Reducing the effect of non-volatile fatty acids (non-VFAs) on polyhydroxyalkanoates (PHA) production from fermented thermal-hydrolyzed sludge

Bidirectional effects of melanoidins derived from thermal Maillard reaction on polyhydroxyalkanoates synthesis by mixed culture: Transient and long-term responses

The synthesis of polyhydroxyalkanoates (PHA) by mixed microbial culture (MMC) using thermal hydrolyzed sludge is a promising strategy. However, the thermal hydrolysis by-product melanoidins may disturb PHA synthesis. In this study, transient-cycle experiments revealed bidirectional effects of melanoidins on PHA production: low concentrations (≤400 mg/L) promoted, high concentrations (≥800 mg/L) inhibited. Low melanoidins induced hormesis response and improved electron transfer, with energy metabolism increasing by 45 % and 15 % at 200 and 400 mg/L compared with no melanoidins. Reversely, high melanoidins caused cytotoxicity, with cell death ratio 4.5 times higher at 2000 mg/L than blank. In long-term operation, the promoting effect of low melanoidins diminished or turned inhibitory, possibly due to more PHA degraded. The melanoidins-tolerant genera such as Hydrogenophaga and Pannonibacter might alter MMC enrichment and PHA-producing capacity. These findings highlight the concentration-dependent effects of melanoidins on PHA synthesis, offering an insight into optimizing sludge-to-PHA recycling.

Enhanced poly(3-hydroxybutyrate-co-3-hydroxyvalerate) production from volatile fatty acids by Halomonas sp. YJ01 with 2-methylcitrate cycle

Volatile fatty acids (VFAs) are suitable substrates for synthesizing poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), wherein propionate is a precursor of PHBV biosynthesis; however, high concentrations are toxic to bacteria. Therefore, VFAs with suitable ratio are needed. Here, with the ratio of acetate: propionate: butyrate being 1:4:2, the maximum PHBV content and the 3HV content were 46.77 wt% and 19.24 mol%, respectively, by Halomonas sp. YJ01. The optimal C/P and C/N ratios for PHBV synthesis were controlled at 800-1000 and 70-90. The carbon source uptake by the strain at higher C/N ratios was mainly used to synthesize PHBV. The metabolic pathway for PHBV biosynthesis with mixed VFAs showed that the 2-methylcitrate cycle (2-MCC) pathway converted propionyl-CoA to pyruvate, which reduced the toxicity of propionate to the strain. Moreover, the strain utilized acetate and butyrate without producing pyruvate, which did not affect the detoxification of the 2-MCC pathway. Real-time fluorescence quantitative polymerase chain reaction (RT-qPCR) results showed that when the 2-MCC pathway was inhibited, phaC expression decreased 2.74-fold, and the expression of prpB and prpC was down-regulated 2-fold and 6.88-fold, respectively; therefore, propionate toxicity exposure resulted in a significant decrease in PHBV content.

Optimization of activated sludge polyhydroxyalkanoates(PHAs) synthesis system by performing sludge activity recovery experiments and varying the initial sludge concentration

Polyhydroxyalkanoates(PHAs) are considered a good alternative to petroleum-based plastics because of their good biodegradability and biocompatibility. The synthesis of PHAs using activated sludge can not only solve the problem of the high cost of pure cultures but also improve the utilization value of activated sludge. In this study, sludge activity recovery experiments were firstly conducted and the effects of different initial sludge concentrations on the activated sludge PHAs synthesis system were further investigated. the initial sludge concentrations were 1#SBR (2800 ± 50) mg/L, 2#SBR (4200 ± 50) mg/L, and 3#SBR (5500 ± 50) mg/L. The results showed that the activity, sedimentation performance and PHAs synthesis capacity of activated sludge were enhanced after the sludge activity recovery experiment. At the initial sludge concentration of 4200 mg/L, the activated sludge PHAs synthesis system was operated stably and the synthesis efficiency of PHAs was enhanced. In contrast, at the initial sludge concentration of 2800 and 5500 mg/L, the steady state of the activated sludge PHAs synthesis system was damaged to different degrees at different times, and the synthesis efficiency of PHAs was greatly reduced.

Role of polyferric sulfate and ferric chloride on anaerobic fermentation of sludge from novel two-stage process for resource recovery

Polyferric sulfate (PFS) and ferric chloride (FC) were compared for their efficiencies in capturing organic carbon and phosphorus, and their effects on the anaerobic fermentation process of sludge from a pilot-scale two-stage reactor were studied. Both PFS and FC promoted organic carbon and phosphorus capture. Further study revealed that PFS-based sludge with a dosage of 18 mg Fe/Lsewage showed a better volatile fatty acids (VFAs) production performance (202.97 ± 2.38 mg chemical oxygen demand (COD)/g volatile solids (VS)) than that of FC-based sludge (169.25 ± 1.56 mg COD/g VS). Besides, the high dosage of PFS effectively promoted the activities of the α-glucosidase and proteases. The dissimilatory iron reduction process enhanced sludge flocs disintegration and the conversion of carbohydrates and proteins to VFAs. Non-hydroxyapatite phosphorus predominated in the total phosphorus of all samples. This study contributes to developing strategies for optimizing iron-based sludge management and high-value product recovery.

Polyhydroxyalkanoate production from rice straw hydrolysate: Insights into feast-famine dynamics and microbial community shifts

Activated sludge contains a versatile microbiome capable of converting wastes into valuable chemicals like polyhydroxyalkanoates (PHA). This study investigated the influence of repeated feast and famine phases on PHA production as well as the corresponding microbial population dynamics using waste activated sludge (WAS) as inoculum. Hydrolysate derived from rice straw was employed as a substrate for PHA production. The 16sRNA analysis results revealed that Corynebacteriaceae (40%), Bacillaceae (23%), and Pseudomonas (5%) were the primary contributors to PHA synthesis. Notably, Bacillaceae and Pseudomonas thrived in all the feast and famine phases. The achieved PHA concentration was 3.5 ± 0.2 g/L, and its structure and composition were assessed using Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR). The analysis revealed that the PHA consists of a copolymer of hydroxybutyrate (HB) and hydroxyvalerate (HV), specifically identified as Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV).

Sewage sludge acidogenic fermentation for organic resource recovery towards carbon neutrality: An experimental survey testing the headspace influence

Volatile fatty acids (VFAs) produced by acidogenic digestion of sewage sludge are very interesting bio-products which can contribute to carbon neutrality of wastewater treatment plants. Studies on the production of VFAs from sewage sludge from fermenters with membrane are limited. In view of above, VFAs from a fermenter pilot plant equipped with a membrane bioreactor and fed with real sewage sludge has been monitored. The effect of headspace volume (HdV) on VFA production was studied for the first time to elucidate the optimal operation conditions. Specifically, three fermenter HdV values (namely, 20, 40 and 60% of the total volume) have been investigated. Results revealed that the HdV of 20% ensured the highest sCOD production (900 mgCOD/L) and VFA/COD ratio (45.4%). High value of HdV (namely, 40 and 60%) strongly decreased the acidogenic fermentation performance in terms of VFA production.

Influence of volatile solids and pH for the production of volatile fatty acids: Batch fermentation tests using sewage sludge

The aim of this work was to study the effect of volatile suspended solid (VSS) and pH on volatile fatty acids (VFA) production from waste activated sludge (WAS) fermentation by means of batch tests. The final goal was to gain insights to enhance VFA stream quality, with the novelty of using WAS with high sludge retention time. Results revealed that the optimum conditions to maximize VFAs and minimize nutrients and non-VFA sCOD are a VSS concentration of 5.9 g/L and initial pH adjustment to pH 10. The WAS bacterial community structures were analysed according to Next Generation Sequencing (NGS) of 16S rDNA amplicons. The results revealed changes of bacterial phyla abundance in comparison with the batch test starting condition.

Co-generation of biohydrogen and biochemicals from co-digestion of Chlorella sp. biomass hydrolysate with sugarcane leaf hydrolysate in an integrated circular biorefinery concept

BACKGROUND

A platform for the utilization of the Chlorella sp. biomass and sugarcane leaves to produce multiple products (biorefinery concept) including hydrogen, methane, polyhydroxyalkanoates (PHAs), lipid, and soil supplement with the goal to achieve the zero waste generation (circular economy) is demonstrated in this study. Microalgal biomass were hydrolyzed by mixed enzymes while sugarcane leaves were pretreated with alkali followed by enzyme. Hydrolysates were used to produce hydrogen and the hydrogenic effluent was used to produce multi-products. Solid residues at the end of hydrogen fermentation and the remaining acidified slurries from methane production were evaluated for the compost properties.

RESULTS

The maximum hydrogen yield of 207.65 mL-H₂/g-volatile solid (VS)_added was obtained from 0.92, 15.27, and 3.82 g-VS/L of Chlorella sp. biomass hydrolysate, sugarcane leaf hydrolysate, and anaerobic sludge, respectively. Hydrogenic effluent produced 321.1 mL/g-VS of methane yield, 2.01 g/L PHAs concentration, and 0.20 g/L of lipid concentration. Solid residues and the acidified slurries at the end of the hydrogen and methane production process were proved to have compost properties.

CONCLUSION

Hydrogen production followed by methane, PHA and lipid productions is a successful integrated circular biorefinery platform to efficiently utilize the hydrolysates of Chlorella sp. biomass and sugarcane leaf. The potential use of the solid residues at the end of hydrogen fermentation and the remaining acidified slurries from methane production as soil supplements demonstrates the zero waste concept. The approach revealed in this study provides a foundation for the optimal use of feedstock, resulting in zero waste.

Carbon Source Applied in Enrichment Stage of Mixed Microbial Cultures Limits the Substrate Adaptability for PHA Fermentation Using the Renewable Carbon

Suitability of different substrates for enriched mixed microbial cultures (MMCs) is of importance to the polyhydroxyalkanoate (PHA) fermentation using renewable carbon. In this study, three enriched MMCs were evaluated for their fermentation features and kinetics with different carbon sources (sodium acetate, glucose, or starch). The results showed that the highly specific bacterial community composition was developed depending on the applied carbon source. Correspondence analysis suggested that the genus affiliated in Gammaproteobacteria_unclassified was related to 3-hydroxybutyrate (HB) synthesis in acetate-fed MMC (relative abundance of 38%) and glucose-fed MMC (relative abundance of 76.7%), whereas Vibrio genus was related to 3-hydroxyvalerate (HV) production in glucose-fed MMC (relative abundance of 0.4%) and starch-fed MMC (relative abundance of 94.6%). The acetate-fed MMC could not use glucose and starch as fermentation carbon sources, showing the limitation of microbial species developed with the specific metabolic substrate. Glucose-fed MMC produced the highest PHA cell content of 64.2% cell dry weight when using sodium acetate as the fermentation carbon. Glucose-fed MMC showed wide resilience and adaptation to various carbon sources. When actual landfill leachate was used for fermentation by glucose-fed MMC, maximum PHA cell content of 45.5% cell dry weight and the PHA volumetric productivity of 0.265 g PHA/(L·h) were obtained. This study suggested carbon sources applied in the MMC enrichment stage had a significant influence on utilization of carbon in the fermentation stage.

Strategies for the valorisation of a protein-rich saline waste stream into polyhydroxyalkanoates (PHA)

Saline Mussels Cooking Wastewater was valorised to produce PHA with Mixed Microbial Cultures (MMC). Due to the high protein content (1.8-5.7 g COD_PROT/L), PHA accumulating capacity was below 10%, so several strategies were tested. In the acidification unit, Na(HCO₃) was added, increasing protein conversion into Volatile Fatty Acids (VFA) from 10.3% to 69.2% and subsequent PHA accumulation from 6.9 to 14.7%. In the enrichment unit, the incorporation of a settling stage after the feast phase provoked a shift in the proteins' oxidation from the feast to the famine phase, where the nitrogen released in the famine is used by the MMC for growth. This increased the biomass concentration and the tolerated COD (from 1.6 to 4.2 g VSS/L and from 2.2 to 4.38 g COD/L). Finally, varying the proteins/VFA ratio for MMC acclimation to proteins allowed increasing PHA accumulation from 8.8 to 41.5%.