Water purification technologies

The chemical composition of potable or raw water drawn from boreholes or direct from mains supplies can vary considerably in terms of the quantity and the variety of dissolved minerals and particulate matter. Because of its variability, utilising the correct water purification techniques is essential to ensure a consistent supply of high purity or ultra-pure water.

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The chemical composition of potable or raw water drawn from boreholes or direct from mains supplies can vary considerably in terms of the quantity and the variety of dissolved minerals and particulate matter. Because of its variability, utilising the correct water purification techniques is essential to ensure a consistent supply of high purity or ultra-pure water.

The impurities in water come from several different sources, and the importance of eliminating the different types of contaminant will vary depending on the specific equipment and processes. For most applications, contaminants can be divided into five main groups: ions; organic materials; bacteria; endotoxin by-products; and dissolved gases.

The required level of purity is based on the measurement approach used to determine the level of these different types of contaminants. Ions increase the ability of the water to conduct electricity, so are measured by determining the conductivity of the water. In purer water, due to the lack of ions, resistivity is used. Organic compounds are measured in terms of total oxidisable carbon (TOC). Particulates are measured by size, and quantity per millilitre (ml).

We use a wide range of proven, advanced water purification technologies in our water purification systems. Some of these are listed below – for more information or to find the best water purification system for your business and application contact us for no obligation technical advice.

Reverse Osmosis

Typically, a Reverse Osmosis (RO) system works by feeding a conventional mains supply under pressure into a module containing a semi-permeable membrane. The membrane removes a high proportion of impurities, including up to 98% of inorganic ions, together with virtually all colloids, micro-organisms, endotoxins and macromolecules, with almost 70% of the feed-water passing through the membrane as a purified permeate, with impurities being removed in a residual concentrate stream that is run to drain.

RO systems generally include a pre-treatment package designed to meet the characteristics of the feed-water. Typically, this equipment includes a base-exchange softener to remove hardness that would otherwise scale the membranes. Further protection being provided by passing the water through activated carbon filters, to remove free chlorine and organic contaminants, with any remaining particulates being removed by a fine filter before the pre-treated water enters the RO plant.

For applications that require even higher purity water, such as Haemodiafiltration, it is normal to use double pass reverse osmosis. This involves processing the permeate through two sets of membrane filters, but produces water that has extremely low levels of endotoxins and bacteria.

Finally, portable and self-contained containerised RO units can be located outside buildings to maximise the space available internally. These systems can be installed quickly and simply, with a minimum of disruption; once in place they have low operating costs, requiring minimal maintenance or user intervention, and can be used to provide a 24/7 supply of high purity water.

Softening

All mains water supplies in the UK contain hardness to some degree, however there are regional variations in water quality.

Hardness in potable water is generally a result of the reaction between rainwater and chalk, leading to the formation of soluble calcium and magnesium bicarbonate. If left untreated, precipitation of scale, insoluble calcium and magnesium carbonate can occur when hardwater is heated or passed through a reverse osmosis system.

Once scaling has occurred the only way to remove it is by treatment with a weak acid solution. This can be both hazardous and costly, and results in a plant being off line for the duration of the clean.

The most cost effective way to remove hardness from water is to use a water softener.

The most basic water softeners comprise a single vessel with individual control valve, filled with a commercially available cationic, ion-exchange resin. During softening, hard water flows down through the resin bed where calcium (Ca2+) and magnesium (Mg 2+) ions are selectively attracted to the resin exchange sites and are replaced by sodium (Na+) ions.

The resin has a finite capacity to exchange calcium and magnesium ions and so will require regeneration. Regeneration can be initiated at specified time intervals or, more cost effectively, on volume of water treated.

Regeneration involves simply passing a diluted salt solution (sodium chloride) down through the resin bed. This has the effect of eluting previously exchanged calcium and magnesium ions from the resin and replacing them with sodium ions. Any unused salt solution is then rinsed clear with mains water.

When using a single column plant during regeneration, the softener will be off line so the production of softened water would cease; if a continuous flow of soft water is always required then a system incorporating two columns would be used. As one column goes into regeneration, the standby column would come on line to maintain production. This type of arrangement is termed “duplexing” and is commonly used for feeding reverse osmosis plant, which generally require an uninterrupted flow of soft water.

Typical applications that require the use of softened water are: low pressure boiler feed, endoscope reprocessing (first rinse stage), industrial hot water boiler feed and pre-treatment to reverse osmosis equipment.

Demineralization/Deionization

Demineralized, or deionized water, is essentially water from which ions, such as cations of Sodium, Calcium, Magnesium and Potassium, and anions such as Chloride, Nitrate and Sulphate, have been removed. Water is often required to be deionized for applications where the elimination of impurities is essential.

Demineralization using ion exchange techniques can produce varying standards of water purity, by means of reusable and rechargeable ion-exchange resins. These are normally supplied in disposable cartridges, for small volume or infrequent use applications, or replaceable cylinders, for higher volume or constant use flow.

In each case, the cartridge or cylinder is generally filled with a specially developed ion-exchange resin, in the form of small beads through which the feed water is passed.

The process works by exchanging Hydrogen and hydroxyl ions from the resin, for cationic and anionic contaminants in the feedwater. Eventually, the cations and anions will have replaced the active Hydrogen and hydroxyl sites in the resins and the cartridges will need to be replaced or regenerated.

Ion Exchange

Ion exchange is essentially a rapid and reversible technique in which the ionic contaminants present in feedwater are exchanged for ions released. This can either be achieved using ion-exchange resins, supplied in disposable ion exchange cartridges or rechargeable cylinders, or by means of a technique known as continuous electro-deionization or CEDi. Cartridges and cylinders have the advantage of being relatively simple and are generally used in applications where purified water is required in small volumes or at infrequent intervals. CEDi is a more complex process but is ideal for applications where a continuous volume of high purity water is required.

CEDI

With the latest technology, it is now possible to achieve deionized water without the use of cation and anion resins regenerated with acid and caustic solutions; this technology is known as continuous electro-deionization or CEDi.

The CEDi cell is constructed from alternating layers of cation and anion permeable membranes; these ion selective membranes prevent the passage of ions, allowing only water to pass through. Cathode and anode electrodes are placed on the outer sides of the stack.

Feedwater flows into the stack and is split, with 90% passing into the Feed/Dilution chamber, 5-10% into the Concentration chamber and <5% into the Electrolyte compartment.

The unwanted ions in the feedwater are exchanged onto the ion exchange resin and the applied DC potential draws the ions towards their respective electrodes. As cations move towards the cathode they pass through the cation permeable membrane and into one of the Concentration compartments. Further movement towards the cathode is halted by anion permeable membranes; the cations are neutralised by anions rejected from the other corresponding chambers. The rejected ions are flushed from the system in the concentrate stream.

Carbon Absorption

The core treatment process of many water purification systems is reverse osmosis. Spirally wrapped, polyamide membranes are used in the vast majority of today’s RO plant. These membranes have a limited exposure life to ‘free Chlorine (approximately 1,000 ppm hours).

Chlorine is added to drinking water supplies by the water authorities to kill any harmful bacteria. The by-product of free Chlorine is Chloramines, which then provide protection to the water supply distribution system as they are more stable and take longer to break down compared with free Chlorine.

For renal dialysis patients, Chloramines, if present in excess, can give rise to conditions such as haemolysis and haemolytic anemia, which cause the destruction of red blood cells. Therefore, within the designs of many water treatment systems, dechlorination forms an integral part of the pre-treatment programme.

One of the most effective media for the removal of both free Chlorine and its by-products is “activated carbon”. Raw materials such as bituminous coal, peat and coconut shells can all be used to produce activated carbon.

Many stages of treatment are required to turn the original raw material into the final product. Firstly, the selected material is ground into small granules, then, to remove any residual organic impurities, the granules are heated to extremely high temperatures and thermally activated with super-heated steam under extreme pressure.

To remove any inorganic impurities the granules are washed in a solution of diluted acid and then neutralised with an alkaline rinse. The resulting product resembles that of a sponge made from pure Carbon, with a surface area of 800-1500m² /gm. The higher the surface area, the higher the activity the carbon will have for adsorption.

Apart from activated Carbon’s ability to dechlorinate water, its porous structure and high surface area are ideal for the removal of dissolved organic compounds, which, in water supplies, can be both man-made or natural in their origins. Compounds such as pesticides, herbicides or micropollutants such as polychlorinated bi-phenyls (PCBs) or trihalomethanes (THMs) would fall under the category of man-made organics, while humic and fulvic acids, derived from decaying vegetation, would be classed as naturally occurring organics.

Activated Carbon (AC) units are typically sited in the pre-treatment train to remove free Chlorine and organics, therefore providing protection for downstream RO plant. For free Chlorine removal, a contact time of six minutes is necessary, but for the more stable Chloramine compounds a contact time in excess of 10 minutes is generally required.

AC units can also be installed in the purified water distribution system to reduce the Total Organic Carbon (TOC) levels. TOC in purified water can be attributed to low molecular weight organics leached from the plastic pipe and solvent adhesives commonly used to construct distribution systems. TOC is generally expressed in µg/l (ppb, parts per billion).

Filtration

Within the design of any water treatment system there will be a requirement for filtration. The mechanics of filtration can be delivered in many forms and range from coarse screening, designed to remove large particulates, typically >100 microns, through to filters that can remove contaminants based on their molecular weight. A summary of the types of filtration that can be used within a water treatment system are as follows:

Cartridge filtration

Nominal rated cartridge filters can be supplied in various lengths from 5” to 40” and with particulate removal ratings typically anywhere between 75 and 1 micron, and are generally constructed from spun polypropylene fibres.

They are regarded as having a nominal rating; i.e. approximately 85% of the particles at the stated filter rating being retained. The process of filtration within these cartridges is achieved by passing water through a depth of media whereby particles are removed by physical sieving and retained by adsorptive forces known as Van der Waals’ forces.

The function of these types of filter is to provide protection to reverse osmosis plant from fouling by particulates and colloidal contaminants originating from the incoming potable mains water supply. The designed operating life expectancy of a cartridge filter is typically two to three months but can vary depending on the quality of the feedwater.

Membrane filtration

The requirements of filtering purified or ultra-pure water are somewhat different compared with those for potable water. Taking into consideration that purified water has already been pre-filtered and processed, typically by reverse osmosis, the only need for further filtration is to remove unwanted bacteria or bacterial fragments. Cartridge filters would not be effective, as they cannot guarantee absolute removal especially at the micron rating required to retain bacteria.

Therefore, filters constructed from thin layers of filtration membrane are used with tightly controlled pore sizes, between 0.1 and 0.2 microns, resulting in an efficiency of bacterial removal that is better than 99.9%; this gives almost absolute retention of bacteria. The typical operating life of a membrane filter is normally between 6 and 12 months.

Ultra-filtration

Ultra-filters (UF) can be obtained either as spirally wrapped membrane devices or as a bundle of hollow fibres. The nominal molecular cut-off (NMWCO) range for ultra-filtration devices is between 5,000 and 150,000 Daltons.

They can be used in both pre-treatment and polishing duties in water purification systems. For pre-treatment, a UF device with a (NMWCO) of between 80,000 and 150,000 Daltons would be used for the removal of colloidal contaminants. For the removal of Endotoxin material from purified water, the pore size would be reduced to between 5,000 and 20,000 Daltons.

To maintain performance, UF membranes will require frequent back-flushing and occasional cleaning using chemicals.  For critical applications, hot water can be used to thermally disinfect the membranes; this will depend on the materials of construction used in the UF membrane.

Thermal Disinfection

It is widely known that boiling water effectively renders it sterile and it is for this reason that distillation equipment is still in use today.

Practically, is not feasible, and would be potentially dangerous to disinfect a system with boiling water; it is also unnecessary to achieve such high water temperatures to kill water borne bacteria efficiently.

Heating water to between 80ºC and 90ºC for a period of not less than two to four hours will achieve the same result.

The use of hot water as a method of disinfection is becoming more common-place as it eliminates both the handling of the hazardous chemicals that have traditionally been used for this purpose, and their subsequent disposal.

Systems can be operated either continuously at over 80ºC or be periodically heated to achieve good bacterial control. It should be noted that when designing a purified water distribution system that runs at elevated temperatures, traditional piping materials may not be suitable; most heated systems are, therefore, constructed from stainless steel, polypropylene or cross-linked polyethylene (PEX).

Ultraviolet

Ultraviolet (UV) light at a wavelength of 254nm is used widely within many water treatment systems as a method of controlling bacterial levels.

UV light @ 254nm deactivates or kills water borne gram negative bacteria through disruption of their DNA, thus preventing them from replicating.

Low pressure mercury lamps are the preferred choice of technology as they do not impart high levels of thermal energy into the water; for an efficient kill rate a minimum dosage of 30-32mJ/cm² is generally required.

The life expectancy of a UV lamp is approximately 8,000 hours or typically one year.

Organic Scavenging

In certain geographical areas, feedwaters can be heavily loaded with dissolved organic contaminants. Generally, the main source of this type of water tends to be surface supplies such as lakes, reservoirs, lochs, etc., with much of the organic contaminants in the water originating from dissolved vegetation. For this reason, the concentrations of contaminants will vary throughout the year and because of changing rainfall levels.

The most noticeable effect of organics in water is in an increase in colour; organic laden waters tend to take on a brown or straw coloured appearance. Due to their high molecular weight and the possibility of forming large colloidal complexes, dissolved organics can pose a fouling problem for both reverse osmosis membranes, ion-exchange resins and sub-micron filtration systems.

The traditional way to remove dissolved organics from any incoming mains water supply is to use an ion-exchange system incorporating a specifically selected macro-porous, anion, ion-exchange resin. The resin is typically contained in a single vessel with a dedicated controller valve, which automatically controls the operation of the system. Water to be conditioned flows down through the bed where the organics are adsorbed and held in the macroporous structure of the resin itself. The resin has a finite capacity to remove organics and so will periodically require regeneration; initiation of regeneration can be controlled by time or by volume of water treated.

During regeneration, a weak solution of brine is passed down through the bed, which has the effect of eluting off the captured organics and replacing Chloride ions on to the resin’s functional groups.

When designing a water treatment system when it is known that the feedwater contains high levels of dissolved organics, the primary aim is to provide protection to any downstream ion-exchange resin or reverse osmosis plant. The efficiency of both processes will suffer if subjected to treating water with high organics. The effects of organic fouling tend to result in a reduction of output flowrate, drop in peak water quality and sometimes an increase in the microbiological levels of the treated water.

By installing an organic scavenger system upstream of this equipment will provide protection from this form of fouling.

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