Purifying effect of biochar-zeolite constructed wetlands on arsenic-containing biogas slurry in large-scale pig farms

Abstract

To explore the purifying effect of vertical subsurface flow constructed wetlands (CWs) on arsenic-containing biogas slurry in pig farms under natural conditions in the field, vertical subsurface flow CWs with water spinach-Pennisetum sinese as the core was constructed in this study. Under the condition of biogas slurry with different dilution ratios (2, 4, 6; v/v) was fed to the wetlands and different substrates (biochar and zeolite) were added to the wetlands, respectively, the removal efficiency of the pollutants in biogas slurry by CWs were investigated. The enrichment of arsenic (As) in wetland substrates and three wetland plants (water spinach, green Pennisetum sinese and purple Pennisetum sinese) was analyzed. The results showed that, the removal rate of ammonia nitrogen (NH4+-N) in biogas slurry at low dilution ratio was always higher in the CWs, while that of total phosphorus (TP), chemical oxygen demand (COD) and As was higher in biogas slurry at high dilution ratio at first and then was just the opposite. The addition of zeolite and biochar could enhance the decontamination effect of CWs. Zeolite had better removal effect on NH4+-N, TP and COD, while biochar showed better removal effect on As. At low dilution ratio, the enrichment of As was high in the wetland sediments and three wetland plants. Adding biochar and zeolite increased the enrichment and absorption of As by sediments, water spinach and purple Pennisetum sinese, which was more significant by adding zeolite than biochar. However, they slightly reduced the absorption of As by green Pennisetum sinese. The detailed effluent quality assessment showed TP and NH4+-N to be significant pollution factors. The CWs treated with zeolite showed a higher purifying effect, while the CWs added without the two substrates had a lower purifying effect. This study provides a theoretical basis for the practical application of CWs in livestock and poultry wastewater treatment.