Sludge minimization in municipal wastewater treatment by polyhydroxyalkanoate (PHA) production

An innovative approach has been recently pro- posed in order to link polyhydroxyalkanoates (PHA) produc- tion with sludge minimization in municipal wastewater treat- ment, where (1) a sequencing batch reactor (SBR) is used for the simultaneous municipal wastewater treatment and the selection/enrichment of biomass with storage ability and (2) the acidogenic fermentation of the primary sludge is used to produce a stream rich in volatile fatty acids (VFAs) as the carbon source for the following PHA accumulation stage. The reliability of the proposed process has been evaluated at lab scale by using substrate synthetic mixtures for both stages, simulating a low-strength municipal wastewater and the efflu- ent from primary sludge fermentation, respectively. Six SBR runs were performed under the same operating conditions, each time starting from a new activated sludge inoculum. In every SBR run, despite the low VFA content (10% chemical oxygen demand, COD basis) of the substrate synthetic mix- ture, a stable feast–famine regime was established, ensuring the necessary selection/enrichment of the sludge and soluble COD removal to 89 %. A good process reproducibility was observed, as also confirmed by denaturing gradient gel elec- trophoresis (DGGE) analysis of the microbial community, which showed that a high similarity after SBR steady-state had been reached. The main variation factors of the storage properties among different runs were uncontrolled changes of settling properties which in turn caused variations of both sludge retention time and specific organic loading rate. In the following accumulation batch tests, the selected/enriched consortium was able to accumulate PHA with good rate (63 mg CODPHA g CODXa−1h−1) and yield (0.23 CODPHA CODΔS−1) in spite that the feeding solution was different from the acclimation one. Even though the PHA production perfor- mance still requires optimization, the proposed process has a good potential especially if coupled to minimization of both primary sludge (by its use as the VFA source for the PHA accumulation, via previous fermentation) and excess second- ary sludge (by its use as the biomass source for the PHA accumulation).