Removal of particulate matter and phosphorus in sand filters treating stormwater and drainage runoff: a case study

Runoff is often delayed and treated in wet ponds to retain particles and particulate substances. To increase the treatment efficiency, a sand filter can be placed before the outlet. The filter material is often renewed after 10–15 years due to presumed clogging by trapped particles, but often it clogs much earlier. Knowledge of how clogging develops over time is therefore important. This study has examined two filters, focusing on particle size, content and placement of particles, organic matter (OM) and phosphorus (P) retained in the filter. The study concludes that both particles and P are retained in the upper few cm’s of the filter, causing clogging after a few years, thus leaving the deeper filter material unused. Even small particles (<63 µm) are efficiently retained as long as clogging is avoided. This is preferable, as runoff is rich in small particles and as particulate P is associated with the small particles.