Wastewater treatment technology at Xian tannery

The Xian tannery is located in the Northwest region of China, one of the most important bases for the leather industry in the country. With enormous natural and agricultural resources throughout the region, it has provided abundant supplies of raw hides and skins to the leather industry and quite a number of the tanneries are located in the region. They have significantly developed their capacity along with the substantial growth of the national economy and the government's new policy of opening up the Northwest region. Today there are over 800 tanneries producing about 40% of the national leather production. Due to the serious pollution problems generated by tanneries, the environmental legislation and enforcement is done through the National Environmental Protection Bureau and tanneries are under pressure to set up complete treatment plants to enable them to meet the discharge limits. In 1995, with the assistance of Unido, a special programme was launched to upgrade the existing effluent treatment plant of the Xian tannery, under the project: 'Assistance in pollution control and treatment of tannery wastes in the selected areas of China'. The Xian tannery was built in 1949 in the Sanxi province. Over 350 people are employed in the tannery which has an annual output of 350,000 pieces of cattle hides, 50,000 buffalo hides and 300,000 goatskins for making shoe upper and sole leather. Due to the highly concentrated, coloured waste liquor generated by the vegetable tanning processes, the pollution loads are high in oil, grease, COD and BOD. The characteristics of the wastewater are given in table 1. The effluent volume discharged by the tannery is calculated at between 2,500-2,700 m³/day. It is pumped into an equalisation tank via mechanised screens. One equalisation tank was installed with a capacity of 800 m³, which allows for a 5.7 hours maximum retention time when the tannery is working at maximum capacity. The water from the equalisation tank flows to a primary sedimentation tank (type Dortmund) which has an operational value of 1.5 m³ per hour. At present no chemical is added to the primary process. After about 4-5 hours settlement, the wastewater is pumped into the biological treatment tank. This has the dimensions 7.55 x 10.2 x 3.5 m, with a total 270 m3 volume. There are also four jet aerators, which were self made by the tannery, (three in operation plus one stand-by unit), each with a capacity of 470 m³/h. The advantage of the jet aerobic treatment technology is the high oxygen transition efficiency. The air absorbed from the atmosphere is mixed intensively with the effluent by utilising high-speed pumping, the mixture of air and wastewater thus facilitates rapid transition of oxygen into the effluent. The oxygen transition rate of normal biological processes is usually about 10-15%, but at Xian the rate with the special jet aerator can reach 20-25%. The operational parameters of the treatment are given in the table 2. Because of the high oxygen transition rate the average retention time is only 2.5 hours. Thus, the energy used is reduced by 40%. An operational cost saving was calculated at US$ 0.15/m³. The effluent treatment plant of Xian tannery has been working regularly since 1995 and meets all the discharge standards and has achieved significant results. Treatment results and removal efficiency are given in table 3.