A mobile fluid treatment system and portable fluid treatment apparatus are described. The portable apparatus is configured to be placed within a mobile tank of fluid for separating solids from the fluid and includes an upper portion, a lower portion, and a number of adjustable attachment members that can removably secure the apparatus to the side walls of the tank, thus allowing the apparatus to be moved from one tank to another. Furthermore, the lower portion of the apparatus can include a flexible hopper that funnels solids separated from the fluid toward a sediment outlet. The flexibility of the hopper allows the apparatus to fit in different sizes of tanks and further facilitates the break-up and removal of sediment from the apparatus, such as through vibration of the hopper due to the action of a sediment withdrawal pump.