Filter & Membrane Treatment

Activated carbon filters are designed to trap other carbon-based impurities (total organic carbon, or TOC) and remove chlorine. Chlorine is normally added to raw water to control microbial growth and oxidize certain metals. The chlorine must be removed prior to the RO membranes. Since carbon filters remove chlorine, there is a strong potential for microbial growth in the filter. If not properly treated, the microbial growth can infect piping and the RO membranes located downstream of the filter.

If your water has both iron and manganese, iron and hydrogen sulfide, or iron bacteria, greensand can be a very effective approach for removal. Using potassium permanganate, manganese and soluble iron are oxidized and precipitated by contact with higher manganese oxides on the greensand granules. The hydrogen sulfide is eliminated by oxidation to sulfate and an insoluble precipitate. Another option is to inject chlorine upstream of the greensand filter, oxidizing the iron and/or manganese, and allow it to be removed as a colloid.

Multimedia filtration is a proven design concept of diminishing media particle size. The top layers of coarse media trap large colloidal particles, and successively smaller particles are trapped in the finer layers of media deeper in the bed. The result is a highly efficient filtering system with removal taking place throughout the entire bed. These filters may contain various types of media, as well as gravel under-bedding for media support. Multimedia filters typically have a 10-micron removal efficiency, which can be increased greatly with the addition of a filter aid. If a multimedia filter is used prior to an RO membrane, the maximum inlet turbidity should be 10 NTU.

Microfiltration membranes have a pore size of 0.1–10 µm. These membranes remove all bacteria and some viral contaminants. Even though viruses are smaller than microfiltration membrane pores, they attach themselves to the bacterial biofilm and are removed along with the much larger bacteria. Microfiltration can be implemented in many different water treatment processes when particles with a diameter greater than 0.1 µm need to be removed from a liquid. For complete removal of viruses and color, ultrafiltration is required. Ultrafiltration membrane pores can remove particles of 0.001–0.1 µm.

Sand filters are used to remove suspended solids. A filter aid is typically employed with sand filters to increase colloidal particle size, which allows the larger “floc” particles to be trapped by the sand. Most solids can be removed using this filter type. Varying grades of sand are available for different applications. ChemTreat has experience servicing every kind of sand media available, and can assist with suspended solids removal in your systems.