Abstract

Organic solid waste streams can be converted to volatile fatty acids (VFA) by acidogenic fermentation and/or wet oxidation. VFA are the preferred substrates of Polyhydroxyalkanoate (PHA) producing organisms. However, PHA production based on a waste stream is yet to be commercialised due to techno-economic challenges, such as lower volumetric productivities necessitating larger plant foot print, lower PHA production from complex organic materials and higher cost of waste transformation. The objective of this study was to investigate the feasibility of using biosolids (municipal wastewater sludge) derived VFA as substrates for open enriched culture PHA production. Wasted activated sludge from a municipal wastewater treatment plant was subjected to anaerobic digestion and non-catalytic wet oxidation, at pilot-scale. This process oxidised 90% of solids in the sludge and created a carbon-rich process liquor. This liquor, containing approximately 7 g/L of acetate, other VFAs and a high concentration of macronutrients, was used as the sole substrate in a two-stage PHA production process in lab-scale reactors. An aerobic sequencing batch reactor was used to acclimatise and grow biomass with the liquor as a feed. The mixed culture in this reactor was enriched with PHA producers by subjecting the biomass to a feast-famine feeding regime. This enriched culture was then batch fed with the liquor for PHA accumulation and the sludge was harvested, freeze-dried and analysed for PHA content. PHA accumulation of 39% (w/w) was obtained within 9 hours of operation. The PHA contained 19% PHV and 81% PHB. The PHA yield on a carbon basis was promising at 0.47 (Cmmol PHA/ Cmmol substrate used). The sludge conversion process resulted in a carbon yield of 0.21 (kg DOC/ kg dry solids). Therefore the overall process yield on a carbon basis was 0.3 kg C in PHA/kg C in biosolids (0.2 kg PHA/kg DS) excluding the carbon consumed in biomass growth stage. A process scheme is proposed for production of PHA, from wastewater and biosolids, in which the wastewater is used as the substrate for biomass growth and the biosolids liquor is used for PHA accumulation. Based on the process scheme and yields, an initial economic assessment revealed that the process scheme holds promise for commercial development.