wash water where dumping of waste products is an unsound procedure because of polution prevention requirements it frequently becomes necessary to minimize the quantity of wash water in order to facilitate the disposal of the waste products or the wash water it is further noted that because of the high cost of energy and materials it is desirable to use the minimum amount of wash fluid consistant with maximum removal of solubles in order to minimize the energy costs associated with separating valuable solubles from the wash fluid it has now been found that one of the limiting factors which takes place in the washing of soluble components from a filter cake during a throughwash operation is the mass transfer limitations of the system and that proper recognition and use of this controlling factor can result in more efficient and economical processes through the conservation of power and throughwash fluid the wash system of the present invention departs from those of the prior art by being an intermittent operation having specifically determined and controlled interruptions when the concentration of salts in the wash water becomes very low the system automatically initiates a series of wash sequences in order to produce a spent wash water having a reasonable consistently high salt concentration the system inherently also makes a highly reliable measurement of the salts remaining in the cake by measuring the conductivity of the water extract after a period in which the wash step is interrupted or after it is terminated it should be noted that while reference is made to wash water any desired fluid can be used with water being the most commonly employed liquid it has now.