NOVELTY - The septic tank (140) comprises two equal-sized compartments separated by a partition wall, and an inlet connected to a pipe that is extended towards a bottom of the tank such that a lower end of the pipe is located at a height of 90 mm from the bottom of the tank. The partition wall is open at the bottom of the tank. The septic tank is further connected to a sludge-withdrawal pit by a lateral pipe located at the bottom of the tank, and further comprises a valve configured to control a flow of wastewater from the septic tank to the sludge withdrawal pit.    USE - The septic tank is useful for treatment of domestic wastewater.    ADVANTAGE - The cost-effective septic tank has simple structure and high degree of removal efficiency of solids and Biochemical oxygen demand, rapidly provides treated water with high quality and reduced risk of pollution of the surrounding environment, and can be easily cleaned without causing any disturbance to the ongoing treatment process using lateral pipes.    DETAILED DESCRIPTION - The septic tank (140) comprises two equal-sized compartments separated by a partition wall, and an inlet connected to a pipe that is extended towards a bottom of the tank such that a lower end of the pipe is located at a height of 90 mm from the bottom of the tank. The partition wall is open at the bottom of the tank. The septic tank is further connected to a sludge-withdrawal pit by a lateral pipe located at the bottom of the tank, and further comprises a valve configured to control a flow of wastewater from the septic tank to the sludge withdrawal pit. The inlet is connected to a down-comer pipe that has a diameter larger than an inlet size so that occasional clogging by solids and larger objects are avoided. The partition wall is positioned parallel to a tank wall, is located at 70-80 mm from the bottom of the tank, and is completely sealed at its joints with the walls of the tank to ensure no leakage of liquid or short-circuiting from the joints and ensure that the wastewater remains in full contact with the sludge accumulated at the bottom of the tank. The compartments of the septic tank are connected with the sludge-withdrawal pit by lateral pipes. Each of the lateral pipes is located at the bottom of the tank, and has a telescopic sludge-withdrawal valve installed on the lateral pipe and located between the septic tank and the sludge withdrawal pit. The lateral pipe is provided with perforation, which has a diameter of 5 mm, on its periphery to facilitate uniform withdrawal of the sludge from the septic tank under gravity and a hydraulic head difference between the septic tank and the sludge-withdrawal pit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic sectional elevational view of a modified septic system.    Tank side wall (104)    Opening (106)    Outlet (108)    Scum layer (120)    Modified septic tank. (140)