Mechanical Filtration

Mechanical Filtration units are either installed as Point of Entry (POE) or point-of-use (POU) systems. They are either placed at the incoming water supply as POE or as POU at the kitchen faucet and used to filter water intended for drinking and cooking purposes only.

Mechanical Filters remove dirt, sediment, and loose scale from incoming water. Mechanical filters may also be effective in removing asbestos and giardia cysts and in treating color, taste, and odor problems associated with solid organic residues. Mechanical filters can neutralize acid water when the filter contains a slowly dissolved liming agent. These filters are often called particulate or turbidity filters.

Sand, filter paper, spun cellulose, rayon, ceramic or compressed glass wool are some of the filter media that serve as a fine sieve to affect removal of suspended materials. As line pressure forces water through the filter fibers particles are trapped in the filter material and removed from the water. Marble chips or a slowly dissolved liming agent can neutralize acid water when it is forced through the filter.

Mechanical filters do not purify or soften water and have little effect on chemical contaminants dissolved in the water. They will not remove nitrates, heavy metals, pesticides, bacteria, or trihalomethane (THM) compounds.

Mechanical Filtration units are effective at filtering :

non-dissolved particles from .22 micron [1/100,000 of an inch] to as big as required depending on the porosity of the media used

Considerations:

Most mechanical filters signal their own need to be replaced or cleaned or backwashed because they restrict line flow when they become clogged with particulates. Consider maintenance.

The most common mistake made when purchasing a filter unit is to size it inappropriately to the application. To keep line flow as unrestricted as possible larger filters are frequently required. If a very fine level of filtration is required, it may be best to consider using two or more filters in series with the first filters graded to trap larger particles. Consider the volume and amount of filtered water you need.

Membrane Filtration

Membrane Filtration provides a barrier with a specified pore size. Flow rates are affected by the filter membrane (barrier). Membrane filtration can be categorized in the following manner:

Nanofiltration: eg. Reverse Osmosis which has a membrane with sufficiently small pore sizes to restrict larger molecules.
Ultrafiltration: eg. Ceramic filters which have pore size sufficiently small to filter bacteria cysts. Ultrafiltration operates relatively high pressures.
Microfiltration: eg. Woven and spun filter media within filter cartridges and bag filtration. Microfiltration works under lower differential pressures than ultra filtration.

Ceramic Filtration

Ceramic filters are made of china fabricated from diatomatious earth with a porosity capable of filtering out microorganisms. The ceramic provides: 100% rejection of Bacteria, Cysts, absolute depth filtration >.9 micron and effective depth filtration of particulate >.5 micron (dirt, asbestos, iron etc), excellent improvement in Taste and Odor, plus the added bonus of Chemical, Lead and Heavy Metal Rejection.

Bag Filtration

Consist of a filter housing and a filter media bag.

Filter pore sizes vary from 1 to 800 micron.

Filtration materials such as: Felt, Polyester, Polypropylene, Nylon, Teflon, Monofilament are used.

Considerations:

The bags must be manually cleaned which creates higher operational costs

Activated Alumina Filters

Activated alumina is a granulated form of aluminum oxide which has been heat treated to allow it to absorb inorganic chemicals such as arsenic, fluoride, lead, and selenium chemicals efficiently.

During the filtration process, water containing the contaminant is passed through a cartridge or canister of activated alumina. The alumina absorbs the contaminant and fresh water continues to the service faucet.

Considerations:

Activated alumina devices may accumulate bacteria, so at times treated water may have higher bacteria counts than raw water. Consider maintenance.

The cartridge of activated alumina has to be replaced periodically. It is important to determine by testing when the contaminant removal capability of the device is exhausted. Consider maintenance.

Like carbon filters once the alumina has absorbed the maximum amount of contaminant, the filter may release contaminant to the water rather than removing it. Consider maintenance.

Sand Filter

The two basic types of sand filtration are classified a rapid and slow sand filtration

Rapid Filtration

The most common example of rapid filtration is the swimming pool filter. This filtration system consists of a bed of granular material (usually sand) which is rests upon a porous surface which allows water to pass through to the drain but with pores small enough to stop the sand from passing through. The particulate capture and containment process occurs throughout the filtration media.

The filtration unit is cleaned by backwashing a technique which forces water backwards through the filter and into a drain. Backwashing frequently requires both substantial water pressure and volume.

Slow Filtration

As its name suggest, slow filtration works with a much slower water flow rate.

A slow sand filter is comprised of a bed of graded sand, which is supported by a layer of gravel. The filter media is confined in a box with openings at both ends allowing water to flow in and out, while operating on a top-down, gravity basis. The filtration process, a form of natural, biological water treatment is used to remove solids, precipitates, turbidity (muddiness) and in some cases bacterial particles that produce bad taste and odour.

Sand filtration is suitable as a pre-treatment process for surface water containing parasites, bacteria and suspended solids. However, pre- screening/settling may be required prior to filtering for water with a high algae concentration or is very turbid. It is also effective at treating groundwater with high iron and sulphur gases. With iron and dissolved gases removed, other processes such as softening and demineralization can be incorporated into an overall in-house treatment process.

These filters use both mechanical and biological actions as part of their process. The biological activity within this type of filtration unit requires that it must be operated year-round.

Cleaning a slow sand filter consists of removing the top layer of sand and detritus from the filter bed.

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