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Water Treatment

Muhammad Fahad Ansari 12IEEM14

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Why do we treat water?

• The main objectives of the water treatment processes are the reduction in contamination or pollution levels present in water.

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• Water treatment describes those processes used to make WATER more acceptable for a desired end-use.

• These can include use as DRINKING WATER, industrial processes, medical and many other uses.

• The goal of all water treatment process is to remove existing contaminants in the water.

• Or reduce the concentration of such contaminants so the water becomes fit for its desired end-use.

• One such use is returning water that has been used back into the natural environment without adverse ecological impact.

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• It also may be necessary to remove nutrients such as N and P, toxic components, non-biologically degradable compounds and dissolved solids.

• Removal of these materials are necessary for the simple reason that discharge to the environment will result in “damage”.

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• The processes involved in treating water for drinking purpose may be solids separation using

• Physical: Such as settling and filtration,

• Chemical: Such as disinfection and coagulation.

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• The damage is a function of the type of

pollutant discharged –

• Heavy metals = toxicity,

• Organic matter = oxygen depletion,

• N or P- eutrophication, etc.

• In the case of water treatment the objective is

to remove contaminants from the water which

can result in health or aesthetic problems.

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• Many pollutants fall into several

categories.

• For example, some biodegradable

organic matter is in the form of

suspended solids.

• So removal of SS sometimes results

in the removal of organic matter.

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• Each solids “type” may require a different

type of treatment process.

• Generally it is tried to remove large very

settleable or screenable particles first and

• Then proceed to the smallest and finally to

the soluble species.

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• Level needed to remove contaminants.

• The degree to which drinking water must be treated depends on the raw water quality and the desired quality of the finished water.

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For example water treatment may

require "secondary treatment" as

shown here:

• BOD5 = 30 mg/L

• Suspended Solids = 30 mg/L

• pH (if industrial input) = 6 – 9

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• The" secondary" standards are just the

“basic” requirements.

• More stringent standards are placed on

effluents which are discharged to

potentially eutrophic lakes, etc. or

whenever there is a known toxic

contaminant in the wastewater (e.g.

industrial discharges).

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• For drinking water treatment the requirements are, of course, much more stringent with many more categories and lower contaminant limits.

Some examples are:

• Turbidity (a measure of suspended solids): less than 0.5 NTU in at least 95% of samples taken each month.

• Lead: 0.005 mg/L

• Copper: 1.3 mg/L

• Total Coliform: no coliform detection in more than 5% of samples collected

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• The contaminants removal from water

Contaminant removal is accomplished by

a series of unit processes or unit

operations.

(The system of integrated unit processes or

unit processes used to treat a water or

wastewater is called a treatment train.

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• Treatment processes are usually divided into

two trains: liquid train, and the solids (sludge)

train.

• The reason for this is that we usually take a

rather dilute waste and through a series of

phase separation processes create a more

concentrated waste (sludge).

• The sludge then has to be treated accordingly.

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An example of a water treatment plant:

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Treatment

The processes below are commonly used in water purification plants.

• 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.

• Screening - 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.

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• Storage - Water from rivers may also be stored in bank side 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.

• Pre-conditioning - Many waters rich in hardness salts are treated with soda-ash to precipitate out.

• Pre-chlorination - In many plants the incoming water is chlorinated to minimize the growth of fouling organisms on the pipe-work and tanks.

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pH adjustment

• Distilled water has a 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, soda ash or sodium hydroxide is added to raise the pH.

• For somewhat acidic, alkaline waters are the cheapest way to lower the pH.

• Lime is commonly used for pH adjustment for municipal water, or at the start of a treatment plant for process water.

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• Making the water slightly alkaline ensures that coagulation and flocculation processes work effectively.

• Also help to minimize the risk of lead being dissolved from lead pipes.

• Acid (HCl or H2SO4) may be added to alkaline waters in some circumstances to lower the pH.

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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 a very small particles but as the water is gently stirred, these particles stick together to form bigger particles - this process is called flocculation.

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• 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 volatile coal).

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Coagulants / flocculating agents that may be used include:

• Iron Hydroxide: This is formed by adding a solution of an iron (III) compound 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.

• Aluminum Hydroxide is also widely used as the flocculating precipitate although there have been concerns about possible health impacts and mis-handling led to a severe poisoning incident

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Drinking water in U.S.• The United States has one of the safest water

supplies in the world.• In recent years, microbial contamination of the

water supply has led to highly publicized outbreaks of disease, causing illness and even death.

• How safe is our water?• Where do these infectious microbes come from?• How is water treated now and what’s being done

to make it even safer?

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Surface Water Treatment Plant

• Water treatment transforms raw surface and groundwater into safe drinking water.

• Water treatment involves two major processes:

• Physical removal of solids and

• Chemical disinfection.

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Water Disinfection• Purpose of disinfection:

• To make Drinking water free of any disease causing bacteria and microbes.

• Methods of disinfection:

• There are 3 mainly used disinfection methods at large scale.

• CHLORINATION• OZONATION• ULTRAVIOLET RADIATION

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CHLORINATION

Chlorine is the most common cost-effective means of disinfecting water in the U.S.

The addition of a small amount of chlorine is highly effective against most bacteria, viruses, and protozoa.

But cysts (durable seed-like stages) formed by parasitic protozoa such as Cryptosporidium and Giardia can survive chlorine.

Chlorine is applied to water in one of three forms: elemental chlorine (chlorine gas), hypochlorite solution (bleach), or dry calcium hypochlorite. All three forms produce free chlorine in water

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OZONATION

OZONE is Strongest oxidant/disinfectant available.

More effective against microbes than chlorination.

But, costly and difficult to monitor and control under different condition.

• Ozonation process:

• Ozone (o3) is generated on-site at water treatment facilities by passing dry oxygen or air through a system of high voltage electrodes.

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ULTRAVIOLET RADIATION When UV radiation penetrates the

cell wall of an organism, it damages genetic material, and prevents the cell from reproducing.

Now a days emerging technology made UV radiation to find a place in both household and large scale drinking water disinfection.

• How is UV light generated?

• Ultraviolet light is most typically generated from a low pressure or a medium pressure lamp generating UV light.

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ULTRAVIOLET RADIATION

COAGULATION

SEDIMENTATION

FILTRATIONDISINFECTION

STORAGE

COMPLETE CYCLE OF WATER TREATMENT:

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Design of a Chlorination Process

• Based on exposing a microorganism for an amount of time with a minimum amount of disinfectant

• Rough calculation of the amount of time spent in a reactor is based on the reactor volume and the flow rate:

• Time = volume/flow rate