TRICKLING FILTERS

Description

A trickling filter is a device for reducing the amount of BOD suspended solids and ammoniacal nitrogen in a liquid effluent. The filter is constructed from a bed of pebble or gravel media with a void volume of about 50%, on a base that incorporates underdrains and air ducts. Rotating or travelling mechanisms distribute the effluent on the surface of the bed so that it percolates down through the media while air rises through it. A complex community of bacteria, fungi, protozoa, algae and invertebrates establishes on the media. The most aerobic zone lies at the base of the bed, where well-treated effluent meets the incoming air, with the least aerobic zone just below the surface of the filter. Alternatively, trickling filters may be constructed with plastic media which with its higher surface area and higher void volume (94-98 %) can accept higher hydraulic loadings.

Trickling filters have a high surface area and in exposed locations the operating temperature may be low. Below 8°C nitrification is limited, and ammoniacal nitrogen may be discharged in the effluent. Trickling filters may also lose biomass in the spring, but if linked to aerated solids contacting tanks the solids discharge can be controlled.

Application

Trickling filters are robust treatment systems and are particularly attractive options where the local conditions allow them to operate under the action of gravity without energy input. They are used for the treatment of settled sewage and industrial effluents, including those from the food and chemical industry. Nitrified effluents can be produced in filters of adequate depth, as long as they are not overloaded.

Sizing

Trickling filters will reduce the BOD and suspended solids of settled sewage by 90 %. Effluent loadings of 05-3 m3/m3/day and BOD loadings of 01-05 kg/m3/day are appropriate. The combination of a trickling filter and a solids contact process will enable 95% of the BOD and suspended solids to be removed. Filters with plastic media can also operate as roughing filters with higher loadings.

Trickling filters may be up to 30 m in diameter and the depth of media is normally about 15-20 m. A depth of 18 m is considered to be the minimum required to provide nitrification.

Most trickling filters operate hydraulically from the head of water pressure imposed by the effluent. However, if the effluent has to be raised or has to be recycled, then electrical pumping will be required. A 5 kW pump might be required for a flow of 100 m3/hour against a 5 m head.