BACKGROUNDMethanotrophs can offer a long-term reliable solution to two of the major problems causing environmental pollution. These microorganisms can convert methane, i.e. second major greenhouse gas (GHG), into biodegradable polymers as polyhydroxybutyrate (PHB). In this research, waste activated sludge was used as a seed for cultivating enrichments utilizing methane and ammonium as their sole carbon and nitrogen sources respectively with high PHB accumulation capacity. Moreover, to test the effect of different parameters, such as the initial ratio between the ammonium and the microorganisms (N/M), food to microorganisms ratio (F/M), the solids retention time (SRT), copper elimination and nitrate, on the selection of PHB accumulating, i.e. Type II methanotrophs. RESULTSAfter establishing a mixed culture dominated by Type II methanotrophs, the enriched culture was kept in the exponential phase of growth and had a stable performance for almost 30 consecutive cycles with a specific growth rate of 0.077 0.005 h(-1) and biomass yield of 0.81 +/- 0.06 mg-VSS mg(-1)-CH4. The PHB accumulation capacity of the enrichment reached 52.9 +/- 4% with a yield of 0.54 +/- 0.12 mg-PHB mg(-1)-CH4. CONCLUSIONSIncreasing the N/M ratio can be employed to ensure the dominance of Type II methanotrophs in mixed cultures having high PHB accumulation capacity while maintaining a high F/M ratio overcomes any inhibition resulting from ammonium co-metabolism. Despite the dominance of Type II, switching the nitrogen source to nitrate caused an invasion of Type I methanotrophs and a deterioration in the PHB accumulation of the enriched culture. (c) 2017 Society of Chemical Industry