As impurities continuously accumulate in the filter medium layer, the head loss (i.e., pressure difference) will keep increasing. When the pressure difference reaches a certain set limit, or the filtration time hits the set value, the system will automatically switch to the backwash state to clean the filter medium layer and remove accumulated impurities. Upon entering the backwash state, the PLC controls change the positions of the inlet and outlet valves (closing the water inlet, opening the sewage outlet, opening the backwash water inlet, and closing the backwash sewage outlet). In a sequence combination, water from other filtered tanks partially enters the tank to be backwashed. Due to system pressure, the filter medium layer of the backwashed unit is lifted by the flow of water, and impurities are discharged through the sewage outlet.

Internally, the specially designed collector in the tank creates an internal circulating flow during the backwash state, causing the filter media to scrub against each other, maximizing backwash efficiency, reducing the required backwash water, and preventing material loss during backwashing. When backwashing ends, the valves return to their filtering state, and the next filtering unit enters the backwash state. In a sequence combination, a system composed of multiple filter tanks (units) performs backwashing in turn, ensuring continuous flow during backwashing.