A successful physical pre-treatment is necessary for a good performance of all Constructed wetlands; exceptions are the FWS and see also Future Trends.

The influent has to free from coarse and floating material probably by screening them out. Unsatisfactory pre-treatment may lead to build-ups in the inflow area, to odour nuisances, to clogging of the filter or to blockages of the soakage links.

The pre-treatment can be realised as primary sedimentation in tanks, for small scale plants typically septic tanks are used. Imhoff tank is a possibility which reduces sludge production.

Ponds may be an option for pre-treatment, often used before a VSF system. The size of wastewater ponds for pre-treatment typically range from 1.5 – 4 m²/PE. A partly reduction of COD, BOD and TSS < 100 mg/l can be achieved (SWAMP 2002).