LAYER is the layer of micro-organisms develops on the granular bed of the trickling filter.There are 2 types of trickling filter:1. Standard rate trickling filter (SRTF)2. High rate trickling filter (HRTF) The recirculation means returning a portion of TREATED sewage to the treatment unit. By doing so, efficiency of trickling filter IMPROVES and it also reduces the load on filter.The recirculation ratio is connected to another term called recirculation factor (F) to obtain the efficiency of filter and it is given as,\(F = \FRAC{{1 + R}}{{{{\left( {1 + 0.1R} \RIGHT)}^2}}}\)The ratio of the Volume of sewage re-circulated (O) to the volume of Raw sewage (I) is called recirculation ratio, R.\(R = \frac{O}{I}\)The low rate trickling filter does not have any recirculation system.⇒ O = 0 ⇒ R = 0, substitute this in above equation ⇒ F = 1