Ejector wells work on the same principle as a well point system but allow water to be drawn from deeper in the ground. An ejector dewatering system consists of an array of wells pumped by jet pumps installed at the base of each well up to depths of 150 metres.
- Operating depth is not limited by suction lift; ejectors are available with an operating depth down to 150m (groundwater control systems usually operate at depths of 30 to 50m).
- Ejectors will pump air and water; this means that at low flows, if the well head and annulus is sealed, the ejector will develop a vacuum in the well, which can provide vacuum assisted drainage in fine grained soils.
- Single pipe ejectors can be installed in the well liners as small as 50mm internal diameter. This leads to a lower unit cost per well, allowing cost effective installation of wells at a close spacing if required
- The capacity of individual ejectors is limited
- Ejectors have relatively low energy efficiency and for large flow rates the power consumption be prohibitive
- Ejector systems can be susceptible to loss of performance from biofouling or nozzle and venture wear. Regular monitoring and maintenance is needed to identify any reduction in performance.
Certain types of ground condition require alternative techniques such as ground where the permeability is very low or where the depth and nature of the excavation precludes the use of wellpoints and centrifugal pumps. One of the techniques commonly used then is the high pressure ejector system. This system works on the principle of forcing water at high pressure down the well and through a nozzle to create a venture effect, which in turn creates a vacuum of sufficiently high level to draw up the water surrounding the bottom of the well and return it to the surface.