Water & Wastewater Treatment Magazine
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40 | april 2017 | WWT | www.wwtonline.co.uk In the know CPD article Pumping station design an example of a traditional pumping station sump, which can suffer from problems relating to the accumulation of sludge and other sewage solids The self-cleansing pump sump designed to transport solids more effectively. The TOpS (The Optimal pumping Station) is patented to Xylem a sump is less regular, in terms of both direction and capacity. This is because most wastewater pumping stations in the network are set to operate inter- mittently, with the pumps kicking in when the wet well is filled to a certain level. In between times, when the pumps are not running, the sump is acting as a settling tank. The solids with an SG>1 settle on the bottom and accumulate in the still areas of the sump. If allowed to remain undisturbed, they may turn septic and cause odour problems as well as increase corrosion and release hazardous gases. They can only enter the pump if they are within the influence of the pump suction where the velocity exceeds the threshold of movement. Stagnant conditions in a sump, which result from intermit- tent operation, may cause granular solids and sludge to agglomerate or otherwise interact to adversely affect the transportation of sediment. Neutral buoyancy solids (SG=1) are easily carried by the flow and, in gen- eral, are easily pumped away. Long, stringy materials, however, may cause pumping problems either by hanging on the impeller blades or by blocking the pump inlet. Most severe clogging is caused by 'ropes' that may be cre- ated in long sewers or possibly in a sump. If caught by a rotating impeller, the longest objects may catch other materials and become entangled with them to form such ropes. Since they seldom pass through the pump, these ropes tend to grow and become much longer than the individual pieces. They tend to hang at a pump inlet when the pump operates, obstructing the pump performance, and fall back to the sump bottom when the pump has stopped. At some point, they may completely block the pump. The floating solids (SG<1) present a different type of problem. Since they float to the surface of the sump, they rise and fall with the changing water level, without ever being drawn into the pump. The combination of fats and floating rags tends to form thick 'raŠs'. If the incoming flow falls onto these raŠs, it will deposit more solids of various densities on their surfaces. These raŠs tend to obstruct level sen- sors, and may cause pump blockage if they break into smaller pieces. The floating solids must also be within the influence of the pump suction to enter the pump, a condition that can be achieved by lowering the water surface below its usual levels. Pumping station dimensions The majority of pumping stations in sewer networks are circular in design and equipped with two pumps. Tra- ditionally, sumps have been designed to provide ample space around the pumps to allow access for service personnel. However, the larger the sump, the more likely it is that it will become a settling tank for sludge and debris. The accumulation of sludge leads to the build-up of harmful gases and requires periodic desludging of the sump, which will increase opera- tion costs. The optimal sump, in this context, should be small enough to prevent the settling of sludge yet large enough to avoid obstructing pump operation. Pump manufacturer Flygt (a Xylem brand) has developed a set of design criteria for a self-cleansing sump by conducting laboratory tests on sediment movement. These tests have established that: -The flat bottom area of a sump must be kept to a minimum and lo- cated directly under the pump suction -All other surfaces must be vertical or steeply inclined – at least 45 de- grees for smooth surfaces (plastics or coated concrete) or at least 60 degrees for concrete -The distance from the pump suc- tion inlet should be less than half the diameter of the inlet -The pumps must occasionally operate to the lowest possible liquid level in the sump – that is, to the point of incipient air ingestion or "snore" -The inflow should be prevented from cascading directly onto the pumps or in the vicinity of the pump inlet -Neither the proximity of the walls themselves, nor the clearance between the two pumps, has any effect on pump performance Sump configuration Sump diameter and bottom configura- tion are the characteristics that most affect the transport of solids. Because the suction effect of the pump inlet decreases rapidly with distance, it is important that the solids be guided towards the inlet. The position of the inflow to the sump is critical, and the free spaces have to be designed to form passageways to facilitate the movement of solids. Nevertheless, dur- ing the idle phase of a pumping cycle, heavy solids will settle to the bottom. To pump them away, the shear force over the bottom, which is induced by the suction of the pump, has to exceed a certain critical value, which is dependent on the sediment type. The closer the pump inlet is to the bottom, the greater the shear force scouring the sediment. Liquid level The floating solids, as with the settling ones, can only enter the pump if they are within the influence of the pump suction. Lowering the liquid level in relation to the position of the pump inlet is, therefore, an important factor. Occasionally running pumps to the minimum water level – or the begin- ning of snore – helps to maintain a sump free from floating solids. Here again, the comparatively large surface area in a standard sump is a disadvan- tage, as the floating solids can still be some distance from the pump inlet. Meanwhile, neutral-buoyancy sol- ids move readily with the flow currents in the sump. They tend to accumulate