| Present status of technology
in small capacity plants in Maharashtra: |
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| Abstract : |
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| At the beginning of the decade 1990-2000, the number of small capacity water treatment plants being constructed in the state increased rapidly. In order to do correct execution and time-bound mass implementation of the plants, the exercise of standardization was taken up. The combination of unit processes was rationalized and uniformity was brought to the specifications. Documents were prepared for dissemination of information about relatively new concepts and practices. The exercise was done to provide guidance to engineers, contractors and operators. This consolidation phase of the developmental work done in the past has resulted in improvement of overall quality of the finished product. Today, in 2011 there are more than 400 plants constructed in Maharashtra. |
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| Introduction : |
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| The Govt. of Maharashtra has undertaken a massive program over the last few years to cover the entire population of rural and urban area with potable water. Most of the drinking water supply schemes in the state have surface water as a source. Irrespective of the size of the community, surface water sources need to be treated throughout the year to provide safe and clean drinking water to the people. The small capacity water supply schemes in the state are classified as having the capacities from 0.25 mld (million liters per day) to 5.0 mld. The communities to which these serve are a village, a group of villages and small cities up to Taluka level places. |
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| Need for Standardization : |
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At the beginning of this decade (1990-2000) the number of small capacity plants being constructed in the state increased rapidly. It was soon realized that speedy and correct execution of small water treatment plants was a prerequisite for delivering quality treated water in a time-bound period.
Since early eighties, the efforts were done by the government of Maharshtra through statutary organization - Maharashtra Water Supply & Sewerage Board (now Maharashtra Jeevan Pradhikaran) to develop small, compact and cost effective plants. These plants were based on high rate unit processes. The mechanical components or the moving parts were kept minimum to simplify the operation and maintenance. The emphasis was to develop small plants which could be managed in the prevailing rural infrastructure. These were known as simplified or unconventional plants.
A field survey of some of the existing plants had indicated that a few of these were not functioning up to the desired potential. As a result, the water quality and quantity was not found up to the satisfaction from some of the plants.
This was primarily to due to the following factors:
1. The simplified or unconventional water treatment technology was comprised of flexible combinations of different unit processes. The lack of rationalized selection criteria and uniform specifications had resulted in unsatisfactory design and execution in some cases.
2. Unlike in the developed countries, the concept of prefabricated units had not gained a foothold in Maharashtra. In-situ construction in concrete demanded that grass-root level engineers of the organization knew the basic concepts and intricacies of the relatively new technologies and concepts thoroughly. This was found to be missing due to lack of dissemination of relevant information.
3. Operation and maintenance of the plant had remained far from desirable. Inability of the local self governing bodies to generate adequate funds remained (and still remains) a major obstacle. As a result, the local bodies which manage the plants did not have the financial as well as technical resources to operate the schemes satisfactorily.
The developmental work done in the decades 1970-90 which had resulted in about 50-60 plants, by far had remained in the isolated pockets. It was essential to strengthen the link between the developmental work done in the past and requirements for correct implementation of plants on a large scale in the future. On a mass scale, the engineers, contractors and operators needed guidance for proper implementation of the projects. It was realized that standardization of small capacity plants and dissemination of the relevant information to the concerned people was urgently required. |
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| Basic guidelines followed for the Standardization : |
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| The standardization of small capacity plants and preparation of the relevant documents were based on the following guidelines. |
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| 1) The selection of unit processes should be in accordance and based on the experiences derived from the Unconventional plants constructed by the M.J.P. in the past (1970-1990). |
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| 2) The unit process and their combination to be employed should be appropriate under the current techno-social-economic scenario prevailing presently in the state of Maharashtra. |
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| 3) The unit processes should be simple and the plants should be with minimum mechanical (moving) parts. The unit processes should be robust as well as flexible to comply with most of the raw waters sources in the state. |
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| 4) The design guidelines should be broadly as per the recommendation of C.P.H.E.E.O. Manual, Edition 1991 but should incorporate additional details based on the quality of water and operational experiences gained in the past. |
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| 5) Irrespective of the specialized technical expertise or experience, the concerned technical staff of the M.J.P. should be able to execute the work of small capacity plant plants correctly to produce the desired water quality. |
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| 6) The design of units as such should ensure maximum reliability and enough redundancy for absolutely safe operation. The arrangement of various units should be based on a modular concept. |
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| 7) The executing agency (contractors) should be encouraged for creativity within the guidelines of the DTP ( Detailed Tender Papers ) prepared by the M.J.P. (For example, the freedom of making better layouts than the suggested, hydraulic and structural aspects should be within the scope of executing agency) |
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| 8) While designing the hydraulic structures and ancillary units, due consideration should be given to the convenience of operator and his movements. |
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| 9) The structure of the water treatment plant and other ancillary works should be architecturally pleasing so that the campus as a whole exerts the feeling of an importance in the community. |
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| 10) The scope of the work and estimated costs should be correlated in details for preparation of Detailed Schedule Rates (D.S.R.) and revision in the estimated costs in future should be a simple exercise related to general escalation of material prices. |
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| Salient features of various units of the plant : |
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| The following unit processes were selected on basis of guidelines described above. |
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| 1. |
Aeration Fountain (Cascade Aerator): |
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The surface water in Maharashtra does not generally contain either iron or manganese. This unit was basically incorporated to have an esthetic appeal to the water treatment complex. It was also felt that the cascade aerator will aid in eliminating the foul odor and compensate for the depleted oxygen if any. |
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| 2. |
Stilling Chamber, Mixing Weir, Mixing Basin: |
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After aeration, a mixing weir (rapid mix unit) is provided, which is preceded by a small stilling chamber. The stilling chamber provides dampening effect to water, which facilitates accurate measurement of flow over the same weir.
The rapid mix weir as the name suggests, is a flash mix unit. The theory indicates that a free-fall of 0.30m to 0.40m over the weir for the plant capacities in consideration generates velocity gradient to the tune of 800 to 1000 sec-1. A small detention chamber on the downstream side of weir (2 to 5 sec) is enough to effectively dessipitate the energy for efficient mixing of coagulants with raw water.
The coagulant (Alum) diffuser pipe is normally provided just on the downstream side of weir for instantaneous mixing of coagulant and raw water. |
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| 3. |
Mechanical Flocculation Tank : |
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To provide effective flocculation to coagulated water prior to settling, electrically operated mechanical slow agitator arrangement were preferred in the flocculation tank. The detention time provided for the process is 25-30 minutes.
The circumstances in Maharashtra today, compared to two decades back have changed a lot. The power supply is now available in the villages to a large extent. The maintenance required for the drive unit is now possible, if not at the village level but at least at the Taluka places.
The mechanical flocculation unit is employed as a "once through flocculation" with a downward flow. The drive unit for the agitator is mounted above the water level on a walkway. The paddles are fixed to the horizontal supports extended from the vertical central shaft.
The flexibility in operation of mechanical flocculator ( Different velocity gradients ) can ensure more reliability for different qualities of raw water sources, provided the equipment is designed accordingly and maintained satisfactorily. |
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| 4. |
Tube Settling Tank : |
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The tube modules enhance the surface area of settling, many times to the plan area of tank. The hydraulics also ensures a laminar flow through the tubes which enhances the separation efficiency of the settling. The counter-current principle of operation greatly simplifies the engineering.
The M.J.P. has gained an enormous experience on design and operation of tube settlers with 50mm x 50mm square tubes. The tube settling tank is the heart of the unconventional plants as any clarification unit is. The solids removal efficiency is recorded up to 90-95% for low turbidity and up to 99.5% for moderately high turbidity. The surface loading adopted for the settlers is up to 5000 lph/sqm. The detention time is 35-40 minutes.
The distribution of flocculated water below the modules ( in the buffer zone ) and uniformity in collection of clarified water ( in the collection zone above the modules ) are two important parameters which should be taken care of properly in the design.
The sludge is required to be drained out of the hopper or hoppers (which is provided at the bottom of the tank) periodically. The operational response of these tube settlers with respect to the fluctuations in raw water turbidity is related to the changes in coagulant dosages and the sludge draining frequency. The unit process is found to be stable under these conditions. |
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| 5. |
Rapid sand gravity filter beds without rate controllers : |
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It has been observed in a number of conventional plants in state that most of the problems related to filter performance occur due to two reasons. |
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1. Failure of rate of flow control mechanism/equipment due to lack of maintenance. |
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2. De-watering of beds due to the design and operational inadequacies. |
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Both the deficiencies of the conventional design are eliminated by selecting constant rate filtration with split flow weirs at inlet. Filter beds are provided with sufficient depth to accommodate headloss of 2 to 2.5m over top of the sand. The de-watering of bed is prevented by raising the level of outlet weir over sand top by 0.15m. There is no control provided over the filter outlet valves. At the beginning of the filtration cycle the water level is minimum over the bed. At the end of the cycle the water reaches the level of inlet split flow weirs.
Another advantage of this control system is that the headloss is directly seen by the operator in terms of water level over bed. Hence the headloss indicator is really not required. Similarly the need for flow indicator for individual bed is also eliminated. The rate of filtration adopted is conservative and is 5000 lph/sqm.
The filter beds are open to sky. The filter underdrain is of mild steel manifold pipe and rigid PVC laterals. For the plant capacities under consideration one or two filter beds are sufficient. The area of individual bed is restricted to 10 to 12 sqm so as to limit the size of piping as well as the capacity of wash-water tank. The beds are designed to be backwashed with hard wash and or air-scour. |
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| 6. |
Overhead wash-water tank : |
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The hard-wash option by providing low-head pumps for backwash for small beds may be found tempting to reduce the capital cost. But it cannot be generalized as the operation and maintenance factors are going to remain a suspect till some time.
If the wash-water tank is constructed overhead the chemical house or as a separate structure, it contributes to 15 to 20% cost of the plant as a whole. Therefore if the wash-water tank is also designed as an elevated service reservoir for the community, then there could be a appreciable cost saving. |
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| 7. |
Ancillary Structures : |
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A double storied structure is provided adjacent to the hydraulic structures with total area of 40 to 60 sqm. The ground floor consists of Filter operating room, a small storeroom for chemicals and bleaching powder dosing tank for post-chlorination.
The First floor consists of chemical dosing room, a small laboratory and space for the office. Two numbers of Alum solution tanks are provided (12 hour rating each) for gravity dosing of coagulant. A manually operated constant flow tank with a 15 degree V-notch is provided to ensure uniform dose from the tanks. For pre-chlorination, a bleaching powder dosing tank is also included in the chemical room.
A few simple laboratory equipments are included to help the operators for the routine operation and control. They include a turbidity rod, a turbidity meter, pH Indicator, Chloroscope etc. |
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| 8. |
General : |
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Most of the water supply schemes in the state involve two stage pumping i.e. raw water as well as pure water. Normally raw water is pumped up to the Aeration Fountain. From then on the ever dependable gravity flow is utilized for the reliable plant operation. A typical hydraulic flow diagram is shown in Fig.1.
The plants are manually operated. All valves are cast iron sluice valves of non-rising spindles and piping is either in cast iron or mild steel. The sizes of open channels are sufficient enough to ensure that they are accessible for the maintenance.
The waste water and sludge from the water treatment plant are disposed off to land irrigation or to a water body.
The layout of the plants is based on the modular concept and the plants are compact in nature. In-situ construction with concrete grade M 200 is prescribed for water retaining structure.
The entire range of plants under consideration is divided into three separate layout configurations.
Catagory 1: 0.50 m.l.d. to 1.25 m.l.d. ..... Fig 2.
Catagory 2 : 1.50 m.l.d. to 2.50 m.l.d. ..... Fig 3.
Catagory 3 : 3.00 m.l.d. to 5.00 m.l.d. ..... Fig 4, Fig 5.
All structures of the plant are inter-linked with walkways or cat-walks. The operator has to travel a minimum distance for the routine operation and maintenance.
For storage of pure water a sump of capacity equivalent to one hour plant flow rate is provided near the plant. The area of pump-house is according to the specific requirements of pumps. |
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| Documents for dissemination of information : |
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Document No.1: Standard D.T.P. |
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As a first step of proper dissemination of information about the standardization effort, a common set of detailed tender papers (D.T.P.) as described earlier was prepared.
To bring the entire state at par, this standard DTP with uniform specifications was circulated to all the Circle offices in the state.
A number of illustrative sketches are included in the detailed specifications to offer guidance to the concerned staff. |
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Document No.2 : Model designs |
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In the second document, model designs for hydraulics and process for the small capacity plants were prepared at the interval of 0.50 m.l.d. with smallest capacity being 0.50 m.l.d. This document was prepared to provide the guidance to the engineers from the M.J.P. for asserting the correctness of the contractor's design. |
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Document No.3: Estimation of costs |
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During the 1980-90 (initial phase of small plants implementation), it was observed that disparity in the scope of the work and the cost put to tender had delayed the administrative approval to the schemes at many places. ( If the bided cost by the contractor is more than a certain percentage of the cost put to the tender, the acceptance of the tender gets considerably delayed due to the administrative approval procedures).
Thus the third document was prepared to bring uniformity to the cost of these plants, Bill of material for mechanical and civil quantities for all the plants from 0.50m.l.d. to 5.00 m.l.d. was prepared in details.
The estimates were prepared on the basis of Detailed Schedule Rates (D.S.R.) of Pune Circle, with 1994-1995 rates as a base. This is of great importance as the cost revisions in the future have now become independent of Bill of Material and are solely linked to Pune Zone 1994-95 D.S.R. rates as a datum.
The D.S.R. is an important document for speedy and correct estimations of costs required for the administrative approval at the Govt. level. The D.S.R. rates thus must reflect the justifiable cost of the works, for speedy execution of projects. |
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| Performance : |
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At most of the plants, it was observed that the raw water turbidity was maximum at the beginning of monsoon season ( 100 to 200 NTU). This figure reduces gradually till the end of the monsoon to less tan 50 NTU. For rest of the year the turbidity is less than 25 to 30 NTU.
For all practical purposes two parameters are monitored on a periodical basis. The residual chlorine level in the pure water sump and the turbidity. The samples for bacteriological testing are sent at a random interval to the District Public Health laboratories.
Under satisfactory operating conditions, the filtered water turbidity is observed to be in the range of 1 to 2 NTU. The settled water turbidity is controlled up to 15 NTU. Presently only alum (Aluminium Sulphate) is used as a coagulant.
Due to the use of stored water as a raw water source the algae and color imparted by the vegetation is becoming an increasing menace in the lean flow period. This is especially true where the balancing tanks are used as a raw water source.
The training of the plant operators on operation and maintenance aspects of the plant will play an important part in the future in achieving the desired level of performance. |
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| Future Development and possible trends : |
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These plants and the incorporated unit processes comply to 90-95% of raw water sources available in Maharashtra. In some cases where raw water is of high quality, only direct filtration preceded by coagulation basin could be sufficient. In case of highly silty water (like Tapti, near Bhusawal), pre-settling tank would be required to trap the excessive silt prior to the treatment plant. So the next logical step will be to monitor and classify the source quality over a time and then accordingly optimize the treatment scheme. This will avoid "under or over" designing of plants from process point of view.
To reduce bulk handling of chemicals and to improve upon the finished water quality, alum lumps are likely to be replaced by coagulants like polyaluminium chloride (PAC). Chlorination by bleaching powder is not very efficient due to the "availability factor" of chlorine in the powder. Some alternatives like on- site generation of chlorine by electrolysis of common salt can be adopted as it is non hazardous and is a clean technology.
Already the trends have set in for prefabricated packaged plants for very small capacities, up to 1.0 mld. In the developed countries, these plants normally are classified as packaged, skid mounted and/or portable plants. Due to high cost of labor for in-situ working, completely standardized modular plants are available even up to 10 to 15 mld capacities. These plants are shop fabricated and are available in partly or fully assembled conditions. Normally the effort is to spend minimum time at site for cost reduction. The shop fabrication ensures high accuracy and good quality of material. Various types of materials like steel, prefabricated concrete panels, plastics (normally corrosion resistant materials) are used for the containers or vessels. In India some manufacturers have started using mild steel as a container for plants. The steel plates are protected with anticorrosive paints or FRP lining. The plants are capable of delivering high quality water.
The changing economic scenario of liberalization coupled with the virtual awakening of this potentially huge market, may attract many firms from across the world their plants will be naturally of proprietary nature. Their popularity will depend on their cost and reliability under Indian operating conditions. |
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| Conclusion : |
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The decision of the Govt. of Maharashtra to implement the water supply schemes on the permanent assured sources has helped indirectly in standardization of the plants to some extent. As the rivers in Maharashtra are not perineal, most of the water supply scheme have to be necessarily on the stored water sources i.e. either dams or reservoirs or balancing tanks. Most of the rivers in Maharashtra have good alkalinity (75 to 150 ppm) and PH (6.8 to 8.0). The detention in the raw water reservoirs has eliminated the turbidity peaks. Therefore at least as far as the turbidity is concerned there are uniform patterns for many of the sources. Most of these sources exhibit a good treatability with units and processes described earlier.
The "packaging" of small plants was done by way of standardization, as any manufacturer across the world would have done to optimize his product. The drinking water treatment is a highly conservative field. The new concepts and technologies which are incorporated will require protection till the engineers digest the basic concepts and update their knowledge. The minimum expectation from the standardization exercise is to ensure that errors are not committed inadvertently till then and in the intermediate future. The overall improvement in the finished product (treated water quality) is a long time goal.
This standardization exercise is not an end in itself. The exercise will be required to be reviewed; technology and methodology will be required to be upgraded regularly. Feedback from the field is essential. However it certainly paves a way for systematic, meaningful and disciplined mass application. The other states in India can be benefited by the Maharashtra example if they are to undertake such program. |
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| Acknowledgements : |
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| The author will like to acknowledge with thanks the efforts and contribution of all senior and junior officers of the Maharashtra Jeevan Pradhikaran who have participated in this exercise. Thanks are also due to the contractors and plant operators on their feed back and suggestions. |
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| References : |
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| 1. The "Manual on Water Supply and Treatment "Edition 1991, published by the Ministry of Urban Development, New Delhi |
| 2. Data collected on performance of small capacity plants in the state of Maharashtra. |
| 3. New Concepts in Water Purification : G.R.Culp & R.L.Culp |
| 4. Surface water treatment for communities in Developing Countries: C.R.Schulz & D.A.Okun |
| 5. Simple methods in Water Purification: J.N.Kardile |
| 6. Basic Water Treatment: George Smethurst |
| 7. Various papers published in the journals of IWWA, AWWA, IWSA |
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