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| Research Paper & Presentation |
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| A new simplified treatment plant at Songad |
| S.J. Kardile, Consulting engineer, K Consultation, Nasik Road, Maharashtra |
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| Introduction: |
| The United Nations organization has decided to cover 100 percent Urban and rural population of the world for providing protected water supply schemes during the international decade for drinking water supply and sanitation.
The conventional methods of providing slow sand, rapid sand and pressure filters are rather costly for construction and also difficult for maintenance particularly in rural areas. The skilled labor and supervision are required for the construction as well as maintenance of the conventional filter plants. Due to these difficulties many towns and villages particularly in the developing countries cannot afford to adopt the same for their water supply schemes. An intensive search is therefore going all over the world for the development of simple and cheap water treatment plants using new technology. The Songad Treatment Plant in Gujarat which has been constructed and put into operation recently has been designed to meet the demands of this challenging problem. The purpose of this paper is to explain the new features which have been adopted in the design of this unconventional treatment plant. |
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| Sanctioned Scheme: |
| The regional rural water supply scheme for five villages near Varangaon was sanctioned at an estimated cost of Rs. 35, 84,740/- in the year 1973. The populations to be served in the immediate and ultimate stages, were 25,620/- and 36,000/- souls respectively. The scheme is designed for the daily water supply of 4.20 MLD. To be supplied in 18 hours in the ultimate stage. Raw water is pumped from River Tapi, the source of supply to this new treatment plant, whence the treated water is pumped to elevated service reservoirs for distribution to the villages.
The conventional type of treatment plant as proposed in the sanctioned scheme was estimated to cost Rs. 6, 50,000/- and included flash mixer, settling tanks, and pressure filters. The actual cost of construction for the conventional treatment plant of this capacity during the year 1974 to 1976 was Rs. 8,00,000/- to Rs. 10,00000/- based on the tendered costs. With the adoption of the new design, the actual cost of construction for the Varangaon Treatment Plant has come down to Rs. 4,00,000/-n. Thus the saving was more than 50 per cent as compared to the cost of a conventional plant of the same capacity. |
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| The new approach: |
| The new design adopted for this treatment plant includes units such as missing channel, PVC module flocculator, tube settling tank and dual-media filter bed, followed by disinfection. The other features provided at this plant are, control room in which T.C.L. solution tanks and pure water pumping machinery are provided. The alum solution and dosing tanks are constructed on the first floor over the control room, where a small laboratory with normal testing equipments is provided. The wash-water tank of 52,000 lit capacities has been provided on the top of the chemical room. With all these arrangements the plant has become very compact, the total area provided being about 30 percent of the area normally required for the construction of a conventional plant of the same capacity. |
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| Design Aspects: |
| The treatment plant is designed for the hourly flow of 75,000/- litres. The hours of working will be eight and sixteen in the immediate and ultimate stages. The raw water from the Tapi River is lifted using submersible pumps, through the intake well to the treatment works situated at a distance of about one k.m. from the river bank near the village. |
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| (i) |
Mixing Channel: |
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The raw water is taken in the small stilling chamber at the start of the mixing channel, where the required alum dose is added. The mixing channel is located on the side of the control room as shown in the Fig.1. The detention period in the mixing channel is one minute and its bed slope is 10 cm. |
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| (ii) |
Pre-module flocculation unit: |
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The hopper chambers have 45o slopes on all sides for the collection and removal of the sludge from bottoms through 75 mm. dia sludge draining pipes. The gravel is spread on the M.S. flat screen placed on the top of the hoppers (Fig. 1). The gravel bed can be cleaned with the raw water by gravity, actual flushing being done for about 5 minutes through 200 mm dia outlet pipe provided just below the gravel bed. During such cleaning operation the outlet valves in the tube settling tanks are closed, to avoid the back flow of water from the settling tanks. |
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| (iii) |
Tube settling tanks: |
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There are two units of tube settling tanks, size 3 x 6m. Each with 3m. Depths over the top of hoppers are provided with 45o slope for removal of sludge through 75 mm dia sludge draining pipes by hydrostatic pressure. A layer of rigid PVC square tubes, size 50 x 50 mm opening and 0.6 m. in height, are provided at 60o angle covering all the surface area. The top level of the tubes is kept one metre below the F.S.L. in the tanks. The P.V.C. square tubes were first fabricated in the module forms of size 3m x 0.5 x 0.5m, and these modules were then lowered and placed on the angle irons fixed to the sides for supporting the modules. The modules being strong enough to resist the necessary bending moment additional bottom supports were not found necessary. The surface loading on the open surface area of the tubes is about 6600 lit/sqm/Hr., while the total detention period in the tube settling tanks is 35 min.
The raw water after passing through the gravel-bed is introduced through four 150 mm dia perforated pipes fixed at the bottom of the tube settling tanks, to distribute the flow uniformity. The water after passing through the tubes in vertical direction is collected through five 100 mm dia P.V.C. collecting pipes with side perforations in the central collection channel. The settled water from the contral channel is then taken on the filter beds. |
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| (iv) |
Dual-media filter beds: |
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There are three units of dual-media filter beds of size 4 x 2.2m. each, being designed for the filtration rate of 6600 lit/sqm./Hr. (i.e. 140 galls/sqft./Hr.). However, the plant observations given in this paper ae based only on two filter beds when run at a higher filtration rate of 10,000 lit/sqm/Hr. (i.e. 210 galls/sqft./Hr.). The filter media consists of 40cm. of crushed coconut shell of size 1 to 2 mm over the fine sand bed of 50 cm. thick of effective size 0.5 mm and uniformity co-efficient of 1.5. The supporting graded gravel-bed, 50 cm. thick is laid over the under-drainage system. The under-drainage system consists of M.S. manifold pipe, 375 mm dia, with P.V.C. perforated laterals, 50 mm dia placed at 20 cms. Centres on both sides. The side-gutters are provided along all the sides of the filter beds, for wash water collection and further draining out through two 300 mm dia outlet pipes with valves. |
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| (a) |
Filter control: |
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The outlet pipes are of 300 mm dia. And only one outlet chamber is provided to give declining rate controlling effect, with only one control valve before the chamber. Rectangular notch at the centre of the chamber controls the flow over the weir. The chamber is covered by glass shutters. Arrangement for chlorination is made on the side of the control chamber. The required chlorine dose is administered in the control chamber after rectangular weir to permit effective mmixing in the filtered water. Wash water and pure water pumps are accommodated in the Control room. |
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| (b) |
Back Wash: |
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Only hard wash is given to these filter beds for about 10 to 12 min. for cleaning the beds, supply being drawn from wash water tank of 75,000 lit. capacity installed on the top of the chemical house. The back wash pipe line is connected to the filter outlet pipe for allowing back wash to any one of the filter beds which is normally done in serial order. |
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| Construction: |
| The plant was constructed by the Executive Engineer, Environmental Engineering Division, Jalgaon, during the years 1975-77 and trial runs were started in April 1977. The net period of construction was about one year. As shown in the Fig. 1. Most of the works are in masonry with only R.C.C. structure for the wash water tank. The work done through a contractor employing local labours. |
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| (i) |
Fabrication of Tube Modules: |
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A special mention has to be made about the fabrication of the tube modules which were adopted probably for the first time in India. As far as the the author is aware, the P.V.C. square tubes of 1.5 mm thickness, and of 50 x 50 mm clear opening size were not manufactured in India before 1975. However special efforts were made to get these manufactured. The modules were then fabricated at the site and were installed in the tube settling tanks. The cost of the tube modules worked out to about Rs. 2,000/- per sq.m. of the plan area. |
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| Plant Observations |
| The plant scale observations from the day-to-day data register regarding the turbidity, head losses, rate of filtration etc. are given in the Table No. 1. The bacteriological results are given in the Table No. 2. The Varangaon Plant is specially designed for the treatment of highly turbid water and from the results obtained it will be seen that this plant has given satisfactory results. |
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| (i) |
Turbidity Observations: |
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Maximum turbidity was seen up to 4000 units from the Tapi River source during the rainy season of 1977. Such a high turbidity was successfully treated to give average filtered water turbidity between 0.5 to 1 unit. The settled water turbidity was generally below 10 units. |
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| (ii) |
Day-to-Day Observations: |
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The Table No. 1 shows day-to-day observations on this plant from 5.9.77 to 17.10.77, which includes nine filter runs. Only two dual-media filter beds were run at a rate of 10,000 lit/sqm/Hr. during the study. The filter beds were washed either, when the head loss reached 2 m. or the filtered water turbidity exceeded one unit. The average length of the filter run was about 40 hours, even with the intermittent working. It is presumed that when the three dual-media filter beds will be operated on the declining rate controlling system, the length of the filter runs may be normally from 60 to 70 hours. |
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| (iii) |
Bacteriological Observations: |
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Table No. 2 shows the bacteriological observations for the period from 28.877 to 17.10.77. The average removal of coliform load during pretreatmentwas about 79 per cent, while during chlorination it was 8 per cent. Thus the removal of bacterial load was found satisfactory. |
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| Performance of the units |
| As stated earlier, this plant consists of two units of tube settling tanks followed by three units of dual-media filter beds. The gravel bed flocculation units and tube settling tanks are the special features of this treatment plant as both these units may have been provided for the first time in India. Therefore, the performances of these two units are discussed below in details. |
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| (i) |
Gravel Bed Flocculators: |
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The author first introduced the gravel bed prefilter unit in the filter at Ramtek (Maharashtra) where the flow is in the upward direction through the gravel bed, where it works as a complete pretreatment unit for the treatment of low turbidity water. The gravel bed unit in the Varagaon plant is adopted as a flocculation unit, where the flow is in the downward direction. The floc size increase and consolidates as the water passes from the top to the bottom through the gravel and flocculated water is then introduced in the tube settling tanks. From the turbidity reduction in the settled water as shown in the Table No. 1, it will be seen that the flocculation process in gravel beds is satisfactory. The actual plant scale results of these gravel beds are far superior to the pilot plant study on a perpex model of the same height, 10x10 cm. size. |
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| (a) |
Cleaning of gravel beds: |
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There is possibility of clogging of the gravel beds, particularly when the sizes of the gravels are of uneven and small or when the raw water contains heavy slit during the floods. From the actual results, it was observed that there was some clogging in the beds, but bed can be cleaned by routine flushing operation and to create adequate velocity for the removal of sludge, a 200 mm dia washout pipe is provided below the gravel bed, with a valve outside. The raw water from the mixing channel can be taken on the bed without alum dose for cleaning the beds. |
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| (b) |
Back Wash: |
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In routine course, gravity flushing is adequate. However, for effective back wash, a pipe from the wash water tank was connected to the 200 mm dia. Washout pipe. Suitable wash water collection and outlet arrangements are provided. |
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| (ii) |
Tube settling tanks: |
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The plant scale results (Table No. 1) show satisfactory performance in the removal of turbidity load, which signify that the gravel bed flocculators as well as tube settling tanks are working in perfect combination. When the flocculated water coming out of the gravel beds passes through the tubes at 60o angle, the floc particles get further consolidated due to large surface contact of tubes and heavy floc particles come in the downward direction to form sludge. In this process, the zone below the tubes also becomes a very active sludge blanket zone, as the heavy floc particles flowing in the upward direction, further accelerate the flocculation process in this zone. The natural sludge blanket thus formed in the tube settlers= is not required to be controlled as in the case of a conventional sludge blanket tank.
Due to the accelerated action of removal of lfoc particles in the tube settling tabks, the surface loading can be adopted in much higher range of 6,000 to 15,000 lit./sqm./Hr. through the tube opening area. The rate adopted at Varangaon is about 6,600 lit/sqm/Hrs. as against the normal of 1200 lit/sqm/Hr. adopted for a conventional rectangular settling tank. Due to the possibility of adopting high surface loading, the detention period is about 30 min. as compared to three hours generally provided in a conventional settling tank. These tanks are also therefore called as "shallow depth vertical flow settling tanks." |
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| (iii) |
Dual Media Filter Beds: |
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The three dual media filter beds are designed for 6,600 lit/sqm/Hr. However, the performance of these filter beds was found satisfactory, even at a much higher rate of 10,000 lit/sqm/Hr. when only two filter beds were run during the study. The lower filter rate of 6,600 lit/sqm/Hr. has been adopted at this Regional Rural Water Supply Scheme, as the operating personnel are not generally trained. Considering the possibility of lower performance of the pre-treatment works, mainly due to inadequate alum dosing etc. the quality of end-product should be acceptable, even at higher settled water turbidity. |
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| Future scope for Varangaon Plant |
Considering the successful performance of the new treatment plant at VArangaon, the Author feels that this type of plant is likely to be adopted in rural and semi rural areas not only in our country, but also in some other developing countries. The process is not only useful for the small capacity plants, but with the change of mechanical flocculation, the tube settling tanks and dual media filters, can also be adopted for plants of big capacities. Further, these techniques will be useful for the augmentation of the existing conventional treatment units.
The proposed techniques should save more than 50 per cent in capital costs as compared to the costs of construction of new conventional plants, while for augmentation of the existing plants, there can be further saving in capital and maintenance costs. The construction of this type of plant is simple due to elimination of mechanical arrangements in mixing, flocculation and settling tanks. The chief Engineer (ENE) and Joint Secretary to Govt. of Maharashtra, Urban Development and Public Health Department has approved number of such proposals and some of these are under construction in the Maharashtra State. |
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| Conclusions |
| (i) |
From the plant scale results of the treatment plant at Varangaon, it is observed that the new techniques adopted should solve some of the important problems in providing simple and cheap water treatment plants for rural and semi rural areas in the developing countries. |
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| (ii) |
Gravel bed flocculation chamber should prove a promising solution to replace the mechanical flocculation unit for the treatment of turbid water sources for small capacity plants. |
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| (iii) |
The tube settling tank (with the use of rigid PVC square tubes of size 50 x 50 mm openings) fives satisfactory performance in the removal of the high turbidity after the gravel bed flocculation chamber, at much higher surface loading as compared to the conventional settling tanks. |
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| (iv) |
Dual-media filter beds with the adoption of crushed coconut shell media over the fine sand bed, shows satisfactory performance for higher filtration rate of 10,000 lit/sqm/Hr. after the pretreatment with gravel bed flocculation and tube settling tank. |
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| (v) |
(v) The reduction in capital cost by adoption of the VArangaon type treatment plant may be more than 50 per cent as compared to the cost of conventional plant of the same capacity. |
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| Acknowledgements |
The author is grateful to the Chief Engineer, (ENE) and Joint Secretary to the Govt. of Maharashtra State Urban Development and Public Health Deptt. For giving the opportunity to design and to put into practice this new treatment plant for the Regional Rural Water Supply Scheme near Varangaon. The Author is also grateful to the Chief Engineer (W.R.) and Joint Secretary to Govt. Irrigation Deptt. And the Director, Maharashtra Engineering Research Institute, Nasik, for giving the encouragement in the adoption of new techniques in the field of filtration. The Author is also grateful to the Superintending Engineer, Environmental Engineering Circle, Jalgaon, under whom the execution of this plant was done.
Further, the Author expresses his thanks to the Executive Engineer. Environmental Engineering Division. Jalgaon, Deputy Engineer, and Junior Engineers, under him and Mr. V.R. Chaudhari, Mr. R.N. Pathak, and Mr. D.D. Ozarkar from Environmental Engineering Research Division, M.E.R.I. Nasik, who have actually participated in the construction of the plant. |
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| Special Notes |
| (i) |
This paper is a part of the Ph.D. study the Author is doing presently on the "Development of simple and cheap water treatment methods for small capacity plants for rural areas and small communities." |
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| (ii) |
The use of crushed coconut shell media in filtration process has been patented inIndia vide patent No. 134979. |
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| References |
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| 1. |
Prof. Ir. L. Huisman, "Trends in the design, construction and operation of filter plants", Special Subject No. 8 of Proceedings of the International Water Supply Congress Vienna (1969). |
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| 2. |
Dr. K.J. Ives, "Problems in filtration", Journal of the Indian Water Works Association, Vol. II No. 4 (1974). |
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| 3. |
G.L. Culp and P.L. Culp, "New concepts in water purification", (1974). |
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| 4. |
J.N. Kardile, "Crushed coconut shell as a new filter media for dual and multilayered filters", Journal of the Indian Water works association, Vol. IV, No. 1 (1972). |
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| 5. |
J.N. Kardile, "Water treatment problems of rural areas", Journal of the Indian Water works association, vol. V, No. 1 (1973). |
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| 6. |
J.N. Kardile, "An unconventional 0.5 mgd Treatment Plant for Ramtek Town, Nagpur", Journal of the Indian Water Works Association, Vol. VI, No. 1 (1974). |
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| 7. |
J.N. Kardile, "One year observations on Filter Plant at Ramtek near Nagpur", Jour. Of Indian Water Works Assn. Vol. VIII, No. 1 (1976). |
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