Trickling Filter


Introduction

A trickling filter is a type of fixed media filter which can be used to replace the aeration basin in location and function in a packaged plant or extended aeration plant.  Trickling filters are very efficient at removing B.O.D. and ammonia from wastewater, and they use a minimal amount of power.  The cost to remove B.O.D. is only a few dollars per ton.

The trickling filter fits into the wastewater treatment system as shown in the flow chart below. 



Initial Treatment

The beginning of the trickling filter treatment process is like treatment in a packaged plant.  The wastewater passes through a bar screen, a grit screen, a comminutor, and a primary clarifier.  Sludge from the primary clarifier goes to the digester while the mostly liquid portion of the wastewater goes on to the trickling filter.  



The Trickling Filter

The liquid portion of the B.O.D. and ammonia from the clarifier, as well as the supernatant from the digester, are pumped to the trickling filter.  Both the B.O.D. and the ammonia are required to produce the right growth of microorganisms on the media to provide good treatment.  



The liquid influent is piped to the spray heads at the top of the trickling filter.  The force of the water causes the spray heads to rotate above the media, acting  like a sprinkler and evenly distributing water across the media.  

The influent trickles down through the media.  The media is covered with a slime of both aerobic and anaerobic microorganisms.  These microorganisms remove the organic matter from the water.  As we have mentioned previously, the microorganisms in a trickling filter are especially efficient at removing ammonia from the water.



Clarifier, Disinfection, and Aeration

As the water passed through the trickling filter, some of the slime was sloughed off of the filter bed.  In addition, there is still some sludge in the wastewater.  These solids are removed when the water flows from the trickling filter through the secondary clarifier.  The solids from the clarifier are sent to the digester.





The supernate from the clarifier (now called effluent) is sent in two directions.  Some of the supernate is re-circulated through the trickling filter to reduce the B.O.D. loading by watering down the influent.  The rest of the clarifier's supernate is chlorinated and de-chlorinated or passed through a UV light.  This disinfection process helps wipe out most of the microorganisms and the coliform bacteria.  

The final part of the treatment process is the step aerator.  As the effluent flows over the step aerator, the dissolved oxygen levels are increased.  The water is now ready to be released into a stream or river.