Microfiltration screen filters

Microporous ultrafiltration and microfiltration membranes used to filter particulates from liquids fall into the two general categories illustrated in Figure 2.31. The first category (a) is the surface or screen filter such membranes contain surface pores smaller than the particles to be removed. Particles in the permeating... 

The two principal types of microfiltration membrane filter in use—depth filters and screen filters—are illustrated in Figure 7.3. Screen filters have small pores in their top surface that collect particles larger than the pore diameter on the surface of the membrane. Depth filters have relatively large pores on the top... 

Depth membrane filters are usually preferred for in-line filtration. As particles are trapped within the membrane, the permeability falls, and the pressure required to maintain a useful filtrate flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particle loading of the feed solution. A typical application of in-line depth microfiltration membranes is final polishing of ultrapure water just prior to use. Screen membrane filters are preferred for the cross-flow microfiltration systems shown in Figure 7.1(b). Because screen filters collect the retained particles on the surface of the membrane, the recirculating fluid helps to keep the filter clean. 

However, the short lifetime of in-line cartridge filters makes them unsuitable for microfiltration of highly contaminated feed streams. Cross-flow filtration, which overlaps significantly with ultrafiltration technology, described in Chapter 6, is used in such applications. In cross-flow filtration, long filter life is achieved by sweeping the majority of the retained particles from the membrane surface before they enter the membrane. Screen filters are preferred for this application, and an ultrafiltration membrane can be used. The design of such membranes and modules is covered under ultrafiltration (Chapter 6) and will not be repeated here. 

Test results starting from pig manure with a solids content of ca. 11% a vibrating screen separates this into a feed stream for the microfiltration containing 6% solids. On this feed, HIC s ceramic 0.1 im membranes reach average fluxes of 80-100 1/m h at filtration temperatures of 80°C. The concentration factor can range between 2.5 and 3. Operating costs are below the DEM 20/m quoted by Meindersma [24]. The combined concentrate of pre-filter and MF is about 55% of the original volume and contains approximately 20% solids the clear permeate of the MF contains approximately 2% solids, typically dissolved substances. 

MicrofQtration with 0.2 im ceramic membranes (HIC) yields average fluxes of 125-150 1/m h at temperatures from 40 to 70°C. Suspended solids and concentration of hydrocarbons are both reduced to less than 10 ppm in the permeate. By RO the COD is reduced to below 100 mg/1. Cleaning interval for the microfiltration installation is once a week. The feed to the microfiltration system has to be filtered over 100 pm screens to prevent clogging of the equipment. 

Screening Equipment Filtration Cake Filters Centrifugal Filters Principles of Cake Filtration Clarifying Filters Liquid Clarification Gas Cleaning Principles of Clarification Crossflow Filtration Types of Membranes Permeate Flux for Ultrafiltration Concentration Polarization Partial Rejection of Solutes Microfiltration... 

---