In-situ sulfite treatment enhanced the production of short-chain fatty acids from waste activated sludge in the side-stream anaerobic fermentation

The enhancement of caproic acid synthesis from organic solid wastes: A review

Organic solid waste (OSW) significantly harms the environment and threatens human health. Producing caproic acid (CA) from OSW presents a cost-effective, sustainable, and resource-efficient solution. This study comprehensively examines the various methods for synthesizing CA from OSW, focusing on waste material selection, pretreatment processes to improve dissolution and hydrolysis of OSW, key substrates, and optimization strategies. Using OSW resources has been extensively studied and applied across numerous industries, presenting a promising solution for reducing environmental pollution. This study provides insights into CA synthesis pathways and substrate selection while emphasizing the optimization of CA production from OSW. It also highlights key areas for future research.

Feasibility assessment and underlying mechanisms of metabisulfite pretreatment for enhanced volatile fatty acids production from anaerobic sludge fermentation

Employing chemical pretreatment for waste activated sludge (WAS) fermentation is crucial to achieving sustainable sludge management. This study investigated the feasibility of metabisulfite (MS) pretreatment for enhancing volatile fatty acids (VFAs) production from WAS. The results show that after 24-h MS pretreatment, the content of soluble organic matter and loosely bound extracellular polymeric substances (LB-EPS), especially proteins, increased significantly. During the fermentation, MS pretreatment under alkaline conditions was more efficient, with VFA peaking on the fifth day, showing a 140 % increase compared to the alkaline control group. Correlation analysis suggests that the dosage of MS, rather than pH, is closely related to the levels of soluble protein, polysaccharides, LB-EPS, and subsequential VFAs production, while alkaline conditions facilitate the dissolution of total organic carbon. Furthermore, sulfite radicals (SO3•-) are attributed to cell inactivation and lysis, while alkaline conditions initially reduce the size of the flocs, further promoting MS for attacking flocs, thereby improving the performance of fermentation. The study also found that MS pretreatment reduced microbial community diversity, enriched hydrolytic and fermentation bacteria (Actinobacteriota and Firmicutes), and suppressed methanogens (Methanobacteriaceae and Methanosaetaceae), making it a safe, viable, and cost-effective chemical agent for sustainable sludge management.

Insights into effects of thermotolerant nitrifying and sulfur-oxidizing inoculants on nitrogen-sulfur co-metabolism in sewage sludge composting

In this study, high temperature thermotolerant nitrifying bacteria (TNB) and high temperature thermotolerant sulfide oxidizing bacteria (TSOB) were obtained from compost samples and inoculated into sewage sludge (SS) compost. The effects of inoculation on physical and chemical parameters, ammonia and hydrogen sulfide release, nitrogen form and sulfur compound content change and physical-chemical properties during nitrogen and sulfur conversion were studied. The results showed that inoculation of TNB and TSOB increased the temperature, pH, OM degradation, C/N ratio and germination index (GI) of compost. Compared with the control treatment (CK), the addition of inoculants reduced the release of NH3 and H2S, and transformed them into nitrogen and sulfur compounds, the hydrolysis of polymeric ferrous sulfate was promoted, resulting in relatively high content of sulfite and sulfate. At the same time, the physical and chemical properties of SS have a strong correlation with nitrogen and sulfur compounds.

Thiosulfate/FeCl3 pre-treatment enhances short-chain fatty acid production and mitigates H2S generation during anaerobic fermentation of waste activated sludge: Performance, microbial community and ecological analyses

Anaerobic fermentation (AF) has been identified as a promising method of transforming waste activated sludge (WAS) into high-value products (e.g., short-chain fatty acids (SCFAs)). This study developed thiosulfate/FeCl3 pre-treatment and investigated the effects of different thiosulfate/FeCl3 ratios (S:Fe = 3:1, 3:2, 1:1, 3:4 and 3:5) on SCFA production and sulfur transformation during the AF of WAS. At a S:Fe ratio of 1:1, the maximal SCFA yield (933.3 mg COD/L) and efficient H2S removal (96.5 %) were obtained. S:Fe ratios ≤ 1:1 not only benefited hydrolysis and acidification but largely mitigated H2S generation. These results were supported by the enriched acidogens and reduced sulfur-reducing bacteria (SRB). Molecular ecological network analysis further revealed that the keystone taxon (g_Saccharimonadales) was found in S:Fe = 1:1, together with reductions in associations among methanogens, acidogens and SRB. This work provides a strategy for enhancing high-value product recovery from WAS and minimising H2S emissions.

In-situ sulfite treatment promotes solid reduction during aerobic digestion of waste activated sludge: Feasibility for small-scale wastewater treatment plants

Aerobic digestion remains the preferred choice for small-scale wastewater treatment plants (WWTPs) in some developing countries, largely due to economic viability and operational simplicity. The escalating production of waste activated sludge (WAS) has prompted small-scale WWTPs to improve efficiency. To address this issue, this study employed an in-situ sulfite treatment as a non-intrusive method to augment aerobic digestion. With sulfite-enhanced solubilization and hydrolysis, a 3.6-fold increase in degradation was achieved. Both sludge dewatering properties and pathogens inactivation were improved. Microbial community analysis revealed a preferential enrichment of Actinobacteriota and Firmicutes during sulfite treatment. The desktop scaling-up estimation suggests that implementing this treatment yielded operational cost savings exceeding 40 %. In summary, in-situ sulfite treatment offers a cost-effective strategy for WAS management in small-scale WWTPs.

Effect of mixing intensity on volatile fatty acids production in sludge alkaline fermentation: Insights from dissolved organic matter characteristics and functional microorganisms

Alkaline fermentation for volatile fatty acids (VFAs) production has shown potential as a viable approach to treat sewage sludge. The hydrolysis and acidogenesis of sludge are greatly influenced by mixing. However, the effects of mixing intensity on VFAs production in sludge alkaline fermentation (SAF) remain poorly understood. This study investigated the impacts of mixing intensity (30, 90 and 150 rpm continuous mixing, and 150 rpm intermittent mixing) on VFAs production, dissolved organic matter (DOM) characteristics, phospholipid fatty acid profiles and microbial population distribution in SAF. Results showed that 150 rpm continuous and intermittent mixing enhanced the hydrolysis of sludge, while 150 rpm intermittent mixing resulted in the highest VFAs production (3886 ± 266.1 mg COD/L). Analysis of fluorescent and molecular characteristics of DOM revealed that 150 rpm intermittent mixing facilitated the conversion of released DOM, especially proteins-like substances, into VFAs. The abundance of unsaturated and branched fatty acids of microbes increased under 150 rpm intermittent mixing, which could aid in DOM degradation and VFAs production. Firmicutes and Tissierella were enriched at 150 rpm intermittent mixing, which favored the maximum VFAs yield. Moreover, Firmicutes were found to be the key functional microorganisms influencing the yield of VFAs during SAF. This study provides an understanding about the mixing intensity effects on VFAs production during SAF, which could be helpful to improve the yield of VFAs.

Sulfite pretreatment enhances the medium-chain fatty acids production from waste activated sludge anaerobic fermentation

Production of high-value medium chain fatty acids (MCFAs) from anaerobic fermentation of waste activated sludge (WAS) has been considered as a promising alternative for renewable energy resources. However, the low biodegradability of WAS greatly limits the anaerobic fermentation performance. This study proposed and demonstrated a novel approach, sulfite pretreatment, to efficiently produce MCFAs through anaerobic fermentation of WAS. Pretreatment of WAS at a sulfite concentration of 100-500 mg S/L for 24 h effectively improved the MCFAs production and MCFAs selectivity and the promotion effect was positively correlated with the sulfite concentration used in pretreatment (Pearson's R > 0.9). The maximum MCFAs production of 6.84 g COD/L and MCFAs selectivity of 39.1 % were both achieved under 500 mg S/L sulfite pretreatment, which accounts for 2.6 times and 2.4 times of the control, respectively (MCFAs production of 2.62 g COD/L and MCFAs selectivity of 16.4 % in the control). Sulfite pretreatment also enhanced the WAS degradation from 25 ± 2 % in the control to a maximum of 39 ± 2 % under 500 mg S/L sulfite pretreatment. The electron transfer efficiency and COD flows from the substrate to products were enhanced by up to 25 % due to the sulfite pretreatment, which supports the enhanced WAS degradation. Sulfite pretreatment also promoted the solubilization, hydrolysis, and acidification processes during the anaerobic fermentation by up to 200 %, 60 %, and 45 %, respectively, which subsequently makes more substrates available for MCFAs production. The findings from this study provide a potential solution of using industrial sulfite-laden wastes for WAS pretreatment, to enhance the MCFAs production at a minimized cost.