The rate at which sewage can be treated on a biological filter will depend on the nature of the sewage and the required effluent quality. Loading rates are best measured in terms of organic loading, kg of BOD per cubic metre per day, often abbreviated to kg/m3/d. The loading rate on conventional biological filters ranges from 0.3 – 1.5 kg/m3/d. Winter/summer performance differences in biological filters can be considerable. A 25% reduction in nitrification and a 10% reduction in BOD removal occurs when the temperature drops by 15ºC as it does from summer to winter. For this reason works are designed to comply under winter conditions. The loading rate also influences the production of sludge in the humus tanks, with a high loading rate increasing the yield of sludge. The calculation of the loading rate is very simple and is based on filter volume, influent BOD and flow rate.

Example:A biological filter treats a waste from a small rural community after primary sedimentation. The works has recently being producing poor quality final effluent. Is the filter being overloaded? Area of filter = 20 m2 Depth of filter = 2 m Daily flow to filter = 10 m3 Influent BOD = 20 kg/m3 OLR = 5 kg BOD/m3/d

This works appears to be overloaded from the above calculation.