The adsorption effect of three minerals on chemical oxygen demand, total nitrogen, total phosphorus and heavy metals in biogas slurry

Abstract

Contraposing the intractable problem of high content of nitrogen, phosphorus and heavy metals in biogas slurry from livestock and poultry farm, the adsorption effect of three minerals (montmorillonite, kaolin and acticarbon) on chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP) and heavy metals (Zn, Cu, Cr and As) have been studied. In addition, the mineral material with good adsorption effect has been screened. Based on the screened mineral material, the adsorption effects of screened mineral material dosage and the initial pH value of biogas slurry on COD, TN, TP and heavy metals have been analysed. The results show that the adsorption effect of montmorillonite (MMT) is obvious, followed by kaolin and acticarbon. Concretely, the removal rates of TP, Zn, Cu, Cr are up to 63.0%, 72.1%, 67.4%, 58.5% respectively adsorbed by MMT; What is more, the removal rate increased by the increase of MMT dosage. Nevertheless, the adsorption of COD and TN gradually reaches equilibrium when the dosage of MMT is 3%. Likewise, the removal rate of TP, Zn, Cu and Cr increase slowly. In comparison, As has a highest removal rate, up to 58.5%. The adsorption of COD, TN, TP and heavy metals has little effect with the increase of pH adsorbed by MMT, while the removal rate of TP sharply increases with the increase of pH, from 29.1% to 77.8%. Specially, the removal rates of TP range from 71.6% to 77.8% when pH is between 8.0 and 9.5. All of the results indicate that MMT is the optimal adsorption material, and the optimum MMT dosage is 3%, the appropriately initial pH of biogas slurry is alkaline.