Water treatment: mimicking earth’s hydrological cycle

 

Water treatment and water treatment technologies are an essential line of defense to remove contaminants and bacteria before the delivery of clean, potable water supplies for consumption. Water sources can be subject to contamination and therefore require appropriate treatment to remove disease-causing agents. Public drinking water systems use a variety of methods to provide safe drinking water for their communities. Depending on the continent, country and region, different water treatment systems may be in operation depending on regional regulations and raw water input. The following article provides an overview of the basic principles of water treatment and the processes and technologies involved.

Water treatment: a guide to water treatment technology

What is water treatment?

Maintaining water treatment to ensure a clean supply to meet growing global populations has been an ongoing challenge throughout human history.

Thanks to significant technological developments in water treatment, including monitoring and assessment, high quality drinking water can be supplied and enjoyed around the world. Replicating the earth’s hydrological cycle in which water is continuously recycled, treatment enables the same water to be cleansed through several natural processes.

To ensure they do no present a health risk, nearly all water sources require treatment before they can be consumed. Most treatment systems are designed to remove microbiological contamination and physical constituents, including suspended solids (turbidity). Following this, a final disinfection stage is nearly always included at the end of the treatment process to help deactivate any remaining microorganisms. If a persistent disinfectant, such as chlorine, is added this can also act as a residual to help prevent biological regrowth during water storage, or distribution in larger systems.

Water treatment consists of several stages. This can include the initial pre-treatment by settling or through using coarse media, filtration followed by chlorination, called the multiple barrier principle. The latter allows effective water treatment and allows each stage to treat and prepare water to a suitable quality for the next downstream process. For example, filtration can prepare water to ensure it is suitable of UV (ultraviolet) disinfection.

Depending on the quality and type of the water entering a water plant, treatment may vary. For example, groundwater treatment works abstract water from below ground sources such as aquifers and springs. This tends to be relatively clean in comparison to surface water, with fewer water treatment steps required.

Surface water treatment works meanwhile take water from above ground sources, such as rivers, lakes and reservoirs. This raw water is subject to direct environmental input. As a result, multiple treatment steps are required and individual process steps are used to different combinations to clean and disinfect the abstracted water.

Some water supplies may contain disinfection by-products, inorganic chemicals, organic chemicals and radionuclides. As a result, specialised water treatment methods may also be part of water treatment to help control formation and removal.

Furthermore, under renewed regulations, tighter limits could be placed on endocrine disrupting chemicals as well as lead limits being halved.

How does the water treatment process work?

Coagulation, flocculation and sedimentation are processes used to remove colour, turbidity, algae and other microorganisms from surface waters and may be used to reduce turbidity and solids in suspension.

Chemical coagulants can be added to the water to cause the formation of a precipitate, or floc, which entraps these impurities. Iron and aluminium can also be removed under suitable conditions. The floc is separated from the treated water by sedimentation and/or filtration, although flotation processes may be used in place of sedimentation.

The most commonly used coagulants are aluminium sulphate and ferric sulphate, although other coagulants are available. Coagulants are dosed in solution at a rate determined by raw water quality near the inlet of a mixing tank or flocculator. The coagulant is rapidly and thoroughly dispersed on dosing by adding it at a point of high turbulence.

Water is then allowed to flocculate and then passes into the sedimentation tank (sometimes known as a clarifier) to allow aggregation of the flocs, which settle out to form sludge. This sludge will need to be periodically removed. The advantages of coagulation are that it reduces the time required to settle out suspended solids and is very effective in removing fine particles that are otherwise very difficult to remove.

The principal disadvantages of using coagulants for treatment of small supplies are the cost and the need for accurate dosing, thorough mixing and frequent monitoring. Coagulants need accurate dosing equipment to function efficiently and the dose required depends on raw water quality that can vary rapidly. The efficiency of the coagulation process depends on the raw water properties, the coagulant used and operational factors including mixing conditions, temperature, coagulant dose rate and pH value. The choice of coagulant and determination of optimum operating three conditions for a specific raw water are normally determined by bench scale coagulation tests.

Thus, while coagulation and flocculation are the most effective treatment for removal of colour and turbidity they may not be suitable for small water supplies because of the level of control required and the need to dispose of significant volumes of sludge.

Six essential Water treatment technologies

A variety of water treatment technologies are needed to work together, in sequence, in order to purify raw water before it can be distributed. Here is a list of basic technologies often used in water treatment works.

  1. Screens

    Effective for the removal of particulate material and debris from raw water, screens are used on many surface water intakes. Coarse screens will remove weeds and debris while band screens or microstrainers will remove smaller particles including fish and may be effective in removing large algae. Microstrainers are used as a pre-treatment to reduce solids loading before coagulation or subsequent filtration.

  2. Gravel filters

    Gravel filters may be used to remove turbidity and algae. A larger gravel filter may consist of a rectangular channel or tank divided into several sections and filled with graded gravel (size range 4 to 30mm). The raw water enters through an inlet distribution chamber and flows horizontally through the tank, encountering first the coarse and then the finer gravel. Filtered water is collected in an outlet chamber. Solids removed from the raw water accumulate on the floor of the filter. Gravel filters can operate for several years before cleaning becomes necessary.

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  4. Slow sand filters

    Slow sand filters, sometimes preceded by microstrainers or coarse filtration, are used to remove turbidity, algae and microorganisms. Slow sand filtration is a simple and reliable process and is therefore often suitable for the treatment of small supplies provided that sufficient land is available. Slow sand filters usually consist of tanks containing sharp sand (size range 0.15-0.30mm) to a depth of between 0.5 to 1.5m.

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  6. Activated carbon

    Activated carbon removes contaminants from water by physical adsorption. This will be affected by the amount and type of the carbon, the nature and concentration of the contaminant, retention time of water in the unit and general water quality (temperature, pH, etc.). Granular activated carbon (GAC) is the most common medium employed although powdered activated carbon (PAC) and block carbon are also sometimes used. The filter media is contained in replaceable cartridges; a particulate filter at the outlet of the cartridge removes carbon fines from the treated water.

  7. Aeration

    Aeration is designed to transfer oxygen into water and remove gases and volatile compounds by air stripping. To achieve air stripping various techniques can be used including counter current cascade aeration in packed towers, diffused aeration in basins and spray aeration. Packed tower aerators are most commonly used because of their high energy efficiency and compact design.

  8. Membrane processes

    Reverse osmosis (RO), ultrafiltration (UF), microfiltration (MF) and nanofiltration (NF) are the most commonly used membranes for water treatment processes. Previously applied to the production of water for industrial or pharmaceutical applications, membranes are being applied to the treatment of drinking water. Membrane processes can provide adequate removals of pathogenic bacteria, Cryptosporidium, Giardia, and potentially, human viruses and bacteriophages. In a notable case study, companies from the Netherlands and Denmark are working on integrating enzymes into membrane technology for the removal of pesticides and pharmaceutical residues from drinking water.

UV water treatment: shining a light on disinfection

Ultraviolet (UV) light is invisible to the human eye but can be used to disinfect microorganisms in water treatment processes. The wavelengths of UV light range between 200 and 300 nanometers (billionths of a meter). Special low-pressure mercury vapor lamps produce ultraviolet radiation at 254 nm, the optimal wavelength for disinfection and ozone destruction. Categorised as germicidal, this means they are capable of inactivating microorganisms, such as bacteria, viruses and protozoa. The UV lamp never contacts the water; it is either housed in a quartz glass sleeve inside the water chamber or mounted external to the water which flows through UV transparent Teflon tubes.

How does it work? When bacteria, viruses and protozoa are exposed to the germicidal wavelengths of UV light, they are rendered incapable of reproducing and infecting.

UV disinfection can be used for the primary disinfection products of potable drinking water, or as a secondary disinfection to provide a barrier against chlorine-resistant microorganisms, such as Cryptosporidium and Giardia.

In addition, UV light (either alone or in conjunction with hydrogen peroxide) can destroy chemical contaminants such as pesticides, industrial solvents, and pharmaceuticals through a process called UV-oxidation.

Although 100% destruction of microorganisms cannot be guaranteed, it is possible to achieve 99.9% reduction in certain applications and with proper maintenance.

Under ideal conditions, a UV unit can provide greater than 99% reduction of all bacteria. However, even with this performance, ultraviolet disinfection has two potential limitations: “point” disinfection and also cells not being removed.

For the former and “point” Disinfection – this can occur if the UV units only kill bacteria at one point in a watering system and do not provide any residual germicidal effect downstream. If just one bacterium passes through unharmed (100% destruction of bacteria cannot be guaranteed), there is nothing to prevent it from attaching to downstream piping surfaces and proliferating.

Secondly, a second limitation can be if bacteria cells are not removed in a UV unit but are converted into pyrogens. The killed microorganisms and any other contaminants in the water are a food source for any bacteria that do survive downstream of the UV unit.

One notable development to UV systems is the scaling up of light-emitting diode technology, known as UV-LED, with 2018 witnessing a tipping point on power density and purchasing price.

Ozone water treatment: harnessing the power of lightning

Ozone water treatment: harnessing the power of lightning Ozone, a form of oxygen, is created when oxygen in the air is exposed to the discharge of a powerful electric current through air, similar to after a lightning storm. While widely used in Europe for many years to treat municipal drinking water, it has not had a similar acceptance in the US.

With excellent disinfection and oxidation qualities, ozone can be added at several points throughout the treatment system, such as during pre-oxidation, intermediate oxidation or final disinfection. Usually, it is recommended to use ozone for pre-oxidation, before a sand filter or an active carbon filter (GAC). Following ozonization these filters can remove the remaining organic matter (important for final disinfection).

Ozonation involves the formation of oxygen into ozone and this occurs with the use of energy. This process is carried out by an electric discharge field as in the CD-type ozone generators, or by ultraviolet radiation as in UV-type ozone generators. In addition to these commercial methods, ozone may also be made through electrolytic and chemical reactions.

In general, an ozonation system includes passing dry, clean air through a high voltage electric discharge, i.e., corona discharge, which creates and ozone concentration of approximately 1% or 10,000 mg/L. In treating small quantities of waste, the UV ozonation is the most common while large-scale systems use either corona discharge or other bulk ozone-producing methods.

Raw water is then passed through a venturi throat which creates a vacuum and pulls the ozone gas into the water or the air is then bubbled up through the water being treated. Since the ozone will react with metals to create insoluble metal oxides, post filtration is required.

Ozone is highly reactive and, as a result, has a very short half-life once dissolved into water. The natural reaction is for ozone to return to its oxygen form, with a reaction time typically taking 10-20 minutes at 20 degrees Celsius.

Advantages to ozone water treatment include the elimination or inorganic, organic and microbiological problems and taste and odour problems. Furthermore, no chemicals are added to the water.

Meanwhile disadvantages include a lack of germicidal or disinfection residual to inhibit or prevent growth. Furthermore, the system may require pre-treatment for hardness reduction.

Types of water treatment chemicals (and why they are used)

Chemical disinfection of drinking-water includes any chlorine-based technology, such as chlorine dioxide, as well as ozone, some other oxidants and some strong acids and bases. Except for ozone, proper dosing of chemical disinfectants is intended to maintain a residual concentration in the water to provide some protection from post-treatment contamination during storage.

Disinfection of household drinking-water in developing countries is done primarily with free chlorine, either in liquid form as hypochlorous acid (commercial household bleach or more dilute sodium hypochlorite solution between 0.5% and 1% hypochlorite marketed for household water treatment use) or in dry form as calcium hypochlorite or sodium dichloroisocyanurate. This is because these forms of free chlorine are convenient, relatively safe to handle, inexpensive and easy to dose.

Chlorine is the most widely used primary disinfectant and is also often used to provide residual disinfection in the distribution system. Monitoring the level of chlorine in drinking water entering a distribution system is normally considered to be a high priority (if it is possible), because the monitoring is used as an indicator that disinfection has taken place. Residual concentrations of chlorine of about 0.6 mg/l or more may cause problems of acceptability for some consumers on the basis of taste.

Chlorine dioxide breaks down to leave the inorganic chemicals chlorite and chlorate. These are best managed by controlling the dose of chlorine dioxide applied to the water. Chlorite can also be found in hypochlorite solution that has been allowed to age.

Proper dosing of chlorine for household water treatment is critical in order to provide enough free chlorine to maintain a residual during storage and use. Recommendations are to dose with free chlorine at about 2 mg/l to clear water (< 10 nephelometric turbidity units [NTU]) and twice that (4 mg/l) to turbid water (> 10 NTU).

Monochloramine, used as a residual disinfectant for distribution, is usually formed from the reaction of chlorine with ammonia. Careful control of monochloramine formation in water treatment is important to avoid the formation of di- and trichloramines, because these can cause unacceptable tastes and odours.

A number of other chemicals may be added in treatment. These include substances such as sodium hydroxide for adjusting pH and, in certain circumstances, chemicals for fluoridation of drinking-water.

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