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

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.

Figure 1

Profiles of SCOD, total VFAs, soluble proteins and carbohydrates (a), main VFAs (b), their uptake rates (c), and nutrients utilization (d) during one cycle in the SBR operated under stability. Red dashed lines mark the end of feast phase. The uptake rate represents the average for the marked period. ‘tVFA’: total VFAs, ‘Ace’: acetate, ‘But’: butyrate, ‘Val’: valerate, ‘SP’: soluble proteins, ‘SC’: soluble carbohydrates. (bars = S.D., n = 3).

Figure 2

Profiles of PHA and biomass concentration during one cycle (a), the specific production rates in feast phase (b), the degradation and utilization rates in famine phase (negative values) (c), and the effects of nutrients on PHA content (d) in the SBR operated under stability. Red dashed lines mark the end of feast phase. Red circles mark the outliers away from the trend lines. Figure d only shows the data in feast phase. (bars = S.D., n = 3).

Figure 3

Profiles of 3HB, 3HV and 3H2MV monomers (a), their production and degradation rates (negative values) (b), the effects of valerate accessibility on 3HV and 3H2MV production in feast phase (c), and the monomers proportion (d) during one cycle in the SBR operated under stability. Red dashed lines mark the end of feast phase. The percentages of 3HB, 3HV and 3H2MV are on molar carbon basis. (bars = S.D., n = 3).

Figure 4

The relative abundance levels of genera in the microbial communities of the original activated sludge (a) and the enriched MMC (b). The genera with abundance less than 1% are classified into ‘others’.

Figure 5

The overall process and efficiency of PHA production with valerate-dominant sludge hydrolysate in conversion of 1 kg excess sludge into PHA.