Abstract: In order to analysis the changes in fertilizer efficiency, heavy metal residues, and sanitary indicators of pig manure before and after biogas fermentation, and to assess its agricultural risks, liquid samples were collected from temporary storage ponds and final sedimentation ponds of three large-scale pig farms in Hunan Province. The changes in heavy metal residues (Cu, Zn, Pb, Cr, Cd, As, Hg), nutrient elements (TN, TP, TK), parasite eggs, faecal coliforms counts, and seed germination index were detected in accordance with relevant industry standards. The single-factor pollution index and Nemerow comprehensive pollution index methods were used to evaluate the agricultural risks of biogas fertilizer. The results showed that the highest residual concentrations of heavy metals in pig manure were Zn and Cu, with maximum levels of 19 793.00 μg/L and 6 521.30 μg/L, respectively, while Hg was undetectable in all samples. After biogas fermentation, the residual concentrations of Zn, Cu, Pb, As, Cr, and Cd significantly decreased. The removal rates were: Zn (81.66%～90.87% ), Cu (86.32%～92.86% ), Pb (85.26%～91.18% ), As (76.07%～83.67% ), Cr (89.46%～90.65% ) and Cd (81.16%～97.56%). Nutrient elements—total nitrogen (TN), total phosphorus (TP), and total potassium (TK)—showed losses (P＜0.01), with concentrations ranked as TK＞TN＞TP. TK content ranged from 164.00 mg/L to 615.00 mg/L, TN from 29.50 mg/L to 116.50 mg/L, and TP from 1.32 mg/L to 8.78 mg/L. The removal rates of hookworm eggs and roundworm eggs reached 100%, while faecal coliforms removal rates were 95.75%～98.62%. The seed germination index of the fermented manure was 76.05%～77.20%. The single-factor pollution index for Pb, Cr, As, and Cd in the biogas manure range was 0.02～0.33, all indicating clean levels with low agricultural risk. However, for Zn and Cu, the single-factor pollution indices were 0.16～2.32 and 0.14～2.06 respectively, with Nemerow comprehensive pollution indices of 2.52 and 2.21, representing moderate pollution, which posed a potential risk for agricultural irrigation. These results demonstrated that after biogas fermentation, the concentration of heavy metals in pig manure water tends to decrease. However, residual heavy metals (Zn and Cu) still posed potential risks for irrigation purposes. It was recommended that the application rate of biogas manure be controlled appropriately when used for land application.

Key words: pig manure, biogas fermentation, heavy metal residues, sanitization, risk assessment