| As a flocculation and dehydration agent,ferric chloride has been widely used in urban sewage phosphorus removal and sludge dewatering processes,but after its addition,a large amount of iron-containing compounds will be deposited in the excess sludge,and these iron-containing substances will affect the sludge.Further anaerobic digestion and resource utilization of sludge will have a certain impact.Therefore,based on the investigation of the effect of different dosage levels of ferric chloride on the methane production efficiency of sludge anaerobic digestion,this paper studies the methanogenesis effect of alkali-heat pretreatment to strengthen iron-containing sludge,and explores the mechanism of pretreatment strengthening.The main findings of this paper are:(1)Ferric chloride is used as a sludge dewatering agent,and its excessive dosage will affect the acid-base buffer system and the oxidation-reduction potential(ORP)in the anaerobic digestion system,and inhibit the methane production efficiency.When the dosage of FeCl3·6H2O is 1500mg/L(the total iron content in the sludge is 20185.97mg·(kg dry basis)-1,which is basically the same as the content of ferric chloride used for sludge dewatering),in the digestive system.The pH range of 6.39-6.62,with excessive addition of FeCl3·6H2O can lead to acidification of the digestive system and increase the oxidation-reduction potential(ORP)of the digestion reaction substrate,which is not conducive to the growth of methanogens.The methane yield was reduced by 11.75%compared with that without ferric chloride(the dosage was 0 mg/L).The effect of ferric chloride dosage on the difference of organic components in the digestive juice was analyzed by three-dimensional fluorescence comparison,and it was found that excessive dosage of ferric chloride could lead to insufficient consumption of organic intermediates such as glycated proteins,and the fluorescence intensity of microbial metabolites weakened.The degradation degree of organic matter in sludge was weakened(1.93%).(2)Alkali-heat pretreatment can improve the methane production efficiency of iron-containing sludge anaerobic digestion,and promote the reduction and stabilization of sludge.Pretreatment can shorten the methane production cycle and increase methane production.Compared with thermal pretreatment and untreated iron-containing sludge,the cycle time to reach the same methane yield was shortened by 5d and 15d,respectively;the methane yield was increased by 42.07%and 20.74%,respectively(running one cycle).Alkali-heat pretreatment can promote the formation of fulvic acid-like substances and provide an easy-to-use substrate for anaerobic digestion.They were 3.76%and 3.7%higher than those of direct anaerobic digestion and heat treatment+anaerobic digestion,respectively;the production rate of humic acid was 23.64%,which were 10.24%and 8.41%higher than those of direct anaerobic digestion and heat treatment+anaerobic digestion,respectively.In the digestion residues pretreated by alkali heat,the fluorescence intensity of fulvic acid-like substances is obviously weakened,and the fluorescence intensity of humic acid-like substances and tryptophan substances is enhanced,indicating that the fulvic acid-like substances are gradually decomposed into free or combined amino acids and other substances.At the same time,with the formation of stable macromolecular substances such as humic acid,the stability of digestive residues is enhanced.(3)Alkaline heat pretreatment can break down the sludge cell wall,accelerate the hydrolysis process of the sludge,increase the content of soluble organic matter,increase the abundance and enzymatic activity of methanogenic archaea,and promote methanogenic efficiency.After alkali-heat pretreatment,a large amount of organic substances were dissolved,and the dissolution concentration of soluble polysaccharide(SC)was 30.61 mg/L,which was 4.24 times that of untreated,and 1.61 times that of low-temperature heat treatment.The dissolution concentration of soluble protein(SP)was 16.04 mg/L.It is 3.08 times that of untreated and 0.95 times that of heat treatment.Alkaline-heat pretreatment changed the microbial community structure.Compared with the untreated group,the relative abundance of bacteria involved in the hydrolysis of macromolecules decreased by 10.45%,the relative abundance of main acid-producing bacteria increased by 21.08%,and the relative abundance of methanogenic archaea.An increase of 10.74%.At the same time,the activity of coenzyme F420 related to methanogenesis was significantly increased,and the peak value of F420 activity was 20.47%higher than that of the untreated group.The expression abundance of protein functions related to energy metabolism and coenzyme production and transport in the archaeal community increased by 1.03%. |
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