The influence of essential growth nutrients on PHA production from waste

Abstract

Volatile fatty acids (VFA) may serve as building blocks for Polyhydroxyalkanoates (PHA) production and can be derived from waste streams. Ideal streams for PHA production contain a high Chemical Oxygen Demand (COD) to nutrient ratio, such as (waste)water from a paper-mill factory or candy-bar factory. The (waste)water generated by these companies are usually treated anaerobically with the final product being methane containing biogas. Usually, the methane is burned to produce either heat or electricity. Potentially, more value can be added to these streams by producing VFA and/or PHA. PHA can be produced using microbial enrichment cultures that can be established by cultivation in a selective environment that favours the growth of PHA producing microorganisms. Some advantages of using open cultures are that no sterilization and expensive equipment is required compared to pure culture biotechnology. Open culture biotechnology can be effectively applied when the right selection pressure for a specific microbial trait is identified. The microorganism that is most effective in the given conditions will win the competition, i.e. the strongest will survive. A selection criteria for PHA productis is consuming substrate very fast by first making a storage polymer (in this case PHA) from the supplied substrate. The PHA producers prefer VFA as substrate, hence it is important to maximize the VFA content in the substrate stream. For the production of VFA in the product chain towards PHA it is important to minimize the solid content in the feedstock for PHA production. The objective of the research described in this thesis was to gain more insight in the two-step upstream process for PHA production from agricultural waste streams. The first step concerns the maximization of the VFA concentration in the feedstock. Optimization of VFA production was investigated using the granular sludge process in order to maximize the volumetric VFA production capacity and to minimize the solids concentration in the effluent. Two process variables were investigated regarding the PHA production process. Firstly, the influence of the presence of nutrients on PHA production was investigated using PHA producing enrichment cultures. Secondly, the production of PHA was investigated using the leachate of the organic fraction of municipal solid waste (OFMSW) at pilot scale.