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Research Paper & Presentation

Improvement in the existing settling tank At Pimplner Village

Dr. J.N. Kardile

Introduction:

At many existing water treatment plants adequate facilities are not provided, with the result the quality of the treated water is not satisfactory. Particularly the turbidity of filtered water is more than the acceptable limits and in rainy season, it is considerably high. At such times, people make complaints about the quality of water, but the operating authorities are unable to give satisfactory water quality due to the inadequate treatment facilities in such plants. This, improvements in the existing water treatment plants is an urgent necessity at many villages and towns in India and may be in the other developing countries also.

A case of improvements in the existing water treatment plants at Pimpalner Village in Dhule District of Maharashtra is discussed in this paper. This is a gravity scheme and the capacity of this old treatment plant was provided previously for water treatment plant was 2.25 MLD. Only settling tank with a mixing channel was provided previously for water treatment. The detention period was three hours. During modification work, the capacity of the plant was augmented to 2.88 MLD for the estimated population of 24,000 souls at the rate of 40 liters per capita. Even though the source of water supply is a dam outlet, the maximum turbidity goes above 1000 JTU at many times during the rainy season. Hence it was not possible to give desired quality of water supply to the villagers. Particularly during the rainy season, when the alum dose was inadequate, the turbidity of settled water was very high (>100 JTU), which was supplied to the villagers. Further, due to unskilled operators employed by the Village Panchayat, the maintenance was poor. So, there was great need of upgrading of the old plant to give desired quality of drinking water to the villagers.

Design aspects of modifications :

There are two units in the existing settling tank and size of each unit is 3.35 m x 13.40 m, with water depth of 3.0 m over the top of the bottom hoppers. During modification work of the settling tank, two units of non-mechanical flocculators, two units of tube settlers and two units of dual media filter beds followed by a control room were provided in the settling tank itself. Hydraulic design calculations are given in Table 1. These modifications were carried out during 1990-91 with simple low cost methods as discussed in the paper.

Table 1

Hydraulic Design Calculations

Capacity :

Existing Capacity : 2.25 MLD
Proposed Capacity: 1, 20,000 lit/hour
120 cum/Hr or 2.88 MLD.

Mixing arrangements:

(i)	Stilling chamber is provided at the inlet end of existing channel with removable RCC precast slab of 30 cm in height.

(ii)	Capacity of the channel is increased by removing existing baffles and providing 100 mm dia A.C. Pipe pieces 20 cm length in vertical positions.

Non-mechanical Flocculation Chambers : 2 Nos.

(i)

Size of each Unit : 3.35 X 3.0 m x 3.0 m height.

(ii)

Detention period for

3.5m average width :	2x 3.0x 3.5 x 3.0
	120
	= 0.515 hr or 32 min

(iii)

Inlet and Outlet pipes : 4 No. of 200 mm dia.

(iv)

Perforated partitions: 4 nos.

Size: 3.50 m (Ave) x 3.0 m depth.

Tube settling Tanks: 2 Nos.

(i)

Size adopted : 3.0 m x 3.5 m x 3.0 m depth

(ii)

Surface area : 2 x 3.0 x 3.5 = 21.0 sqm

(iii)

Surface loading :	1,20,000
	21
	= 5700 lph/m2

(iv)

Detention time:	= 2 x 3.0 x 3.5 x 3.0 x 60
	120
	= 32 minutes.

(v)

Tube settlers:

Rigid PVC tube modules of size 3.0m x 0.27m x 0.5m in height to cover all surface area of the tanks are provided. M.S. angles of size 75 mm x 75 mm on both sides are fixed to RCC partitions walls to support tube modules. Collector pipes of 200 mm dia PVC pipes with side perforations are provided.

(vi)

Sludge drains:

Existing 100 mm dia sludge drain pipes from hopper bottoms both for flocculation chambers and tube settling tanks are adequate.

Dual media filter beds: 2 beds

(i)

Size of each bed: 3.50m x 2.30m = 8sqm.

(ii)

Area of filter beds: 2 x 8=16.0 sqm.

(iii)

Rate of filtration:	1,20,000 = 7500 lph/m2
	16

Control Room:

This is provided by dismantling central masonry wall in the last two units of settling tank and providing RCC partition walls between filter beds and control room. size of room is 9.0m x 3m. Control chamber size: 1.0m x 0.6m x 1.0m.

Other arrangements:

Details of backwash, head loss, filter media, under drainage, chemical dosing arrangements and various levels are as described in the paper.

Mixing Channel:

In the existing mixing channel baffles were provided for the mixing of alum dose. It was necessary to reduce head loss in the mixing channel due to baffles for maintaining the same top water level (TWL) in the settling tank for the increased flow. For this purpose, the baffles were removed and A.C. pipe pieces of 20 cm length were embedded in vertical position in staggered form to improve the mixing action after addition of alum dose. A stilling chamber is constructed at the inlet side of the existing mixing channel by providing 30 cm high RCC precast slab. This RCC slab can be removed when required for lowest water level in the canal to get required flow by gravity. This will facilitate to let out the required discharge over the measuring weir from the old new gravity mains connected to the stilling chamber from the intake well near the canal. The alum solution dose is added just on the downstream of the weir, where the turbulence is maximum for quick dispersion of alum dose for effective mixing action. RCC roof is constructed on the portion of the control room and two alum solution and dosing tanks are provided to give required dose.

Non-mechanical Flocculation Chambers:

There are two non-mechanical flocculation units of size 3.0 x 3.33 m each with 3.0 m water depth over hopper top. The existing hopper slopes at the bottom were corrected to 45% with brickbats in cement motor. Smooth cement plaster is provided to hopper sides to remove sludge effectively through the existing 100 mm dia sludge drain pipes and valves by hydrostatic pressure. The detention time in the flocculation units is about 32 min. For non-mechanical type flocculation purpose, four perforated partitions are fabricated with 90 mm dia half-cut rigid PVC pipes by fixing with nuts and bolts with 50mm gaps in M.S. fame work with angle at the top and flats on three sides.

At the inlet end, four numbers of 200mm dia rigid PVC perforated pipes are provided in each unit for uniform flow of water throughout the vertical section of the unit. Side perforations of 25mm dia are provided at 10mm centres on both sides with 50mm dia bottom hole for sludge removal. At the top inlet, pipes are connected to mixing channel by 200 mm are submerged below TWL in the flocculation unit. At the outlet end similar arrangement is provided. However, 200 mm dia PVC bends are provided at the bottom with pipe outlets in the tube settling tanks at about 30cm above the top of hopper level. The top level of both the inlet and outlet pipes is kept about 10 cm above the TWL in the flocculation chambers. This arrangement will not only create uniform flow through the flocculation chambers, but will also improve the flocculation action.RCC partition wall is provided between the flocculator and tube settlers.

Tube settling tanks:

There are two units of tube settling tanks of size 3.0m x 2.35m each with 3.0m water depth over the top of hopper level. The existing hopper slopes were corrected to 45o as explained for flocculation chambers. RCC partition wall is constructed between the tube settling tanks and filter beds. In the tube settlers, rigid PVC square tubes of 50mm x 50mm size and 60mm in lengths are provided at 60o angles in opposite directions in the form of tube modules. The size of the modules is 3.0 m x 0.27 m x 05. M in height and tubes are fixed with PVC solvent cement. All the surface area of the tanks is covered by lph/m2, while detention period is about 32 min. The raw water after passing through the flocculators and then through the tube settlers in the upward direction is collected through three numbers of 200 mm dia at 15 cm centres are provided to collectors for uniform collection of water. The settled water is then introduced in the side gutters of the filter beds with 200 mm dia pipe outlets through the partition wall.

Dual media filter beds:

There are two units of dual media filter beds of size 3.50 m x 2.30 m each. The filters are designed for the filtration rate of 7500 lph/m2. The filter media consists of 40 cm thick crushed coconut shell of 1 to 2 mm size and is provided over fine sand media having effective size of about 0.5 mm and uniformity coefficient between 1.5 to 1.6 for 40 cm depth.
The supporting graded gravel bed is provided for 60 cm depth over the bottom under drainage system. The under-drainage system consists of a central mild steel circular manifold of 300 mm dia with 50 mm dia rigid PVC pipe laterals 6 kg. Tested at 200 mm centres on both sides. The laterals have 6 mm dia perforations at 40 mm centres in staggered positions at 90o angle at the bottom side of laterals. Two side gutters of 40cm x 40cm size are provided on both sides of filter beds for wash water collection and disposal through 300 mm dia pipe line and valves. The side gutters of 40cm x 40cm size are provided on both sides of the filter beds for wash water collection and disposal through 300 mm dia pipe line and valves. The side gutters are constructed by brick masonry work on both sides. The bottom hoppers in the filter bed compartments and in the control room are filled up with rubble masonry from the dismantled rubble. Cement concrete is provided for 150 mm thick for the bottom slab over the rubble masonry. RCC partition wall of 250 mm thick is provided between the filter beds and control room. RCC wall sides are anchored in the side masonry walls so as to have proof joints.

Control room:

A control room of about 9.0m x 3.0m size is provided adjoining the filter beds for providing control chamber and bleaching powder solution and dosing plastic tanks. Head loss measuring arrangements consisting of two sets of two glass tubes of 12 mm dia and showing difference in water levels in the TWL of filter bed and before the control valve are provided on the RCC wall of filter side. The control room is covered partially by RCC roof. One staircase is provided to enter into the control room from the outside. A master control valve is provided over the rectangular weir. This acts as declining rate control of flow. Chlorination dose is given in the outlet side in the control chamber. Outlet pipe of 300mm dia from the chamber is taken out through the end masonry wall to connect the existing pipe line to introduce filtered water in the existing pure water sump.

Backwash arrangements:

Only hard wash is given for about 8 to 10 minutes from the backwash tank of 50,000 lit capacity constructed near the plant. Backwash pipe line and filter outlet piping and valves area of 200 mm dia with M.S. piping. Backwash is given so as to create 30% to 40% expansion in the top filter media for effective cleaning of filter beds. Washed water outlet line of 300 mm dia is connected to the existing drain line for sludge disposal.

Cost Aspects:

The cost of the modification work is about Rs. 2, 80,000/- on the basis of tendered cost. This is considerably low in view of augmented capacity (2.88 MLD) of filtered water supply.

Plant Performance:

The plant is maintained by the Village Panchayat. The maximum raw water turbidity is about 1000 JTU while average turbidity is between 20 to 30 JTU. The settled water turbidity is maintained between 5 to 10 JTU by adjusting alum dose. The filtered water turbidity is maintained below 1.0 JTU with occasional increase up to 2.0 JTU. Filters are required to be washed after 40 to 50 hours of operation. The operators have been given adequate training for effective operation of the plant. It will be desirable to maintain or supervise the rural W.T. plants by State W.S. & S Boards of Z.P. for efficient maintenance.

Suggestions:

It was possible to carry out the modification of the existing settling tank into a complete filtration plant due to the adoption of the new low cost methods. This has not only improved the filtered water quality within acceptable limits but also augmented the capacity of the plant. As the designed loadings are in medium range, it is possible to overload the plant by about 50% when required as it is a gravity scheme, without affecting the quality of the filtered water. For high turbid water sources, it will be advisable to adopt PVC angle floc modules for 2.0 m depth in place of perforated partition as adopted in this plant. All these new techniques are now explained in revised “Manual on Water Supply and Treatment” (3rd Edition-March 1991) published by the Ministry of Urban Development of Govt. of India.

There are hundreds of existing rectangular settling tanks in this country and the performance of many tanks is not seen satisfactory. The main reasons are alum dosing and mixing methods, non-provision of effective flocculation arrangements, poor sludge removing facilities from settling tanks, unnecessary partition walls, defective inlet & outlet arrangements, etc. The performance of the existing settling tanks can be improved by these low cost techniques. Further, it is possible to augment the capacity of the settling tank and also to incorporate filter beds in some cases as explained in this paper. This will not only improve filtered water quality, but will save large amounts when the plants are augmented due to the adoption of new low cost techniques. Even though these improvements are carried out in rural plant, the new techniques are equally useful for adoption in the small and medium capacity urban treatment plants.

Conclusions:

The aim of this paper is to bring forward the new low cost methods for modification of existing rectangular settling tanks and also to convert the settling tanks into complete filtration plants in some cases as explained in this paper. It will be possible to improve the quality of settled water and also filtered water by adoption of these simple low cost techniques. In many cases it will be possible to augment the capacities of the existing settling tanks with improvement in the quality. There will be considerable saving by adoption of these low cost techniques when the existing plans are required to be augmented for increased capacities.
There is great need for adoption of these new low cost methods for hundreds of villages and small towns for the treatment of turbid water sources, so as to give potable drinking water supply where it is needed. Their costs of construction and maintenance are considerably lower than the costs for conventional treatment plants.
The views expressed in the paper are entirely personal.

References:

1.	Kardile, J.N. Report on “Improvements to Pimplner water supply scheme” prepared for execution for ENE Divn Dhule of Mah. W.S. & S. Board (Nov. 1990).

2.	Kardile J.N. “Development of simple and economic filtration methods for rural water supplies”, Journal of the International Water Supply Association, “Aqua” No. 1, 1981. Paper presented at the specialized conference on “Low cost technology”, held in Berlin in April, 1981, “Wasser Berlin 1981”.

3.	Kardile J.N. “Simple Methods in water purification”, India, 1987.

4.	Ministry of urban development of Govt. of India, Publication of CPH & EEO “Manual on water supply and treatment” (Third edition), March, 1991.

5.	Schulz, C.R. and Okun, D.B. “Surface treatment for communities in developing countries”, John Wiley and Sons, New York, 1984.

6.	Culp, J.N. and Culp P.L. “New concepts in water purification”, (1974).

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