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Research Paper & Presentation

Doubling the capacity of existing Clariflocculators : A case study at TISCO, Jamshedpur

Abstract :

Enhancement in the original capacity of most of the clarification or settling units can be achieved by retrofitting these with tube settlers. Settler modules are placed in the clarification zone at an appropriate location and depth. Augmentation of Clariflocculator (with rotating bridge) primarily becomes complicated due to interference of vertical supports of the scrapper rake with the module layer. However recently at a large steel manufacturing units in India, all technical hurdles were overcome to enhance the capacity of existing Clariflocculators from 67.50mld to 144mld with minimum structural modifications. The work was executed in six months and at half the cost of new plant of same capacity. During high turbidity period from July to September 2000, the upgraded unit performed consistently to produce the desired clarified water quality.

Introduction :

One of India's largest private sector steel production company, TISCO in the state of Jharkhand has a water clarification plant, with river Suvarnarekha as a raw water source. The existing plant was capable of treating 15.00 mgd flow ( million gallons per day) roughly about 67.50 mld ( million litres per day ). The treatment units consisted of a Receiving Chamber, Elevated channel with Parshall Flume, Flash mixing tank, Clariflocculators (2 x 50%), a downstream Parshall Flume and a clarified water tank. ( Fig.1 & Fig.2). The chemical dosing facility consisted of Alum, Lime and PAC tanks. The post-chlorination arrangement was provided for the treatment plant. The average turbidity of river is less than 50 NTU for eight to nine months. The turbidity reaches up to 300-600 NTU during the monsoon. Existing treatment facilities were constructed in the year 1990-91.

As a result of expansion program of the mills and due to the requirement for their captive power plant, additional demand for clarified additional water of about 17.00 mgd (76.50 mld) was envisaged by month of April 2000. Thus the total supply requirement became 32.00 mgd (144.00 mld). Competitive bids were invited for a new clarification plant of capacity 17.00 mgd. The owner's requirement for clarified water turbidity was 10 to 15 NTU. Alternative Offers were also called to augment the existing units by new and advanced technologies with minimum structural modifications. After evaluation of the offers with respect to the technical merits and the price, the authorities accepted the proposal to augment the existing Clariflocculators with tube settlers. The actual work of modification was started in the December 1999. The plant with nearly double it's original capacity was commissioned in April 2000.

Proposal in nut-shell :

Approach : The design parameters of various units of existing plant are affected by enhancing the flow, in this case by nearly 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. Principally the two important unit processes, flocculation and clarification were analyzed in the following way.

a) The detention time in the flocculation tank for the existing flow rate was 40 min. By doubling the flow rate it would reduce to 20min. But there was a scope to maneuver the velocity gradient ( G value) by modifying the agitator to achieve the desirable GT value. Therefore effective flocculation at enhanced capacity was possible.

b) In the clarification zone of Clariflocculator, enough plan area was available to introduce tube modules with the desired loading rate (5000 to 6000 lph/sqm) with double the flow rate. The depth of 4.0m was adequate for incorporation of tube modules. The radial flow path in the clarification zone made settler modules extremely compatible with the existing equipment.

Details of Modifications : ( Fig.3 & Fig.4)

The detailed modifications to the treatment plant are discussed in three parts as follows.

1) Modifications to units upstream of Clariflocculators

2) Modifications to the Clariflocculators

3) Modifications to downstream units of Clariflocculator

Modifications to units upstream of clariflocculators:

The hydraulic elevations in the existing inlet works were affected due to the additional losses generated in the interconnecting piping between flashmixer and Clariflocculators. This was the direct implication of doubling the flow. The rise in the water elevation was more than 1.0m. To accommodate the extra depth, the complex nature of work ruled out the possibility of structural modifications to existing inlet works. Therefore it was decided to discard the existing inlet works and to construct a new inlet works.

The new inlet works was designed for flow rate of 6000 cum/hr (144 mld). A Receiving Chamber of 1.0 min detention time (DT) with bottom entry was constructed to receive the flow from the raw water mains. The receiving chamber was provided to dissipate the velocity head and to dampen the turbulence of raw water.

At the outlet to the receiving chamber, a sharp crested weir was installed as a rapid mix device. It has also a dual use of measuring flow. On the downstream side of weir a mixing basin with 5 sec DT was provided for effective mixing of coagulants with raw water. The weir is designed with a free fall of 0.40 to 0.50m. This would generate velocity gradients of 800 to 1000 m/sec/m. The coagulant diffuser pipes were provided immediately on the downstream side of the weir.

The mixing basin (a channel of width 5.0m) was then connected to the existing flashmixer unit. The flashmixing chamber was no more required and was converted as a distribution chamber (to the 2 nos of Clariflocculators).

The existing machinery of flashmixing chamber was dismantled. The top slab of the unit was dismantled and height of the sidewalls was increased to make a proper connection with the new mixing basin. A baffle wall was constructed in this distribution chamber to alter the flow path from flow distribution point of view.

The existing flashmixer (now a distribution chamber) had two nos of isolation/flow distribution gates discharging the flow to the clarifier inlet chambers. The elevation of the gates was raised suitably as a result of new water elevations.

No modifications were proposed to the inlet pipe of Clariflocculators. The diameter of each pipe is 700mm RCC P2 class. The pipes were inspected from inside by isolating the system. The hydraulic analysis of these buried pipes was done as pressure pipes functioning as 'U' tubes.

As a result of these modifications the water elevation (FSL) in new receiving chamber was 1.65m higher than the existing Chamber. The raw water pumps were capable of taking this head without sacrificing the flow rate.

2.	Modifications to existing Clariflocculators :

	Each of the existing Clariflocculator was originally designed for flow rate of 1425 cum/hr ( 50% of the total flow 2850 cum/hr). In the augmented plant each was designed to handle flow rate of 3000 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 inside the outer wall. The diameter of flocculation zone was 17.50m and that of the existing clariflocculator was 40.00m. The side water depth (SWD) was provided as 4.00m. For the designed flow of1400cum/hr, flocculation zone had DT of 40 min. The clarification zone had DT of 3.00hr and surface loading rate of 33.50cum/sqm/day (1400 lph/sqm).

	Modifications in the clarification zone :

	To enhance the surface area, modules manufactured out of 50mm x 50 mm Rigid PVC tubes (length of each tube 600mm, angle of inclination 60 degrees) were introduced in the clarification zone of the each Clariflocculator. 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 area of this annular portion is 560 sqm, which would ensure the net surface loading up to 5600 lph/sqm. This modified or retrofitted version of Clariflocculator is termed as Klarisettler. The modules were supported on thirty two 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 sixty-four number 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 scrapper arm of the Clariflocculator is 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 DT of flocculation zone was reduced to 20 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.

3.	Modifications to downstream units of Clariflocculator :

	In the existing system, the flow path from both the clariflocculators was led to a common channel with Parshall Flume. The flume was provided to measure the flow of outgoing clarified water. The channel had a outlet chamber, from which the clarified water was conveyed to the storage tank by a 900mm dia MS pipeline. The water elevation difference between that of Clariflocculator and clarified water tank was 1.85m. The distance between the tank and clarifiers is about 125 to 150m. The hydraulic analysis had shown that the existing arrangement was not capable of handling the double flow. It was proposed to retain the existing arrangement as it is. In addition two new MS pipelines of 700mm diameter each were laid in parallel to convey the additional flow. The RCC launder of existing clariflocculator was not capable of handling the double flow (3000 cum/hr). Therefore, a separate opening ( with a outlet chamber ) was provided to the launder . The location of the opening is diametrically opposite to that of the existing opening. Thus each semi-circular half of the channel caters to only half of the flow (1500 cum/hr) which almost same as the original capacity. The above said 700mm dia MS pipelines were laid from the new outlet chambers up to the clarified water storage tank. In all three pipelines, one of diam 900mm and two of diam 700mm convey the entire flow of 6000cum/hr to the storage tank without reducing the storage capacity. All the pipelines discharge into a flow measuring chamber at the inlet of the storage tank . A sharp crested weir was provided for measurement of the total clarified water flow.

	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 laying of new dosing lines up to the point of dosing was done under this works.

	Sludge drainage and disposal arrangement :

	There were two sludge storage tanks provided in the existing plant. The sludge was stored and the supernatant water was allowed to be overflowed back to the river over a period of time. The dried sludge is disposed off for land filling. No modifications or alterations were suggested to this arrangement.

	Hydraulics : ( Fig.2 & Fig.4)

	The total headloss through the existing plant was (136.60m-134.00m) 2.60m. The total headloss through the augmented plant is (138.25m-134.00m) 4.25m. The entire additional headloss is on the upstream side of Clariflocculator. The inlet pipe to Clariflocculator itself generates about 0.60m of additional headloss. Laying of additional inlet pipeline (buried) to Clariflocculator would have been impractical if not an impossible task. It would have added a certain degree of difficulty in the execution of the works. The increase in head (of the upstream units) is relatively a less price to pay to overcome this hurdle. While on the downstream side of Clariflocculator, the parallel streams (pipelines) approach was adopted as the headloss between Clariflocculator and storage tank was fixed. The flow was required to be conveyed to the storage tank without reducing the tank capacity.

Salient features :

The cost of this modification works is Rs.16 million (1.60 crores), which is 50% to 60% of the cost of new plant of same capacity. In addition there is a saving of precious land. For operation and maintenance, no additional manpower is required. The electrical power consumption is only be marginally higher. The contract involved overhauling of existing machinery, so the augmented plant could function smoothly.

The time required for completing the job was six months. December to February being a fair weather season, the Clariflocculators were bypassed one at a time during that period and the internal modifications were completed. The construction of inlet works, laying of additional pipelines were independent activities and were carried out simultaneously.

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. This low difficulty factor was mainly due to the healthy and robust design of the existing plant. 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.

Start-up, commissioning and Performance of the plant :

a]	The shutdown period of the plant, when raw water was directly bypassed to the clarified water storage tank, was limited to 4 days to enable structural connection of the new inlet works to the distribution chamber (existing flashmixer). This was planned and executed during April 2000 when raw water turbidity was low (less than the required outlet water turbidity of 20 NTU).

b]	Individual Klarisettlers were operated up to flow capacity of 3000 m3/hr (originally 1450 m3/hr) each. The entire system was also operated at flow rate of 6000 m3/hr for a few days. The head loss in the system was found to be as per the design predictions.

c]	During monsoon (July to Sept 2000) the average raw water turbidity was found to be 100-200 NTU. It had peaked upto 400-600 NTU for a short duration. The raw water turbidity patterns were varying and were directly dependant on the rain received in the upper catchment area of the river Suvarnarekha. The pH was about 7.5; the total alkalinity was above 100 ppm.

d]	Jar Tests were conducted for a period of 7 days to finalize the chemical dosages. As per the owner's requirement, the primary coagulant used was Poly Aluminium Chloride (PAC). Alternatively, combination of PAC with PE (Catfloc-T) was tried to study the possible savings in chemicals.

e]	The PAC dosages were found to be 1/3rd to � that of Alum dosages. As the volume of handling was less, the logistics of control during varying & high turbidity was better.
f]	For raw water turbidity of 100-600 NTU, clarified water turbidity up to 5 to 10 NTU was achieved. The optimum PAC dosage for this range was found to be 10 to 25 ppm. The turbidity samples were collected on an hourly basis to know the behavior of the augmented system.

g]	The system response to the varying turbidity was observed to be much quicker, compared to the original clariflocculators as per the plant operators.

h]	De-sludging of Klarisettlers was done on a regular basis of 6 to 8 hour's interval, depending on the raw water turbidity. The plant behavior was found to be extremely stable.

i]	During October 2000, when the raw water turbidity was very low (less than 50 NTU), slight increase in PAC dosage was observed. This may be attributed to the changed composition of suspended solids from predominantly inorganic nature in monsoon. For low turbidity of 30 NTU, clarified water turbidity of 5-6 NTU was achieved.

Conclusion :

Till date in India, circular tanks, hopper bottom tanks, rectangular tanks, Clarifiers with fixed bridge are augmented with these techniques successfully. 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 and abroad. 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. M/s Thermax Ltd., Pune were the principle contractors for the TISCO project. Ten years have been completed after the completion 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,AWWA.

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