Water Treatments Plant Guide

Operation and Maintenance manual of Water Treatments plant consisting of features of Operation and Maintenance of individual components of the water treatment supply system is necessary for adopting in the O & M of the water treatment systems. The Operation and maintenance features of the following water treatment  components are discussed in the following subsections.

 Sources :

1. surface water sources and
2. groundwater sources

• Intake works, pumps and machinery · Transmission systems including raw/treated water pumping mains, gravity mains, treated water transmission main from treatment works to service reservoirs.

• Water treatment plant

• Service Reservoirs

• Distribution system

• Water supply systems management including metering, UFW and leak detection.

1.Raw Water Quality

Nutrient

Moderate or large quantities of nutrients such as phosphates, nitrates and organic nitrogen compounds may act as a fertilizer in a reservoir to stimulate the growth of algae which may cause algal bloom.

The problems related to algal blooms are:

• Taste, odor and color,

• Increased pH

• Shortened filter runs of treatment plants,

• Dissolved Oxygen variation,

• Organic loading.

Thermal Stratification

Thermal stratification develops in lakes and reservoirs when the surface water begins to warm.

The warm surface waters expand and become lighter than the lower waters. The water temperature difference causes variation in water densities, which create resistance to mixing. This ultimately results in anaerobic conditions in lower zones.

Test for water quality

Regular analysis of raw water quality preferably once in a week in large water treatment plants is necessary to identify and correct the problems likely to be faced in the water treatment.

• If the fluctuation in quality of water is rapid, the analysis should be undertaken at short intervals.

• Turbidity is not a special problem as the dosage of coagulant is adjusted on a daily routine.

On the other hand a sudden rise in chloride content will indicate pollution due to sewage.

In such cases, more confirmatory tests should be taken such as for nitrogen in its various forms, dissolved oxygen, oxygen absorbed and chlorine demand to decide the occurrence of pollution and to fix the dose of pre-chlorination needed.

In case of lake as a source the periodical physical and biological examination of samples will indicate if there is any need for control of algae, which may cause taste and odour problems or clogging of filters.

2 Flow Measuring Devices of Water Treatment Plant

The preventive maintenance procedures related to Flow measuring devices are listed below:

1. Float sumps should be periodically cleaned to see that silt does not accumulate, which may affect the proper functioning of the float.
2. Charts and pen recorders should be stocked adequately
3. Annual or more frequent calibration of these devices is necessary.
4. Annual servicing and checking of the instrument is imperative.

3 Chemical Feeding Unit of Water Treatment Plant

The preventive maintenance procedures related to Chemical feeding unit are listed below:

• Alum and lime solution tanks are to be painted annually with anti-corrosive paint.

• V-notch weirs, and floats arrangements should be cleaned daily. Setting of the V-notch should be checked periodically.

• Adequate spares for mechanical mixing device should be stocked.

• The optimum dosing of coagulant should be determined periodically, in particular there is a change in water quality based on a proper laboratory study including jar test.

• The chemical feeding rate should be controlled depending upon the needs from time to time.

4 Flash Mixer of Water Treatment Plant

The preventive maintenance procedures related to flash mixer are listed below:

• Adequate spares for the mechanical mixing equipment should be kept in ready stock for timely replacement when necessary.

• Life of equipment such as metallic blades and shafts can be prolonged by periodical painting the equipment with anti-corrosive paints.

5 Flocculator of Water Treatment Plant

The preventive maintenance procedures related to Flocculator are listed below:

• Flocculator paddles should be operated continuously to avoid sludge building up.

• All metallic parts of the equipment should be painted with anti-corrosive paints once in a year.

• Mechanical devices should be lubricated and worn out parts replaced.

• In non-mechanical type flocculator with baffles, removing sludge once in six months is necessary.

6. Clarifier or Sedimentation Tank

The preventive maintenance procedures related to clarifiers or sedimentation tanks are listed below:

• Annual overhauling and repainting of the unit should be done a month or two prior to monsoon.

• Sludge line should be kept free of chocking. The line should be flushed with high-pressure water if chocking is noticed.

• The telescopic sludge discharge device, when provided, should be checked for vertical movement and O-rings replaced when leaky.

• The traction wheels of the moving bridge should be checked for alignment and rubber wheels replaced, if required.

• The units should be worked continuously to protect the mechanical parts from ill effects of corrosion, malfunctioning etc., as well as problem of sludge build-up.

• Outlet weirs should be kept clean at all times.

• Algaecide or bleaching powder may be used for controlling biological growth on weirs

7 Rapid Sand filters of Water Treatment Plant

The preventive maintenance procedures related to Rapid sand filters are listed below:

Defective Gauges

Rate of flow and loss of head gauges frequently get out of order. The operator should be conversant with the working of these gauges and should be able to do minor-repairs.

Inadequate sand media on Filter bed

Expansion of sand bed during back washing should be kept within the limits to avoid carry over of sand to wash water trough. Sand bed should never be depleted more than 10 em from the original thickness, when it is more than 10 em the sand media has to be replenished. The entire bed should be taken out and additional sand media mixed to give the required effective size and uniformity coefficient.

Air binding

This is caused due to the negative head and formation of air bubbles in the filter sand. This could be overcome by more frequent back washing during these periods. Provisions should be made wherever possible to increase the depth of water by about 15 to 30 em. The maintenance of depth of water of at least 105m over sand may eliminate air binding problem. If air binding persists, loss of head may be limited to 1.5 m, which will discourage air binding and ensure reasonable length of filter run.

Incrustation of media

This problem of incrustation of sand particles may arise as in the case of water softening with lime soda when sand particles get coated with material that is difficult to remove by normal backwash. Remedy lies in washing the filter occasionally with sodium hydroxide (10 kg/ m2 area of bed) or bleaching powder (20 kg/ m2 area of bed).

Cracking of sand beds

This occurs when water is lowered below the surface of the sand. Cracks in a filter bed under water may also arise due to cementing of grains by some materials in the applied water. The vulnerable portion is near the walls, since the sand is drawn away from the walls. The rate of flow increases through cracks allowing heavier deposits of solids at these points resulting in unequal distribution of the wash water. This can be avoided by the use of hand rake or by draining of bed and removing clogged sand.

Bumping of filter beds

Sometimes careless or indifferent operation of the filter bumping or lifting of the filter beds when switching on the back wash cycle is adopted. This type of inadequate cleaning process should be discouraged.

Mud-balls

These are caused by the general build-up of materials not removed in back-wash. Mud balls accumulate at or near surface and in course of time clog the entire sand media. Proper coagulation and settling of feed water could considerably reduce mud-ball formation. Surface wash or surface raking at intervals helps reduce mud ball formation. Compressed air scouring for more than 3 minutes also effectively decreases mud ball concentration.

Sand boils

Sand boils are caused when disproportionately large discharges of wash water rush towards expanding the sand and displacing the gravel. This situation arises mainly due to poor distribution of wash-water from under-drain system.

Slime growths

When slime growths are noticed on filters, the bed is cleaned in the normal way and water is lowered to the level of the sand bed. Then common salt is distributed evenly over the surface of sand using 7 kgl m2 of filter area, after which the wash water valve is opened until water rises about 15 cm above the sand level. The water is allowed to remain for 2 hours to dissolve the salt and then lowered to the bed level to be retained for 24 hours after which it is thoroughly back-washed before placing into service.

Backwashing

The waste wash water drains should be kept free of clogging or sediment. The requisite up-flow velocity of backwash water should be maintained at the design rate for proper cleaning of the sand.

Backwashing of filters in water treatment should not be based on arbitrarily fixed time scheduled but the frequency should be in accordance with the filtrate quality and head loss measurement. Duration of cleaning should be dependent upon the turbidity of the wasted water. .

8 Chlorinators of Water Treatment Plant

The preventive maintenance procedures related to  water treatment  chlorinator and chlorine house are listed below:

The chlorine demand of the filtered water is to be satisfied and optimum free residual chlorine maintained to render water completely safe.

The operator should be careful in administering calculated doses accurately.

Periodically chlorine dose should be monitored weekly to ensure adequate chlorine residual at the drinking tap; assuming adequate filtration and chlorine contact time, the residual of total chlorine should be 0.5 mg/L or the residual free chlorine should be 0.2 mg/L;

Bubbling the chlorine gas through the filtered water stored in the clear water reservoir by dipping rubber tubes connected to chlorine cylinder must be avoided. Chlorine application should be done through a chlorinator only.

Chlorinators require periodical adjusting; The chlorinator should be maintained properly.

If the unit is out of order, it should be repaired immediately.

Since chlorine gas is heavier than air it is likely to sink to the floor. Hence providing and maintaining arrangement for forcing air from top and driving chlorine gas from the floor through ventilators located at the floor level is necessary.

Safety requirements

Whenever dealing with gaseous chlorine, safety is an important issue. Ammonia should be kept handy for checking for leaks. Chlorine cylinders storage building should be well ventilated. It the operator must walk through an area with chlorine in the air, he or she should use a breathing apparatus. If no breathing apparatus is available, the operator should keep his head high. since chlorine is 2.5 times as heavy as air and will tend to sink to the ground.

9 Clear Water Storage Reservoirs

The preventive maintenance procedures related to clear water reservoirs are listed below:

• Water level recorder should be kept in working order at all times.

• During the pumping of water the water level in the reservoir should be maintained at the lowest level as possible so that overflow and wastage of water when the pump is suddenly shut down could be prevented.

• Roof of the clear water reservoir is always kept at least 60 cm above the surrounding ground level to prevent rainwater flooding over the roof.

• Roofing should be periodically checked to ensure that no leakage is there so that pollution can be prevented. Top of roof should be sloped in such a way to prevent stagnation of rainwater;

• Ventilator outlets should be regularly checked and cleaned to guard against mosquito breeding and bird droppings.

• Cleaning of sump and reservoirs should be done regularly at least once in six months.