Domestic and Industrial



KNOWLEDGE

     Pressure Sand filters. are used to remove turbidity. Sand filter media quality is crucial to the operation and longevity of the filter. To assure good wastewater treatment, sand media free of fine particles, with the proper sand uniformity and effective size must be used with the appropriate sand filter type and hydraulic loading rate.


     Activated carbon (AC) is similar to ion exchange resin in density and porosity. It absorbs low molecular weight organics and reduces chlorine or other halogens from water, but does not remove any salts. These filters must be changed periodically to avoid bacterial growth, but are not easily reactivated in the field. Accumulated solids require frequent backwashing of the filter unless installed after reverse osmosis or ultra-filtration. Installation of these filters is usually done for the removal of offensive tastes and odour, colour, chlorine and potentially dangerous and carcinogenic organic compounds, which may be present in a water system as a result of chlorination or industrial pollution. Activated carbon act by adsorption of gas and volatile organic compounds on its surface. The efficiency of any activated carbon filter is dependent on the "useful flow rate of the filter and estimated filter lifetime, which are determined mostly by the filter size and the amount of carbon in the filter.

     Cartridge filters can now be described two general ways: as depth filters or surface filters.

     Depth cartridge filters. In a depth cartridge filter, the water flows through the thick wall of the filter where the particles are trapped throughout the complex openings in the media. The filter may be constructed of cotton, cellulose, synthetic yarns or "blown" micro-fibers such as polypropylene. The best depth filters have lower density on the outside and progressively higher density toward the inside wall. The effect of this "graded density" is to trap coarser particles toward the outside of the wall and the finer particles toward the inner wall. Depth cartridge filters are usually disposable, cost-effective, and are in the particle range of 1 to 100 microns. Generally, they are not an absolute method of purification since a small amount of particles within the micron range may pass into the filtrate.

     Surface filtration–pleated cartridge filters. Pleated cartridge filters typically act as absolute particle filters, using a flat sheet media, either a membrane or specially treated non-woven material, to trap particles. The media is pleated to increase usable surface area. Pleated membrane filters serve well as sub-micron particle or bacteria filters in the 0.1 to 1.0 micron range. Newer cartridges also perform in the ultrafiltration range: 0.005 to 0.15 micron. Ultrafiltration cartridge filters. Point-of-use ultrafiltration cartridges are used to remove pyrogens and other macromolecular compounds from ultrapure water. They are built in a spiral-wound configuration. This allows a crossflow mode of operation to help keep the surface clean.

     Water softening. The ion exchange water softener is one of the most common tools of water treatment. Its function is to remove scale-forming calcium and magnesium ions from hard water. In many cases soluble iron (ferrous) can also be removed with softeners. A standard water softener has four major components: a resin tank, resin, a brine tank, and a valve or controller. However, water softening is disadvantageous when high quality water is required since sodium ions will be present after the ion exchange process.

     De-mineralization / de-ionization. Ion exchange deionizers (Dl) use synthetic resins similar to those used in water softeners. Typically used on water that has already been pre-filtered, DI uses a two-stage process to remove virtually all ionic material remaining in water. Two types of synthetic resins are used, one to remove positively charged ions (cations) and another to remove negatively charged ions (anions). Resins have limited capacities and must be regenerated upon exhaustion.

     Mixed-bed deionizers. The two basic configurations of deionizers are two-bed and mixed-bed. Two-bed deionizers have separate tanks of cation and anion resins. In mixed-bed deionizers, the anion and cation resins are blended into a single tank or vessel. Generally, mixed-bed systems will produce higher quality water with a lower total capacity than two-bed systems. De-ionization can produce extremely high-quality water in terms of dissolved ions or minerals, but they do not generally remove organics and can become a breeding ground for bacteria.

     Reverse osmosis (RO) was the first cross-flow membrane separation process to be widely commercialized. RO removes virtually all organic compounds and 90 to 99% of all ions. A large selection of reverse osmosis membranes are available to meet varying rejection requirements. RO can meet most water standards with a single-pass system and the highest standards with a double-pass system. RO rejects 99.9+% of viruses, bacteria and pyrogens.

     Pressure, on the order of 200 to 1,000 psig (13.8 to 68.9 bar), is the driving force of the RO purification process. It is much more energy efficient compared to heat-driven purification (distillation) and more efficient than the strong chemicals required for ion exchange. No energy-intensive phase change is required.

     Water then flows from higher energy to lower energy. In this case, from the more concentrated

The rate of water transport is a function of:

  • The pressure applied.
  • The apparent or differential osmotic pressure between the solutions. (Differential osmotic pressure is the difference between the absolute osmotic pressures of the two solutions.)
  • Area and characteristics of the membrane.
  • The solution temperature.

     Nano filtration (NF) equipment removes organic compounds in the 300 to 1,000 molecular weight range, rejecting selected salts (typically divalent), and passing more water at lower pressure operations than RO systems. NF economically softens water without the pollution of salt-regenerated systems and provides unique organic desalting capabilities.

     Ultra filtration (UF) is a similar process to RO and NF, but is defined as a cross flow process that does not reject ions. UF rejects contaminants in the range of 1000 dalton (10 angstrom) to 0.1 micron particles. Because of the larger pore size in the membrane, UF requires a much lower operating pressure: 10 to 100 psig (0.7 to 6.9 bar). UF removes organics,bacteria, and pyrogens while allowing most ions and small organics, such as glucose, to permeate the porous structure.