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A polyhydroxyalkanoate (PHA) accumulating microbial enrichment was established on a volatile fatty acids (VFA) 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 VFA were completely consumed in less than 50 minutes during stable periods of operation. No pH control was applied 2 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 m diameter) beta-proteobacteria appeared dominant in the culture. A method for estimating the fraction of PHA accumulating active biomass in the total VSS was established: results indicated an increase of this fraction from 25 to 56% after implementing two modifications in the operational protocol: (i) a pretreatment of the substrate removing virtually all settleable solids and (ii) a settling phase in the enrichment reactor after the feast phase, selectively removing non-settleable 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 favourable but technically difficult.

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