| 1. |
Introduction : |
| 1.1 |
The urgent need |
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Numbers of water treatment plants are likely to be constructed for the supply of potable water to the villages in the developing countries during the next decade. As the conventional water treatment methods are fairly costly in their construction and maintenance, an intensive search is going all over the world for the development of low cost water treatment methods with the use of new techniques.
Even though the slow sand filters are recommended for village water supplies, there are some problems in providing these filters for turbid raw water sources. With the provision of storage basins, coagulation and sedimentation before slow sand filters, the cost of construction and maintenance goes very high. In the case of rapid gravity filters due to various mechanical arrangements adopted for mixing, flocculation and clarification in the pretreatment units for treatment of turbid waters. Further there are maintenance problems for all these treatment methods in the small capacity plants. Hence the development of simple and economic filtration methods for the small capacity rural plants has become an urgent need. |
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| 1.2 |
Development of new methods |
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Considering the various problems in the design and construction of the conventional treatment plants, the author has developed three new designs for the construction of simplified water treatment plants in his Ph.D. thesis (1978), which are explained in this paper. For the treatment of low turbidity water sources, Ramtek plant has been designed, while for the treatment of turbid water sources Varangaon and Chandori plants criteria for these new methods are given in Table 1 |
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| 2. |
Ramtek Plant : |
| 2.1 |
Design and construction |
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The plant was constructed in 1973 for a population of 0-02 million with a capacity of 2-4 MLD. The source of supply is an irrigation tank and the average raw water turbidity Is within 10 to 20 J.T.U. with an occasional turbidity of 300 to 500 J.T.U. The treatment plant is designed for an hourly pumping rate of 0.1 mlph, which supplies intermittent water supply to the town. The plant comprises of two separate units and each unit consists of one gravel bed prefilter chamber followed by one dual media filter bed. Both the units are open to sky with a control room on the outlet side. The flow diagram of one unit is shown in Fig.1, and the plant is shown in Photo 1. |
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| 2.2 |
Prefilter Chamber |
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Raw water, after addition of alum dose and taking through the mixing channel is introduced through the bottom of the prefilter chamber. The dimensions of the prefilter chamber are 2.0m x 3.5m with 3.0m water depth. The under drainage system at the bottom of the chamber consists of one mild steel manifold of size consists of one mild steel manifold of size 200 mm x 300 mm, with 50 mm dia. Are placed for a depth of 1.7 m from the bottom to the top. The direction of flow in the prefilter is upward and the surface loading rate is 6.75 mph. The gravel bed provides ideal facilities for flocculation, and gets consolidated, and when it reaches at the top of the bed can be drained out through the perforated draining pipe system provided at the top of the gravel bed. This can be operated periodically
depending on the raw water turbidity, and the same can be judged from colour of the drained sludge. In addition to this draining facility, the sludge settled in the voids and at the bottom of the gravel bed can be removed by draining out the same by hydrostatic pressure through the under drainage system. Further, a back wash can be given periodically with pressure to clean the gravel bed effectively. |
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| 2.3 |
Dual media filter bed |
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Water from the top of the prefilter chamber introduced on the top of the dual media filter bed is then filtered in the downward direction. The dimensions of the filter chamber are the same as those given for prefilter chamber, including the under drainage system at the bottom. The dual media consist of the top coarse coconut shell media of average 1 mm to 2 mm size, which is provided for 35 cm depth over the sand bed. The fine sand media having effective size of 0-45 mm and uniformity coefficient of 1-5 is provided for 55 cm depth, over the supporting graded gravel bed of 45 cm depth at the bottom.
The filter bed is cleaned by giving hard wash for 7 to 8 minutes, when the expansion of the filter media is achieved at 30% to 40%. The rate of filtration is 675 mph which is controlled by a manually operated sluice valve before a ‘V’ notch chamber in the control room. A simple gravity chloronome is provided to give the dose of gaseous chlorine in the control chamber. |
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| 2.4 |
Plant observations |
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| i] |
From the actual plant observations the turbidity after prefilter was generally within 20 J.T.U. even when the raw water turbidity was between 300 to 500 J.T.U., with occasional increase up to 1000 J.T.U. The head loss in the prefilter chamber was negligible. The filtered water turbidity was maintained below one J.T.U. during the filter runs for a limiting head loss of two meters. Even though the filter bed was designed for a filtration rate of 6.75 mph, one dual media bed was operated for a higher rate of 9.65 mph for one year. The average length of filter run for this higher rate of filtration was found to be 88 hours for a maximum head loss of two metres, when the plant was operated intermittently for 4 to 8 hours daily. The wash water consumption was less than one percent. |
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| ii] |
Use of the crushed coconut shell media. |
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This media was used for the first time for filtration in the high rate dual media filter beds at Ramtek, and the general performance of this new media was found to be very satisfactory. Even though the media is organic in nature, there is no sign of deterioration of the media after a period of six years of its use in the filter beds at Ramtek.
The specific gravity of the coconut shell media is about 1.43 when it is soaked in the water. Its colour is brownish when dry but turns to black when it is soaked in the water. The media is angular, hard & tough and microscopic
observations show a compact & uniform structure. Its solubility in 20% HCL is 0.7% in 24 hours, and the durability tests of the media shows about 2.5% loss in weight, when the media was continuously backwashed for 100 hours. During laboratory study with different media the coconut shell media was found to be superior. Further the cost of this media may also be cheaper than the other coarse media, as the coconut shell is a waste material and it is mainly used as fuel. Though the coconut shell is available for use in some other coastal countries. However, its availability for large scale use may have to be ascertained. |
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| 3. |
Varangaon Plant: |
| 3.1 |
Design and construction |
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The plant was constructed in 1977 for a population of 0.035 million in five villages. The source of supply is the Tapi River and the maximum turbidity during rainy season is more than 3000 J.T.U. while the average turbidity is between 30 to 50 J.T.U. The plant is designed for a pumping rate of 0.175 mlph with a capacity of 4.2 MLD. The plant comprises of two pretreatment units in parallel and each unit consists of one gravel bed flocculator chamber and one tube settling tank, which are followed by three dual media filter beds. Raw water after addition of alum dose is taken through the mixing channel to the top of the gravel flocculator chamber. All the beds are open to sky with a control room on the outlet side. The control room provides the declining type rate control system, with one master control valve before weir chamber. The pure water pumping machinery is also installed in the control room. Alum solution and dosing arrangements are provided on the first floor, while the back wash tank is provided on the top of the chemical room. The flow diagram is shown in Fig. 2 and the plant is shown in Photo 2 on Page 3. |
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| 3.2 |
Gravel bed flocculation chambers |
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The size of each chamber is 3.0m x 3.0m with 2.5 m depth of gravel, with 0.3 m water depth on the top. Graded gravel of 60 mm to 20 mm sizes are placed from the bottom to the top on mild steel grating, for supporting the gravel on the top of hopers. Two hoppers are provided at the bottom with 45 o slopes, to drain out the sludge by hydrostatic pressure. The direction of flow is downward and the surface loading rate is 9.7 mph. Arrangements are provided to desludge the gravel bed by draining out water to waste. For cleaning the bed effectively, back wash arrangements are also made, for giving the back wash periodically. |
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| 3.3 |
Tube settling tanks |
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The flocculated water from the gravel bed is introduced at the bottom of the tube settler and just above the top of the hoppers, through 150 mm dia. Four perforated distribution pipes. The size of each tank is 3.0 m x 6.0 m with 3.0 m water depth above the top of the hoppers. A layer of 50 mm X 50 mm size rigid PVC square tubes, 60 cm in length is provided to cover all surface area. The modules of PVC
tubes were fabricated by fixing the tubes at 60o angle in opposite directions, and were then installed in the tanks. The direction of flow is upward and the flow through rate is 6.6 mph through open area of the tubes, with detention period of about 35 minutes in the tube settling tanks. The settled water is collected through the perforated collecting pipes of 100 mm dia. In the central collecting channel, and is then introduced on the dual media filter beds. Four hoppers are provided with 45o slopes in these tanks, and sludge is drained out by hydrostatic pressure by opening respective sludge valves. |
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| 3.4 |
Dual media filter beds |
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There are three filter beds and size of each bed is 4.0 m X 2.2 m with 3.0 m water depth. The under drainage system consist of 300 mm dia. Mild steel manifold with 50 mm dia. PVC pipe perforated laterals placed at 20 cm centers. The filter media consists of 40 cm depth of coarse coconut shell media of average size 1 mm to 2 mm over the fine sand bed of 50 cm depth. The effective size of fine sand is 0.5 mm with uniformity coefficient of 1.5, and the media is supported by graded gravel bed of 0.5 m depth. Even though the designed rate of filtration is 6.6 mph, two filter beds were operated at a higher rate of 10 mph for one year, which showed satisfactory quality of filtrate. The filter beds are cleaned by hard wash with 30% to 40% of expansion of media for about 8 to 10 minutes. The chlorine dose is given in the control chamber. |
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| 3.5 |
Plant observations |
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The actual plant performance with pretreatment consisting of gravel bed flocculator, followed by tube settling tanks was found to be very satisfactory even for higher turbidities above 3000 J.T.U. The settled water turbidity was generally below 20 J.T.U. while the filtered water turbidity was between 0.5 to 1.0 J.T.U. The average filter run was seen for 40 hours for a maximum head loss of 2 metres. The wash water consumption was about two percent. |
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| 4. |
Chandori plant : |
| 4.1 |
Design and construction |
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The plant was constructed during 1979-80 for a population of 0.015 million of the chandori village. The source of supply is the Godavari River and the maximum turbidity is more than 3000 J.T.U., while the average turbidity is between 30 to 50 J.T.U. The plant is designed for a pumping rate of 0.40 mlph. The plant comprises of one unit of pretreator followed by one unit of dual media filter bed. Both the units are open to sky with a control room on the outlet side. The pure water pumps are also installed in the control room. The elevated service reservoir is used for giving back wash. The flow diagram is shown in Fig. 3 and the Chandori type is shown on page 6. |
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| 4.2 |
Pretreator unit |
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Raw water, after addition of alum dose and passing through the mixing channel is introduced through the bottom of the pretreator unit. The dimensions of the pretreator are 4.0m x 2.2m with 3.6m water depth. It is gravel bed flocculator – cum- tube settler unit. Graded gravel of 60 mm to 20 mm sizes are placed for 1.5 m depth over the under drainage system. The PVC tube settler modules are provided for 0.5m depth and covering all the surface area over the gravel bed, but keeping a clear spacing of 0.9m below the tube settler zone. Three 150 mm dia. Perforated C.I. pipes settled water collectors are provided at 0.6m above the top of tube settler. The surface loading rate on the gravel bed is 4.5 mph, while for actual tube opening area it is 5.7 mph. The total detention in the pretreator unit is about 45 min. The direction of flow is upward and the raw water after passing through the gravel bed and the tube settling zone is introduced on the filter bed through the collector pipes at the top. The cleaning of the gravel bed is similar to the procedure explained for the prefilter bed in the Ramtek plant. |
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| 4.3 |
Dual media filter bed |
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The dimensions of the filter chamber are 4.0m x 2.2m with 3.6m water depth. The details of the dual media filter bed are similar to those given for the Varangaon filter bed. One weir chamber is provided in the control room where chlorine dose is given. |
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| 4.4 |
Plant observations |
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The plant was put into operation from October 1980 and various observations are being conducted. The pilot plant results are satisfactory. The chandori plant has special advantage for two stage construction. For lower turbidity sources the tube settler in the pretreator and the coconut shell media in the filter bed can be omitted in the first stage. However the same can be introduced at a large stage for augmentation of the plant output, or for obtaining longer filter runs with improvement in the quality of filtrate. |
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| 5. |
Some common aspects : |
| 5.1 |
Design aspects |
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All these three plants have been constructed in the gravity masonry side walls with RCC roof only on the control room. This type of structure was adopted mainly to utilize the local material and unskilled labour in the villages. This type of massive structure is generally found leak proof. Further there is advantage of providing mixing channels and walkways on the top of the side walls. However, the R.C.C. structure can be cheaper for these designs, if such facilities are available in the villages. All these designs can also be adopted with some modifications for
fabrication of package plants, which will be cheaper and will have some more advantages.
All these plants have been found considerably simple for construction and maintenance, the main reasons being, the absence of mechanical equipments, plants having compact designs with higher surface loading rates, and these can be built with local material and labour. Even though the dual media filter beds with coconut shell media have been adopted in all these designs, rapid sand bed can also be adopted. The use of gravel bed flocculation and tube settlers in pretreatment and the use of coconut shell media in the dual media filter beds are the new techniques adopted in the development of these small capacity unconventional treatment plants. |
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| 5.2 |
Cost aspects |
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The cost of construction for Ramtek plant was Rs. 1, 29,100/-, for Varangaon plant was Rs. 4, 13,000/-, for Chandori plant was Rs. 1, 54.875/- and these costs were between 30% to 50% of the construction costs for the same capacity conventional plants. Table 2 showing the comparative costs of construction of the simplified plants as developed by the author, and the conventional treatment plants is enclosed. The costs of RCC and package plants can still be reduced. Further the overall costs can be reduced if the elevated service reservoirs are utilized for back wash facility. |
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| 6. |
Conclusions : |
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Considering the proposed “International Drinking Water Supply and Sanitation Decade” during 1981 to 1990, the provision of safe drinking water supply facilities will have to be provided on the priority basis. The new treatment methods as explained in this paper have shown satisfactory results for number of such plants constructed in the Maharashtra State in India. Table 1 giving the recommended general design criteria based on the actual plant observations, shows that there is considerable flexibility in the design of such small capacity plants for village water supply schemes. The new treatment plants are found simple for construction and maintenance and are also found considerably cheaper as compared to the costs of conventional plants. It is therefore felt that these new methods may be able to help in solving some of the important problems in providing simple and low cost water treatment plants for adoption in the rural and semi-rural areas, particularly in the developing countries. |
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| 7. |
Acknowledgements : |
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The author is grateful to Mr. P.K. Nagarkar, Chief Engineer and Director, Maharashtra Engineering research Institute, Nashik, Mr. S.T. Khare chief Engineer and Member Secretary, Maharashtra Water Supply and Sewerage Board, Bombay and Dr. A.G. Bole, V.R.C.E. Nagpur, for giving their valuable encouragement during the Ph.D. study on this work. |
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Fig. 1: Flow diagram of Ramtek Filter
(All Dimensions in Millimeters) |
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Fig.2: Flow diagram of Varangaon Treatment plant
(All Dimensions in Millimeters) |
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Flow Diagram of Chandori Treatment Plant
(All Dimensions in Millimeters) |
