Polishing Methods: Distillation and Reverse Osmosis
A polishing barrier will increase the safety of water used for human consumption. Distillation and reverse osmosis filtration are two ideal polishing barriers. These processes are very effective in removing impurities that the pre-treatment barriers did not remove. An animated version of this section is available below.
Distillers work by heating water to create steam. The steam is then cooled to collect a purified water. Unwanted microorganisms are killed in the process. Dissolved minerals settle as a scale. Sometimes distillers use additional treatment to remove any organic chemicals that might re-condense in the steam. Distilled water has a very low mineral content, which some consider as a disadvantage to nutrition.
What does osmosis mean?
Osmosis is the movement of water from one side of a membrane or barrier to the other side. Since the water in the less concentrated solution seeks to dilute the more concentrated solution, the water passage through the membrane generates a noticeable head difference between the two solutions. This head difference is a measure of the concentration difference of the two solutions and is referred to as the osmotic pressure difference.
What does reverse osmosis mean?
A pressure can be applied to the concentrated solution which is greater than that of the osmotic pressure. The direction of water passage through the membrane is reversed and the process that is called reverse osmosis is established. That is, the membranes ability to selectively pass water is unchanged. But only the direction of water flow is changed.
In order to keep the membrane from fouling it is important to continually flush the brine side. As the water is squeezed through the membrane, leaving most of the salts behind, the brine side solution becomes more concentrated. Without the reject flow to drain, the brine side mineral concentration would eventually exceed the solubility limits of the salts present and they would precipitate, forming a scale on the membrane.
Reverse Osmosis System
One of the most common methods of personal use water treatment is an undercounter or point-of-use reverse osmosis system. A home reverse osmosis system is a series of components or stages that are designed to work together as a unit. Most systems are designed to allow easy maintenance or upgrading of the individual components.
A typical home reverse osmosis system consists of a sediment filter, two carbon filters, a membrane vessel, and a storage tank. Depending on the water source, additional treatment devices such as water softeners, chemical injectors, media filters may be required to pretreat the source water so it meets the minimum water quality parameters required by the manufacturer. A pump may be required where pressure is low.
It is fairly common to see membranes advertised as having several stages; a system with more stages does not necessarily produce a better quality water since the additional stages are not usually filters but antiscalants, pH adjusters or disinfectants which extend the membrane life.
The spiral wound membrane
At room temperature, a water molecule is relatively small compared to the most common "contaminants" dissolved in the water. By making a membrane fabric which just allows the water molecule to squeeze through, a very pure water can be produced.
The contaminants which are left behind on the membrane must be removed or the membrane will become clogged. To keep a clean membrane, most water entering the membrane moves across the surface and carries the contaminants to waste. Most units produce filtered water by rejecting 75 to 90% of the raw water. The quantity of this flow is controlled by a flow regulator which restricts the waste to a portion of the product flow.
All membranes require protection against large sediment particles: the most common sediment filter will filter particles which are 5 microns or greater (a human hair is roughly 100 microns in diameter) but may range from 1 to 30 microns depending on the source water. If the prefilter clogs rapidly then it likely indicates a problem with either the water source or the pre-treatment process.
In reverse osmosis system installed on municipal water systems which deliver chlorinated water, the membrane is usually cellulose triacetate (CTA) which requires the presence of an oxidant such as chlorine to continuously disinfect the membrane. Because chlorine is required to prevent the bacteria from consuming the membrane, the precarbon filter is omitted since it would remove the chlorine.
In reverse osmosis systems installed on non-chlorinated water, the membrane is usually a thin film composite (TFC) which has several advantages over CTA like higher flow rates, better contaminant rejection and wider. As direct exposure to chlorine or natural oxidants in water will destroy TFC membranes, a coarse carbon filter is added between the sediment filter and the membrane as an oxidant barrier and to reduce taste and odour compounds in the water.
Carbon filters can process between 6,000 and 12,000 litres of water before exhaustion. This translates to an element replacement once every three to six months for typical household demands. The sediment filter should be replaced with the carbon filter, while extending the use of the membrane filter beyond recommended limits can lead to very high bacterial counts in the product water.
Quality of the Product Water
Virtually all reverse osmosis systems have a carbon filter located after the membrane. This serves to reduce undesirable taste resulting from the installation of new filters or in situations where very small organic compounds are present after the membrane. This carbon filter usually contains finer particles than the pre-filter to improve removal of these compounds which have bypassed the initial filters.
Because reverse osmosis systems produce water at relatively low rates, most systems store water. The common storage vessel is a 10 to 20 litre pressure tank with a low pressure bladder. Choosing a large tank can impact on water quality as the quality of the water produced decreases as the pressure builds while filling in the tank. In addition it is better to completely empty and refill the storage daily to inhibit bacterial growth.
It is becoming more common to disinfect the product water prior to its use. Chlorine may be used where a long lasting disinfectant is required but many people dislike the taste. Other options include ozonation or ultraviolet lamps which don’t affect taste but aren’t long lasting.
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