved at pH 9 and 7 in both high VS and FS bioreactors. In another experiment, sludge from ammonia removal, digester feed, and digested sludge with and without sawdust additive were used in bioreactors producing VFAs. The highest VFAs yield was in digester feed bioreactor, which reached 171.6 ± 6.3 mg/g VS. PHA accumulating microorganisms for the inoculum could be cultivated aerobically from the activated sludge. In the production of PHAs, the bacteria accumulating in the 5-liter fermenter were supplement five times. Final PHA yields reached 40-70 % of volatile solids.