Trickling filters are sometimes classified by the organic loading rate applied. The organic loading rate is expressed as a certain amount of BOD applied to a certain volume of media. In other words, the organic loading is defined as the pounds of BOD or chemical oxygen demand (COD) applied per day per 1000 cubic feet of media — a measure of the amount of food being applied to the filter slime.

To calculate the organic loading on the trickling filter, two things must be known: the pounds of BOD or COD being applied to the filter media per day and the volume of the filter media in units of 1000 cubic feet. The BOD and COD contribution of the recirculated flow is not included in the organic loading.

Example:

A trickling filter that is 60 ft in diameter receives a primary effluent flow rate of 0.440 MGD.Calculate the organic loading rate in units of pounds of BOD applied per day per 1000 ft3 of media volume. The primary effluent BOD concentration is 80 mg/L. The media depth is 9 ft.

0.440 MGD x 80 mg/L x 8.34 lb/gal = 293.6 lb of BOD applied/d
Surface area = 0.785 x (60)2 = 2826 ft 2
Area x depth x volume = cu ft
2826 ft2 x9 ft = 25,434 cu ft (TF volume)