Abstract: The addition of polymeric flocculant or other chemical dewatering modifier to a suspension flowing through a service line to a dewatering plant is controlled automatically in response to automatic determination of a filtration rate parameter. This may be determined using novel apparatus which includes a filtration assembly comprising a chamber with a filter element mounted across its open end and water sprays for washing the filter element. In one embodiment it is preferred to assist back-washing with an air purge while in another embodiment there are means for disassembling the filtration assembly to permit cleaning.