Passivating effect of passivating agent on heavy metal As in biogas residues from anaerobic fermentation of pig manures

Abstract: Due to the extensive use of feed additives and drugs, heavy metal residues in pig manures of large-scale farms increased. In order to reduce the pollution of heavy metals in pig manures, the influence of the passivating agent on the content and morphological transformation of As in the fermentation process was clarified, so as to provide a scientific basis for the safe and rational utilization of the biogas residue. The study focused on the heavy metal As, pig manure fermentation raw materials, the anaerobic fermentation technology. The orthogonal tests with three factors and 3 levels of each factor were designed, using BCR extraction method to analyze the heavy metal morphology. Different types of passivating agents (fly ash, diatomaceous earth, activated carbon), addition ratios of passivating agents (2.5%, 5%, 7.5% in pig manures) and fermentation temperatures (25, 30, 35 ℃) were studied. The influence of heavy metal passivation results showed that: 1) After anaerobic fermentation of pig manures, most of the heavy metal As remained in the biogas residue, and the content range in the biogas residue was between 5.50-6.56 mg/kg. The effects of passivating agents and their adding ratio and fermentation temperature on the content of As in biogas residue were not significant. In the 9 groups of experimental treatment, under 2.5% fly ash and fermentation temperature of 25 ℃, the proportion of oxidizable state could be increased to the maximum, which accounted for 7.94%, and was 5.14% higher than that before fermentation. Under the conditions of activated carbon and fermentation temperature of 35 ℃, the residual state of As accounted for the most, which was 84.27%, and was 21.66% higher than that before fermentation. Therefore, addition of different kinds of passivating agents during anaerobic fermentation for pig manure could decrease the effectiveness of heavy metal As. 2) The order of influencing factors on the passivation effect of heavy metal As were follows: passivation agent, addition ratios of passivating agent and fermentation temperature. The passivation effect of passivating agent types on the effective state of heavy metal As in biogas residue was significant, and the passivation effects of passivation agent addition ratio and fermentation temperature were not significant. Therefore, adding passivation agent with optimal content during the anaerobic fermentation of pig manure could transform the effective state of As to a stable state. 3) The treatment with the best passivation effect of As was 7.5% activated carbon and the fermentation temperature of 30 ℃. According to the verification test, the passivation effect could reach 62.34%, which was not significant different with the best treatment (61.46%) in the 9th group of the orthogonal test. The addition of passivating agent during the anaerobic fermentation of pig manures can improve the passivation effect of heavy metal As in biogas residue, which can effectively promote the sustainable development of biogas project.