| 1. |
Introduction : |
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Hundreds of filter plants are under design and construction, particularly in the developing countries for the supply of filtered water in cities and villages. As the conventional filter plants are fairly costly in their construction and maintenance. An intensive research is going on all over the world for the development of cheaper filter plants with the use of improved pretreatment methods including the use of coagulant aids and higher rates of filtration through dual and multimedia filters. In addition to this extensive automation for the various controls in the filter plants are under development in the developed countries to reduce the operational cost. Due to high cost of construction and maintenance in adopting the conventional filter plants, many towns and villages particularly in the developing countries cannot afford to adopt the same for their water supply schemes. Thus the design and construction of simple and cheap high rate water treatment plants is a challenging problem before the Engineers in the world. |
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| 2. |
New techniques for design of simplified treatment plants: |
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The basic approach in designing a filter for the rural areas lies in its extreme simplicity, economy in capital and maintenance cost, consistent with ability to produce filtrate of acceptable quality. From this view point the conventional pretreatment units viz. flash mixing, mechanical flocculation and clarification associated with the rapid sand filters may have to be replaced with suitable simplified methods. Similarly simplified high rate filter units need to be provided where possible after the simplified pretreatment units.
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 mainly for the rural and medium capacity water supply schemes, Ramtek plant has been developed while for the treatment of turbid water sources varangaon and chandori plants have been developed. The principal design criteria for these new methods are given in Table 1 enclosed at the end. The details of these new designs are given below. |
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| 3. |
Ramtek Plant |
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| 3.1 |
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. MLD. The source of supply is an irrigation tank and the average raw water turbidity is within 10 to 20 JTU with an occasional turbidity of 300 to 500 JTU. 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. |
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| 3.2 |
Prefilter Chamber: |
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Raw water, after addition of alum dose ad taking through the mixing channel is introduced through the bottom of the prefilter chamber of size 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 200mm x 300mm, with 50 mm dia, side perforated pipe laterals provided at 20 cm centres.
Graded gravel of 50 mm to 10 mm dia is 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 the floc formed due to continuous recontracts get consolidated, and when it reaches at the top of the bed, it settles at the top due to sudden drop in the upward velocity.
The sludge thus settled at the top of the bed can be drained out through the perforated draining pipe system provided at the top of the drained sludge. In addition to this draining facility, the sludge settled in the voids and at the bottom of gravel bed can be removed by draining out the same be hydrostatic pressure through the under drainage system. Further, a back wash can be given periodically with full pressure to clean the gravel bed effectively. |
| 3.3 |
Dual media filter bed |
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Water from the top of the prefilter chamber is introduced on the top of the dual media filter bed and 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 305 TO 40%. The rate of filtration is 6.75 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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| 3.4 |
Plant Observations: |
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| 1) |
From the actual plant observations the turbidity after prefilter was generally within 20 NTU even when the raw water turbidity was between 300 to 500 JTU with occasional increase up to 1000 JTU. The head loss in the prefilter chamber was negligible. The filtered water turbidity was maintained below one JTU 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 forthishigher rate of filtration was found to be 88 hours for a maximum head loss of two meters, 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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| 2) |
Use of the crushed coconut shell media
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 twelve years of its use in the filter beds at Ramtek.
The specific gravity of the coconut shell media is about 1.40 when it is soaked in the water. Its color is brownish when dry but turns to black when it is soaked in water. The media is angular, hard and tough and microscopic observations show a compact and uniform structure. |
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| 4. |
Varangaon Plant : |
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| 4.1 |
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 JTU, while the average turbidity is between 30 to 50 JTU. 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 setting tank, which are followed by three dual media filter beds. Raw water after addition of alum does 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. |
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| 4.2 |
Gravel bed flocculation chambers. |
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The size of each chamber is 3.0m x 3.0m. With 2.5m depth of gravel, with 0.3m water depth on the top. Graded gravel of 60 mm 20mm sizes are placed from the bottom to the top n mild steel grating, for supporting the gravel on the top of hoppers. Two hoppers are provided at the bottom with 45o 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 periodically. |
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| 4.3 |
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 50mm x 50mm 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 the 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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| 4.4 |
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 water depth. The under drainage system consists of 300 mm dia mild steel manifold with 50 mm dia, PVC pipe perforated laterals placed at 20 cm centres. 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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| 4.5 |
Plant observations. |
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The actual plant performance with pretreatment consisting of gravel bed flocculator, followed by the settling tanks was found to be very satisfactory even for higher turbidities above 3000 JTU. The settled water turbidity was generally below 20 JTU. The average filter run was seen for 40 hours for a maximum head loss of 2 meters. The wash water consumption was about two percent. |
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| 5. |
Chandori Plant : |
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| 5.1 |
Design and construction : |
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The plant was constructed during 1979-80 for a population of 0.015 million of chandori village. The source of supply is Godavari river and the maximum turbidity is more than 3000 JTU is designed for a pumping rate of 0.04 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. |
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| 5.2 |
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 4.0 m x 2.2 m with 3.6 m water depth. It is gravel bed flocculator � cum- tube settler unit. Graded gravel of 60mm to 20mm sizes are placed for 1.5 m depth over the under drainage system. The PVC tube settler modules are provided for 0.5 m depth and covering the entire surface are over the gravel bed, but keeping a clear spacing of 0.9m below the tube settler zone. Three 150mm 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 the actual tube opening area it is 5.7 mph. The total detention in the pretreater unit is about 45 min. The direction of flow is the tube settling zone is introduced on the filter bed through the pretreator bed is similar to the procedure explained for the prefilter bed in Ramtek plant. |
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| 5.3 |
Rapid sand filter bed. |
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In the approved scheme dual media filter bed was provide. However as the rate of filtration is 4500 lph/m2 one rapid sand filter bed was provided. The size of the filter bed is 4.0 m x 2.2 m with 3.6 m water depth. The filter media consists of fine sand bed of 80cm thick of effective size 0.5mm and uniformity coefficient of 1.5. The supporting gravel bed is provided for 50 cm thickness over the under drains. The filter bed is cleaned by giving hard wash from the E.S.R. The chlorine dose is given in the control chamber. |
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| 5.4 |
Plant observations. |
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The plant observations were conducted for the period from 16-7-81 to 7-11-81 for intermittent filter runs. The actual plant found to be very satisfactory even for the higher turbidities. The settled water turbidity was generally less than 1.0 JTU. The average filter run was for 60 hours. |
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| 5.5 |
Special advantage. |
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Chandori plant has special advantage for two stage construction. For low 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 later stage for augmentation of the plant capacity or improvement in the quality of filtrate. |
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| 6. |
Some common 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 structures 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. |
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| 7. |
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 Chadori plat was Rs. 1,54,875/- and these costs were between 30% to 50% of the construction costs for the same capacity conventional plants. The costs for the same capacity conventional plants. The costs of RCC and mild steel 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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| 8. |
Augmentation and improvements of existing plants : |
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The new techniques as explained above can also be utilized for improvements as well as augmentation of the existing Municipal big capacity plants. The expansion of the pretreatment works can be done by installation of tube settlers in the existing settling tanks. While the existing conventional rapid sand filter beds can be converted into high rate dual media filter beds by the use of crushed coconut shell media over sand layer. The existing plant capacity can be increased by 200 to 300 precent by adoption of these new techniques in a short time and in considerably cheaper cost as compared to conventional methods. |
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| 9. |
Conclusions |
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The new treatment methods as explained above have shown satisfactory results for number of such plants constructed in Maharashtra State in India. Table 1 giving the recommended general design criteria based on actual plant observations, shows that there is considerable flexibility in the design of such small capacity plants for village water supply schemes.
The new techniques can also be adopted for improvements and augmentation of the existing medium & big capacity water treatment plants. It is therefore felt that these new methods may be able to help in solving some of the important problems in providing simple & low cost water treatment plants for adoption in the rural and urban areas, particularly in the developing countries. |
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| TABLE 1 |
| Recommended design criteria for the simplified filtration plants |
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Design criteria
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Ramtek
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Varangaon
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Chandori
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| I Raw water turbidity: |
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| (i)Average range in J.T.U. |
For low turbidity
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For high turbidity
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For moderate
turbidity sources |
| (ii) Average range in J.T.U. |
10 to 30 |
30 to 100 |
30 to 100 |
| (iii) Maximum range in J.T.U. |
300 to 500 |
1000 to 5000 |
1000 to 2000 |
| II Pretreatment: |
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| (1) Mixing unit |
Mixing channel |
Mixing channel |
Mixing channel |
| (2) Type of gravel bed units |
Prefilter |
Flocculator |
Pretreator |
| i) Direction of flow |
Upward |
Downward |
Upward |
| ii) Surface loading in Iph/m2 |
4000 to 7000 |
4000 to 10000 |
4000 to 7000 |
| iii) Volumetric loading in Iph/m3 |
2000 to 3500 |
2000 to 5000 |
2000 to 4000 |
| iv) Depth of the gravel bed in m. |
1.5 to 2.0 |
2.5 to 3.0 |
1 to 1.5 |
| (3) Tube settling tank |
Not adopted; |
Tube settler |
Gravel bed cum
tube settler |
| i) Surface loading in Iph/m2 |
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5000 to 8000 |
4000 to 7000 |
| ii) Detention period in minutes |
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30 to 50 |
30 to 50 |
| iii) Depth of the tank in m. |
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3 m above hopper |
3.5 to 4.0 |
| iv) Direction of flow |
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Upward |
Upward |
| v) Size of PVC square tubes |
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50mm x 50mm |
50mm x 50mm |
| (vi) Depth of tube settler |
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0.5 to 0.6m |
0.5 to 0.6m |
| III Dual media filter bed : |
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| (i) Surface loading in Iph/m2 |
4000 to 7000 |
5000 to 10000 |
4000 to 7000 |
| (ii) Dual media details |
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a) Coconut shell media depth
Average size in mm. |
30 to 40 cm.
1.0 to 2.0 |
30 to 40 cm.
1.0 to 2.0 |
1.0 to 2.0
1.0 to 2.0 |
b) Fine sand media depth in cm.
effective size in mm.
Uniformity coefficient |
40 to 50
0.45 to 0.55
Below 1.5 |
40 to 50
0.45 to 0.55
Below 1.5 |
40 to 50
0.45 to 0.55
Below 1.5 |
| (iii) Back wash method |
Hard wash |
Hard wash |
Hard wash |
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| Conversion : 1000 lph/m2 = mph. |
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