Removal and recycling of pollutants from Hong Kong restaurant wastewaters

Most restaurant operators in Hong Kong run their business in commercial buildings. Many of them fail to meet oil and grease (O & G) limits. The O & G and Chemical Oxygen Demand (COD) content of the effluents can be reduced to meet government prescribed standards using a combination of compressed air and chemical treatment in existing two-chamber grease traps. A mean level of 41 mg/L and a mean removal of 92.7% of O & G can be achieved for a typical fast food restaurant. COD reduction can further be improved by dissolved air flotation enhanced with chemicals. Sludge generated from flotation treatment of the wastewaters can be recycled as a fertilizer to give a mean total nitrogen content of 13,217 mg/kg and a mean total phosphorus level of 1133 mg/kg. Moreover, it can also be utilized as a biofuel to give a mean calorific value of 6690 Cal/g which is equivalent to that of coal.