A process for producing PHA comprises obtaining biomass produced in the course of biologically treating a first wastewater source containing RBCOD. The biomass is to be exploited with a second wastewater source having a different RBCOD content from the first wastewater source in order to accumulate and thereby produce PHA. Before subjecting the biomass to a PHA accumulation process, the biomass PHA accumulation potential is enhanced via an acclimation process with the second wastewater source. During acclimation, the biomass is subjected to repeated feast-famine periods. During each feast period, the biomass is exposed to a fraction of the second wastewater source. The RBCOD uptake and/or biomass respiration rate is directly or indirectly measured during each feast period. The famine period is maintained for a period of time that is at least two times greater than the length of time of the proceeding feast period. After at least two feast-famine acclimation periods or after one or more measured parameters reveal an increased RBCOD relative uptake or respiration rate of the biomass during a subsequent feast period, the biomass is subjected to a PHA accumulation process using the second wastewater source.