| The effects of different substrate ratios of citrus waste and waste activated sludge on acid production by anaerobic co-fermentation were investigated for the problems of poor stability and long fermentation cycle of waste activated sludge alone and poor degradation performance and low treatment and disposal rate of citrus waste.The effects of pH and zero-valent iron(ZVI)on the solubilization,hydrolysis,acidification and methanogenesis stages of the anaerobic co-fermentation system of citrus waste and waste activated sludge,as well as on the microbial abundance,diversity and species composition of the anaerobic co-fermentation system,were investigated by regulating the pH value of the anaerobic co-fermentation system and by adding zero-valent iron.To provide reference and technical support for achieving the conversion of organic matter to VFA in citrus waste and waste activated sludge and its resourcefulness,reduction and harmlessness.(1)The optimal substrate ratios for anaerobic co-fermentation of mixed substrates to produce VFA were investigated by controlling the VS ratios of citrus waste and waste activated sludge.The results showed that the addition of citrus waste could effectively improve the content of degradable organic matter in the co-fermentation system and enhance the anaerobic fermentation performance,and the SCOD concentration,VFA production and cumulative biogas production in each experimental group were higher than those in the blank group.The VFA yield reached 232.66 mg COD/g VS when the optimum VS ratio of citrus waste and waste activated sludge was 1:1,which was 19.22 times higher than that of the blank group.The anaerobic fermentation system showed acetic acid type fermentation,but there was a trend of conversion of acetic acid to nbutyric acid in the co-fermentation system with the addition of citrus waste.(2)The effects of pH regulation on acid production in anaerobic co-digestion system under the optimum substrate ratio of citrus waste and waste activated sludge were investigated by both continuous pH regulation and initial pH regulation.The results showed that the VFA production and cumulative biogas production were 287.39 mg COD /g VS and 420.60 m L,respectively,when the pH of the anaerobic cofermentation system was continuously regulated to 6,which were 1.36 and 1.19 times higher than those of the blank group.The mechanistic study of pH on the cofermentation system showed that the continuous regulation of pH 6 could promote EPS conversion and increase the content of dissolved organic matter in the system,but slightly inhibit the hydrolysis process and promote the acid and methane production processes.The microbial community structure analysis showed that constant pH 6control increased the relative abundance of Fimicutes and Clostridium and decreased the relative abundance of Olsenella.The increased abundance of Methanothrix spp.promoted the consumption of acetic acid and increased the production of methane.(3)The effects of different concentrations of zero-valent iron on the acid production of anaerobic co-fermentation system were investigated by adding different concentrations of ZVI in the optimal substrate ratio of citrus waste to waste activated sludge.The results showed that the addition of zero-valent iron could promote the acid production and gas production of the co-fermentation system,and the VFA production and cumulative biogas production were 321.53 mg COD /g VS and 619.00 m L,respectively,at the ZVI addition of 5 g/L,which were 1.48 and 1.37 times higher than those of the blank group.The ZVI dosing increased the pH value and decreased the ORP value of the system,providing a favorable anaerobic environment for microorganisms.The mechanistic study of ZVI on the co-fermentation system showed that the ZVI dosing promoted the solubilization,hydrolysis,acid production and methanogenesis processes in the anaerobic co-fermentation system,while increasing the key enzyme activities.Microbial community structure analysis showed that ZVI dosing increased the relative abundance of Clostridium＿IV,Methanobacteria and Methanothrix,and promoted methanogenic processes.(4)Both continuous pH control and ZVI dosing could effectively improve the performance of the anaerobic fermentation system,while the maximum acid production by dosing 5 g/L ZVI was 1.12 times higher than that by continuous pH control of 6.The dosing of ZVI could also enhance the acetic acid-based fermentation of the cofermentation system.ZVI dosing could enhance the performance of all stages of anaerobic fermentation system by consuming H+ and raising the pH and lowering the ORP of the anaerobic fermentation system,thus effectively enhancing the hydrolysis process and acid production process of anaerobic fermentation.In addition,ZVI as a conductive material enhanced the electron transfer efficiency between hydrolytic fermentation bacteria and methanogenic bacteria,which accelerated the anaerobic fermentation rate of the co-fermentation system. |
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