HOW IS WATER TREATED?

The water treatment processes developed in the 19^th century and refined during the 20^th century are simple in nature. However, engineers have since developed ways of making these processes happen faster, in a smaller area and in a more controlled way at lower cost.

These earlier technologies are referred to as traditional or conventional technologies to distinguish them from technologies developed more recently.

There are a great variety of water treatment processes, although only a few are applied in most situations. A summary of each of the main treatment processes is given below.

In traditional water treatment, certain chemicals are added to raw water to remove impurities. While some particles will spontaneously settle out from water on standing (a process called sedimentation), others will not. To cause particles that are slow to settle or are non-settling to settle out more readily, a soluble chemical or mixture of chemicals is added to the water. Such a chemical is called a coagulant and the process is called coagulation.

The coagulant reacts with the particles in the water, forming larger particles called flocs, which settle rapidly.

Flocs can also be effectively removed by passing the water through a filter. The process is controlled so that the coagulant chemicals are removed along with the contaminants.

Coagulation/flocculation processes generally use aluminium sulphate (alum) or ferric chloride as the coagulant.

A combination of coagulation/flocculation/sedimentation and filtration is the most widely applied water treatment technology around the world, used routinely for water treatment since the early part of the 20^th century.

Coagulation/flocculation processes are very effective at removing fine suspended particles that attract and hold bacteria and viruses to their surface. Research has shown that these processes alone are capable of removing up to 99.9 per cent of the bacteria and 99 per cent of the viruses from water supplies.

These processes also remove some of the organic matter washed from soil and vegetation as water travels across the landscape, from raindrop to river. It is usually this natural organic matter that is responsible for any brown discolouration in water. However, not all of this natural organic matter (what water scientists call NOM) is removed by coagulation: certain taste and odour problems may remain.

One of the oldest and simplest processes used to treat water is to pass it through a bed of fine particles, usually sand. This process is called sand filtration. In its simplest form, the water is simply passed through the filter with no other pre-treatment, such as the addition of a coagulant. Usually this type of filter will remove fine suspended solids and also some other particles such as larger microorganisms.

Sand filtration is even more efficient when the water being treated passes through the sand filter very slowly. Over time the sand particles become covered with a thin surface layer of microorganisms. Some might refer to this layer as a slime but water scientists call it a biofilm. Even very small particles stick to this biofilm and are held, while water of greatly improved quality passes out through the filter.

First operating in London in the 19^th century, slow sand filters are still widely used throughout the world today. Although very effective, they require a large area of land to achieve the sort of flows required by a large modern city. Additional processes may also be needed to achieve adequate water quality.

In the early 20^th century, engineers developed rapid sand filters, which use high rates of water flow and sophisticated backwashing of the filter bed to remove trapped contaminants.

Because the sand filtration processes become less effective at removing fine suspended particles at higher water flow rates, the water must be pretreated – coagulated and flocculated – before passing through the filter bed. Such high rate direct filtration processes are widely applied to raw water with low levels of suspended matter. A good example is the water treatment plant at Prospect in Sydney.

The development of plastics has led to a new range of filter materials and methods. Processes based on these new filter materials are now increasingly used to treat water for urban and industrial purposes.

In membrane filtration, water is filtered through tiny holes (usually referred to as pores) in a membrane wall rather than a bed of sand. The smaller the pore size, the greater the proportion of material the membrane retains as the water passes through.

Processes of membrane filtration are categorised by the pore size in the membrane. Pore size can vary from 0.1 microns (1000 microns is equivalent to 1 millimetre) for microfiltration down to 0.001 microns for nanofiltration.

The most common form of microfiltration membrane is a one-metre long bundle of thin, thread-like hollow fibres. A microfiltration water treatment plant would contain many such bundles.

A cross-section of a single hollow fibre is shown below (in yellow). Particles (in brown) are retained on the outer surface of the membrane while the purified water (in blue) passes into the central channel from where it flows lengthwise along the hollow fibre.

Previously too expensive to use in many circumstances, recent advances have reduced the cost of membrane filtration to a level approaching that of conventional water treatment processes.

While membrane water treatment plants are simple and reliable in operation, especially in small to medium-sized applications, there are some disadvantages. High energy costs are involved in pumping the water through the membrane. If a lot of natural organic matter is in the water, the membrane tends to block easily. This is referred to as membrane fouling. If cleaning cannot reverse the membrane fouling, the life of the membrane will be significantly shortened. This increases the cost of water treatment, since replacing membranes regularly is expensive.

Microfiltration will remove most of the fine suspended solids in the water and almost all protozoa and bacteria but is not able to remove the dissolved part of the natural organic matter in the water. It is this dissolved part of the natural organic matter that is frequently the cause of colour, taste and odour problems.

The microfiltration process is becoming increasingly popular for small-scale water treatment plants supplying smaller communities in rural and regional Australia. It has become the most widely used membrane water treatment process in Australia.

Ultrafiltration membranes have smaller pores than those used in microfiltration and can therefore remove finer particles from the water. This process is capable of removing almost all the viruses (the microorganisms most difficult to remove) and improving colour.

Because of the relatively high levels of natural organic matter found in raw waters in Australia, ultrafiltration technology has not found wide application here at this stage of its development.

Nanofiltration uses membranes with even smaller holes than for ultrafiltration, so requires a high operating pressure to force the water through the membrane. This results in high energy and operating costs.

However, nanofiltration is more effective than other filtration methods at improving water quality. For example, it is capable of removing all virus particles and most of the NOM. However, it also removes some natural minerals from the water, which can cause pipes to corrode. To reduce corrosion in these circumstances, stabilising chemicals, such as lime, must be added to the treated water.

The cost involved in using this technology, and the fact that backwashing of the membrane can consume a significant proportion of the water produced, limits its use to specific circumstances.

There are no working examples of a nanofiltration plant in Australia at present, but the process is in operation elsewhere, including Europe, where it is used to treat surface waters contaminated by herbicides and insecticides.

While coagulation processes and/or filtration remove most of the troublesome contaminants from water, they usually do not remove all of the dissolved (or soluble) material. This includes low concentrations of dissolved organic matter that microorganisms in the water can use as a food supply and perhaps algal toxins and associated taste and odour compounds.

If water contains undesirable contaminants, additional treatment processes are required, like adsorption and oxidation.

Adsorption refers to the process by which chemicals are attracted to and held by a solid surface and is quite different from the similarly sounding process of absorption.

In water treatment, specialised adsorbent materials are used. Examples are activated carbon and ion exchange resins. These adsorbants can be used to remove purely soluble contaminants from water.

Activated carbon is the most widely used adsorbent material in water treatment, because it is highly effective in removing taste and odour compounds and algal toxins. It can be used as a powder or in granular form.

In Australia, there has only been limited use of granulated activated carbon. In this treatment process, the activated carbon is usually placed in a column or filter and the water percolated through the bed of carbon granules. After some time the activated carbon will become saturated with the adsorbing material and will need to be replaced or regenerated. Current technology to regenerate the carbon granules involves heating in a high temperature furnace. Because of the cost of this regeneration process, it has not been used in Australia.

If water contamination occurs only occasionally, a better approach is to add powdered activated carbon to a conventional coagulation/flocculation process when the problem arises. The carbon is collected in the filters and then discarded with the normal water treatment plant sludge. Such intermittent dosing of activated carbon powder is used in Australia at numerous locations that have problems with blue-green algal blooms.

The use of activated carbon is a very costly and can be justified only when there are particular problems with toxins or taste and odour compounds.

Ion exchange resins can also remove soluble materials from water by exchanging ions (charged atoms or molecules) in the water and on the resin. This form of treatment is more often used for industrial purposes in industries that require very pure water for specialised processing, for example in computer chip manufacture. It has also found general application in the treatment of boiler feed water to reduce the problem of scaling.

With new developments in the technology, ion exchange resins are also being used to treat urban water supplies. For instance, the Water Corporation of Western Australia has established the biggest ion exchange water treatment system of its type in the world at the Wanneroo Groundwater Treatment Plant to remove intermittent odour problems occasionally experienced in some of Perth’s groundwater supply schemes. This plant uses an Australian invention, MIEX (magnetic ion exchange) resin manufactured by Orica Watercare.

Another treatment technology commonly used in Europe but only now appearing in Australia is oxidation with chemicals such as ozone or chlorine dioxide. These are strongly reactive chemicals able to oxidize a range of substances in water.

Ozone in particular is a strong oxidizing agent and is used as a disinfection agent (see below) and as a means of destroying soluble contaminants such as algal toxins, taste and odour compounds and (particularly in Europe) trace levels of insecticides. It is quite often used in combination with a column of granular activated carbon, as any soluble organics remaining after the chemical oxidation stage are biologically degraded by the film of microorganisms that develops in the activated carbon bed.

Experience with the process in Europe has been very good, with consumers reacting positively to the improved taste of the water produced. However, the technology is more expensive than standard coagulation and is suited to applications only where taste and odour problems are becoming severe. For example, Grampians Water, supplying water services in the Wimmera region of Victoria, has installed such a plant at Edenhope to overcome problems caused by algal contamination of the local water source.

Some raw water supplies are not stable, becoming acidic or alkaline depending on which material they are in contact with. This condition often leads to corrosion in piping systems and hot water services and can result in dissolved metals appearing in the water. For example, where copper corrosion occurs, a telltale bluish stain can appear where a tap drips on to a surface.

To prevent such corrosion problems, many waters are chemically stabilised before distribution by the addition of lime and sometimes carbon dioxide. The addition of lime (calcium carbonate) will make the water slightly harder by increasing the level of calcium in the water. Here, hardness refers to the characteristic of the water that prevents soap from lathering. In contrast, soft water will allow soap to form a lather easily.

Disinfection is carried out to kill harmful microorganisms that may be present in the water supply and to prevent microorganisms regrowing in the distribution systems.

Good public health owes a lot to the disinfection of water supplies. Without disinfection, waterborne disease becomes a problem, causing high infant mortality rates and low life expectancy. This remains the situation in some parts of the world.

There can be no higher priority in any water supply system than effective and safe disinfection of the water. The only possible exception to this rule occurs with secure groundwater supplies, where harmful microorganisms are prevented from entering the underground water source or contaminating the water when it is brought to the surface. Such water supplies need to be inspected and tested regularly to make sure that they remain safe.

The two most common methods to kill the microorganisms found in the water supply are oxidation with oxidising chemicals or irradiation with ultra-violet (UV) radiation.

The most widely used chemical disinfection systems are chlorination, chloramination, chlorine dioxide treatment and ozonation.

Key factors considered by a water authority in selecting a disinfection system are:

• Effectiveness in killing a range of microorganisms.

• Potential to form possibly harmful disinfection byproducts.

• Ability of the disinfecting agent to remain effective in the water throughout the distribution system.

• Safety and ease of handling chemicals and equipment.

• Cost effectiveness.

A summary of each of the main disinfection processes is given below.

Chlorination is the most widely used disinfectant for drinking water in Australia. Its introduction a century ago removed the threat of cholera and typhoid from Australian cities.

It is cheap, easy to use, effective at low dose levels against a wide range of infectious microorganisms, and has a long history of safe use around the world.

Chlorine is a strongly oxidising chemical and may be added to water as chlorine gas or as a hypochlorite solution.

Chlorine’s main disadvantage is a tendency to react with naturally occurring dissolved organic matter to form chlorinated organic compounds.

The substances formed by the disinfectant reacting with the natural organic matter in the water are referred to as disinfection byproducts.

In the 1970s, as scientific instruments capable of measuring lower and lower concentrations of substances were developed, trace quantities of chloroform and other similar chemicals were identified as disinfection byproducts in chlorinated water supplies.

While the concentration of these disinfection byproducts is usually very low (a typical figure might be 0.1 part per million), some have been identified as potential carcinogens. As a precaution, many countries limit the allowable level of chlorinated disinfection byproducts in the water. The Australian Drinking Water Guidelines also suggest maximum values for a range of byproducts (for example, 0.25 part per million for chloroform-type compounds).

Studies have compared the health risk from microbiological contamination of drinking water with the potential chemical risk from chlorination byproducts. The conclusions so far are:

• The risk of death from pathogens is at least 100 to 1000 times greater than the risk of cancer from disinfection byproducts.

• The risk of illness from pathogens is at least 10,000 to one million times greater than the risk of cancer from disinfection byproducts.

The Australian Drinking Water Guidelines encourage action by water authorities to reduce organic disinfection byproducts in water supplies but not in a way that would compromise the proper disinfection of the water.

The likelihood of such byproducts forming can be greatly reduced by treating the water to lower levels of dissolved organic matter before chlorine is added for disinfection purposes.

Some Australian examples of chlorinated water supplies are those of Melbourne, Adelaide, Perth, Canberra, Hobart and Townsville.

Chloramines are produced when ammonia and chlorine are added to water together. They are less effective than chlorine in killing microorganisms because they are not as chemically active. However, chloramines maintain their disinfecting capability longer than chlorine and are ideal for very long distribution systems or for water supplies with long holding times in service reservoirs. For example, the disinfected water supplied to some Australian communities may travel through the distribution system for more than a week before use as drinking water from someone’s tap.

Chloramines also react less with dissolved organic matter in the water and so produce fewer disinfection byproducts.

Chloramination is a common disinfection system in Australia and many examples of its use can be found in regional Australia.

Chlorine dioxide is about 10 times more expensive than chlorine and its use in Australia is very limited. Its most significant use is by the Gold Coast City Council in Queensland.

The choice of chlorine dioxide in this application was primarily to prevent an aesthetic water quality problem caused by naturally occurring manganese compounds in the raw water. The problem is sometimes described as "black water" and can result in black stains on customers’ washing. When "black water" occurs, the material being washed effectively acts as a filter for the tiny black particles during the rinse cycle of the washing machine.

Chlorine dioxide is a strong oxidant that can be used in low doses. It is a highly reactive, unstable gas that must be generated at the water treatment plant from sodium chlorite. Its use does not lead to the formation of chlorinated disinfection byproducts, but other possible byproducts of oxidation, such as chlorate and chlorite ions, can be a public health concern.

Ozone (O₃) is the most powerful disinfectant used in water treatment. It is even effective against the difficult to treat protozoan parasites, Cryptosporidium and Giardia.

Ozone, which only recently began to be used in Australia, destroys soluble contaminants such as algal toxins, taste and odour compounds and trace levels of insecticides.

Ozone is an unstable gas that must be generated at the water treatment plant. This is done by passing an electric discharge through clean, dry air or oxygen.

Because it is so reactive, ozone decays quickly in water. For this reason, it is usually used together with a small dose of chlorine or chloramine to ensure that some residual disinfection capacity is maintained in the water supply distribution system to prevent regrowth of microorganisms.

The use of ozone does not lead to chlorinated disinfection byproducts. However other possible oxidation products, such as bromate formed from the naturally occurring bromide found in some water sources, are a potential health concern.

Ultraviolet radiation (UV) is a component of sunlight. Sunlight achieves disinfection by ultraviolet irradiation naturally. In water treatment, an appropriate level of UV irradiation, produced by mercury lamps, can kill bacteria and viruses. However, there is some uncertainty surrounding the effectiveness of UV irradiation against Cryptosporidium and Giardia.

UV irradiation adds no chemicals to water and uses equipment that is relatively simple to operate and maintain. However, impurities in the water that cause colour and turbidity can severely reduce the effectiveness of the process because UV radiation cannot penetrate the water effectively.

UV irradiation has no lasting effect and a further disinfectant such as chlorine or chloramine is usually added to ensure that some residual disinfection capacity is maintained in the water supply distribution system to prevent regrowth of microorganisms.

The cost of UV treatment of water supplies is becoming increasingly affordable, especially for small water supply systems where the raw water is clean and cold.

UV irradiation may also be chosen where the water source is close to the customers, allowing only a short time between when the water is disinfected and when it is consumed.