A rotary distributor or trickling filter as it is also known is a low cost, low tech biological treatment device. The trickling filter spreads incoming flow evenly, as a thin layer, over the surface of a media filled basin. An electrical motor or the hydraulic force of wastewater leaving the distributor slowly rotates the assembly. Flow passes over the media absorbing oxygen from the air in the voids of the filter bed. The air circulates through the bed by upward convection from the open underdrain system under the media bed.

In this aerobic environment an aerobic fixed film microbial growth develops on the media fed by organics in the wastewater.

Temperature can have an effect on performance. Rising wastewater temperature can increase the rate of metabolism whilst decreasing temperatures slow it down. The seasonal impact is more noticeable where extreme changes occur.

In conventional use there are two types of trickling filters, low-rate filters and high-rate filters. Low rate trickling filters depend on incoming flow for operation, a device called a dosing siphon can be used to regulate flow onto the filter bed. High-rate filters incorporate recirculation.

Trickling filters may have two, four or six arms depending on the range of inflow rates experienced. Machines with four or more arms may use staged (primary/secondary) flow regimes. Materials of manufacture are either galvanised mild steel or stainless steel.

The main advantages of a rotary distributor/trickling filter is its basic simplicity, low power consumption, ease of operation and low maintenance.

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