Overall process of using a valerate-dominant sludge hydrolysate to produce high-quality polyhydroxyalkanoates (PHA) in a mixed culture

Jiuxiao Hao,Xiujin Wang,Hui Wang

doi:10.1038/s41598-017-07154-3

Abstract

The overall process of polyhydroxyalkanoates (PHA) production in a mixed culture fed by thermophilic fermented valerate-dominant sludge hydrolysate with high-level soluble organics (proteins and carbohydrates) and nutrients (nitrogen and phosphorus) was investigated in this study. The valerate-dominant hydrolysate was fed to enrich a PHA culture with an increasing concentration, and the enriched culture displayed a strong PHA-producing capacity under feast-famine conditions. Valerate in the feedstock was preferentially utilized over acetate and butyrate, and its uptake correlated with the production of 3-hydroxyvalerate (3HV) and 3-hydroxy-2-methylvalerate (3H2MV). The maximum PHA content (42.31%) was highest to date in a mixed culture with complex feedstock, and the PHA consisted of 3-hydroxybutyrate (3HB), 3HV, 3H2MV at 68.4, 23.7, 7.9 mmol C%. PHA production was inhibited when the nutrients exceeded a certain limit. Microbial analysis revealed that valerate-dominant feedstock caused Delftia (53%) to become the prevailing group over other PHA-producing bacteria. For long-term operation, 75% of the biomass at the end of feast phase was collected for PHA recovery, and the entire process exhibited a potential to produce 5 g PHA from 1 kg sludge. These findings indicate that the complex valerate-dominant sludge hydrolysate can be used to stably produce PHA containing high 3HV and 3H2MV.

Highlights

Polyhydroxyalkanoates (PHA) are biopolyesters synthesized by a wide range of bacteria and can functionally replace the conventional petroleum-based plastics[1]
The results revealed that valerate-dominant sludge hydrolysate was suitable to produce high-quality PHA containing 3HV and 3H2MV monomers
After being assimilated into the cell, valerate is converted to acetyl-CoA and propionyl-CoA, which are the precursors for 3HV and 3H2MV production[21]

Summary

Introduction

Polyhydroxyalkanoates (PHA) are biopolyesters synthesized by a wide range of bacteria and can functionally replace the conventional petroleum-based plastics[1]. The resultant copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) yielded 48 wt% content, consisting of 47 mol% 3HB and 53 mol% 3HV19 Except for this investigation, there has been no study exploiting the valerate-dominant fermented hydrolysate for PHA production, either for enrichment or for synthesis, and valerate has only been reported in studies using sole valerate or a valerate-containing mixture[8, 20, 21]. Few studies used the raw fermented streams with non-VFAs and excess N and P for PHA production[24, 30], the feedstocks in these studies were all characterized by dominant acetate, and the accumulated PHA contents were relatively low In this respect, the direct utilization of valerate-dominant sludge fermented hydrolysate remains unknown

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