In operation, the monitoring index to determine whether the filter needs backwashing is not the turbidity of the effluent water quality, but the head loss of water flowing through the filter layer, which is because the change regularity of the effluent turbidity is not strong, and it is difficult to detect; while the head loss changes obviously, and the pressure detection is relatively simple.

When the head loss (i.e. pressure difference) of the filter reaches a certain value, it should stop running and carry out backwashing. The main reason why the filter can't run until the pressure difference is too large is that when the head loss is large, the inlet pressure must be increased to achieve a certain output. This will easily cause the breakdown of the filter layer, a large amount of water flow will pass through the cracks, destroy the filtering effect, and make the quality of the effluent deteriorate quickly.

The velocity of water flow in backwashing can be expressed by "backwashing intensity" in L/(m2.s), which indicates the amount of backwashing water flowing through the filter section per square metre per second (liter). The strength of backwashing should be properly controlled. The filter layer should be fully loosened so that particles can collide and friction with each other, while washing away mud and small filter debris, and not taking away normal particles. Generally, the backwashing strength of quartz sand is 15-18 L/(m2.s), while that of anthracite is 10-12 L/(m2.s) because of its low density. The backwashing time is usually 5 to 10 minutes.