spray header 94 which prefereably surrounds each of the process tanks 10 21 a plurality of conventional elongate spray nozzles 96 are mounted on the spray header 94 in order to provide a stream of spray to the process tanks 10 21 referring to fig3 a second pipe network 100 is provided for draining the chemical solution from each of the process tanks 10 21 and disposing of the chemical solution or returning it to its original supply tank each of the supply tanks 30 36 receives chemical solution discharged from each of the process tanks 10 21 through a network preferably comprising a return pipe 102 which communicates with a supply tank and a principal discharge pipe 104 the principal discharge pipe 104 is in turn connected to each of the process tanks 10 21 by a series of discharge connector pipes 106 butterfly valves 108 are preferably mounted on each of discharge connector pipes 106 at the exit point of the process tanks 10 21 selective activation of the butterfly valves 108 allows for routing of particular chemical solution in a particular process tank 10 21 to the desired return location the return location is normally the original supply tank that provided that chemical solution however some or all of the chemical solution being discharged from a particular process tank may be routed for disposal through waste pipe 110 since each of the butterfly valves 108 acts independently of the others it is possible to selectively discharge the solution from any one or more of the process tanks 10 21 to any one selected supply tank 30 36 or to the waste pipe 110 referring.