Impurities are blocked by filter screen when water pass, which may increased on the screen with time lasting out. When the impurities accumulate to certain degree, the pressure difference between the entrance and the outlet reaches set value i.e.0.05MPa,the installation enters backflushing mode, and differential pressure switch sends signal to PLC controller, which starts booster pump and opens sewage value. The impurities on the screen may be brushed off by rotated brushes and discharged through sewage outlet. When the pressure difference lowers down to the minimum, backflushing ends and the system restores to original filtering mode and waits for next backflushing. In case of backflushing, filtering keeps running and only less amount of water is used for backflushing, which makes online cleaning possible.