Study On Bioaugmentation Of Caproate Production Process From Sewage Sludge Fermentation

A large amount of sewage sludge is produced as by-products in the process of wastewater treatment plants.Anaerobic digestion is widely used in reduction,resources recycling and harmless treatment of sewage sludge.At present,the products of sewage sludge anaerobic digestion are mainly biogas and short chain fatty acids(SCFAs).SCFAs can be converted into Medium-chain fatty acids(MCFAs)with higher added-value by the chain elongation reaction based on the reverse β oxidation pathway.MCFAs have higher energy density,stronger hydrophobicity and is easy to separate from the fermentation system,thus MCFAs production from sewage sludge have great research value and industrial application potential.To improve the low efficiency of MCFAs production from sludge anaerobic fermentation,and develop and optimize the one-step process for MCFAs production from sewage sludge,study on the bioaugmentation for caproate production process from sewage sludge fermentation was studied in this paper.Samples from different sources had been collected for enrichment and screening of chain elongation mixed culture,and their growth condition was optimizated.Then,the chain elongation enrichments with the highest caproate conversion rate was compounded with SCFAs-producing sludge to construct a microbial consortium,and the application of the microbial consortium for caproate production from sludge anaerobic fermentation was carried out.Finally,the chain elongation enrichments were inoculated into a continuous-flow UASB reactor to study the caproate production rate from sludge fermentation liquid.The results can provide reference and guidance for MCFAs production from organic wastes,and have important research value and guidance significance for resources recycling of organic wastes.The main results and conclusions are as follows:(1)Chain elongating microbiome existed in diverse anaerobic environment such as rumen,anaerobic reactor,natural water and chinese baijiu cellar mud.The chain elongation enrichments with the highest caproate conversion rate was screened from bovine rumen.p H of 7.0,ethanol/acetate molar ratio 6:1,of ethanol/butyrate molar ratio 1.5:1 and without hydrogen accumulated in headspace favoured caproate production from ethanol and acetate or butyrate by chain elongation enrichments.Compared with the initial samples,the bacterial community composition of the enrichments from different sources tended to be similar.Clostridium genus was the main bacteria closely related to the chain elongation.The result of function prediction shows that the relative abundance of acetate Co A-transferase and3-hydroxybutyryl-Co A dehydratase can be considered as functional indicators of chain elongation microbiome.(2)A microbial consortium was constructed by chain elongation enrichments and SCFAs-producing sludge to bioaugment for one-step caproate production process from sludge fermentation.The addition of chain elongation enrichments significantly improved the efficiency of caproate production.The existence of ethanol as electron donor is a necessary condition for efficient caproate production.Thermal alkali pre-treatment greatly improved the efficiency of fatty acids production from sewage sludge fermentation.SCFAs production and chain elongation occurred sequencely of fermentation process increased caproate production rate from sludge sewage.The maximum caproate concentration was up to 8.99 g/L when thermal alkali pre-treated sludge was fermented by the microbial consortium with the addition of 12 g/L ethanol at Day 7.Adding activated carbon and nano zero valent iron further enhanced caproate production.Maintaining the stability of the microbial consortium would be the key to stable and efficient caproate production from sewage sludge fermentation by bioaugmentation.(3)Chain elongation enrichments were inoculated to continuous flow UASB reactor and chain elongation process was operated to produce caproate from sludge fermentation liquid.With sludge fermentation liquid as influent,the caproate maximum concentration in effluent was 3.94 g/L under 20000 mg/L s COD of sludge fermentation supernatant,ethanol/acetate ratio 3:1 and p H of 7.0.The bioaugmentation during the reactor operation was affected by the existence of methanogens and fermenters,which led to the reduction of caproate productivity.Inhibition of methanogenesis by adding 2-bromoethanesulfonic sodium salt(BES),the decrease of p H to 5.5 and the removal of fermenters from sludge fermentation liquid recovered caproate production.