It is possible to combine the soakaway and sediment-trap in instances of low-level sludge load by using an inlet filter module.

The inlet filter is comprised of, to give an example, a 3 metre long plastic pipe DN 200, which is fitted vertically to the coupling of an outlet pipe. The lower section (about 1 metre) of the pipe is wrapped with several layers of activated carbon mats. The pipe is perforated where the mats are so that rainwater accumulated in the sediment-trap is conducted horizontally through the activated carbon filter layer into the plastic pipe, flowing through the floor of shaft and on into the ground.

The height of the filter module is aligned with the depth of the shaft in order for it to be easily accessed using the overlying manhole. This allows the hassle-free exchange of filters without having to enter the module. Water samples can also be taken through the top opening.