MODEL RELEASED. Effluent water purification. Worker holds two beakers containing effluent water before and after biological purification treatment at a paper production mill. In the background is some equipment used in effluent purification. Effluent water runs into bacterial ponds, where bacteria digest chemical pollutants and leave the water clean. The water may them be re-used by the mill or is clean enough to run out into local rivers without harming wildlife.