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One hundred mm diameter sewers were used with the interceptor tank systems in Kulakarai. |
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4.2.6
Dimensions of Interceptor Tank Chambers |
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Information is available on the interceptor tanks incorporated into three of the schemes considered in this chapter, those in the Peshawar Cantonment, Kulakarai and Thirukatchur. Those used in the Peshawar schemes are circular in plan with an internal diameter of 0.9 m and a depth of just over 2 m. The design was based on the perception that there would be limited maintenance and that there would be a reluctance to desludge the tanks at regular intervals. The size would provide sufficient capacity for tanks to work for several years (calculations suggest between 4 and 5 years) before desludging became necessary and it is as yet too early to say what will happen when they are full of sludge. However, it is interesting to note that communal septic tanks, receiving flows from the WCs of up to about 20 houses, which exist in the Civil Lines area of Peshawar Cantonment, are possibly 50 years or more old and are still operating, although it does not appear that they have ever been desludged. These septic tanks discharge to open drains and it is likely that they no longer perform as designed, but it may be that they are still carrying out the main function of an interceptor tank, to remove gross solids from the sewer. |
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In complete contrast, the tanks used in Kulakarai were a mere 800 × 600 mm in plan with a total depth of 650 mm. Of this depth, only about 350 mm was below the invert level of the outlet pipe, giving only about 6 months sludge retention. When the scheme was inspected, some 9 months after it had been commissioned, it was working satisfactorily, but it would be interesting to carry out a follow-up investigation now, some 5 years after commissioning. The tanks on the Thirukatchur scheme were 1000 × 600 mm in plan and about 650 mm deep below the invert of the outlet pipe. The available sludge storage volume is thus about 0.39 m3, intermediate between those in Peshawar and Kulakarai. |
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One of the sewers implemented in Faisalabad is also designed to include interceptor tanks which are similar to the septic tanks commonly provided in Pakistan to provide some treatment prior |
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