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

Abstract

An innovative approach has been recently proposed in order to link polyhydroxyalkanoates (PHA) production with sludge minimization in municipal wastewater treatment, 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 effluent 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 mixture, 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 electrophoresis (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(-1) h(-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 performance 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 secondary sludge (by its use as the biomass source for the PHA accumulation).

Unveiling PHA-storing populations using molecular methods

“…The operation time ranged between 28 and 84 days. Through DGGE, the authors observed huge changes in the community composition, shown by Dice similarity indexes between DGGE profiles at the beginning and the end of the runs, in the range of 60-38 %, run I and run IV, respectively (Valentino et al 2015). Faster changes were observed, performing moving window analysis, during the first days of operation alongside with oxygen profiles, showing clearly the acclimation process of the MMC.…”

“…At lab scale, Valentino et al (2015) conducted a MMC selection stage using a synthetic medium mimicking municipal wastewater similar to an activated sludge process. The intent, besides MMC selection, was to provide a preliminary wastewater treatment (COD removal).…”

“…Later, Morgan-Sagastume et al (2015) investigated the scale-up of the process. Valentino et al (2015) operated six independent SBR runs fed with synthetic medium (to simulate the soluble fraction of a typical municipal wastewater) and with 2.14 h and 1.5 days of HRT and SRT, respectively. The operation time ranged between 28 and 84 days.…”

Value-Added Products Derived from Waste Activated Sludge: A Biorefinery Perspective

“…Literature-reported experimental (laboratoryscale) conditions for PHA production from WAS-derived media are summarised in Table 5. At present, sequencing batch reactors are reported to be promising for enhanced PHA production, due to their highly flexible operation, ease of process control and biomass growth under transient conditions [95,96]; however, full-or pilot-scale studies are rarely reported in the literature [50]. The major technical challenge associated with bioplastics production from WAS remains the economically lower yields [97], which requires extensive studies on the optimisation of organic loading rate and efficacy of utilising mixed substrates (e.g., sewage sludge and agroindustry wastewater sludge) with mixed cultures.…”

Composition of mixed culture PHA biopolymers and implications for downstream processing

“…The carbon source or feedstock that is used for the enrichment biomass production and for PHA accumulation does not need to be the same. In fact, some wastewater feedstocks are found to be adequate for producing a biomass rich in the PHA-storing phenotype, but are of poor quality when it comes to accumulating PHA to the limits of the biomass capacity (Valentino et al 2014). …”