Introduction

The purpose of filtration is the removal of suspended solids from water. Filtration is the process of passing water through a bed of granular or powdered medium or through a porous, fibrous medium such as paper, cloth or synthetic membrane to remove the suspended particles. These solid particles may be finely divided sand, silt, solid organic matter such as plant debris, precipitated (not soluble) iron, algae, bacteria and a host of other materials found in both surface and underground water supplies.

To understand how a mechanical filter works, a number of factors need to be taken into consideration such as:

pore size of the filtering medium
total filtering area available
capacity of the filter
quality of the water to be filtered
required flow rates
construction/design of the filter

Surface vs. Depth (deep bed) Filters

Single mechanical filters can be grouped into two basic types:

1. Surface filters uses sieves or screens to reject particles on the basis of size. The     process is characterized by the eventual formation of a continuous surface layer of     film composed of the removed particles. The surface layer is then the portion of the     filter that does the actual filtering. The openings in the surface layer itself control     the size of particles subsequently removed. Surface filters tend to clog quickly.
2. Depth filters (deep bed) use a tortuous path to entrap particles deep within the     depth of the medium. Particles are deposited in the upper part of the bed first, but     once a layer of particles is deposited, additional particles simply flow past them to     find a spot deeper in the bed. This is possible because the passage through the     irregular maze of channels or paths in a granular bed is larger than the particles to     be removed. Some of the particles are strained out while others are attracted to     the medium because of electrostatic and intermolecular forces. Depth filtration can     be a single medium (sand) or several different media either in stratified layers or     mixed format.

Factors Affecting Filtration

Filtration is an inter-relationship of straining and the ability of a particular medium to retain the particles and other factors such as the chemical characteristics of the water being treated and the nature of the suspension (characteristics of the solid particles and amount in suspension).

The following is a list of the factors that effect filtration:

  - Water Temperature
  - Forces on Suspended Particles
  - Surface Charge
  - Effects of Time on Filtration
  - pH

Particle Size

Mechanical filtration can remove a wide variety of particles from suspension in water such as clay, bacteria, asbestos fibers, cysts and algae. The filtration processes do not readily reduce particles under about 0.1um. To remove these small particles, they must be reduced by other treatment methods or the contaminants must be inactivated. For example, disinfections by ozonation, chlorination, ultraviolet radiation, ultra-filtration, reverse osmosis or distillation may be used.

Particles between approximately 0.1 and 5 microns such as bacteria are most effectively reduced by micro-filtration, ultra-filtration, absolute rated cartridge filter and distillation. These are not practical alternatives for treating large quantities of water and are more suitable for residential treatment.

Pre-coat filtration (such as diatomaceous earth filters) is suitable for reducing particles greater than about 1 um in size, such as asbestos fibers and silt particles.

Residential and commercial granular media filtration is generally used to remove particles greater than 20 microns. It may also be used to reduce particles in the range of 5 to 20 microns with proper pretreatment. Granular filters should not be relied upon for complete reduction of bacteria, cysts and viruses.

Reverse osmosis, also known as hyperfiltration, is the finest filtration known. This process will allow the removal of particles as small as dissolved individual ions from a solution. Reverse osmosis is used to purify water and remove ions and dissolved organic molecules. It can be used to purify fluids such as ethanol and glycol, which will pass through the reverse osmosis membrane, while rejecting other ions and contaminants from passing. The most common use for reverse osmosis is in purifying water. It is used to produce water that meets the most demanding specifications that are currently in place.

OSMOSIS	REVERSE OSMOSIS
If two aqueous solutions of different salinity are separated by a semi-permeable membrane, osmosis will cause water to pass through the membrane in the direction of the more concentrated solution, therefore diluting it. By applying sufficient pressure to the more concentrated liquid, the direction of osmosis can be reversed. In this way, we can mechanically reverse the flow and separate the concentrated solution into its constituents: the water and the dissolved solids. One part is called the permeate, or filtrate, and the other is the reject stream, or concentrate.

Reverse osmosis uses a membrane that is semi-permeable, allowing the fluid that is being purified to pass through it, while rejecting the contaminants that remain. Most reverse osmosis technology uses a process known as crossflow to allow the membrane to continually clean itself. As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected species away from the membrane, in a concentrated brine reject water. The process of reverse osmosis requires a driving force to push the fluid through the membrane, and the most common force is pressure from a pump. The higher the pressure, the larger the driving force. As the concentration of the fluid being rejected increases, the driving force required to continue concentrating the fluid increases.

Reverse osmosis is capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, and other constituents that have a molecular weight of greater than 150-250 daltons. The separation of ions with reverse osmosis is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected

Ultraviolet (UV) disinfection uses a UV light source, which is enclosed in a transparent protective sleeve. It is mounted so that water can pass through a flow chamber, and UV rays are admitted and absorbed into the stream. When ultraviolet energy is absorbed by the reproductive mechanisms of bacteria and viruses, the genetic material (DNA/RNA) is rearranged and they can no longer reproduce. They are therefore considered dead and the risk of disease has been eliminated.

UV-rays are energy-rich electromagnetic rays that are found in the natural spectrum of the sunlight. They are in the range of the invisible short wave light having a wavelength ranging from 100 to 400 nm (1 nanometre = 10-9m).

UV, like distillation, disinfects water without adding chemicals, and therefore possesses some of the same benefits as distillation. It does not create new chemical complexes, nor does it change the taste or odor of the water, and does not remove any beneficial minerals in the water.

Ultraviolet devices are most effective when the water has already been partially treated, and only the cleanest water passes through the UV flow chamber. Use both a sediment and a carbon filter to clean the water prior to passing it through the UV light, to provide complete water quality solutions.

Ultraviolet light is a natural, cost effective, environmentally friendly disinfection process for use in homes where healthy water is a concern.