Strategies for advanced PHA production in a sequencing batch reactor operated with uncoupled carbon and nitrogen feeding

Polyhydroxyalkanoates (PHA) are completely biodegradable polyesters and it is well known the ability of mixed microbial cultures (MMC) to produce them by using renewable resources (e.g. waste organic streams) as feedstock. MMC-PHA production typically involves a multi-stage process including the selection of PHA-storing microorganisms from the mixed culture. This usually occurs in sequencing batch reactors (SBR) operated under dynamic feeding regime whereby microorganisms undergo periods of high (feast) and low or none (famine) concentrations of external organic substrate. The establishment of the “feast and famine” regime allows for a good microbial selection and, in order to guarantee a sufficient and stable growth of PHA-storing microorganisms, a proper addition of nutrients (e.g. nitrogen and phosphorus) is required. This is particularly relevant when dealing with nutrient-deficient feedstock (such as paper mill and olive oil mill wastewaters). Along this line, research has been currently focusing on the investigation of the optimal strategies for nitrogen supply. In a previous study (Silva et al., 2016) the impact of nitrogen feeding regulation on the PHA-production process has been evaluated in a lab-scale SBR with a cycle length of 6h and fed with a synthetic mixture of acetic and propionic acids (at an overall organic load rate of 8.5 gCOD L-1 d-1, corresponding to 2.125 gCOD L-1cycle-1) as carbon source and ammonium sulphate as nitrogen source. Two SBR runs were performed with the carbon source fed at the beginning of the SBR cycle simultaneously to the nitrogen source (coupled feeding strategy) or with the latter fed at the end of the feast phase (uncoupled feeding strategy). As a main result, it was found that PHA production was more than doubled (up to about 1300 ± 64 mgCOD L-1) when nitrogen was separately fed from carbon source. According to these results, in the present study the effect of both the SBR cycle length and the applied organic load rate (OLR) has been investigated with the uncoupled feeding strategy. More in detail, the SBR has been operated with a 12 h cycle length and at two OLRs. Initially, in order to maintain an identical OLR per cycle, the reactor was fed with a synthetic mixture of acetic and propionic acids at an overall OLR of 4.25 gCOD L-1d-1. With respect to the previous research (Silva et al., 2016), it has been observed a significant decrease (from 35% to about 21%) of the ratio between the length of the feast phase and the overall cycle length (Figure 1). This also resulted in a significant increase of the PHA content in the biomass (from 28 % to 40 %, mgPHA/mgVSS). Finally, maintaining the SBR cycle length at 12 h, the OLR has been doubled and, consequently, also the PHA content in the biomass almost doubled (up to 76 %, mgPHA/mgVSS) (Figure 1). Taken as a whole, the obtained results clearly highlight the possibility to significanly advance the PHA production in an SBR operating with uncoupled carbon and nitrogen feeding by finding the optimal operating conditions.