Dewatering is the removal of

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

from a location. This may be done by wet classification,

centrifugation Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...

filtration Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filte ...

, or similar solid-liquid

separation process A separation process is a method that converts a mixture or a solution of chemical substances into two or more distinct product mixtures, a scientific process of separating two or more substances in order to obtain purity. At least one product mi ...

es, such as removal of residual liquid from a filter cake by a filter press as part of various industrial processes. Construction dewatering, unwatering, or water control are common terms used to describe removal or draining groundwater or surface water from a riverbed, construction site, caisson, or mine shaft, by pumping or evaporation. On a construction site, this dewatering may be implemented before subsurface excavation for foundations, shoring, or cellar space to lower the

water table The water table is the upper surface of the phreatic zone or zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater, which may be fresh, saline, or brackish, depending on the loc ...

. This frequently involves the use of submersible "dewatering"

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

s, centrifugal ("trash") pumps, eductors, or application of

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

to well points. The international business research company Visiongain valued the global dewatering pump market at $6.4 billion in 2018.

Processes

Deep wells

A deep well typically consists of a borehole fitted with a slotted liner and an electric submersible pump. As water is pumped from a deep well, a hydraulic gradient is formed and water flows into the well forming a cone of depression around the well in which there is little or no water remaining in the pore spaces of the surrounding soil. Deep wells work best in soils with a permeability of k = m/s to m/s; the amount of drawdown that a well can achieve is limited only by the size of the fish pump. Deep wells can be installed in a ring around an excavation to lower the water level and maintain a safe, dry site. Several equations can be used to design deep well dewatering systems, however many of these are based on empirical data and occasionally fail. Practice and experience, along with a firm understanding of the underlying principles of dewatering, are the best tools for designing a successful system. Some dewatering situations "are so common that they can be designed almost by rule of thumb". Deep wells are also used for aquifer testing and for groundwater drainage by wells.

Wellpoints

Wellpoints are small-diameter (about 50 mm) tubes with slots near the bottom that are inserted into the ground from which water is drawn by a vacuum generated by a dewatering piston pump. Wellpoints are typically installed at close centers in a line along or around the edge of an excavation. As a vacuum is limited to 0 bar, the height to which water can be drawn is limited to about 6 meters (in practice). Wellpoints can be installed in stages, with the first reducing the water level by up to five meters, and a second stage, installed at a lower level, lowering it further. The water trickling between the deep wells may be collected by a single row of well point at the toe. This method ensures a much thicker width free from seepage forces. Wellpoint spears are generally used to draw out groundwater in sandy soil conditions & rock condition and are not as effective in clay . Open pumps are sometimes used instead of spears if the ground conditions contain significant clay .

Horizontal drainage

The installation of horizontal dewatering systems is relatively easy. A trencher installs an unperforated pipe followed by a synthetic or organic wrapped perforated pipe. The drain length is determined by the drain diameter, soilconditions and the water table. In general drain lengths of 50 meters is common. After installation of the drainpipe a pump is connected to the drain. After the water table has been lowered, the intended construction can start. After the construction is finished the pumps are stopped, and the water table will rise again. Installation depths up to 6 meters are common.

Control of pore pressures

Whilst engineers can use dewatering to lower a groundwater table, or to drain soils, they can also use the process to control pore pressure in soils and avoid damage to structures by base heave. High pore pressures occur in soils composed of fine silts or clays. Since these soils have a very low permeability, dewatering in a traditional sense (gravity flow into an abstraction well) may prove very costly or even futile. Instead, a

-assisted dewatering scheme, such as ejector wells, or vacuum-sealed deep wells may serve to draw water into a well for abstraction.

Applications

Construction

Dewatering is often a critical component of construction projects. Dewatering of a site improves safety by preventing the formation of mud and eliminating hazards to electrical equipment posed by water. Removing water also improves the stability of soils and mitigates erosion.

Wastewater treatment

wastewater treatment Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on ...

, dewatering may be used to remove solids during the treatment process for separate disposal. This may take the form of thickening, where only some of the water is removed, or full dewatering.