| Abstract : |
| The paper highlights the need for employing shallow depth sedimentation techniques for small capacity water treatment plants. |
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| Key words : |
| Tube settlers; small water treatment plants |
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| Introduction : |
The decade 1980 to 1990 was celebrated as the International Decade for Drinking Water Supply and Sanitation to provide safe drinking water and minimum sanitation facilities to all the population of the world. The majority of the population in the Developing countries is spread in the rural areas and is dependent on the surface water sources for drinking water supplies.
In India & other developing countries, the water purification facilities were mainly provided for the urban population until about fifteen to twenty years back. The Decade has helped to focus attention on the rural population. There is a growing awareness in the recent years to provide safe and clean drinking water to the rural masses also. |
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| Problems of small capacity plants: |
Adequate treatment has to be given to most of the surface water sources irrespective of the size of the community. The conventional technology which is represented by rapid sand gravity filters, slow sand filters, related pre-treatment units like sedimentation tanks, clariflocculators, etc. are either uneconomical or have technical limitations when applied to small capacity rural plants especially with turbid water sources. Therefore, there is an urgent need of simple and cheap appropriate water treatment technology for such rural water supply schemes.
Shallow depth sedimentation technique is one of such developing technologies which has enormous potential for application in small capacity plants, and is slowly emerging as an alternative to the conventional sedimentation unit processes over the last two to three decades across the world. It is found to be one of the most effective solid-liquid separation systems and offers some unique advantages over the conventional clarification processes. |
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| Horizontal flow sedimentation basin and Hazen�s interpretation |
The paths traced by discrete particles in a rectangular basin are shown in Fig.1. All particles having velocity Vs>VO will be removed in the basin. The portion of particles having velocity Vs <Vo will be removed in the ratio Vs/Vo. The efficiency of the basin is thus directly proportional to Vs/ (Q/A). In other words, the efficiency of the basin is solely a function of setting velocity of the particles, surface area of the basin and the rate of flow. It is independent of the detention time of the basin.
In order to increase settling efficiency, either Vs (settling velocity of the particle) or a (surface area of the basin) has to be increased. To increase settling velocity of the particles or flocs, some techniques like use of polyelectrolytes or ballasted flocs are available. These techniques are not suitable for mass application in the Developing Countries as it involves a lot of chemistry, continuous monitoring and skilled supervision of the process.
For these reasons, increasing surface area of the basin remains the only attractive and practical alternative to increase the settling efficiency.
From Fig. 1, it can be seen that the particles with velocity Vs<VO could be completely removed if trays were introduced at certain ideally fixed intervals. The grater the number of trays or smaller the spacing, more the particles with smaller velocity than VO could be arrested in the basin.
This theory was proposed by Hazen in 1904. It points out that doubling the surface area by inserting one horizontal tray would be roughly double the capacity of the basin. (Figs. 2A, 2B). The theory indicates that the use of very shallow setting basins enables detention time of the settling processes to be reduced to only a few minutes in contrast to the conventional design which use 1-4 hr detention. |
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| Earlier Attempts |
| Based on the Hazen�s theory, some new ideas like multi-storeged basins were introduced earlier. The increase in the capacity of the basin was obtained by introducing additional number of floors. It was found out that such basins were unwieldy and cumbersome from construction, operation and maintenance point of view. Moreover, there was a constructional restriction on the number of floors that could be constructed. |
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| Modern Practice |
In modern practices, the surface area of the basin is increased by parallel tubes (tube modules) or by parallel plates. These are generally fermed as lamellar modules. These modules serve two purposes, firstly to extend the total settling area and secondly to obtain a laminar flow (to reduce the turbulence component which hinders settling). Under conditions of laminar flow, Hazen�s theory is fully justified and a very high separation performance could be achieved.
The two basic shallow depth settling systems are shown in Figs 3A & 3B. In horizontal tube configuration, the separate sludge remains on the surface of the tubes and tubes tend to clog-up during the operation. The tubes are required to be cleaned by forcing the water in the reverse direction. For this drawback, this type of system has found limited applications essentially in very small capacity packaged plants.
In steeply inclined tubes, where angle of inclination to the horizontal plane is greater than 45 degrees, the separated solids do not accumulate but move down along the tube fig.4. A counter current of flow pattern of upward flowing clarified water and downward flowing stream of concentrated solids is established in a short time during the operation. The separated solids move continuously in the downward direction and are flushed out at the tube end. The continuous removal of sludge during the course of operation eliminates the need dislodging or back for flushing the tube. This is a single great advantage of the such settlers over the horizontal tube configuration. The steeply inclined tube settlers coupled with hydrostatic sludge draining, offer an enormous potential for application in designing compact low cost water treatment plants and for increasing the capacity of the existing settling basins. |
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| A typical cross-section of tube settling tank |
| The configuration shown in Fig. 5 is being widely adopted in India for small capacity water treatment plants. The depth of a typical tube settling tank can be roughly divided in four zones. |
i) Buffer Zone
ii) Tube Zone (modules)
iii) Collection zone
iv) Hoppers. |
| i) |
Buffer Zone: |
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This is a zone between top of the hoper to the bottom of the tube modules. The depth of this zone is limited from 1.3 m to 1.8 m. The function of this zone is to provide a cushion in between the incoming flocculated water and the tube modules. The turbulence and eddies generated at the inlet due to the velocity are dampened in this zone. This helps the tubes t change the flow pattern into a laminar range over a short distance to enhance separation.
This zone also provides opportunity for the incoming flocculated water to interact with continuously descending solids from tube modules. This aspect is particularly useful in case of raw water of low turbidity as the flocculation is enhanced in this zone. |
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| ii) |
Tube Zone: |
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The tube modules, supported on the side supports (corbels) provided on the side walls, cover the entire plan area of the tank. The size of tubes is 50mmx50mm, with length, 0.6m. The angle of inclination is 60 degrees. The tubes are of rigid PVC plastic and are black in colour to prevent the growth of algae in fair weather season. Each module consists of five-six layers of tubes pasted together. The depth of this zone (modules) is 0.5m. The clarified water moves in the upward direction and the separated solids in the downward direction. The sludge is eventually flushed out in the buffer zone. |
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| iii) |
Collection Zone: |
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This zone is extended from top of the tube modules up to the settled water collection pipes. The settled water collectors are normally provided 0.6m to 1.0m above the top of the modules. The spacing between the collectors closely approximates their distance above the modules. The collector pipes or troughs are provided with collection orifices on both sides. The function of the collector pipes is to collect settled water uniformly over the plan area. This ensures that uniform surface loading is imparted over the tube modules. This is one of the most important aspects for effective operation of this process. |
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| iv) |
Hoppers (Sludge Zone): |
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Hopper or multiple hopper system is provided at the bottom of the tank. The function of this zone is concentrate the sludge for effective desludging operation. The hoppers are provided with slope of 45 to 50 degrees. The effective hydrostatic head required for draining of sludge is found to be 3.0m. The dia of the drain pipe is 100 to 150 mm. It is provided from bottom of the hopper upto the hopper top and then is taken out. A sluice valve is provided on the outside for periodical draining of sludge. With this arrangement, it is possible to further dispose of the raw under gravity. The frequency of draining depends on the raw water turbidity and usually is once or twice a day. The draining of hopper is done during the normal operation of the plant. |
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| Application of tube settlers for small capacity plants |
The small capacity plants are generally constructed for villages, towns or for small communities, mainly in the rural or semi-urban areas. For mass application of the technology, the plants should have a low capital cost and operation and maintenance should be minimum. The plants should be simple in concept, design and construction to overcome the rural constraints. The various unit processes should involve minimum mechanical equipment and electrical consumption should be low. The operation & maintenance should be simple enough so that even unskilled persons should be able to operate and maintain the plants. The remedial measures, if required, should be possible locally.
The tube settling process qualities for most of the above mentioned qualities to be an appropriate technology for application in the small capacity plants. The area required for the tube settling tank is one third that of a clariflocculator and one fifth that of a sedimentation tank. Therefore, the capital cost involved in the construction is proportionately less. With hydrostatic draining arrangement, effective disposal of sludge is possible without any mechanical or electrical equipments. The tubes can be manufactured over small extruders. The modules can be easily fabricated with cutting and pasting of square PVC tubes. It can be transported over long distances without any appreciable packaging. The erection of the modules is done with the help of local labour and no specialized tools or tackles are required. Three or four persons can easily stand on a module for cleaning and maintenance of the tank. Thus the erection of the system is possible in the given rural infrastructure.
With proper selection of precending units (to tube settling tank) like a mixing weir (for alum solution), mixing channel, gravel bed/floc module type non-mechanical flocculators and succeeding units like rapid sand/dual media filters with declining rate controls, it is now possible to design compact non-mechanical type simplified water treatment plants for mass-application. By designing proper layouts, the plants can be made cost-effective and attractive. The first such simplified plant was constructed in India at Varangaon in 1977. Since then, a number of such plants have been constructed in the country.
It is found that the capital cost of these plants is about 70% to 80% that of conventional plants of same capacity. Operation and maintenance cost is much less. Three such case studies of representative plants are reported in Table 1.
Various hydraulic units of these plants are as follows: |
i) Stilling chamber and mixing weir
ii) Mixing channel
iii) Gravel bed / Module flocculator
iv) Tube settling tank
v) Dual media filter beds
vi) Control room
vii) Washwater tank and
viii) Alum and TCL dosing tanks
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| The simplified water treatment plants described above are completely non-mechanical type. |
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| Functions of various units of simplified plant |
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| Stilling chamber and mixing weir : |
| The function of this unit is to receive raw water from the main, to dampen the turbulence at inlet and to still the water before passing it over the weir. The mixing weir is of 0.6m. Height. Alum solution is added on the downstream side of the weir in the zone of maximum turbulence for instantaneous mixing with raw water. It is also used to measure the flow. |
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| Mixing channel : |
| It receives the water from mixing weir. Instead of conventional baffles, pipe pieces are embedded in the bed of the channel in staggered fashion for a thorough mixing of alum and raw water without causing any appreciable headloss. |
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| Gravel Bed flocculator : |
The coagulated water is passed through a bed of gravel in the downward direction. The gravel size is from 30mm to 70mm from top to bottom. The coagulated water while passing through the gravel receives multiple recontacts. This facilitates the building of floc. At the bottim, perforated header pipes are provided to collect floc uniformly and to curtail short-circuiting.
The modules flocculator utilizes rigid PVC floc modules instead of gravel. The drawback of gravel is that it tends to clogup during high turbidity period. Therefore, a separate back-wash arrangement is required to be provided. To avoid this maintenance problem, floc modules, fabricated out of PVC angle sections, to increase recontacts of solids are devised. The void space in the module is designed to avoid accumulation of solids.
The depth of gravel of floc modules is kept about 1.8m to 2.25m. detention period is 20 to 30 minutes. Normally a hopper is provided below the bed to drain out sludge which gets accumulated during the non-operational period. |
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| Tube settling tank : |
When a non-mechanical flocculation system lie gravel/module flocculator is adopted as a preceding unit, the header pipes from the flocculators are extended in the tube settling tank for uniform distribution of flocculated water. These distributor pipes are laid above the hpper and are perforated along the length. The flocculated water travels in the upward direction through the layer of tube modules. Sludge is separated and settles down to the hopper. The clarified water is collected by uniformly spaced pipes or troughs and then is conveyed to the filter beds.
This unit is the heart of the plant as 90% to 95% turbidity load is removed in this unit. The surface loading normally employed is 4 to 6 m/hr. The detention time of the tank is 35 to 45 min, and in the tubes it is 6 to 8 min. |
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| Dual Media Filter Bed : |
crushed coconut shell (1.0mm to 2.0mm, depth 0.4m) is the coarse media and quartzite sand (E.S.0.5mm, depth 0.4m) is the fine media with supporting graded gravel at the bottom. Rigid PVC laterals and mild steel manifold pipe are used for the underdrain system. Only harshwater with rate of 45m/hr is employed for back-washing. In case of more than one filter bed, constant head declining rate system is adopted for filter control. A rate control chamber with a weir is provided to measure the filtered water flow. The filtrate quantity is controlled by a master control valve. The maximum allowable headloss is 2.0m.
The rate of filtration adopted is from 6 m/hr to 10 m/hr. The geometry of the filter bed is similar to that of a rapid sand filter bed. |
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| Control Room : |
| A control room of adequate size is provided to house filter conrol piping, rate control chamber, and TCL powder dosing tanks, washwater pumpsets and a small office. |
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| Washwater Tanks : |
| A washwater tank of adequate capacity is provided overhead the control room building. If the elevated service reservoir is located nearby, then the washwater for filter is tapped from the same. The minimum effective head at the filter underdrain is required to be 8.0m. |
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| Alum and TCL Tanks : |
| The common coagulant used is the filter alum. In solution tanks, hand perated agitators are provided to keep solution at uniform strength. The stress is on the weaker does of 1% to 2%. The uniform flow of alum is ensured through a constant dosing tank with a 15 degree v notch. The alum tanks are located on the roof of the control room so as to feed the solution to raw water by gravity TCl solution dosing tanks are located in the control room. |
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| Performance of the Plants |
The performance of all the three plants, being operated and maintained by their respective organizations, is found to be satisfactory. The physical, chemical and bacteriological tests were carried out from time to time. The settled water turbidity was less than 15 to 20 NTU. The filtered water turbidity was less than 2 NTU. The average filter run was found out to be 48 to 72 hr. The treatment plant at Trimbakeshwar has catered to the need of lakhs of pilgrims during the �Kumbh-mela�, a Hindu religious festival. A strict vigilance on the water quality was kept by several govt. health agencies.
One point to be noted here is that the gravel bed flocculator should be adopted for low turbidity / low raw water. For highly turbid water, the bed tends to clogup and has to be washed periodically. Instead, a module flocculator with floc modules is the better choice for highly turbid water.
Recently, mechanical flocculators are also provided prior to tube settling units for large capacity plants. They should be adopted only where there is an assurance of uninterrupted power supply and skilled maintenance.
Conclusion
The tube settling system, as described earlier, is the heart of such simplified water treatment plants. It is possible to further improvise on the flocculation and filtration units upto a certain extent in the near future. From the feedback, received from the field, the technology is required to be updated from time to time. It has to be adapted to the new concepts and ideas to further simplify the operation and maintenance factors. What we have now is the �core� technology for small capacity plants. It has proved to be quite effective and robust to withstand for last twenty years in the Indian rural environment.
A number of such plants are being constructed today. For the effective implementation of the projects, there is need for the standardization and the engineers and effective operation and maintenance of the plants. The local bodies to which the plants are handed over, have neither financial resources nor the technical competence to maintain the plants. In the opinion of the Author, as far the Developing Countries are concerned, the small capacity rural plants should be operated and maintained by the relevant Govt. organizations for at least three to five years. |
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| Acknowledgements |
| It must be noted gratefully that Maharashtra Water Supply and Sewarage board has introduced and nursed this technology over a period of two decades. The Organisation has played a significant role to take this technology to the masses and to it�s present status. |
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