Polyhydroxyalkanotes Production By Two-Stage Process And Three-Stage Process

Polyhydroxyalkanoates (PHAs) are a class of polymers, which can be accumulated as internal carbon sources by part microorganisms under adverse circumstances. PHAs are biocompatible and biodegradable and can also possess the similar properties with the petro made plastics, which enables them to substitute the current plastics to reduce solid wastes. However, PHAs in market are all commercially produced by pure cultures, which bring about high costs and hampered their large-scale application. PHA production by mixed microbial cultures can be completed in open reactors and future more, more cheap organic wastes can be used. This would make cost reduction possible.Two-stage PHA production process includes acidogenic fermentation of organic wastes and PHA production, and the latter is composed by sludge acclimation and PHA accumulation. In sludge acclimation, nutrients are balanced while in PHA accumulation, nutrients are unbalanced. In this study, results showed that direct limitation of ammonia by 80% (compared with balance level) in influent could better stimulate PHA accumulation than gradual limitation. When there was no substrate left in every cycle, little difference was observed in PHA accumulation between anaerobic-aerobic and aerobic operation with substrate as acetate or sludge alkaline fermentation liquids. It can also be drawn that long sludge retention time would guarantee longterm stability of the reactor, while low sludge retention time would bring about sludge bulking. Especially, when sludge retention time was lower than 5 days, the PHA storage capacity would be damaged. VFAs could be uptaken rapidly and the PHA accumulation depended highly on the level of ammonia when the substrate was sludge alkaline fermentation liquids.Three-stage PHA production process includes acidogenic fermentation of organic wastes, culture selection and PHA accumulation, and stage of culture selection is the most important. It was observed that bulking sludge was easily established when selecting cultures in SBR with actate as substrate. Under SRT of 1d and high organic loading rate (6.6 g COD/L/d), the bulking was more severe with a great deal of foam and poor PHA storage ability. While, under SRT of 10 d and low organic loading rate (2.7 g COD/L/d), bulking sludge possessed high PHA storage capacity. After 102 days'operation, sludge from SBR could accumulate PHA to 53% of TSS, under ammonia starvation, with average storage rate of 0.19 mg COD/mg X/h and yield of 0.76 mg COD/mg COD. However, another SBR operated in parallel with anaerobic-aecobic pattern suddenly failed after 55 days'operation.When using cane sugar to simulate molassess as the substrate, after anaerobic fermented, for PHA production, the CSTR gradually stablized towards ethonal-type fermantation one month after startup. The effluent was clarified with hollow fiber membrane and then was used for culture selection and PHA accumulation, thus coupling bio-hydrogen production with PHA production system was achieved. The TSS in SBR rised up to more than 8000 mg/L from 3300 mg/L with organic loading rate of 4.2-4.5 g COD/L/d and SRT of 10 d. Although DO was maintained above 3 mg/L in feast phase, sludge bulking still happened after 30 days'operation. This may be caused by high sludge concentration. Bulking sludge exhibited higher rates in substrate uptake, PHA storage and biomass proliferation than well-settling sludge, about 2 times than the latter. When SBR run 25 days after inoculation, the whole system could produce 16 L H2 and 0.1 kg COD PHA using 1 kg COD cane sugar. The HV weight proportion of PHA was 24% approximately.