How drinking water is purified?

Question

hey i have an assignment for science and i have to do it on How drinking water is purified? but i am having major trouble with it could u please help me out?? i've googled it and everything

thanks for ur help

Answer 1

Pre-treatment

   1. Pumping and containment - The majority of water must be pumped from its source or directed into pipes or holding tanks. To avoid adding contaminants to the water, this physical infrastructure must be made from appropriate materials and constructed so that accidental contamination does not occur.

   2. Screening (see also screen filter) - The first step in purifying surface water is to remove large debris such as sticks, leaves, trash and other large particles which may interfere with subsequent purification steps. Most deep groundwater does not need screening before other purification steps.

   3. Storage - Water from rivers may also be stored in bankside reservoirs for periods between a few days and many months to allow natural biological purification to take place. This is especially important if treatment is by slow sand filters. Storage reservoirs also provide a buffer against short periods of drought or to allow water supply to be maintained during transitory pollution incidents in the source river.

   4. Pre-conditioning - Many waters rich in hardness salts are treated with soda-ash (Sodium carbonate) to precipitate calcium carbonate out utilising the common ion effect.

   5. Pre-chlorination - In many plants the incoming water was chlorinated to minimise the growth of fouling organisms on the pipe-work and tanks. Because of the potential adverse quality effects (see chlorine below), this has largely been discontinued.[citation needed]

Widely varied techniques are available to remove the fine solids, micro-organisms and some dissolved inorganic and organic materials. The choice of method will depend on the quality of the water being treated, the cost of the treatment process and the quality standards expected of the processed water.

[edit] pH adjustment

Distilled water has an average pH of 7 (neither alkaline nor acidic) and sea water has an average pH of 8.3 (slightly alkaline). If the water is acidic (lower than 7), lime or soda ash is added to raise the pH. Lime is the more common of the two additives because it is cheap, but it also adds to the resulting water hardness. Making the water slightly alkaline ensures that coagulation and flocculation processes work effectively and also helps to minimize the risk of lead being dissolved from lead pipes and lead solder in pipe fittings. If the water is alkaline, acid (HCl) and carbon dioxide (CO2) are commonly added to lower pH. Having an alkaline water does not assure that deposits on the pipe i.e. lead or copper are not released into the water. Changing the electrolytic properties of the water are more indicative to release lead or copper into the water.

floc floating at the surface of a basin

floc floating at the surface of a basin

Mechanical system to push floc out of the water basin

Mechanical system to push floc out of the water basin

[edit] Flocculation

Flocculation is a process which clarifies the water. Clarifying means removing any turbidity or colour so that the water is clear and colourless. Clarification is done by causing a precipitate to form in the water which can be removed using simple physical methods. Initially the precipitate forms as very small particles but as the water is gently stirred, these particles stick together to form bigger particles - this process is sometimes called flocculation. Many of the small particles that were originally present in the raw water absorb onto the surface of these small precipitate particles and so get incorporated into the larger particles that coagulation produces. In this way the coagulated precipitate takes most of the suspended matter out of the water and is then filtered off, generally by passing the mixture through a coarse sand filter or sometimes through a mixture of sand and granulated anthracite (high carbon and low valoatiles coal). Coagulants or flocculating agents that may be used include:

       1. Iron (III) hydroxide. This is formed by adding a solution of an iron (III) compound such as iron (III) chloride to pre-treated water with a pH of 7 or greater. Iron (III) hydroxide is extremely insoluble and forms even at a pH as low as 7. Commercial formulations of iron salts were traditionally marketed in the UK under the name Cuprus.

       2. Aluminium hydroxide is also widely used as the flocculating precipitate although there have been concerns about possible health impacts and mis-handling lead to a severe poisoning incident in 1988 at Camelford in south-west UK when the coagulant was introduced directly into the holding reservoir of final treated water.

       3. Aluminium Hydroxychloride is an artificially produced polymer and is one of a class of synthetic polymers that are now widely used. These polymers have a high molecular weight and form very stable and readily removed flocs but tend to be more expensive in use compared to inorganic materials.

[edit] Sedimentation

Water exiting the flocculation basin may enter the sedimentation basin, also called a clarifier or settling basin. It is a large tank with slow flow, allowing floc to settle to the bottom. The sedimentation basin is best located close to the flocculation basin so the transit between does not permit settlement or floc break up. Sedimentation basins can be in the shape of a rectangle, where water flows from end to end, or circular where flow is from the centre outward. Sedimentation basin outflow is typically over a weir so only a thin top layer - furthest from the sediment - exits.The amount of floc that settles out of the water is dependent on the time the water spends in the basin and the depth of the basin. The retention time of the water must therefore be balanced against the cost of a larger basin. The minimum clarifier retention time is normally 4 hours. A deep basin will allow more floc to settle out than a shallow basin. This is because large particles settle faster than smaller ones, so large particles bump into and integrate smaller particles as they settle. In effect, large particles sweep vertically though the basin and clean out smaller particles on their way to the bottom.

As particles settle to the bottom of the basin a layer of sludge is formed on the floor of the tank. This layer of sludge must be removed and treated. The amount of sludge that is generated is significant, often 3%-5% of the total volume of water that is treated. The cost of treating and disposing of the sludge can be a significant part of the operating cost of a water treatment plant. The tank may be equipped with mechanical cleaning devices that continually clean the bottom of the tank or the tank can be taken out of service when the bottom needs to be cleaned.

[edit] Filtration

After separating most floc, the water is filtered as the final step to remove remaining suspended particles and unsettled floc. The most common type of filter is a rapid sand filter. Water moves vertically through sand which often has a layer of activated carbon or anthracite coal above the sand. The top layer removes organic compounds, which contribute to taste and odour. The space between sand particles is larger than the smallest suspended particles, so simple filtration is not enough. Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles. Effective filtration extends into the depth of the filter. This property of the filter is key to its operation: if the top layer of sand were to block all the particles, the filter would quickly clog.

To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called backflushing or backwashing) to remove embedded particles. Prior to this, compressed air may be blown up through the bottom of the filter to break up the compacted filter media to aid the backwashing process; this is known as air scouring. This contaminated water can be disposed of, along with the sludge from the sedimentation basin, or it can be recycled by mixing with the raw water entering the plant.

Some water treatment plants employ pressure filters. These work on the same principle as rapid gravity filters, differing in that the filter medium is enclosed in a steel vessel and the water is forced through it under pressure.

Advantages:

    Filters out much smaller particles than paper and sand filters can.

    Filters out virtually all particles larger than their specified pore sizes.

    They are quite thin and so liquids flow through them fairly rapidly.

    They are reasonably strong and so can withstand pressure differences across them of typically 2-5 atmospheres.

    They can be cleaned (back flushed) and reused.

Membrane filters are widely used for filtering both drinking water and sewage (for reuse). For drinking water, membrane filters can remove virtually all particles larger than 0.2 um--including Giardia and cryptosporidium. Membrane filters are an effective form of tertiary treatment when it is desired to reuse the water for industry, for limited domestic purposes, or before discharging the water into a river that is used by towns further downstream. They are widely used in industry, particularly for beverage preparation (including bottled water). However no filtration can remove substances that are actually dissolved in the water such as phosphorus, nitrates and heavy metal ions.

[edit] Slow sand filters

Slow sand filters may be used where there is sufficient land and space as the water must be passed very slowly through the filters. These filters rely on biological treatment processes for their action rather than physical filtration. The filters are carefully constructed using graded layers of sand with the coarsest sand, along with some gravel, at the bottom and finest sand at the top. Drains at the base convey treated water away for disinfection. Filtration depends on the development of a thin biological layer, called the zoogleal layer or Schmutzdecke, on the surfa

Answer 2

by boiling and distilation to remove bacteria and bad odur

by ionising to remove virus

by chlorinating to remove bacteria and to reduce hardness

Answer 3

Once water is received from the source it needs to be treated before we can use it.  In the previous sections of this module, you learned about microorganisms that need to be filtered out of drinking water.  Now we’ll take a look  at how a typical water treatment plant purifies our drinking water.  The four processes commonly used to treat water are screening, coagulation and settling, filtration, and disinfection.

When water is taken from the source (Like a lake or river), wood, fish, and plants may still be present.  These things are screened out as the water is drawn into the plant.  If the source happens to be groundwater, the screening process isn’t as vital since the layers of the earth that the water travels through act as a natural screen, removing large contaminants.

When the water reaches the plant it is coagulated and settled.  In this process, water is mixed with aluminum sulfate (alum) and chlorine.  The alum forms sticky globs, which attract bacteria and other impurities.  The chlorine kills germs and improves taste and odor.  The water and the globs then flow into a sedimentation basin where the globs "settle" to the bottom and are removed.

Next, the water flows through special filters made of layers of sand and gravel.  The gravel layer of the filters is about 1 foot deep and the sand layer is about 2 ½ feet deep!  This filtering removes any remaining particles left in the water.

During disinfection, disinfecting chemicals are added and chlorine is used again.  This  process kills any surviving germs and keeps the water clean over time.  In some water treatment systems that use ground water, this is the only method needed to treat the water!

Answer 4

It depends on the source.

If it is out of a borehole then they dose a bit of chlorine and it's ready to drink.

If it is from a reservoir it is a very complicated process involving ASG (anthracite/sand/garnet) filtration, granulated carbon filtration, ozone bubbling, chlorine dosing, Sodium Bisulphite dosing... Basically you need to remove the solids, kill anything that's alive, and check it doesn't taste too chemically.

Answer 5

To remove particulate matter:

Settling; or

Filtering.

To remove pathogens:

Filtering;

Boiling;

Chemical treatment using tincture of iodine or chlorine or other chemical; or

Ultraviolet light.

A lot of pathogens are removed simply through filtering.

To make fresh water out of saline water:

Distillation; or

Reverse Osmosis.

Answer 6

A water purifier works through the process of reverse osmosis. Water purifiers cleanse water by passing it through a series of filters. This helps remove large amounts of contaminants and other harmful chemicals.

The purifier also forces the water through a special plastic covering. The special pores of the plastic covering can block the atoms and molecules of pollutants.

Answer 7

Drinking water is kept in a large reservoir in it there are hundreds of trouts to make sure that the water is good then the water is passed through pipes and get pure and is pumped to the large main to supply houses with drinking water