Microfiltration usually serves as a pre-treatment for other separation processes such as ultra filtration, and a post-treatment for granular media filtration. The typical particle size used for microfiltration ranges from about 0.1 to 10 µm. In terms of approximate molecular weight these membranes can separate macromolecules generally less than 100,000 g/mol. The filters used in the microfiltration process are specially designed to prevent particles such as, sediment, algae, protozoa or large bacteria from passing through a specially designed filter. More microscopic, atomic or ionic materials such as water (H2O), monovalent species such as Sodium (Na+) or Chloride (Cl-) ions, dissolved or natural organic matter, and small colloids and viruses will still be able to pass through the filter.
2. Ultra filtration
Ultra filtration is a membrane filtration process similar to Reverse Osmosis, using hydrostatic pressure to force water through a semi-permeable membrane. Suspended solids and solutes of high molecular weight are retained in the so called retentate, while water and low molecular weight solutes pass through the membrane in the permeate. This separation process is used in industry and research for purifying and concentrating macromolecular solutions, especially protein solutions. Ultra filtration is not fundamentally different from microfiltration or nano-filtration except in terms of the size of the molecules it retains – it is defined by the Molecular Weight Cut Off (MWCO) of the membrane used. Common applications for ultra-filtration systems are food & beverage processing, pharmaceutical use, municipal, borehole and surface water treatment.
3. Reverse Osmosis
Explaining the process:
Reverse osmosis is a complicated process which uses a membrane under pressure to separate relatively pure water (or other solvent) from a less pure solution. When two aqueous solutions of different concentrations are separated by a semi-permeable membrane, water passes through the membrane in the direction of the more concentrated solution as a result of osmotic pressure. If enough counter pressure is applied to the concentrated solution to overcome the osmotic pressure, the flow of water will be reversed.- Water molecules can form hydrogen bonds in the reverse osmosis membrane and fit into the membrane matrix. The water molecules that enter the membrane by hydrogen bonding can be pushed through under pressure. Most organic substances with a molecular weight over 100 are sieved out, i.e., oils, pyroxenes and particulates including bacteria and viruses.- Salt ions, on the other hand, are rejected by a mechanism related to the valence of the ion. Ions are repelled by dielectric interactions; ions with higher charges are repelled to a greater distance from the membrane surface. The nominal rejection ratio of common ionic salts is 85 – 98%. – The majority of the commercially manufactured Reverse O osmosis (RO) membranes are usually made from cellulose acetate, polysulfonate, and polyamide. The membrane consists of a skin about 0.25 microns and a support layer about 100 microns. The skin is the active barrier and primarily allows water to pass through.- The amount of dissolved solids in water produced by reverse osmosis is approximately a constant percentage of those in the feed water. For example, when the feed water contains 300 ppm total dissolved solids (TDS), the product water may have 15 to 30 ppm (95% and 90% rejection ratio respectively). A RO system design is based on a certain range of feed water TDS, the percentage of rejection and percentage of recovery desired. For a given system, the higher the percentage of recovery or the lower the percentage of rejection, the poorer the quality of product water becomes.
A Reverse Osmosis Membrane pore is 200’000 smaller than the diameter of a human hair
Various methods exist for effective pre-treatment. The objective of pre-treatment is to remove contaminants from raw water that will affect the stability and performance of the main treatment process and to remove contaminants that will affect performance of clients main process.