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Dewatering involves removing unwanted water from various sources, such as groundwater, process water, or water used to transport solids. Equipment choices depend on factors such as operation type and the percentage of water to be removed. Flocculants can aid in dewatering sludge, and various methods, such as conveyor belts, screw conveyors, and cofferdams, can be used to remove water. Weir tanks, gravity bag filters, and screen filters are also options. Wellpoints use pipes to drain water, and pumps can supplement the flow if necessary.
Dehydration refers to the removal of unwanted water. This can be the removal of groundwater, process water, water entrained or carried by oil or gas, or water used to carry solids. The choice of equipment is based on factors such as a permanent operation versus a temporary operation, a batch process versus a continuous process, and the percentage of water that needs to be removed.
The water used to transport a solid is called sewage. Coal slurry pipelines, for example, conduct ground coal particles that are carried in a stream of water. Gold panning was another operation that exploited the transport possibilities of water. Materials carried by water are referred to as solids. Once the solids are concentrated, they are referred to as sludge or cake.
In many slurry operations, it may be necessary to extract the water or filter out the solids. Agents that promote the aggregation of solids, called flocculants, are often added to sludge to improve dewatering. Dewatering conveyor belts, which can also include heat and vibration, have perforations in the belt that drain water as the sludge is moved. Belt presses squeeze out water by passing the slurry between a lower dewatering conveyor belt and a ballasted upper belt. Screw conveyors, mixing tanks or similar equipment mix temperature and humidity controlled air with the sludge and the water is transported with the air stream.
In the drilling and mining industries, dewatering refers to the removal of groundwater from mines and boreholes. In addition, road construction and any excavation activity may require the temporary alteration of the water table or the redirection of groundwater flows. The water can be pumped out, or drains and catch basins can be built. Cofferdams, which are temporary dams made from metal plates, are used to temporarily hold back water. Crude oil and natural gas are dehydrated to remove trapped water by passing the stream through physical separators.
Construction sites, industrial plants and similar environments often use water to control dust, clean equipment or as a by-product of a process. If this water builds up, it must be removed from the site without contaminating groundwater. Metals can be recovered from mine waste ponds by floating the metal away from the water using flotation agents. The agents can be chosen to obtain economically viable concentrates of individual metals.
Weir tanks are used to deposit solids. They consist of long tanks with separators under which the water must flow or above. Simple but limited in application, a gravity bag filter is a long tube that water is pumped into and allowed to escape. Screen filters at the inlet of any downstream dewatering process remove solids of a given particle size distribution. Filters with sand, carbon or other materials can produce potable quality water, but these are generally considered water treatment stages.
Wellpoints consist of a series of pipes placed below the excavation level. The pressure created by the water table helps drain the water. The pumps pull the initial vacuum to start siphoning the water and supplement the water flow if the pressure is insufficient.
