| 1. |
Introduction : |
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It is generally seen that desired water quality is not obtained from water treatment plants particularly in villages and small towns. Small capacity plants serving them are normally operated by untrained operators under the control of municipal or village panchayat authorities in most of the States in India. In rainy season, the operators have to face turbidity problem and at many places consumers complain about the poor water quality. Further, outbreaks of water-borne diseases are also seen at many such places even with the provision of water supply schemes. In order to operate and maintain such small capacity plants efficiently, some important aspects of these are discussed in this paper based on the experience of the author. |
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| 2. |
Important operations: |
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The important items in the day-to-day operation and maintenance of a water treatment plant are given below: |
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(i) Flow measurement
(ii) Turbidity measurement
(iii) Alum dosing
(iv) Maintenance of pre-treatment units for mixing, flocculation and settlement
(v) Maintenance of filter beds
(vi) Chlorination
(vii) Records
(viii) Improvements
The following paragraphs describe in brief the care to be taken to achieve good results while carrying out these operations. |
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| 3. |
Flow measurement: |
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At many places, the operators do not know the actual flow of raw water received at the plants. Without the knowledge of the flows passing through the plants, it is not possible to adjust the correct doses of alum and chlorine so as to obtain desired quality of water. Rectangular weirs or V notches are simple economical devices which can be used for measurement of flows. There are to be installed in the inlet and outlet channels of water treatment plants. Table 1 shows the flow in liters per second for rectangular weirs and �V� notches. From these figures, the flows can be calculated for adjusting the alum and chlorine doses. Suitable markings or scales in centimeters (cm) can be made on the sides of weir and �V� notch to find the flows at various depths. |
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Table 1
Discharges over sharp crested weirs and 90o V notch |
| Length of weir |
0.3m |
0.6m |
0.8m |
1.0m |
1.5m |
2.0m |
90o V Notch |
| Head in centi-meters |
Discharges in litres per second |
| 5.0 |
6.26 |
12.52 |
16.69 |
20.87 |
31.30 |
41.73 |
0.803 |
| 6.0 |
8.21 |
16.41 |
21.88 |
27.36 |
41.03 |
54.71 |
1.257 |
| 7.0 |
10.33 |
20.66 |
27.54 |
34.43 |
51.65 |
68.86 |
1.836 |
| 8.0 |
12.62 |
25.23 |
33.64 |
42.06 |
63.08 |
84.11 |
2.551 |
| 9.0 |
15.06 |
30.12 |
40.16 |
50.20 |
75.30 |
100.40 |
3.409 |
| 10.0 |
17.65 |
35.31 |
47.08 |
58.85 |
88.27 |
117.69 |
4.420 |
| 12.0 |
23.26 |
46.53 |
62.04 |
77.55 |
116.32 |
155.10 |
6.935 |
| 14.0 |
29.41 |
58.83 |
78.43 |
98.04 |
147.06 |
196.09 |
10.167 |
| 16.0 |
36.07 |
72.14 |
96.19 |
120.23 |
180.35 |
240.46 |
14.169 |
| 18.0 |
43.21 |
86.43 |
115.23 |
144.04 |
216.07 |
288.09 |
19.000 |
| 20.0 |
50.83 |
101.65 |
135.54 |
169.42 |
254.13 |
338.84 |
24.719 |
| 22.0 |
58.89 |
117.79 |
157.05 |
196.31 |
294.47 |
392.62 |
31.359 |
24.0 |
67.40 |
134.81 |
179.74 |
224.68 |
337.02 |
449.36 |
38.973 |
26.0 |
76.35 |
152.69 |
203.59 |
254.49 |
381.73 |
508.98 |
47.606 |
28.0 |
85.71 |
171.43 |
228.57 |
285.71 |
428.57 |
571.43 |
57.306 |
30.0 |
95.50 |
191.00 |
254.66 |
318.33 |
477.49 |
636.66 |
68.106 |
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| 4. |
Turbidity Measurement: |
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This is an important work to be done daily for adjusting the alum dose. Particularly in rainy season, the raw water turbidity changes are frequent and record of raw, settled and filtered water turbidities gives valuable information about the defects in the plants or faults in operation. Costly photoelectric turbidity meters may not be properly used and maintained at such small plants. A turbidity rod to measure raw water turbidity and a simple turbidity comparator to measure settled and filtered water turbidities as shown in Fig. 1 will serve the purpose. Such turbidity comparators can be prepared locally or can be purchased from known suppliers. Standard turbidity suspension bottles of 1,2,5,10,20 and 30 JTU can be kept at the plant to measure settled and filtered water turbidities. These standard suspensions are required to be changed once in a year and preferably before rainy season. The settled eater turbidity should be kept below 10 JTU and should not exceed 20 JTU. The filtered water turbidity should be kept below 1.0 JTU but occasionally can be acceptable up to 2.5 JTU as per Manual of water supply standards. |
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| 5. |
Alum dosing: |
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General: |
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It is observed at many plants that the alum dose is not given either continuously or according to the changes in the raw water turbidity. This is usually because proper alum solution and dosing arrangements are not provided. For want of proper alum dose particularly in rainy season, the settled water turbidity remains high and due to this, the filter beds get clogged up earlier than the desired period. |
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Preparation of Alum solution: |
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A simple method for preparing alum solution and constant dosing arrangement is shown in Fig.2. The size of PVC mixing pipe or tank can be adopted after considering the probable consumption of alum for a day for small plants and for 8 to 12 hours for bigger size plants. Mixing arrangement can be prepared by fixing two tees and one bottom cap to a rigid valve as shown in Fig. 2 for small plants. Supporting graded gravel is placed at the bottom with a perforated pipe piece 25 to 50 mm dia for connecting to the inlet pipe to introduce water from elevated tank. At the outlet end, suitable PVC pipe connections are made up to the solution tank.
Required quantity of alum broken into pieces is placed in the mixing pipe or tank from the top on the gravel bed. Water from elevated tank is then introduced through a gate valve (and water meter) from the bottom of the pipe. The concentrated solution of alum is taken from the outlet at the top to the solution tank. For preparing standard solution with pure water a separate pipe line with a valve is connected to a dilution pipe (100 to 150 mm) placed in the solution tank. For mixing concentrated solution with water, a few rounded gravel pebbles may be placed in this pipe in the solution tank at the inlet side (not shown in Fig. 1). An overflow pipe (25mm) can be kept at T.W. level, so as to let out the excess solution in the dosing tank. All such arrangements can also be made in the inlet side of the RCC tanks for big plants. |
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Constant head dosing tank: |
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As shown in Fig.2, an outlet pipe of 25 to 50 mm dia from the solution tank, after providing a PVC gate valve, is connected to the inlet of a float controlled constant level dosing tank. For giving continuous dose of alum solution during the operation, dosing tank of 30 to 50 lit capacities can be provided at suitable level. The concentration of alum need not be more than 5%, but it is desirable to make weak solution of 2% to 3% where possible to give an economical and effective dose. The alum solution can be added through a perforate half-cut PVC pipe 100 to 150 mm dia fixed on the downstream of the weir of the stilling chamber in the mixing channel. Thus the alum solution enters just before the maximum turbulence zone in the mixing channel which causes effective and complete mixing of alum dose. |
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Material for Alum solution and dosing tanks: |
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It is desirable to use corrosion resistant material such as PVC, FRP or GRP for these tanks. Periodic cleaning of these tanks should be done and a sludge drain hole with a PVC gate valve be provided at the bottom side. Masonry or RCC tanks need good anticorrosive paint application periodically to avoid corrosion and possible leakages. |
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Adjustment of alum dose: |
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A small weighing balance (20 kg cap.) is provided at the treatment plant and daily dose and consumption of alum should be recorded in the register. An example of working out the alum required for raw water flow of 50,000 lph for sixteen hours daily working is given below:
Considering 10 mg/l (ppm) dose of alum |
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| Daily alum consumption |
= |
50,000 x 10 x 16 |
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1000 x 1000 |
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= |
8.0 kg. |
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Storage facility of alum for a minimum stock of four months, considering rainy season requirement, be provided, as transport and supply of chemicals during this period will be difficult. |
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Maintenance of pre-treatment units: |
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Pre-treatment units generally consist of stilling chamber, mixing channel, flocculation chamber and settling tank. Primary sedimentation tanks are also provided where raw water turbidities are very high. Important points in the maintenance of these units are discussed below. |
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Stilling chamber and mixing channel: |
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Stilling chamber is generally provided at the inlet side in the mixing channel itself. The stilling chamber reduces the turbulence of raw water entering into the flow measuring weir. The height of the weir is generally 20 to 40 cm. Both the stilling chamber and the mixing channel should be kept clean by removing silt, sand deposited and the algae growth at the bottom and sides. |
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| 6. |
Non-mechanical flocculators: |
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Gravel bed Flocculator: |
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The top of the gravel bed should be kept free from accumulation of silt, sludge and other floating matter. If the raw water is likely to contain silt and fine sand, it is necessary to remove top layer of gravel of 10 to 15 cm thickness, wash it outside and replace the same periodically. For routine cleaning, the gravity desludging operation for removing the sludge in the bed and in the hopper bottom is carried out at the end of day�s work for 3 to 5 minutes by opening the drain valves. The bed should be filled with raw water to the normal level at the end of this operation. The tube settling tanks are required to be closed during the gravel cleaning operation.
For high turbid water, backwash arrangement is generally provided with inlet and outlet valves. If backwash arrangement is not provided and there is excess of silt load in water, part or full gravel in the bed has to be removed outside for cleaning and replacing periodically. Alternatively, rigid PVC half-cut pipes modules or perforated partitions can be introduced in place of gravel for proper functioning of the flocculator. |
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Flocculator with PVC half-cut pipes perforated partitions: |
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Generally 63 mm dia black colour half-cut rigid PVC pipe are used to form perforated partitions. The PVC half round pipes are kept clean by periodic manual cleaning to remove algae and floating matter. The sludge settled in the bottom hoppers is drained periodically. In order to keep uniform horizontal flow through the bed, adequate perforated pipes are provided in these tanks at the inlet and outlet ends. |
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Flocculator with PVC Angle or Half-cut pipes Floc Modules: |
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This is the best method for non-mechanical flocculation. The top surfaces have to be cleaned periodically to remove algae and floating matter. The sludge in the bottom hoppers is drained by hydrostatic pressure periodically. The operators should not walk over the modules during the cleaning operation. |
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| 7. |
Settling Tanks: |
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Tube settling tanks: |
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The side perforations on the collector pipes or channels are cleaned periodically to remove algae and floating matter. When there is visible algae growth on the top of the tubes, the water level in the tank can be lowered by 5 to 10 cm below the tube surface for cleaning the surface area manually. It is very much necessary to get uniform flow through all the connector pipes or channels. For this purpose, minor corrections, such as readjustment of levels or increasing perforations etc. are made to improve the performance. The sludge in the bottom hoppers is drained periodically by hydrostatic pressure. |
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Plain bottom settling tanks: |
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It the tanks have plain or inadequate sloping bottoms, then the tanks have to be cleaned manually after draining the water. This cleaning operation is done generally before and after the rainy season. If there are no proper inlet and outlet arrangements, the same is provided for proper functioning. |
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Mechanical clariflocculators: |
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Periodic cleaning of all surfaces under and above the water level has to be done for removing algae and floating matter. The scraper blades at the bottom are checked for effective sludge draining and new blades are provided where necessary. Continuous or periodic sludge draining be carried out as per specified method to keep the desired settled water turbidity. Mechanical and electrical parts require periodic cleaning, oiling and specified maintenance. Minor repairs be got done before the rainy season. Anti-corrosive paint is applied periodically to all metallic parts including moving bridge. |
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Maintenance of filter beds: |
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Rapid sand filter beds with hard wash arrangement:
The top surfaces of gutters and side walls are kept free from growth of algae, particularly for the filter beds constructed open to sky. The most important thing to get good results is to give effective backwash. To confirm effective backwash, the expansion of the filter media be checked periodically by using an expansion stick as shown in Fig. 3. For rapid sand filter beds minimum expansion of 10% to 20% is considered adequate. The period of backwash is generally 7 to 10 minutes depending on the depth of media and head of wash water. Where there is effective backwashing, the head loss after the wash does not show sudden or early increases. The other check is the observation of bed surface, which should be uniform and leveled and free from cracks and holes. If mud balls are observed, these are removed manually and after washing the bed, clean sand is placed. If size, due to wear and tear, the same be removed periodically and replaced by fresh good sand of proper grain size. |
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Rapid sand filter beds with air and water wash: |
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These are generally provided for large size filter beds with special air wash systems. Where air wash is given before water wash, it should be uniform in pressure and quantity throughout the bed. Filters get seriously damaged by slugs of air introduced haphazardly during filter washing. In such cases, the supporting gravel overturns and gets mixed with the sand media, which requires complete removal and replacement of all the media after proper repairs. |
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Dual media filter beds: |
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Many of the problems in these filter beds are observed due to the ineffective backwashing. In the conventional rapid sand filter beds, most of the impurities in water are removed and stored in the top 15 to 20 cm, while in the dual media filter beds, these are also stored at deeper layers and within a few centimeters of the filter media. Therefore, (higher) backwash flow rates of 700 to 900 lpm/m2 are required to be provided for a (longer) period of 10 to 15 min. 30% to 40% expansion of filter bed is considered adequate to suspend the bottom grains. These filters are generally provided with only hard water wash to avoid loss of top coarse media. It should be seen that the bottom of the gutters is above the top of the expanded media to prevent loss of the media during backwashing. |
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Measurement of head loss: |
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Filter bed should be generally washed when the head loss reaches its prescribed limiting value (say 2.0 m) If turbidity of the filtered water is seen to be above 2.0 JTU are as prescribed, - even before the normal period of washing � the filter bed should be washed. This increase in the filtered water turbidity is due to either ineffective backwash or breakthrough of floc through the cracks or holes in the filter media. It is very necessary to keep records of head loss in the filter beds to know their performance. If the head loss development is earlier than the normal period, it can be usually due to ineffective backwashing. For head loss measurement, transparent plastic or glass tubes of 12 mm dia can be used. Fig. 3 shows the typical head loss measuring arrangement in a control room. |
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Filter rate control: |
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In many existing plants, the filter effluent rate control arrangement is seen not working properly or is completely inoperable. A simple arrangement with a control chamber with rectangular weir and manually operated control valve is better for small plants. Fig. 3 shows such an arrangement. The control chamber should be kept clean particularly from inside. A wooden cover with a glass window can be kept for observing the discharge over weir and to collect samples. Constant rate of flow is generally adjusted over the weir by a control valve with the help of a scale fixed at the side of the weir. Chlorine dose is generally given in the outlet side in this chamber. |
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Chlorination: |
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The solution and dosing tanks as shown in Fig. 1 can also be provided for TCL or bleaching powder solution and giving required dose. Some manual mixing device can be made for mixing or churning for the TCL powder. Proper sludge draining arrangement has to be provided for the tank at the bottom. PVC, FRP or GRP tanks can be used for both solution and dosing tanks. Calculations of TCL powder requirement can be made as described for alum dosing, considering the available proportion of chlorine in the powder. Required precautions should be taken for safe storage and use of bleaching powder bags or drums and liquid chlorine cylinders as described in the manuals. Liquid chlorine cylinders are generally used at large size plants, as the chlorine gas is more effective and cheaper as compared to bleaching powder. However, this needs very careful handling. Residual chlorine of 0.2 to 0.5 mg/l has to be kept in distribution system for effective disinfection of water.
Simple residual chlorine comparator kits available in the market can be used for visual comparison and records of these is maintained in the register. TCL solution dosing directly by mixing in a bucket and pouring it in the pure water sump should not be adopted. For effective mixing and action, proper type of dosing apparatus is used. |
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Records: |
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In the day-to-day maintenance of the water treatment plants, it is
Very important to keep daily records of the plant operation, which is generally seen to be neglected. The plant performance cannot be known without such records. Table 2 shows a general proforma for keeping the daily records. Turbidities of raw, settled and filtered water samples, adjustment of alum dose, measurement of head losses and chlorine dose are the important items for keeping daily records. In addition to these, records for daily use of water, backwashing dates, consumption of water for backwash, dates of water samples taken and sent for chemical and bacteriological tests are also kept in the remarks column of the register.
A separate register be kept to record the results of chemical and bacteriological tests, as prescribed in the �Manual on Water Supply and Treatment� of Govt. of India, or as decided by authorities. |
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Suggestions for improvements: |
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It will generally be seen that operators at most of such small plants are not trained. Further, there is little quality consciousness either with the water supply authorities or the people. Hence the quality of drinking (treated) water at most of the places is below the prescribed standards. In order to improve these conditions and considering the aim of the U.N. Decade of �Health for All� by the year 2000, it is suggested that all the small and medium capacity water treatment plants be brought under the direct or indirect control/supervision of the PHE departments or Water Supply Boards of the state Governments till the year 2000. Extensive programme of training of personnel at all levels and improvements in the working of the water supply systems are undertaken during the next five years to achieve this goal. When a few state governments, like Kerala and Punjab are doing this, the other states also can do so by adopting a positive approach to achieve the targets set. |
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