Vacuum filter funnel is the driving force to form negative pressure at the outlet of filtrate. The filter can be divided into intermittent operation and continuous operation. Intermittently operated vacuum filters can filter suspensions of various
concentrations. Continuously operated vacuum filters are suitable for filtering thick suspensions containing more solid particles.

Intermittently operated filters have been developed for their automatic operation, and the area of filtration has become larger and larger. In order to obtain filter residue with low moisture content, mechanical press filter has been developed. The
containers are separated into upper and lower chambers by filter media, which constitutes a simple filter. The suspension is added to the upper chamber, and under the action of pressure, the suspension enters the lower chamber through the filter medium to form the filter liquid. The solid particles are intercepted on the surface of the filter medium to form the filter residue (or filter cake).

During the filtration process, the filter residue layer accumulated on the surface of the filter medium gradually thickens, and the resistance of liquid passing through the filter residue layer increases, and the filtration speed decreases. When the filter
chamber is full of filter residue or the filtering speed is too small, stop filtering, remove the filter residue, and regenerate the filter medium to complete a filtering cycle.

The liquid must overcome the resistance through the filter residue layer and the filter medium, so there must be a pressure difference on both sides of the filter medium, which is the driving force to achieve filtration. Increasing the pressure
difference can accelerate the filtration, but the deformed particles are easy to plug the pore of the filter medium when the pressure difference is large, and the filtration is slowed down.

There are three ways of suspension filtration: slag layer filtration, deep layer filtration and sieve filtration. Slag layer filtration refers to the formation of the initial filter slag layer after the initial stage of filtration, after which, the filter slag layer plays a major role in filtering, when large and small particles are intercepted; deep filtration refers to the filter medium is thicker, suspension contains fewer solid particles, and particles are smaller than the channel of the filter medium, when filtering, particles are absorbed in the channel after entering the filter; The retained solid particles are larger than the pore of the filter medium. The filter medium does not absorb solid particles. For example, the rotary filter filter filters the coarse impurities in the sewage.

In the actual filtration process, the three ways often appear simultaneously or successively. The processing capacity of the filter depends on the filtering speed. When the solid particles in the suspension are large and the particle size is uniform, the pore of the filter slag layer is relatively smooth, and the speed of the filter through the filter slag layer is relatively high. The application of coagulant to assemble fine particles into larger aggregates is conducive to improving the filtration rate.

For the suspension with fast settling speed of solid particles, it is applied to the filter which feeds on the upper part of the filter medium to make the direction of filtration consistent with the gravity direction. The first settling of coarse particles can reduce the clogging of the filter medium and the filter slag layer. The filtrate slag layer can be loosened by mixing coarse solid particles such as diatomite and expanded perlite into the suspension (such as colloid) which is difficult to filter. When it is large, the suspension can be heated to reduce the viscosity. All these measures can speed up the filtration.