a water recycling unit and system for dental operatories for use to supply operating priming water to liquid seal type vacuum pumps and including a conduit connectable to a municipal water supply , a solenoid valve in said conduit operated by a timer to open the valve for very short intervals periodically to furnish fresh makeup water to an air - water separator mounted above the level of the pumps and provided with a discharge to drain that is regulated to maintain a constant supply of contaminated water withdrawn from a patient &# 39 ; s mouth and a small amount of the makeup water to comprise priming water delivered to the vacuum pump by gravity flow . the only moving element in the basic system is the solenoid valve and it only passes fresh municipal water free of impurities to insure complete closing of the valve and thereby obviate possible partial opening due to impurities lodged between the valve and its seat .