Short notes on the Methods of Water Treatment

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Need for water treatment:

Getting samples of the water analyzed for its chemical and bacteriological contents and comparing the same with acceptable standard. However, water samples from cities having a treatment plant may not need any treatment at all. Supplies from sources should be closely examined and the method of treatment decided accordingly.

Coagulation –

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Suspended particles, clay, sand and to some extent color and odor are re­moved by adding a chemical coagulation agent and rapidly mixing them. The smaller particles from a flock, bigger and heavier and easily settle down

Coagulation is achieved either by mechanical mixing or by other processes

Commonly used coagulating agent is:

(a) Alum (Hydrated aluminum sulphate)

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(b) Ferric chloride, Ferric sulphate

(c) Poly electrolytes

(d) Special chemicals

(e) Lime

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It is important to ensure that the flocculated water is first settled in a separate tank; ensure constant removal of settled flocks and efficient decantation of the clear water for further treatment.

Sedimentation:

In large water supply systems, the settling of flocks can be achieved by prop­erly designed sedimentation tanks.

Sometimes, water supply from open wells, tube wells and natural springs may pot require coagulation and sedimentation, but this can be determined only after the water has been analyzed,

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Filtration:

Suspended particles in water supplies, including flocks generated by coagulation (within small limitation), are removed when the water is passed over a bed filter media.

Filter media generally comprise of gravel and sand of different sizes laid in layers.

Diatomaceous earth, cellulose cartridges and pads, ceramic cylinders, plastic granules and a variety of materials are also used as filter media.

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(a) Pressure Filters –

Small and medium-sized filtration plants are basically pressure filters having down flow or up flow system. The media is supported on a flat bed with collecting nozzles.

A conventional filter will have four layers of graded, aggregate, coarse and fine sand.

Pressure filters are designed on the basis of the amount of turbidity present. Turbidity in excess of 50 mg/liter (of silica) must be removed by pretreatment with coagulation. Filtration rates vary from 4500 lph/sqm to 22,500 lph/sqm of the filter surface area.

Depending on the turbidity and flow rate, pressure filters have to be backwashed periodically to wash out the solids retained in the filter. Backwashing is achieved by reversing the flow in the filter and allowing the water to drain out for some time.

Back-wash flow rates vary from 15000 to 25000 lph/m2 of the filter area. In addition, the filter media is agitated by blowing compressed air through a blower at about 50 cum/hr/sqm of filter area.

(b) Porous Ceramic Candle:

These are used in hotel. It contains two container- Upper container, filtered water reservoir, lower one, unfiltered water is stored in upper container shall have one/two holes for fixing the filter candles. The lower container shall have a hole near the base on the side for fixing a tap for draining filtered water.

Filter candle is fired, unglazed, porous, ceremonious and subsequently suitably treated chemically so that fine silver is embedded in the body. The filter candle renders water free from suspended matters and water bacteria

It is necessary to back wash the filter media or ceramic Candles from time to tim to keep the filter container clean to obtain good quality water

(c) Granulated on line filters –

Fixed directory on a tap.

Filters use Granulated filter media Activated carbon & crystals of silver ivied

(d) Domestic ultraviolet purification system –

To ensure that domestic water purification system are provided with a ceramic candle. The water is passed through a filter containing a layer of activated carbon which is an excellent agent in removing dissolved gases and foul odours including that of chlorine gas from the water. The resultant water is sparkling and clear and suitable for passing through ultraviolet treatment unit.

The water is then passed through the UV tube exposing it to ultraviolet radia­tion. The rate of flow of water is controlled to ensure complete disinfection.

To prevent untreated water from passing through the system, an electrically operated automatic shut off valve is provided.

It requires high pressure (15 to 20m head) to operate the purification system efficiently and supply from overhead tanks placed at low levels is ineffective.

The system is expensive. Requires power supply, high pressure water and regular maintenance to keep it in operation however it provides safe drinking water. It is used in hotels.

Ultraviolet rays generated from UV lamps.

Sterilization occurs when UV rays of sufficient intensity irradiate at thin film of water. When exposed to UV rays most living bacteria & Virus either die or loss the capability to reproduce in the same water.

Maintenance of water Supply System –

During maintenance of a water supply system the following points shall be attended to:

(i) Storage tanks shall be regularly inspected and cleaned as necessary.

(ii) Record drawings showing the pipe line layout and valve positions with their diameter shall be kept up to date.

(iii) Any temporary attachment fixed to a trap or outlet shall never be left as such

(iv) All the valves shall be periodically opened and checked.

(v) All overflow pipes shall be examined and kept free from obstruction.

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