The simplest way to filter polluted rainwater in soakaways is to use Aktofix activated carbon filter mats. These are laid in at least 3 layers on the water-permeable shaft floor and fixed to the shaft walls by means of metal straps. The polluted rainwater is thus filtered over a large area and uniformly by the activated carbon. Pollutants are retained in the macropores.

The robust Aktofix activated carbon filter mat consists of 2 outer filter textiles with an activated carbon intermediate layer weighing approx. 2.5 kg/m². It can be made up to suit a wide variety of shaft sizes and shapes. The Aktofix activated carbon filter mat has been successfully used in many thousands of seepage manholes for more than 25 years.

The outer textile filters retain accumulating sludge and protect the activated carbon. In case of higher sludge accumulation, it is recommended to install an inexpensive, easily replaceable and sludge-resistant pre-filter mat with 3-d step filter above the filter layer. Furthermore, sand or gravel layers or sludge traps can also be used in advance.