Study The Influence Mechanism Of Fe³⁺ And Ni²⁺ On Anaerobic Digestion

Trace metal elements,as one of the significant factors to maintain the stable operation of biogass engineering,can improve the efficiency of anaerobic digestion.But the addition of trace metal elements is suffering from the challenges,such as the types,optimization and the bioavailability.In the article,the effects of trace metal elements on the process of anaerobic digestion were investigated when Fe³⁺ and Ni²⁺ exist alone and in combination.At the same time,the bioavailability of Fe³⁺ with the addition of Ethylene Diamine Tetraacetic Acid?EDTA?was also explored by batch experiments.The influence of Fe³⁺ on the performance of the Upflow Anaerobic Sludge Bed?UASB?reactor was investigated by Fe³⁺ and EDTA long-term addition experiments.The main results are as follows:?1?The efficiency of anaerobic methane production improved with the increase of Fe³⁺ concentration?from 0 mg/L to 100 mg/L?.At Fe³⁺ of 40 mg/L,compared with the control,the cumulative methane production and COD removal rate increased by 36.32%and 29.98%,respectively.The concentrations of dehydrogenase and F420 were 27.23 ?g/L and 54.85 ?g/L,respectively,which indicated that the presence of Fe³⁺ in anaerobic system enhanced the conversion efficiency of volatile fatty acid?VFA?to methane,and the activity of anaerobic methanogenic bacteria was influenced by regulating the enzyme concentration in methanogenic process.When the concentration of Fe³⁺ increased from 40 mg/L to 100 mg/L,the promoting effect of anaerobic methanogenesis also gradually reduced.?2?By increasing the concentration of Ni²⁺ in anaerobic reaction system,which showed Hormesis effect on the anaerobicmethane producing process.When the concentration of Ni²⁺ increased from 0 mg/L to 10 mg/L,the promotion of anaerobic-methane production was most significant.And compared with the control,the cumulative methane production and COD removal rate increased by 28.26%and 24.37%,respectively.At Ni²⁺ of 1.0 mg/L,the concentration of acetic acid and coenzyme F430 in anaerobic reaction system increased by 35.14%and 26.33%,which indicated that the presence of Ni²⁺ promoted the mutual conversion of volatile fatty acids and increased the substrate concentration in the methanogenesis of acetic acid,thus improving the efficiency of methane production.At Ni²⁺ of 5.0 mg/L,the presence of Ni²⁺ inhibited the anaerobic methanogenic efficiency.The methane production and the COD removal rate decreased to 50%and 24.75%compared with the control.?3?At Fe³⁺ of 50 mg/L and Ni²⁺ of 20 mg/L,respectively,the cumulative methane yield and the COD removal rate increased to a greater extent than that of Fe³⁺ and Ni²⁺ alone.It showed that the anaerobic system had synergistic effect with simultaneous presence of Fe³⁺ and Ni²⁺,and the toxicity of high-concentration metal elements could also be mitigated.?4?The cumulative methane production is of positive relevance with the concentration of EDTA.At Fe³⁺ of 40 mg/L and EDTA of 20 mg/L,the cumulative methane production increased by 18.32%and the concentration of dehydrogenase increased to 119.36%of the control group.The utilization ratio of water-soluble and ion-exchangeable of Fe³⁺ in anaerobic system increased by 90.36%and 54.55%,respectively.Therefore,the addition of EDTA could increase the content of available Fe³⁺ in anaerobic system and promote the bioabsorption and utilization of bioavailabl;Fe³⁺ by microorganisms,and thus improve the bioavailability of Fe³⁺'?5?As the Fe3' exist,it was found that the stability of the reactor could be improved after 240 days of operation,and the daily gas production and methane percentage of the reactor increased.The contents and functional groups of Extracellular Polymeric Substance?EPS?in granular sludge were analyzed by means of Excitation-Emission Matrix spectroscopy?EEM?and Fourier Transform Infrared Spectroscopy?FTIR?.It was found that the long-term addition of Fe³⁺had a certain effect on anaerobic granular sludge and functional microorganisms.The dominant populations of methanogenic bacteria and ancient bacteria translated into methanogenic bacteria of acetic acid type.The results showed that Fe³⁺affected the methanogenic efficiency of anaerobic fermentation by affecting the microbial community structure of anaerobic granular sludge.