Lesson 15:

Trickling Filters




In this lesson we will answer the following questions:
  • What do trickling filters consist of?
  • How is oxygen controlled in wastewater systems?
  • How are the total flow, hydraulic loading, and organic loading calculated?


Reading Assignment

Along with the online lesson, read Chapter 6: Trickling Filters in your textbook Operation of Wastewater Treatment Plants Volume I .


Fixed Media Filters

Introduction to Fixed Media Filters

There are many ways to treat sewage and plant designs vary greatly from one site to the next.  But no matter how a plant is designed, the results are the same.  Some type of mechanical or biological treatment of the wastewater occurs. The supernate is drawn off, treated for bacterial contamination, and aerated before being released.  Solid waste is produced from sludge and is graded for land application or dumping.  

In nearly all types of wastewater treatment, the organic waste is consumed by microbial action.  This microbial action can be divided into two categories: free swimming or attached.

Packaged plants and oxidation ditches are an example of the first type of microbial action.  The microorganisms are free-swimming in the water, so they must be cycled through the system.  After being used to break down B.O.D., they are removed from the wastewater in a clarifier and returned as RAS to the aeration chamber or oxidation ditch.  

In contrast, fixed media filters use microorganisms attached to a medium (rocks, plastic, metal, etc.)  The microorganisms stay in place and do not need to be cycled through the system.  Instead, wastewater is circulated past the fixed microorganisms.  

A fixed media filter mimics the treatment method used in a healthy stream in which microorganisms produce a slick coating on rocks and pebbles.  This coating of microorganisms is able to trap and consume B.O.D. and ammonia in the water.  



Trickling Filters


In most wastewater treatment systems, the trickling filter follows primary treatment and includes secondary settling tank or clarifier as shown below. The process is a fixed film biological treatment method designed to remove BOD and suspended solids.


The trickling filter consists of several major components including distribution system, media, underdrains, effluent channel, secondary settling tank, and recirculation pumps and piping. Each of these components has one or more purposes.

In operation, wastewater is distributed evenly over the surface of the trickling filter media. As the wastewater flows over the surface of the media the organisms in the slime remove the organic matter from the flow.

The organisms aerobically decompose the solids producing more organisms and stable wastes, which either become part of the zoogleal slime or are discharged back into the wastewater flowing over the media. The wastewater continues through the filter to the underdrain system where it is collected and carried out of the filter. At the same time air flows through the filter (bottom to the top or top to bommtom depending on temperature). Oxygen is transferred from the air to the wastewater and slime to maintain the aerobic conditions. Periodically the slime on the media becomes too heavy and portions will be released. This material known as sloughings is carried out of the filter with the wastewater flow and is removed in the settling tank following the filter.

Filter media showing biological activities that take place on surface area.


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 beginning of the trickling filter treatment process is like pretreatment in a packaged plant.  The wastewater passes through a bar screen to remove large objects.  Then the influent is passed through a grit screen or chamber to remove inorganic materials such as eggshells, corn, sand, and tissue. The comminutor shreds the solid material into smaller pieces, which allows the solids to enter the plant without causing mechanical problems or clogging the pumps.  



Separator and Digester

At this point, treatment begins to differ from the typical packaged plant treatment.  We have previously discussed treatment systems which use only one clarifier, which is often called a secondary clarifier.  But in many treatment systems, two clarifiers are used.  The primary clarifier follows the grit chamber while the secondary clarifier follows the aerator, oxidation ditch, trickling filter, or other type of biological treatment.  

In the case of a trickling filter system, the water from the grit chamber enters a separator, which acts as a primary clarifier.  Here, organic solids are separated out of the water using changes in velocity.  The mostly liquid portion of the organic matter goes on to the trickling filter while the mostly solid portion goes to the anaerobic digester.  

The anaerobic digester is a closed tank in which anaerobic organisms slowly digest the organic matter in sludge.  These organisms produce methane, hydrogen sulfide, and carbon dioxide in the process.  The solids are removed from the tank and sent to a drying bed for eventual land application while the supernatant is drawn off and sent to the trickling filter.  



The liquid portion of the B.O.D. and ammonia from the separator, as well as the supernatant from the anaerobic 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 are sent to the anaerobic 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-chlorinate 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.  





Factors Influencing Slime Growth

Every organism requires a certain environment in order to grow and thrive.  The microorganisms which make up the slime on a fixed media filter are no exception.  

Factors which influence the growth of these microorganisms include pH, concentration of food, and concentration of oxygen.  Their growth can also be restricted by overpopulation, which will result in a buildup of enzymes and waste in the water and in a lack of space.  

Different microorganisms require different environments. For example, the microorganisms which are best for B.O.D. removal are most efficient at a pH of approximately 7, while the microorganisms which remove ammonia are most effective at a pH of about 8.



Water Recirculation

An important consideration in the operation of a trickling filter is that the microorganisms (the zoogleal slime) does not move. They just hang onto the media as slime waiting for the food to trickle by. To give the microorganisms another opportunity to eat food they missed, operators recycle clarified effluent. This is called recirculation.

Recirculation is used to reduce the organic loading, improve sloughing, reduce odors and reduce or eliminate filter fly or ponding problems. The amount of recirculation is dependent on the design of the treatment plant and the operational requirements of the process. Recirculation flow may be expressed as a specific flow rate (2.5 MGD). In most cases, it is expressed as ratio (2:1). The recirculation is always listed as the first number and the influent flow listed as the second number. Since the second number in the ratio is always 1.0, the ratio is sometimes written as a single number, namely the initial number, with the 1.0 being dropped.

Trickling filter flows can be recirculated from various points following the filter to various points before the filter. The most common form of recirculation removes flow from the filter effluent or settling tank and returns it to the influent of the trickling filter.





Total Flow

A number of calculations are useful in the operation of a trickling filter. For example, along with determining total flow, determining hydraulic loading and organic loading are also important. For the settling tank, calculating the detention time, surface settling rate, hydraulic loading, and the sludge pumping rate are important.

The formula below can be used to calculate the total flow, including recirculation:

Total Flow, MGD = Influent Flow (Recirc. Rate + 1.0)


The trickling filter is currently operating with a recirculation rate of 1.5. What is the total flow applied to the filter when the influent flow rate is 4.25 MGD?

Total Flow, MGD = 4.25 MGD × (1.5 + 1.0)

Total Flow = 10.63 MGD

Let's consider a plant with an influent flow of 72 mgd and a recirculation of 0.009.  The total flow would be calculated as follows:

Be sure to convert to the correct units, if necessary.  I converted 72 mgd to 72,000,000 gpd.  

You may have also noticed that I added 0.009 to 1 before I multiplied the total by 72,000,000 gpd.  It is very important to always perform any operation within parenthesis before simplifying the rest of the equation. Otherwise, you will get the wrong answer.  


Now, let's take the same formula, but instead of total flow, let's determine the recirculation rate.

The influent flow of a plant is 30 MGD. The total flow is 42 MGD. What percent of the water is recirculated?




Hydraulic Loading

Now that you know how to calculate the total flow, you can calculate the hydraulic loading.  The hydraulic loading is one of the factors which will influence the performance of the filters.  It is defined as the amount of wastewater applied per day over the surface area of the media.  So its units are gallons/day/square foot.  

The following formula can be used to calculate the hydraulic loading of a filter, including recirculation:

Given a total flow of 3 MGD and a filter area of 7,850 ft2, the hydraulic loading would be:

*You must first convert MGD to gpd:

So the hydraulic loading is 382 gpd/ft2.


Organic Loading

The organic loading also influences the performance of the filter.  The organic loading is the amount of B.O.D. applied to the filter per day per volume of filter media.  So the units of organic loading are typically pounds B.O.D./day/1000 ft3 of media.  

The organic loading can be calculated using the formula below.  This formula does not include recirculation, so the flow used is the influent flow rather than the total flow.

The B.O.D. is the concentration of B.O.D. in the filter influent.  The units of the filter volume are "1000 ft3", so a filter with a volume of 7,800 ft3 would have to have the units converted from "ft3" to "1000 ft3".  This conversion is accomplished by dividing the filter volume by 1,000, as shown below:

So let's calculate the organic loading of a 23,500 ft3 filter with an influent B.O.D. of 200 mg/L, and a flow of 3 mgd.

So the organic loading of the filter is 213 lbs/day/1000 ft3.





Fixed media filters use microorganisms attached to a medium, mimicking the water treatment method of nature in a healthy stream.  These filters are very efficient at removing B.O.D. and ammonia from the water.  

 The typical wastewater treatment process for trickling filters includes a screen, grit chamber, comminutor or grinding pump, separator or primary clarifier, trickling filter, aerobic or anaerobic digester, secondary clarifier, chlorination, dechlorination, and aeration.  

Some cleaned water is recirculated back from the secondary clarifier into the fixed media filter.  This helps control the microorganisms' environment by decreasing the concentration of food and increasing the D.O. in the water.  


New Formulas Used

To calculate total flow:

To calculate hydraulic loading:


To calculate organic loading:



Complete the math worksheet for this lesson and return to instructor via email, fax or mail.. Each question is worth 25 points.




Answer the questions in the Lesson 15 quiz When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and submit your grade directly into the database for grading purposes.