Explanation of the mechanism of deep filtration of diaphragm

  When the diaphragm filter filters on the surface, a small amount of sediment will form a high resistance layer, and once formed, the filter unit must be cleaned or replaced.In contrast, deep filtration of the diaphragm filter provides a large particle interception capacity, which as a third treatment of sewage, generally able to obtain a filtrate containing suspension of no more than 5MGL.The diaphragm filter can obtain a very clear filter fluid when combined with the condensation process.Unlike surface filtering and cake filtration using screen or fiber fabric, diaphragm filter deep filtration is a process of filtering using the pores between the medium, characterized by diaphragm filter occurring inside the medium layer and each void has the possibility of retaining particles from the suspension.Since these particles are much smaller than the void size, when the liquid is laminar, there must be certain works for the particles to cross the streamline in contact with the filter medium, an effect commonly known as the "migration mechanism".

  According to the theory of migration mechanism of diaphragm filter, there are three main forces acting on particles and passing through the flow line:

1：The diffusive force, similar to Brownian motion, occurs when particles are less than 1 μM;
2：The gravity force of Stokes settlement theory usually occurs when the particles are greater than 5 μM and the concentration is higher than water;
3：The force of fluid motion, which is a force that crosses the flow field due to the flow velocity distribution in the medium pore and due to the shape of the particle itself.

  Both the intercept efficiency and the function of the particle diameter are approximately curved.The intercept efficiency is lowest at a particle size of 1 μM because this critical value is too large for the diffusion effect and too small for the gravity and intercept effects.Therefore, the diaphragm filter often has the interception efficiency of submicroparticles than the particles with larger particle size.Control the particle movement, in addition to the migration mechanism, there are the attachment mechanism, shedding mechanism and other comprehensive effects.When the particle moves to the dielectric surface, the nature of the binding force determines whether it can attach.Both diaphragm filter particles and medium surface have a low potential in water, generally less than 20MV and negative.According to the ion conditions dissolved in water, particles cannot attach if repulsion between the particle and the medium.