Trickling Filters

After wastewater is treated in the primary clarifiers, it flows to trickling filters.  The word trickling “filter” is misleading because there is no straining or filtering action.  Actually, a trickling filter is a process that brings wastewater into contact with slime (biological) growths (bacteria, protozoa, algae, fungi, worms, and insect larvae). The slime growths utilize the nonsettleable and dissolved solids in the wastewater for food, and in the process, converts those solids to settleable solids.

A trickling filter consists of three main parts; filter media, distributors, and the underdrain system.  The filter media consists of 1” or 4” stones, approximately 6’ deep inside concrete walls. The stone media provide surface area for the slime growths (zoogleal mass) to attach and provide voids for air to move through the filter supplying oxygen to the biological growths. The distributors provide a mechanism for “trickling” the wastewater over the rock media, and can be fixed nozzles or circular rotating distributor arms. 

One fixed nozzle filter bed and four rotary trickling filters are located at the wastewater treatment plant. The fixed nozzle filter bed consists of 1” diameter stone media and the rotary filters utilize 4” media.  In both types of filters the wastewater is discharged or dosed to the media.  A large dosing tank fills with wastewater and is then discharged to the media.  This provides periodic application of wastewater to the filter bed media. The dosing tank mechanism works by a siphon.  The circular trickling filter distributor arms distribute wastewater over the entire filter and rotate by the propulsion of the water exiting the distributor arms, thus providing intermittent discharge to the filter bed media.

As the wastewater passes through the filter bed media, the slime growth thickens from the utilization of food in the wastewater.  Eventually, some of the growth will “slough off” the media, become a settleable solid, and be washed through the media with the wastewater into the underdrain system.  The underdrain system assists in providing ventilation (oxygen) to the zoogleal mass growing on the stone media.  The underdrain system takes the wastewater/settleable solids mixture to an intermediate or final clarifier.