Organic materials and microbial granule are useful for the natural treatment of wastewater containing heavy metals. Characteristics and metal removal efficiency of organic materials such as microbial granule, pure cow manure and the cow manure containing straw or sawdust were investigated. The up-flow anaerobic reactors composed of gravel, limestone and different organic materials were used to detect the effective organic material for the treatment of heavy metals and the position and forms of the precipitated heavy metals. To study the heavy metal removal mechanism of cow manure and microbial granule, fed-batch experiments were carried out.
Among the organic materials used, the cow manure containing sawdust had the highest alkalinity of 19 g $CaCO_3$/kg and removed the largest quantity of heavy metals in the synthetic wastewater by adsorption and ion exchange. In anaerobic reactor operation, Fe in effluent solution was removed to a level lower than 1 mg/L in the flow rate of 0.5 L/day up-to 100 days, and the effluent concentration rose very rapidly during the 40 days after the flow rate was increased to 1.3 L/day. During the entire operation process, all reactors completely removed Cu and Cd in the influent, kept the effluent concentration of Al and Zn lower than 1 mg/L, and the sulfate reducing rate was more than 4.5 mmol/day. Most of heavy metals were precipitated as metal sulfide form and accumulated in the lower part of reactor. The reactor, which includes granule and the cow manure containing sawdust, showed highest metal removal efficiency. In the fed-batch experiment with organic materials, the granule kept sulfate reducing efficiency in high concentration of Fe and Cd, and effectively removed acidity and heavy metals. Because of the high alkalinity, the cow manure containing sawdust removed Fe more effectively than pure cow manure. Some Cd was removed as cadmium sulfide form through the replacement reaction of iron sulfide.
In conclusion, in the up-flow heavy metal removal anaerobic reactor containing cow manure and granule, most of heavy metals was removed in the lower part of reactor as metal sulfide forms, and among the tested cow manure, the cow manure containing sawdust showed highest efficiency of heavy metal removal.