Abstract: A system, which controls the content of an impurity in a crude oil stream, includes an injector which injects a demulsifier into a stream of produced wet crude oil in response to a control signal. A three-phase separator receives the produced crude oil with demulsifier and partially separates produced water and gas from the crude oil to provide a separated wet crude oil stream. Apparatus mixes wash water in with the separated crude oil stream in accordance with a control signal to provide a crude oil/wash water stream. A dehydrator removes substantial amount of water from the crude oil/wash water stream to provide a dehydrated crude oil stream. A monitor monitors the content of a predetermined type of impurity in the dry crude oil stream and provides a corresponding impurity content signal to control apparatus. The control apparatus provides the first and second control signals in accordance with the signal from the monitor so as to control the impurity's content of the dry crude oil stream.