UNITANK®fOr Municipal Wastewater Treatment

　　Abstract：The altemative biological treatment (UNITANK?) is a new combination of advantages of traditional activated sludge process and SBR. It consists of one rectangular reactor which is divided in three compartments (basin A,B,C). Each compartment is equipped with an aeration system. The process works according to a cyclic operation. In compartment B aerated continuously, in compartment A and C ether aerated or settled, the waste water is fed to compartment A, B and C alternatively, the cleaned water is discharged form compartment C and A. There are not primary settling tanks and sludgereturn facilities that are installed in traditional process.
　　The wastewater treatment plant of the ShijlaZhuang Economic Zone is the first plant using UNITANK? in China except Marco. It can treat 100,000 cubic meters municipal wastewater every day and is 7.2 ha area.
The influent characteristics of the plant are as follows: BOD5≤400mg/L,SS≤400mg/L,COD≤600mg/l; The effluent discharge values will be: BOD5≤30mg/l, SS≤30mg/l, COD≤120mg/l.
　　The general description for the process is as follows: the raw wastewater is collected in the inlet pump station, big suspending materials will be retained by means of coarse. After inlet pump lift, the raw water flow through fine screens, small suspending materials removed. In the grit chamber, the sand is reactions of micro-organisms, the substances such as BOD, are degraded. After the grit chamber, the sand is removed from the raw water. Then, the water flow in the UNITANK? . Using the metabolic reactions of micro-organisms, the substances such as BOD, are degraded. After biological treatment, the sludge settles by gravity, the clean water is disinfected seasonally and finally discharged to Wangyang River. The excess sludge is pvmped to the sludge storage tank in which the sludge is stored temporarily, and then is thicken by means of mechanical sludge dewatering.
　　There are six lanes for this project. each of them consists of three compartments which size is 35 meters long, 35 meters wide and 7 meters high, and 6 meters water deep. In compartment A and C, there are overflow weirs around basins. Each lane has a average capacity of 0.193m3 of waste water per day. The average sludge concentration is 4000mg/l. The average sludge loading is 0.113kgBOD/kgMLSS/d. The sludge age is 14 days. Maximum surface loading is 0.74m3/2h when the compartment A and C are used as settling tank.
For this project, the actual oxygen demand is 60000kg/d. The aeration system consists of surface aerators and submersible mixers. The aerators areate aerobic conditions, the mixers keep sludge suspending and enhance oxygen uptake. Because the aerators are floating type,so they can adapt to the water surface up and down. and easy to install and maintenance.
　　UNITANK? have their particluar advantages: First, it is construction-compact, spacesaving and costeffective. Second, it is small kinds of equipment and easy to maintenance. Third, control in time can performs the anaerobic, anoxic and aerobic conditions in a same compartment, and realizes or enhances biological nutrient removal. Fourth, it has high usage ratio for equipment and construction volume.
Though the process has many advantages, it also has its own usage scope. When selecting the process, we suggest to consider the following items:
　　First, if the influent BOD value is very high, it is better to select two-stage UNITANK?system. The first stage is operating under high loading and anaerobic or aerobic conditions, the second stage is operating under low loading and aerobic conditions.
　　Second, if there is effluent phosphorus demand, it should be pay attention to select the process, because it is very difficult to produce the ideal anaerobic condition for biological phosphorus removal.
Third, if the treatment capacity is pretty high, the operation control system for the process will be very complex.
　　About all, UNITANK? has many particular advantages. It is suitable for wastewater treat ment plant on a small or medium scale. In certain scope, it can take place of many other activated sludge processes.

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