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Research Paper & Presentation
 
Doubling the plant capacity from 150 MLD to 300mld: A case study at Navi Mumbai Municipal Corporation (NMMC)

Author : Shirish J. Kardile, Consulting Engineer, Nasik Road,
Maharashtra, India.
 
  Introduction :
  The second water treatment plant has capacity of 300 MLD and is constructed in the year 2008. This treatment plant is further divided in to two streams (Stream A and B) of 150 MLD each. Each stream consist of Receiving Chamber (1 no), Parshall Flume (1no), Flash Mixer (1 no), Clariflocculators (3 no), Rapid sand gravity filter beds (20 no). The common facilities consist of a Chemical House, Chemical Store, dosing system, Chlorination arrangement and Washwater tank. The sludge from clarifiers and waste water from filters is discharged in to a near by nallah. There is a sever space constraint at site.

The treated water from both the plants is conveyed to the common Pure Water Sump and then is pumped to the Master Balancing Reservoir (M.B.R.) the average turbidity of raw water is less than 20 NTU for eight to ten months. The turbidity peak reaches up to 50 NTU during the monsoon.
   
   
  Proposal in Nut shell :
   
 
   
  In order to cater to the increasing water demand of the city of New Mumbai, it was proposed to augment the treated water capacity from the complex by further 150 mld in the year 2010 (Existing 300mld to 450 mld). There was severe space constraint at site. There was no space available to construct a new plant of 150 MLD capacities. Hence it was proposed to enhance the capacity of treatment plant by retrofitting the various units of existing treatment plant by appropriate new technology with minimum structural modifications.

It was proposed to increase the capacity of Stream B from Second plant by 150 MLD (6250 cum/hr) to 300 MLD (12500 cum/hr) by using the plant augmentation technology. The stream B consisted of 3 no of Clariflocculator and associated 20 no of Rapid sand gravity filter beds.

Accordingly it was proposed to double the capacity of existing clariflocculators by retrofitting these with tube settlers (150 MLD to 300 MLD). The inherent deficiencies in the existing sand filter beds made the conversion into dual media filters a complex process. Hence it was decided to construct new dual media filters of 150 mld capacity in the little space that was available at site.The balance flow of 150 mld was to be treated by existing filters.

The design parameters of various units of existing plant are affected by enhancing the flow, in this case by doubling the flow. These deficiencies are overcome either by manipulating the inherent redundancy in the system, or by introducing higher derivative technologies. Both these aspects are required to be complimented within the practical limitations of the hydraulics.

Detailed investigation of treatment units and facilities was carried out to verify process, hydraulic and structural parameters of the existing plant. From process point of view, there was enough redundancy in the various existing units from application of intended unit processes. The design of existing plant was found to be compatible for augmentation with minimum structural modifications.
   
  Details of Modifications :
  The detailed modifications to the treatment plant are discussed in three parts as follows.

1) Construction of New Inlet Works
2) Modifications to the Clariflocculators
3) Construction of new Dual Media Filter Beds
   
  1. Construction of New Inlet Works :
   
  The existing Receiving Chamber, Parshall Flume, Flash Mixer and Feed channels to clariflocculators were just barely sufficient to cater to the flow of 150 MLD. Secondly they were in structurally dilapidated condition. To increase the sizes to accommodate additional flow of 150 MLD would involve extensive modifications and shut-down of the plant for a long time.

Hence it was proposed to construct a new Inlet works for the capacity of 300 MLD. The existing Inlet Works would be discarded eventually. The space was available by the side of existing Receiving Chamber.

The new Inlet Works comprised of a Receiving Chamber (DT 1 min) and three no of Flash Mixers (DT 1 min each). The flow diversion baffles were incorporated at the inlet and outlet of Flash Mixer Units. Each Flash Mixer was designed for capacity of 100 MLD each and would feed the water to three no of clariflocculators the agitator mechanism consisted of a radial turbine, shaft, gearbox, pulley and electrical motor.

The raw water gravity main of diam 2050mm was extended from the existing Receiving Chamber up to the New Receiving Chamber. After the Flash Mixers three pipelines of diam 1200 mm each were provided to convey the water to the existing feed chambers of respective clariflocculators.

The increased flow would cause additional headloss in the feed pipe from Flash Mixer to Clariflocculator. The desired headloss was available in the existing plant. Therefore it was possible to keep the Top Water Level (TWL/FSL) of New Receiving Chamber same as per the existing plant. This was extremely important as the raw water flows by gravity from Morbe dam to the treatment plant.

The chemicals, Chlorine and Alum were proposed to be dosed in to the Flash Mixing Chamber. On the outlet side of Flash Mixers, stop-log type gates were provided for isolation of each clariflocculator. The raw water bypass arrangement was provided to convey raw water directly to the Filters from the Receiving Chamber.
   
  2. Modifications to existing Clariflocculators :
   
 
   
  Each of the existing Clariflocculator was originally designed for flow rate of 50 MLD (2083 cum/hr). In the augmented plant each was designed to handle flow rate of 100 MLD (4166 cum/hr).
   
  Brief description of existing Clariflocculator :
   
  The existing Clariflocculator had a rotating scrapper bridge driven by the end carriage drive. It was supported on the central RCC pier (shaft) over a turntable. It had four numbers of flocculators (slow agitators) suspended from the four radial arms in the flocculation zone. The flocculation zone partition wall was constructed in RCC. The clarified water collection launder was peripheral and was located outside the outer wall. The diameter of flocculation zone was 18.30m and that of the existing clariflocculator was 49.70m. The side water depth (SWD) was provided as 3.15m. For the designed flow of 50 MLD (2083cum/hr), flocculation zone had DT of 30 min. The clarification zone had Detention Time (DT) of 3.20hr and surface loading rate of 29.80cum/sqm/day (1240 lph/sqm).
   
  Modifications in the clarification zone :
   
  To enhance the surface area of clarification, modules manufactured out of 50mm x 120 mm Rigid PVC chevron tubes (length of each tube 600mm, angle of inclination 60 degrees) were introduced in the clarification zone. These were placed in the flow path in a form an annular ring adjacent to the outer RCC wall of the Clarifier. The inner wall of the annular ring was fabricated out of the mild steel (MS) sheets. The diam of annular partition was 39.80m. The area of this annular portion is 695 sqm, which would ensure the net surface loading up to 6000 lph/sqm. This modified or retrofitted version of Clariflocculator is termed as Klarisettler.

The modules were supported on forty eight numbers of radial trusses. The trusses were designed as cantilevers and were supported from the outer vertical wall of the existing structure. The MS partition wall too was supported from end of the trusses.

For uniform collection of the clarified water ninety six numbers of radial troughs were provided. The dead end of the trough was rested on the partition wall and the discharging end was fixed to the existing RCC collection launder. The length of each trough was 4.95m.

The scrapper arm of the Clariflocculator was supported from the vertical steel sections projecting from the rotating bridge. In order to the sweep sludge from the floor below the modules and to avoid obstruction to the modules, the vertical supports were suitably shifted. As a result the sweeper portion of the scrapper below the modules became a structural cantilever and was adequately strengthened.
   
  Modifications in the flocculation zone:
   
  At double the flow rate, the Detention Time (DT) of flocculation zone was reduced to 15 minutes. The existing flocculator drive was capable of generating velocity gradient (G) of 40-45 m/sec/m. The product GT for the existing system was 1.05 x 10^5. Without making changes to the drive, this value would have been half. Additional G was supplemented by providing additional paddle area to the agitator. The calculations confirmed that the drive unit was capable of sustaining G value up to 55-60 m/sec/m without requiring any changes to the gearbox or motor.

The existing outlet ports of the RCC central shaft were enlarged to maintain the velocity of incoming raw water up to 0.60m/sec. If required, a diffuser shield of MS plate was designed to contain the entry turbulence if any.
   
  Modifications to collection launder of clariflocculator :
   
  The RCC launder of existing clariflocculators was not capable of handling the double flow (4166 cum/hr). Therefore, a separate opening (with an outlet chamber) was provided to the launder. The location of the opening was diametrically opposite to that of the existing opening. Thus each semi-circular half of the channel catered to only half of the flow (2083 cum/hr) which is same as the original capacity.

The existing outlets from clarifier launder conveyed the water to existing sand filter beds. From new outlet chambers 1000mm dia MS pipelines were laid to New Filter House (Dual media). Enough headloss was available between New Filter Hose and modified clariflocculators to convey this discharge.
   
  3. Construction of New Dual Media Filter Beds :
   
 
   
  The existing Filter House had capacity of 150 MLD. The additional Filtration capacity required by the plant was 150 MLD. After the study of existing filter houses, it was concluded that the depth of filter bed was inadequate to convert them in to high rate dual media filters. The existing filter pipe gallery also required extensive modifications. Therefore it was proposed to construct a new Filter House in the available space to cater to additional flow of 150 MLD. There was just enough space at site to accommodate this unit.

The new dual media Filter House consisted of 10 filter beds. Each bed will had two sections, separated by a central gutter (gullet). The rate of Filtration was 10,000 lph / sqm. Five no of Filter beds were arranged on the each side of pipe gallery. The filters were �Open to sky� type. Only pipe gallery was covered with a roof slab, 3.50m above the upper level walkway.
The filters were designed on the operation logic of �Constant rate Filtration with influent splitting weirs. The additional 150 MLD water was distributed to filters by means of a suitable inlet channel. Each filter takes equal amount of water. There was no control provided on outlet side. The outlet weir of filter bed was provided 0.15m above the top of filter media. At the beginning of Filtration cycle, filters start operating at low water level. As the bed gets clogged, the water depth in filter bed increases ultimately reaching the height of 2.0m which is terminal headloss. The total height of Filter bed was 4.15m. Thus highly maintenance prone �Rate of Flow Controllers� was eliminated by this simple control arrangement.

The filter media consisted of graded supporting gravel, silica sand and crushed coconut shell. The size of gravel is 2 to 50mm and has depth of 0.60m. The silica sand has E.S. of 0.50mm and U.C. of 1.7. The depth of sand is 0.35m. The crushed coconut shell is 1 to 2mm having a depth of 0.40m. The filter underdrain consisted of concrete manifold and rigid PVC laterals. The cross troughs were provided in mild steel. The central disposal gutter had height of 2.1m.

After the filtration through the media, the filtered water was conveyed to pure water channel through the outlet chambers. From Pure water channel the filtered water was taken to pure water sump by means of 1500mm diam pipeline.

The total available headloss in the filter beds was 2.0m. The clogged filter beds are cleaned by means of air scour and hard (water) wash. The filter control (outlet) piping in the pipe gallery was provided in mild steel. All the valves were double flanged butterfly valves. The filter control valves are provided with electrical actuators for semi-automatic operation. The operation desk (kiosk) for each filter bed is located on the upper level walkway. The operation is manual by operating push buttons designated for each valve. Each kiosk has operating push buttons for Filter Inlet, Filter Outlet, Wash Water Inlet, Wash water Disposal, Air-scour isolation valve and air blower on-off. Option of manual override was provided for each valve by means of an extended rod and hand operating wheel.

Two numbers of air blowers (roots type) with motors and accessories were be provided for air- scour operation. The air-scour rate was 36m/hr. The wash water is conveyed from the existing back wash tank (of existing filter beds). A pipeline of diam 500mm was laid from new filter house to existing back wash tank for the purpose. Each section of filter bed is back washed separately. The hard-wash rate is 50 m/hr.
   
  Chemical House :
  No major modifications were suggested in the Chemical house. The existing dosing tanks were capable of doubling the dosing rate. As Alum and PAC tank flows are by gravity, the elevations of the tanks with respect to the changes in elevations of the dosing points were verified and were found to be capable of gravity dosing. Rerouting and lying of new dosing lines up to the point of dosing was done under this works. New chlorinators were provided for additional flow of 150 MLD. Both pre and post chlorination practices are followed at the plant.
   
  Salient features :
 
1. The cost of this modification works is Rs.180 million (18 crores, 2010), which is 50% to 60% of the cost of new plant of same capacity.
   
2. This project of augmentation was mainly possible due to unavailability of space at the site. No alternate site was possible due to logistics of incoming raw water main and pure water distribution arrangement.
   
3. For operation and maintenance, no additional manpower is required.
   
4. The electrical power consumption is only being marginally higher.
   
5. The contract involved overhauling of existing machinery, so the augmented plant would function smoothly. Its longevity was increased.
   
6. The time required for completing the job was one year. The clariflocculators were bypassed one at a time during that period and the internal modifications were completed. The construction of inlet works, and New Filter House were independent activities and were carried out simultaneously.
   
7. During performance trial for raw water turbidity of 20 NTU, Clarified (settled) water turbidity varied in the range of 2-5 NTU. The filtered water turbidity was found to be in the range of 0.1 to 0.2 NTU.
   
8. on the imaginary scale of 1 to 10 of "degree of difficulty" in execution of such projects (assuming degree of difficulty 1, for construction of new plant), the difficulty factor for this project was 3 to 4. It will be a good idea to provide sufficient selective leniency in the design parameters while constructing the new plants, so that in the future they can be augmented economically.
   
  Conclusions :
  All over the world the plant augmentation technology is gaining the ground fast for the obvious reasons mentioned above. This is probably for the first time, that the Clariflocculators were augmented on such a large scale. These techniques have a huge potential for application in this country. The resource crunch by default will be a single largest factor influencing in favor of such cost effective and efficient techniques in the near future. Six months have been completed after the commissioning of this project and the augmented plant is running very satisfactorily.
   
  References :
 
1. New Concepts in water purification : Culp & Culp
       
2. Handbook of public water systems : Culp, Wesner, Culp
       
3. Simple methods in water purification : J.N.Kardile
       
4. Water treatment plant design : AWWA Handbook
       
5. Manual on water supply & treatment : Ed 1999, C.P.H.E.E.O.
       
6. Papers published in the Journals of IWWA, IWA, and AWWA.    
       
7. Manual on Augmentation and up-gradation Of existing treatment plants (M 5) : IWWA
 
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