Pilot-Scale Polyhydroxyalkanoate Production from Organic Waste: Process Characteristics at High pH and High Ammonium Concentration

Abstract

Polyhydroxyalkanoate (PHA) accumulating microbial enrichment was established on volatile fatty acids (VFAs) containing leachate derived from the organic fraction of municipal solid waste (OFMSW). The enrichment was based on a 12-h feast-famine batch cycle and an exchange ratio of 50% in which VFAs were completely consumed in less than 50 min during stable periods of operation. No pH control was applied in the system, and the pH went as high as 9 due to the presence of amongst others, ammonia [500 mg·L−1 total ammonia nitrogen (TAN) on average]. The degree of enrichment was evaluated with fluorescence in situ hybridization (FISH), and a yet unknown genus of large (3–5 μm diameter) beta-proteobacteria appeared dominant in the culture. A method for estimating the fraction of PHA accumulating active biomass in the total volatile suspended solids was established, and the results indicated an increase of this fraction from 25% to 56% after implementing two modifications in the operational protocol: (1) a pretreatment of the substrate removing virtually all settleable solids; and (2) a settling phase in the enrichment reactor after the feast phase, selectively removing nonsettleable solids and slowly degradable substrates. The PHA accumulation potential of the culture was 77±18 wt% PHA (n=3) after 3 h in batch accumulation experiments. The results suggest the potential feasibility of PHA production under conditions that were previously considered economically favorable but technically difficult.