Date Published: June 12, 2018
Publisher: Springer Berlin Heidelberg
Author(s): Shanwen Zeng, Fuzhong Song, Peili Lu, Qiang He, Daijun Zhang.
With volatile fatty acids as substrates, the typical polyhydroxyalkanoates (PHAs) production by mixed culture always includes two steps: PHA-storing culture enrichment via aerobic dynamic feeding strategy and PHA accumulation under nutrient-limited condition. To simplify the PHA-production steps, the enrichment and accumulation step were coupled in a SBR. At start-up period, to investigate the effect of settling selection, one acetate-fed SBR was operated by settling selection-double growth limitation (SS-DGL) strategy, while the other was operated by DGL strategy. The results showed that the stable operation in SBR1 was obtained at about 21, 12 days faster than SBR2, implying the settling selection accelerated the start-up process. After omitting the settling selection under the stable operation, the SBR1 was run above 15 days. The results showed that the performance was not substantial altered. Therefore, the settling selection affected the start-up process but not the stable operation. At operational period, based on the sharp decreasing of oxygen uptake rate (OUR), the poly-β-hydroxybutyrate (PHB) content was improved 13%, from 70 to 83% by feed-on-demand control-double growth limitation (FD-DGL). And the harvested volumetric productivity was 5.0 gPHB/L/day, almost 1-folder improvement. That was to say, the PHB production in a SBR of coupling the enrichment and accumulation step was improved by feed-on-demand control. Meanwhile, the FD experiment can keep steady running for 10 SRTs. Therefore, the SS-DGL/FD-DGL strategy was a promising method for PHA production.
The widely use of conventional plastics in daily life causes unfriendly effect on our environment, due to those low biological degradation, incompatibility with the environment and non-renewability (Fradinho et al. 2014; Khanna and Srivastava 2005). Therefore, a type of environmentally friendly new materials, possessing similar physicochemical characteristics to conventional plastics, are urgent needed. Polyhydroxyalkanoates (PHAs) are microbial storage polymers, served as an intracellular carbon and energy reserve (Tan et al. 2014). The physicochemical properties of PHAs are similar to the conventional plastics, but biodegradable and renewable (Kourmentza and Kornaros 2016; Laycocka et al. 2014; Prieto et al. 2016; Reddy et al. 2017), which attracted more and more attention of researchers.
The initial inoculated sludge in the two SBRs were both from an aeration tank of a WWTP in Chongqing. And both the initial concentration of MLSS and PHB in the two SBRs were the same. The only difference was whether adding a settling selection. A fast settling and withdrawal in SBR1 caused a slight more sludge discharge, therefore, the concentration of MLSS in SBR1 was slightly less than the one in SBR2 in enrichment process (Fig. 1a). A small amount of sludge with less settling ability was withdrawn at the end of feast phase through settling selection, which was assumed as the bacteria with non-capacity of storage (Chen et al. 2015). In enrichment system, bacteria was divided to two categories, bacteria with PHA accumulating ability (B1) and non-PHA accumulating bacteria (B2) (Chen et al. 2015; Jiang et al. 2012; Marang et al. 2014). The result (Fig. 1) showed that the start-up process in SBR1 (21 days) was faster than SBR2 (33 days), implying that the enrichment process just depending on metabolic selective pressure was relatively slow. The enrichment process depending on the conjunction of metabolic selective pressure and physical selective pressure was more pointed, due to the difference of the cell density at the end of feast phase (Chen et al. 2015). B1 would be heavier at the end of the feast phase, due to more PHA stored in the cells. Through a fast settling and withdrawal, B2 was weeded out. In addition, the result showed that omitting the settling selection had no obvious effect on the SBR performance. The nature of selection was to weed B2 out and maintain B1 growth. And the main goal of steady state was to maintain B1 growth and avoid B2 existing. After omitting the settling selection, the double growth limitation (DGL) strategy was still able to achieve this goal and maintain the system steady running. Therefore, the SBR performance was not altered after the omitting of the settling selection under stable operation.