Polyalkanoates Synthesis From Real Waste By Microorganisms In Excess Sludge

Plastic product is applied widely in the daily life and industrial and agricultural manufacture. In response to the problem and harmful effects of the plastic wastes on the environment, there is considerable interest in the development of biodegradable plastics. Among the various biodegradable polymer materials, polyhydroxyalkanoates (PHA) provide a good fully degradable alternative to petrochemical plastics. The properties of PHA are very similar to those of polyethylene (PE) and polypropylene (PP). PHA is fully biodegradable, and transformed from renewable raw materials. Thus, PHA is a kind of environmental- friendly plastics and a good substitute to the traditional plastics, therefore having the potential to be widely applied in the future.Recently, PHA has been industrially produced mainly by pure cultures as inoculation. Wider use of PHA is prevented mainly by their high production cost compared with the oil-derived plastics. Otherwise, the excess sludge generated from the world-wide municipal wastewater treatment plants is plenty and the disposal cost for the excess sludge is very high. Choosing cheap, reusable and common waste material as the carbon source, using the populations in the activated sludge as inoculation, and optimizing the PHA production process parameters is attractive to produce PHA with significant reduced cost, to reduce and reuse waste material, and to decrease both the environment harm and the consumption of the non-renewable resource resulting from the traditional plastics.The aim of this study is to realize the reduction of both costs for PHA production and the amount of real waste, as well as the reuse of excess sludge. The main content and results are listed as following:The PHA production by excess sludge process parameters influencing the PHA yield, PHA productivity, carbon source transform rate and the copolymer composition was discussed from the lab-scale experiments utilizing synthesized wastewater. Results showed that, adjusting these process parameters is essential for increasing PHA yield and regulating the monomer composition of the PHA copolymer. Aerobic, high ratio of carbon to nitrogen and dynamic feeding pattern was helpful to increase the PHA yield. Under these conditions, the yield of PHA produced by activated sludge was similar to that obtained from pure culture. Moreover, the specific PHB production rate was one order of magnitude higher than that reported for pure cultures. Certain monomer composition of PHA could be obtained by control the process operation parameters. Aerobically, the fraction of the hydroxyvalerate monomer in Poly(β-hydroxybutyrate-co-β-hydroxyvalerate) (PHBV) depends on the carbon atom number of the short chain fatty acids. The variation of dissolved oxygen (DO) concentration influenceed the metabolic pathway of PHA production by activated sludge significantly. The increase of pH value and the decrease of DO led to the increase of HV% in PHBV, which was independent of the type of carbon source. The sludge source as inoculation and the ratio of carbon to nitrogen also influenced the composition of PHBV.According to the experiment results of the above studies and the mechanism analysis of both metabolic and kinetic modeling for PHA production by activated sludge, a new process was developed to optimize the yield of PHA and the monomer composition using real wastewater as carbon source. One of the real wastewater was excess sludge fermentation liquid generating from alkaline anaerobic thermophilic sludge digestion, and the other was food waste. The excess sludge as inoculation was not acclimated before being used to synthesize PHA. To increasing the ratio of carbon to nitrogen in the sludge fermentation liquid, the excess ammonia and phosphate was recovered by adding Mg2+ to form the deposition of strive. Alkaline and thermophilic condition could enhance the yield of volatile fatty acids (VFAs) and the removal rate of the ammonia in the excess sludge fermentation liquid. The main component of the VFAs in the sludge fermentation liquid was acetate. The maximum yield of PHA produced by excess sludge from sludge fermentation liquid was 56.5% (fraction of the Volatile Suspended Sludge, wt%). Malt waste was the most popular carbon source for PHA production by excess sludge and the pure culture, which means malt waste led to the maximum PHA yield by these bacterial. The PHA product generated from real waste by excess sludge was analysized to penetrate their physical-chemical characteristics.To regulate the PHA copolymer composition, the metabolic pathway for PHA synthesis by activated sludge was simulated based on the metabolic mechanism analysis of PHA synthesis by pure culture. To optimize and forecast the yield and productivity of PHA produced by activated sludge, based on the modification of the Activated Sludge Modeling Number 3 (ASM3), a simple kinetic mathematical model was developed for PHB production process by mixed cultures with sufficient accuracy for supporting model-based optimization studies. The mechanism for the acetate uptaken from the outside was transformed anaerobically to be form hydroxyvalerate (HV) was explained as such: intracellular acetate was metabolized through a converse reaction in TCA cycle as"succinyl-CoA→propionyl-CoA"to become propionyl-CoA, which is the prior of HV in PHBV. The change of process operation conditions influenced the community structure and metabolic pathway of the mixed cultures in the activated sludge, led to the variation of hydroxyalkanoate (HA) composition and yield. The kinetic model presented here for PHA production by activated sludge could forecast the PHA yield, PHA productivity, the carbon source transform rate and the biomass yield for accurately, which was improved by lab-scale experiments.The industrial-scale production of PHA from sludge fermentation liquid by excess sludge establishs the sustainably economical and social value, due to the realization of the reduction and reuse of real waste and excess sludge, as well as the wide application potential of PHA product as common package material. Comparing with the PHA production by pure culture and by activated sludge from synthesized wastewater, the PHA production cost by excess sludge as inoculation and sludge fermentation liquid as carbon source was low enough. The saving cost of the PHA production by excess sludge from sludge fermentation liquid includes, the carbon substrate cost, the PHA production process configuration and operation cost, the excess sludge disposal cost, the bacterial selection and enrich cost. The strive is the byproduct during operation.