There are a variety of solenoid valves used within the water treatment industry including valves of plastic, brass and stainless steel construction. They may be used for applications such as tank filling, as service valves on water softeners, piloting larger actuated valves with air or water as well as a variety of inlet valves for equipment. The choice of material depends upon the requirements of the application and its operating environment. Whilst all three valve body materials are suitable for mains water use some consideration must be given to the application parameters such as the ambient temperature, the degree of exposure to shock or other physical impacts.
The most commonly used valves are 2/2 normally closed and 2/2 normally open. The operation of these valves is either as a direct acting, where the force to operate the valve depends entirely on the magnetic field generated by the solenoid coil, assisted lift or servo assisted, where the pressure difference between the inlet and outlet of the valve is used to move a flexible diaphragm. With the range of applications within the water treatment industry as you can expect there are a huge range of connections available in both BSP and JG connections too.
Although there are several seal types offered by valve manufactures by far the most popular for use within water applications are NBR and EPDM. Standard NBR is widely used within general industrial  water applications, however EPDM tends to be preferred for use in drinking/potable water applications due to the perceived absence of taint in the taste in the water. Another consideration for the seal type is temperature. Whilst NBR seals fine for temperatures of up to 60°C or even 80°C, EPDM seals are able to withstand temperatures of up to 150°C.  N.B. If higher temperatures are required we would advise using a valve specifically designed for steam valves.
Mains water is generally supplied to domestic consumers at between 2 to 4 bar however in some areas where the pressures are pumped this can exceed 10 bar.