Use of Trickling filter in Sewage treatment

Trickling filters can be used for complete treatment for domestic waste and as roughing filter for strong industrial waste prior to activated sludge process. The primary sedimentation tank is provided prior to trickling filter so that the settleable solids in the sewage may not clog the filter. The trickling filter is followed by secondary settling tank for removal of settleable biosolids produced in filtration process.

Trickling Filter (Image: wikipedia)

Sloughing of filter

As the wastewater trickles through the filter media, as it may consist rocks of 40 to 100 mm size or plastic media. A biological slime consisting of aerobic bacteria and other biota builds up around the media surface. Organic material in the sewage is absorbed on the biological slime, where they are partly degraded by the biota, thus increasing the thickness of the biofilm. Eventually there is a scouring of the biofilm and fresh biofilm begins to grow on the media. This phenomenon of detachment of the biofilm is called sloughing of the filter.

Low Rate and High Rate Filter

The trickling filters are classified as low rate and high rate depending on the organic and
hydraulic loadings. Low rate filters are designed for hydraulic loading of 1 to 4 m3/m2.d and
organic loadings as 80 to 320 g BOD/m3.d. The high rate trickling filters are designed for hydraulic loading of 10 to 30 m3/m2.d (including recirculation) and organic loading of 500 to
1000 g BOD/m3.d (excluding recirculation). Generally recirculation is not adopted in low
rate filter and recirculation ratio of 0.5 to 3.0 or higher is used in case of high rate trickling
filters. The depth of media varies from 1.0 to 1.8 m for high rate filters and 2.0 to 3.0 m for
low rate filters. The bed of trickling filter is provided with slope 1 in 100 to 1 in 50.

The under drainage system consists of ‘V’ shaped or half round channels, cast in concrete floor during its construction. Revolving distributors are provided at top with two or four horizontal
arms of the pipe having perforations or holes. These rotating arms remain 15 to 25 cm above the top surface of the media. The distribution arms are rotated by the electric motor or by back reaction on the arms by the wastewater, at about 2 rpm. The head of 30 to 80 cm of wastewater is required to rotate the arms.