Water & Wastewater Treatment Magazine
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www.wwtonline.co.uk | WWT | APRIL 2017 | 41 A back-to-back test using sand and plastic bead deposits over 40 pumping cycles revealed that the standard sump (le ) was le with 94kg of sediment. The TOPS sump (right) had only 4kg in the stagnant zones, and can be- come easily trapped by xed objects, such as pipes and cables. Once more, the smaller the volume of the sump, the more e ective the movement of the solids. As with settling and • oating solids, tests have shown that most of the neutral-buoyancy solids enter the pump when the water level in the sump is at its lowest, just before the pump switches o . A controller module can be used to programme the pumps to run down to the minimum level in the sump. Depending on the in• ux of solids, and thus the need for sump cleansing, this controller can force the pumps to QUESTIONS To complete this CPD, read the article and then answer the questions online at wwt.me.uk/cpdpumps Closing date: 1st May 2017. CPD credits: 1 hour 1. Which of these solids found in sewerage system would be most likely to be categorised as SG>1? a. A ball of wet tissues bouncing along the bottom of the sewer b. A twig or match fl oating on the surface c. A sanitary product travelling in suspension in the fl ow d. Sand or silt rolling along the bottom of the sewer 2. Which of these statements is NOT true about the behaviour of SG>1 solids in a sump? a. They rise and fall with the changing water level b. They can cause odour problems and corrosion c. They can only be drawn into the pump if they are within range of suffi cient suction d. They settle on the bottom 3. Which of these statements is true of the optimal design of the fl at bottom of a sump? a. Its surface area should be as large as possible b. Its surface area makes no difference to performance c. Its surface area should be kept to a minimum d. Its surface area must be identical to that of the wall of the chamber 4. And how close should the pump suction inlet be to the bottom the sump? a. As close as possible, without touching b. Less than half the diameter of the inlet c. A distance equal to the diameter of the inlet d. At least double the diameter of the inlet 5. What is the best way of ensuring SG<1 solids do not accumulate in a sump? a. Use a fl ush valve to stir up the bottom of the sump b. Send in operatives to remove them manually as part of a scheduled maintenance regime c. Ensure both pumps in a station run continuously at the same level d. Programme the pumps to periodically run the water level down to the minimum operate to the point of snore as many times as required. A • ush valve - a piece of equipment which stirs up water in the sump at the start of each pump operating cycle – can also be used to make settled solids easier to pump away. Flygt's tests have shown that such valves can enhance the transportation of solids signi cantly. Conclusion The need to transport wastewater, including all solids, through a sewer- age system to treatment plants can only be met if pumping stations are as e ective in handling sewage solids as the sewers themselves. Pumps that can handle solids without becoming clogged are obviously required, but pump sumps must also be constructed in such a way as to enhance the trans- port of solids. The following points are particularly important: -Sumps can be designed with much smaller • oor areas and with smaller clearances to the sump bottom and walls than conventional designs. This will reduce sedimentation and facili- tate the ingestion of solids by a pump. -Running pumps to the minimum water level from time to time will help to keep the sump free from • oating solids.