| 1.1 |
Introduction : |
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The village Chandori is situated on the bank of Godavari River in Nasik District of Maharashtra State. Chandori village water supply scheme was sanctioned for the estimated cost of Rs. 9, 82,000/- in the year 1978, under the accelerated programme of village water supply is 14700 souls. The rate of water supply is 40 lpcd. The scheme is designed for the daily water supply of 0.6 ml in the ultimate stage with daily 16 hours of pumping. The river Godavari is the source of water supply and raw water is pumped through a combined jack well for the water supply schemes for Saikheda and Chandori villages.
The raw water from the river is pumped through 150 mm dia pumping main 2300 m long from the jack well to the treatment site at an hourly pumping rate of 36,750 litres. The filtered water is pumped to the E.S.R. of 3, 00,000 litres capacity situated near the treatment plant. Water is distributed through the E.S.R. to Chandori village through stand posts.
In view of the high cost of construction of a conventional plant for such small capacity water treatment plant, a new simplified filtration plant for the treatment of turbid water sources has been specially designed for this scheme as discussed in this chapter. |
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Quality of the raw water |
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The source of the water supply scheme is the Godavari River which has very high turbidity throughout the rainy season of four months. The maximum raw water turbidity is likely to be 5000 JTU, while the average turbidity will be 10 to 50 JTU, during the remaining seasons. The quality of the raw water source for high turbidity and moderate pollution as discussed in Chapter 1. And simple treatment plants for the small capacity rural water supply schemes for turbid water sources. |
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| 1.2 |
Design Aspects : |
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The new design proposed for this unconventional treatment plant includes mixing channel, one unit of pretreator followed by one unit of a rapid sand filter bed. The pretreator unit is totally new feature of the design of this plant and may have been provided for the first time in the field of water treatment plant as explained in Chapter 2 & 3 of this book. The pretreator is a combination of the prefilter of Ramtek & Varangoan treatment plants as explained in Chapter 2 and 3 of this book. The pretreator is a combination of the prefilter of Ramtek plant and the tube settler of Varangaon plant. It is a flocculator -cum-tube settler unit. Such a pretreator may be able to treat moderate turbid water sources at higher surface loading rates. The pretreator units as well as the rapid sand filter unit for this new plant have been designed for surface loading of 4500 lph/m2. However, for the low turbidity water sources the plant can be designed for higher surface loading.
Figure 4.1 shows the flow diagram and figure 4.2 shows the photograph of a Chandori type plant. The detailed plan and section of Chandori filtration plant are shown in Fig. 4.3. The detailed plan and section of Chandori filtration plant are shown in Fig. 4.3. The detailed hydraulic design calculations are given in Table 4.1. The design aspects of the plant are discussed below. |
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| 1.3 |
Mixing Channel: |
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A mixing channel is provided on the top of two side walls of the plant as shown in Figure 4.3. The width of the channel is kept 60 cm and bottom slope of 20 cm is given to the channel to avoid flooding of water. Asbestos Cement pipe pieces of 100 mm dia and of 30 cm height are fixed in vertical positions in the bottom concrete of the channel in staggered positions as shown in Figure 4.3 o accelerate mixing action.
A small stilling chamber is provided near the inlet pipe by constructing a weir wall in the channel. The alum solution and dosing tanks are provided near the stilling chamber and alum dose is introduced just on the downstream of the weir, where maximum turbulence is available. |
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| 1.4 |
Pretreator Unit : |
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There is one unit of pretreator of size 4.0m x 2.2m with 3.6 m water depth. The pretreator is a gravel bed flocculator-cum-tube settling unit. Graded gravel of 50 mm to 20 mm sizes are provided at the bottom of the unit for 1.5 m depth. The gravel is directly placed on the under drainage perforated pipe laterals as shown in Figure 4.3. The PVC tube settler modules are provided for 50 cm depth covering all the surface area over the gravel bed and keeping a clear space of 90 cm below the tube settlers. Side gutters are provided on all sides of the bed with the top of gutters 10 cm above the top of the tube settlers. Three 150 mm dia perforated cast iron pipe settled water collectors are provided at 60 cm above the top of the tube settlers. The side walls above the gutter levels are provided at 60o angle from the inside face, so as to reduce the velocity of flow towards the collecting pipes. The surface loading on the gravel bed is 4500 lph/m2 while the volumetric loading is 3000 lph/m3. The total detention period in the pretreator is about 30 minutes. The tube settlers zone consists of a layer of rigid PVC square tubes of size 50 mm x 50 mm opening and 0.6 m in length which are fixed at 60o angle in the form of modules as explained in chapter 3 in connection with the tube settling tank of Varangaon plant.
The raw water after passing through the mixing channel is introduced at the bottom of the bed through the under drainage system and then flows in the upward direction through the pretreator unit. The water after passing through the gravel bed and the tube settles is collected in the perforated pipe collectors of 50 mm dia, from where the settled water is introduced on the rapid sand filter bed as shown in Figure 4.3.
At the bottom of the pretreator the under drainage system consisting of 300 mm dia manifold and 50 mm dia PVC pipe perforated laterals are provided at 20 cm centre to centre as per details shown in Figure 4.3. The laterals have perforations of 6 mm dia at 4 cm centre in the staggered positions at 90o angle in the bottom of the laterals. |
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| 1.4.1 |
Sludge removal from top of gravel bed |
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For removal of sludge from the top of the gravel bed, a perforated pipe system is provided. This includes 100 mm dia central G.I. pipe with side PVC pipe laterals of 50 mm dia, having side perforations of 6 mm dia at 10 cm centres on both sides. The operating valve for sludge withdrawal is provided in the control room. The sludge draining operation is done periodically depending on the turbidity of raw water. |
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| 1.4.2 |
Cleaning of gravel bed |
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The gravel bed is cleaned with the settled water at the top by gravity desludging operation through the under drainage system for a period of 3 to 5 min. after the day’s work. This is generally adequate for cleaning the gravel bed. However to clean the gravel bed effectively a back wash can be given periodically for 8 to 10 min. so as to remove any clogged material in the gravel bed. The back wash can be given once a week or a fortnight for effective cleaning of the gravel bed. |
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| 1.5 |
Rapid sand filter bed : |
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In the approved scheme dual media filter bed was proposed to be constructed. However, as the rate of filtration is 4200 lph/m2, one rapid sand filter has been adopted. The size of the filter bed is 4.0 x 2.2 m. The filter media consists of fine sand of 80 cm thick of effective size 0.5 mm and uniformity co-efficient of 1.5. The supporting gravel bed is provided for 50 cm thickness over the under drains. The under drainage system is similar to that provided in the pretreator as explained earlier. The side gutters are provided in the pretreator as explained earlier. The side gutters are provided on all sides of the filter bed for wash water
collection and further draining out through 300 mm dia outlet drain pipe with a sluice valve. The depth up to the top of the gutter is 2.1 m while the total water depth over the filter bed is 3.5m. Water after flowing in the downward direction through the filter bed is taken in to the control chamber. |
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| (i) |
Control Room |
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The outlet pipe gallery of 200 mm dia with valves is provided in the control room as shown in Figure 4.3. A rectangular notch is provided at the centre of the control chamber to adjust the flow over the weir. In addition to this two pure water pumps have also be accommodated in the control room. With this arrangement a separate pump house can be avoided. |
| (ii) |
Back wash |
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Only hard wash is given to the filter bed for about 10 to 12 minutes to clean the filter bed effectively. The back wash is given through 200 mm dia main from the E.S.R. of 3, 00,000 litres capacity constructed by the side of the treatment plant. |
| (iii) |
Head loss measurement |
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The head loss measuring arrangement is made by providing two plastic tubes showing the water levels in the filter bed and before the outlet control valve. The head loss in the pretreator is negligible. However, a minimum drop of 30 cm at the end of mixing channel has been provided between the F.S.L. in the pretreator and the bottom of the mixing channel at the outlet end of the channel. |
| (iv) |
Chlorination |
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The chlorination arrangement is provided by the side of the control chamber. The required TCL dose in the form of solution is given in the control chamber after rectangular weir, so as to mix effectively in the filtered water before taking it to the pure water sump. |
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| 1.6 |
Construction of the plant : |
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The plant was constructed during the year 1979-80 and was put into trial runs from March 1981. As shown in Figure 4.3 most of the works are gravity masonry walls, with only RCC roof slab over the control room. The work was got executed through a local contractor by employing local labors. The PVC tube modules were fabricated departmentally as per details explained for Varangaon plant. |
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| 1.7 |
Plant Observations : |
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The plant observations were conducted from day-to-day data recorded in the register kept at Chandori Plant for the period from 16-7-81 to 07-111-81 for ten filter runs. Actual observations were conducted for intermittent filter runs ranging from 4 to 8 hours per day according to the supply of filtered water to Chandori village. Considering the intermittent nature of working of the treatment plants in rural areas, it was considered observations were conducted for the actual pumping discharge of the installed pumping machinery of 200,000 lph only, where as lph as given in Table 4.1. The observations collected for total hours of daily working, turbidities of raw, settled and filtered water samples, and head losses during the period of study are given in Table 4.2.
During the period of study samples of raw, settled, filtered and tap water were collected weekly for bacteriological and chemical analysis, and their results are given in Tables 4.3 and 4.4 respectively. The performance of the pretreator and filter units are discussed below. |
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| 1.7.1 |
Cleaning of gravel bed |
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| (i) |
The novel idea : |
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This pretreator unit is provided for the first time in the field of filtration and being a novel idea, a patent (No. 150448) has already been granted to the author in India. The unit gives a complete pretreatment at the same rate as that for the filter bed and is a flocculation-cum-tube settler unit. The head losses observed through this unit were negligible. The turbidity after pretreator unit was generally below 20 JTU even when the raw water turbidity increased of the range of 50 to 1000 JTU during this period of observation. The average turbidity in the other season was in the range of 10 to 30 JTU when the settled water turbidity was generally below 10 JTU. The removal of higher turbidity is due to the large surface area available in the gravel bed and the PVC tube settlers introduced in the pretreator unit. Thus the general performance of the pretreator unit was found to be satisfactory. |
| (ii) |
Sludge draining from the top of the gravel bed : |
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This is an important operation in the pretreator unit. The floc and the sludge settled at the top of the gravel bed were periodically drained out under hydrostatic pressure through the special perforated pipe assembly provided at the top of the gravel bed. During very high turbidity of raw water the sludge from the top of the gravel bed can be continuously drained during the working of the plant, so as to get acceptable settled water turbidity during such periods. |
| (iii) |
Cleaning of the pretreator bed : |
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The cleaning of the pretreator bed by gravity desludging operation and periodic back washing of the bed are very important operations for the proper functioning of the pretreator. The unit was generally cleaned daily for 3 to 4 minutes by gravity desludging operation with the settled water available above the top of the tube settlers. The pretreator bed was cleaned by giving a back wash for 7 to 8 minutes through the under drainage system once in a week during the rainy season and once in fortnight during low turbidity period, for cleaning the bed effectively. As this is an important process for control of the settled water turbidity the operator has to be trained properly for this purpose. |
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| 1.7.2 |
Cleaning of gravel bed |
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| (i) |
General : |
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The filter bed was run for a lower rate of filtration during the trial runs, as explained above. Though the plant was operated intermittently as per daily water requirements, the headloss observations were taken at the beginning and at the
End of the daily filter run. Similarly turbidity of raw, settled and filtered water samples was noted daily. |
| (ii) |
Turbidity Observations : |
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From Table 4.2 it can be seen that the turbidity of filtered water was generally in the range of 0.5 to 1.0 JTU. Thus the turbidity removal was satisfactory. |
| (iii) |
Lengths of filter runs : |
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From the observations in Table 4.2 it can be seen that the maximum lengths of filter run was kept for 96 hours while the average filter run was kept for 60 hours. The filter bed was geneally washed after 15 days to avoid the algae growth on the bed even when the head losses were not reached to the maximum limit. |
| (iv) |
Headloss observations : |
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The headlosses were measured through the simple arrangement of plastic transparent tubes. The head losses in the filter bed were considerably low and were in thw range of 50 cm to 60 cm as against the allowable headloss of 2.0 m for the reasons given in the above para. |
| (v) |
Back wash observations : |
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The filter bed was washed by only hard wash method, once in a fortnight during the observations period,. The consumption of the wash water was less than 1% during the period of study. The expansion of the filter media during the back wash was between 10% to 20% and the bed was found to be effectively cleaned. |
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| 1.7.3 |
Bacteriological and chemical results |
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Table 4.3 shows the bacteriological results of the raw, settled, filtered and tap water samples. From these results it is seen that average bacterial reduction after pretreator was 73% and after filtration there was further reduction of 17%. It can be seen that the pretreator bed is not only effective in turbidity removal but also in bacterial removal to a considerable extent. |
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| 1.7.4 |
Maintenance Observations |
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From the plant scale observations as discussed in this chapter it is seen that Chandori treatment plant is giving satisfactory performance. Due to simplicity in day-to-day operations of the plant particularly in alum dosing, manual filter rate control, sludge draining, desludging operation, hard washing and TCL solution disinfection arrangements, one operator can maintain the plant efficiently. The operator with only primary education knowledge was trained at site for alum and TCL dosing, filter rate control, sludge draining and back wash operations. He can measure daily turbidities and sends water pumps to fill the elevated service reservoir, and also operates pure water pumps to fill the elevated service reservoir, and also operates the valves for the distribution of water to the village. Due to all these simple arrangements provided at Chandori plant, the maintenance of the Plant is trouble free, efficient and considerably cheaper as compared to the maintenance of a conventional treatment plant. |
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| 1.8 |
Special Advantages : |
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Chandori treatment plant has special advantages of two stage construction as explained below: |
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| (i) |
For low turbidity sources, the tube settlers in the pretreator unit and the crushed coconut shell media in the filetr unit can be omitted in the first stage. However, the same can be provided at a latter stage for better quality if found necessary or for increasing the plant capacity. |
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| (ii) |
For turbid water sources the plant can be designed for lower loading rate of 4500 lph/m2 in the first stage, for increasing the plant capacity or quality of filtered water the craushed coconut shell media can be provided for 40 cm thickness over 40 cm thickness of fine sand in the filter bed so as to convert into a dual media filter bed. |
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| 1.9 |
Guide lines for designing Chandori type treatment plant : |
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| (i) |
General : |
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The general design criteria for Chandori type simplified plant are given in Table 1.1. The guide lines for designing a new Chandori type plant are given below which will be
useful in practice. The plant is generally recommended for an individual village or small town. |
| (ii) |
Raw water quality : |
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This plant is generally recommended for medium turbidity water sources such as rivers, reservoirs, canals, etc. The average raw water turbidity may be in the range of 10 to 50 JTU while the maximum turbidity range may be 1000 o 2000 JTU. |
| (iii) |
Surface loading : |
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The capacity of such a plant with two chambers will generally be in the range of 0.5 to 2.0 mld. Both the pretreator and dual media filter chambers are designed for the surface loading in the range of 4000 to 8000 lph/m2. Lower surface loading is recommended when the raw water average and maximum turbidity ranges are likely on the higher side also. Lower surface loading is also recommended in the first stage design of this plant when the tube settlers in pretreator chamber and coconut shell media are not adopted. However, during the second stage, higher surface loading can be adopted by introducing tube settlers in the pretreator bed and covering rapid sand bed into dual media filter bed. |
| (iv) |
Pretreator chamber : |
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Generally the same size and surface loading are adopted for pretreator and the filter beds. However, lower surface loading in the range of 3000 to 5000 lph/m2 may be adopted when only rapid sand bed is to be provided after pretreator and raw water turbidity is lkely to be on the higher side. In such cases the pretreator bed can be adopted of larger size. The width of the chamber may be generally 2.0 m to 2.3 m and should not be adopted more than 2.50m. |
| (v) |
Other details : |
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For dual media filter beds, back wash, headloss, control arrange,emts, and construction details guide lines given at the end of Chapter 2 can be followed. |
| (vi) |
Hydraulic design and drawing : |
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These can be prepared on the guide lines of hydraulic design calculations (Table 4.1) and detailed drawing (fig. 4.3) enclosed at the end of this chapter. |
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Table 4.1
Hydraulic design calculations for Chandori filtration plant |
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| (1) |
General Proposal : |
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The capacity of the plant is 36,750 lph.
A new simplified high rate treatment plant is designed for this scheme for a capacity of 40,000 lph as per design given below. |
| (2) |
Mixing channel : |
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It is provided on the sides of pretreator unit and dual media filter unit as shown in Figure 4.3 |
| (3) |
Pretreator unit : |
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This is based on the combination of gravel bed flocculator and tube settler with flow in upward direction |
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| (i) |
Considering the supply rate of 40,000 lph |
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Adopt pretreator unit of size 4.0 m x 2.2 m. |
| (iii) |
Area= 8.80 m2 |
| (iv) |
Surface loading on the gravel bed |
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= |
40,000 |
= 4500 lph/m2 |
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8.8 |
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| (v) |
Volumetric loading on the pretreator |
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= |
40,000 |
= 3000 lph/m3 |
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8.8 x 1.5 |
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| (vi) |
Net surface loading on the open tube area considering 80% effective open area as per
Standard fabrication of tube modules |
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= |
40,000 |
= 5700 lph/m2 |
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0.8 x 8.8 |
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| (4) |
Filtration unit : |
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Provide one unit of rapid sand filter bed of size 4.0 m x 2.2 m |
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| (i) |
Area of filter bed = 8.8 m2 |
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Rate of filtration for above supply rate will be |
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= |
40,000 |
= 4500 lph/m2 |
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8.8 |
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Thus one unit of pretreator of size 4.0 m x 2.2 m is provided which will be followed by a rapid sand filter bed. Both units are designed for surface loading of 4500 lph/m2. The direction of flow in pretreator unit is upward as that of the upflow filters, while for rapid sand filter unit the flow direction is downward. Only hard back wash is provided for washing the filter bed. For pretreator unit gravity desludging operation will be generally adopted. Hard wash will be given occasionally.
Under drains: Central manifold 300 mm dia mild steel pipe and 50 mm dia Rigid pipes laterals have been provided with 6 mm perforations at 40 mm centres staggered and in 90o angles in the bottom of the laterals. |
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| Fig. 4.1 Flow diagram of Chandori Treatment Plant |
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| Table 4.2 : Observation on Chandori filtration plant |
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| Table 4.3 : Bacteriological results at Chandori filtration plant |
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