Trickling Filter Hydraulic Loading

A trickling filter is not a filter at all — it is a bed of rock or plastic media carrying a biological film, over which wastewater is sprinkled so the bugs can eat it in passing. Hydraulic loading is the total flow, recirculation included, spread over the plan area of the media: 40 L/s onto a 200 m² bed is 3456 m³/d over 200 m², or 17.28 m/d. Standard-rate rock filters run 1–4 m/d, high-rate rock filters 10–40 m/d, and plastic-media towers 15–90 m/d.

Recirculation belongs in this number because it is doing real hydraulic work. Beyond diluting a strong influent, the returned flow keeps the media continuously wetted — a rock filter that dries out between doses loses its biofilm, and one that never dries out never sloughs and eventually ponds. Below about 1 m/d nothing flushes the accumulated growth off the rock and the bed floods; that is why standard-rate filters are dosed intermittently by a siphon rather than continuously. The other historical hazard is the one the 1920s literature called the filter fly, Psychoda, which breeds in the moist upper media and can carry for hundreds of metres downwind: continuous high hydraulic loading, or periodic flooding of the bed, is the classic control.