Water & Wastewater Treatment Magazine
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www.wwtonline.co.uk | WWT | APRIL 2017 | 39 The TOPS sump is an example of a pre-engineered, pre-assembled pumping station, consisting of station, tank, piping system, and monitoring and control equipment tries, wastewater contains between 100mg/l and 500mg/l of sanitary sol- ids and between 50mg/l and 1000mg/l of stormwater solids. Although these average levels may appear low, the gures can hide signi cant variations. Moreover, it is important to recognise that with this level of overall solids in the system, any disturbance in the • ow can lead to a rapid build-up of solids which can quickly spiral out of control. A useful, albeit simpli ed, way of thinking about solids is to put them into categories based on their Speci c Gravity (SG) and hence their buoy- ancy: -Settling solids (SG>1): includes most inorganic materials such as grit, sand, silt, and some heavy organic matter. - Neutral-buoyancy solids (SG=1): includes most organic matter and sanitary and other refuse such as paper, plastic, rags and string. - Floating solids (SG<1): includes fats, plastics, hollow objects, and light organic matter. These solids move within the sewer system in very di‰ erent ways. Fine, light material tends to travel in sus- pension, while heavier material travels in a rolling, sliding, and tumbling mode as bed load. The exact mode of travel depends on three groups of factors: the material properties of the solids (e.g. density, particle size and shape, size distribution, and cohesive properties), the properties of the sewer (e.g. size, shape, slope, and surface roughness), and the • ow conditions (e.g. velocity, level of turbulence, and depth). The heaviest solids, such as sand and gravel, are the main concern in the design of sewers, because they re- quire the highest velocity in the waste- water and the highest shear forces (lat- eral force on the sewer wall) in order to get them moving. When designing a sewer system, a widely-used gure is that a wastewater velocity of 0.75m/s should be achieved at least once daily, and that 1.0m/s should be achieved at full capacity – although these gures are somewhat arbitrary as they are not adjusted for quantity of sediment or pipe size. Useful gures for shear stress are 1-2 Pa for foul sewers or 3-4 Pa for surface water sewers. Other lighter materials, such as rags and sanitary waste, move more readily, so are not typically a concern for sewer designers. However, rags tend to agglomerate and become en- tangled when they roll on the bottom for a long distance, forming 'ropes' or 'balls' that may become a major hazard to the pumps further down the line. Sediment movement in pump sumps Sediment movement in a pump sump di‰ ers from that in a sewer: the • ow in