Drinking water treatment

The water then goes through a rigorous set of processes at our water treatment works before being pumped into the network to your home or business.The treatment process varies according to the type and quality of the source of water.  Drinking water treatment has two goals, both improving taste and ensuring safety.

The treatment process varies according to the type and quality of the source of water；

Groundwater tends to be relatively clean in comparison to surface water, and fewer treatment steps are required.

From above ground sources such as rivers and reservoirs.Raw water from these sources is open to direct environmental input, and multiple treatment steps are required to clean the water.

The treatment process

A variety of methods are used to treat drinking water, often in combinations.

•       Filtration: Many water treatment facilities use filtration to remove all particles from the water. Those particles include clays and silts, natural organic matter, precipitates from other treatment processes in the facility, iron and manganese, and microorganisms.

gravity media filtration uses filters containing layers of sand or coal,

membrane filtration used can include microfiltration, ultrafiltration, microfiltration or reverse osmosis.

Sedimentation

As the water and the floc particles progress through the treatment process, they move into sedimentation basins where the water moves slowly, causing the heavy floc particles to settle to the bottom. Floc which collects on the bottom of the basin is called sludge, and is piped to drying lagoons. In Direct Filtration, the sedimentation step is not included, and the floc is removed by filtration only.

Conventional Coagulation-Flocculation-Sedimentation

Coagulation and flocculation practices are essential pretreatments for many water purification systems.A chemical coagulant, such as iron salts, aluminum salts, or polymers, is added tosource water to facilitate bonding among particulates. Coagulants work by creating a chemical reaction and eliminating the negative charges that cause particles to repel each other.

 In conventional coagulation-flocculation-sedimentation,acoagulant is added to the source water to create an attraction among the suspended particles. The mixture is slowly stirred to induce particles to clump together into “flocs.” The water is then moved into a quiet sedimentation basin to settle out the solids，These processes agglomerate suspended solids together into larger bodies so that physical filtration processes can more easily remove them.Particulate removal by these methods makes later filtering processes far more effective. The process is often followed by gravity separation (sedimentation or flotation) and is always followed by filtration.The ultimate effectiveness of coagulation/flocculation is also determined by the efficiency of the filtering process with which it is paired.

Flocculation-Chlorination

A system that incorporates coagulation-flocculation followed by chlorination has been developed as a point of use technology, 

It uses a small packet of powdered ferrous sulfate (a common flocculent) and calcium hypochlorite (a common disinfectant).  The combination of particle removal and disinfection appears to produce high removal rates of bacteria, viruses, and protozoa, even in highly turbid waters.

•Ion Exchange

When water is too hard, it is difficult to use to clean and often leaves a grey residue. An  ion exchange process can be used to soften the water. in the process of softening ,exchange resin is the main mateirals.

Calcium and magnesium are common ions that lead to water hardness. To soften the water, positively charged sodium ions are introduced in the form of dissolved sodium chloride salt, or brine. Hard calcium and magnesium ions exchange places with sodium ions, and free sodium ions are simply released in the water， It can also be used to remove arsenic, chromium, excess fluoride, nitrates, radium, and uranium.

Clarification (and floatation)

Chemicals – usually liquid aluminium sulphate (alum) – are added to help bacteria and small solid particles stick together, forming larger particles. Mechanical settling basins called clarifiers use gravity to separate out solid particles and remove colour

A chemical coagulant is dosed into the water, and acts to bind together fine suspended material such as silt and mud particles.The coagulant forms a 'floc', which traps the clumps of material, and is then removed from the water by settlement or flotation.  where a diffuser creates fine bubbles that stick to the particles. This causes them to float to the surface of the water where they can be removed.

Dissolved Air Flotation

Dissolved air flotation is a form of coagulation-flocculation technology that is used as a pretreatment.Dissolved air flotation is an alternative to sedimentation.

Dissolved air flotation systems also add a coagulant and flocculate ,such as iron salts, aluminum salts, or polymers, facilitate bonding among particulates. Coagulants work by creating a chemical reaction, eliminating negative charges that cause particles to repel each other; The action of these bubbles forces clots or flocs of particles to the water surface where they can be skimmed off.It performs a similar task by a diametrically opposed method—forcing contaminant clumps to the surface rather than allowing them to settle out on the bottom.

Dissolved air flotation is particularly well-suited for the removal of algae, unwanted coloring, and lighter particles that resist settling out of treated source water. Employing this technique before water filtration reduces the clogging that causes maintenance problems of downstream filtration.

•       Granular Activated Carbon (GAC)

Water is directed into GAC vessels, which contain highly porous carbon particles.These particles are able to adsorb organic compounds , unwanted coloring, and taste- or odor-causing compounds can stick to the surface of granular or powder activated carbon and are thus removed from the drinking water.

•       Disinfection :

Water is often disinfected before it enters the distribution system to ensure that potentially dangerous microbe and microbial organisms (including bacteria).Chlorine, chloramine (chlorine and ammonia) are used to destroy any disease-causing bacteria. UV treatment and ozonation are safe, nonchemical methods of disinfection.  

Fluoridation

Fluoride is added in small amounts – less than one milligram per litre – to help prevent tooth decay. This is a legal requirement under the Health

pH correction

Lime, caustic soda or soda ash is added to neutralise the pH of the water as chlorine and fluoride are slightly acidic. The minerals in lime also help prevent household pipes and fittings from corroding. 

For the chemical treatment of water a great variety of chemicals can be applied. Hoo chemtec sum up  the different types of water treatment chemicals below.

•       Algaecides, copper sulphate, iron salts, rosin amine salts and benzalkonium chloride

•       Antifoams, modified polydimethylsiloxane, polydimethylsiloxane fluid

•       Biocides,

•       Boiler water chemicals

•       Coagulants, aluminium sulphate,ferric chloride,ferrous sulphate

•       Corrosion inhibitors,

nitrite, phosphate and molybdate, silicates and morpholine and hydrazine, dicyclohexylamine, cyclohexylamine and hexamethylene-amine

•       DisinfectantsChlorine,· Chlorine dioxide,· Ozone,· Hypochlorite

•       Flocculants, cationic polymers,  anionic polymers,  ions and polyampholytes

•       Neutralizing agents, sodium hydroxide, calcium carbonate, or lime suspension (Ca(OH)2)

•       Oxygen scavengers, hydrazine (N2H4) , carbohydrazine, hydroquinone, diethyl hydroxy ethanol, methylethylketoxime, but also non-volatile salts, ,sodium sulphite (Na2SO3)

         Resin cleaners ,sodium chloride, potassium chloride, citric acid and chlorine dioxide.•

•       Scale inhibitors hosphate esters, phosphoric acid and solutions of low molecular weight polyacrylic acid.