Trickling Filters & Rotating Biological Contactors

Trickling filters and rotating biological contactors are attached-growth wastewater treatment processes. Instead of keeping most microorganisms suspended in an aeration basin, these systems allow biological growth to develop on solid media surfaces.

Wastewater passes over or through the media, where microorganisms remove biodegradable pollutants. Operators must understand biofilm condition, hydraulic and organic loading, oxygen availability, recirculation, solids sloughing, secondary clarification, and mechanical equipment.

Attached-Growth Treatment

In attached-growth treatment, microorganisms grow as a biological film on a surface.

This film is commonly called biofilm.

The biofilm contains microorganisms that use organic material and other wastewater constituents for growth and energy.

Attached Growth Versus Suspended Growth

The basic distinction is:

• attached-growth organisms grow on media;
• suspended-growth organisms remain largely suspended in the mixed liquor.

Trickling filters and rotating biological contactors are attached-growth processes.

Trickling Filter Fundamentals

A trickling filter distributes wastewater over a bed of media covered with biofilm.

Wastewater flows across the media surfaces while microorganisms remove biodegradable material.

Trickling Filter Components

Major components can include:

• influent distribution system;
• filter media;
• underdrains;
• ventilation passages;
• recirculation pumps and piping;
• secondary clarification downstream.

Trickling Filter Media

Media provide surface area for biological growth.

Media can include:

• rock;
• plastic modules;
• other engineered materials.

Media Surface Area

More available media surface generally provides more area for biofilm growth.

However, good treatment also depends on:

• wastewater distribution;
• oxygen;
• hydraulic loading;
• organic loading;
• biofilm condition.

Wastewater Distribution

Wastewater should be distributed over the filter media as evenly as practical.

Poor distribution can create:

• dry areas;
• hydraulic overloading in other areas;
• uneven biofilm growth;
• reduced treatment efficiency.

Rotary Distributors

Many trickling filters use rotating distributor arms.

The arms spread wastewater across the filter surface.

Operators should inspect:

• rotation;
• nozzle condition;
• flow balance;
• bearings;
• arm alignment.

Distributor Rotation

Distributor rotation can be driven by:

• hydraulic reaction;
• mechanical drive;
• other plant-specific systems.

Poor rotation can create uneven treatment.

Clogged Distributor Nozzles

Blocked openings can result in:

• uneven distribution;
• localized overloading;
• dry media;
• reduced treatment performance.

Underdrains

Trickling filter underdrains collect treated wastewater and provide pathways for air movement.

Blocked underdrains can cause:

• ponding;
• reduced ventilation;
• hydraulic problems.

Ventilation

Attached-growth organisms generally require oxygen for aerobic treatment.

Trickling filters can receive oxygen through natural or forced ventilation.

Natural Ventilation

Air movement can occur because of temperature differences between wastewater, air, and the filter structure.

Good open passages through the media and underdrains support ventilation.

Poor Ventilation

Poor ventilation can contribute to:

• low oxygen conditions;
• odors;
• reduced biological activity;
• undesirable biofilm conditions.

Hydraulic Loading

Hydraulic loading relates wastewater flow to filter surface area.

A simplified relationship is:

Hydraulic Loading = Flow ÷ Filter Area

Hydraulic Loading Example

A trickling filter receives 1,500,000 gallons per day and has a surface area of 5,000 ft².

Hydraulic Loading = 1,500,000 ÷ 5,000

Hydraulic Loading = 300 gpd/ft²

Organic Loading

Organic loading describes the mass of biodegradable material applied to the process.

A common operator relationship is:

Loading, lb/day = Flow, MGD × Concentration, mg/L × 8.34

Organic Loading Example

A trickling filter receives 1.8 MGD with BOD of 140 mg/L.

BOD Loading = 1.8 × 140 × 8.34

BOD Loading = 2,101.7 lb/day

Hydraulic and Organic Loading Are Different

Operators should not confuse:

• hydraulic loading, which is related to water flow;
• organic loading, which is related to pollutant mass.

A high flow with low BOD can produce high hydraulic loading but moderate organic loading.

Recirculation

Recirculation returns a portion of treated or partially treated wastewater to the trickling filter.

Recirculation can help:

• improve wetting of media;
• reduce extreme concentration changes;
• increase hydraulic loading;
• improve distribution;
• support process stability.

Recirculation Ratio

A simplified relationship is:

Recirculation Ratio = Recirculation Flow ÷ Influent Flow

Recirculation Example

Plant influent flow is 1.0 MGD and trickling filter recirculation flow is 2.0 MGD.

Recirculation Ratio = 2.0 ÷ 1.0

Recirculation Ratio = 2.0

This is commonly described as a 2:1 recirculation ratio.

Total Filter Flow

When recirculation is used:

Total Filter Flow = Influent Flow + Recirculation Flow

Operators should use total hydraulic flow when evaluating hydraulic loading on the filter.

Biofilm Growth

Biofilm develops as microorganisms attach to the media and grow.

Biofilm thickness depends on:

• organic loading;
• oxygen;
• hydraulic conditions;
• temperature;
• nutrients;
• shear forces.

Sloughing

Sloughing occurs when portions of biofilm detach from the media.

Sloughing is a normal part of attached-growth treatment, but excessive sloughing can increase solids loading on downstream clarifiers.

Why Biofilm Sloughs

Possible causes include:

• biofilm becoming too thick;
• hydraulic shear;
• changes in organic loading;
• temperature changes;
• normal biological turnover.

Secondary Clarification After Trickling Filters

Detached biological solids must be separated from treated wastewater.

A secondary clarifier downstream of the filter removes sloughed biomass.

High Solids Carryover

If sloughing increases, the secondary clarifier may receive a higher solids load.

Operators should monitor:

• clarifier blanket;
• effluent TSS;
• hydraulic loading;
• solids withdrawal.

Ponding

Ponding occurs when wastewater accumulates on the trickling filter surface rather than draining freely.

Possible causes include:

• excessive biofilm growth;
• clogged media;
• debris;
• poor underdrain flow;
• excessive hydraulic loading.

Why Ponding Is a Problem

Ponding can create:

• uneven flow;
• poor oxygen transfer;
• odor;
• reduced treatment;
• insect problems.

Filter Flies

Small flies can develop around trickling filters under favorable conditions.

Control can involve:

• good hydraulic loading;
• proper recirculation;
• media wetting;
• housekeeping;
• plant-specific control practices.

Odors

Odor around a trickling filter can indicate:

• poor ventilation;
• septic influent;
• ponding;
• overloading;
• stagnant areas.

Rotating Biological Contactors

A rotating biological contactor, or RBC, uses a series of closely spaced media discs mounted on a horizontal shaft.

Biofilm grows on the disc surfaces.

RBC Operation

The discs rotate slowly so the biofilm is alternately exposed to:

• wastewater;
• air.

This alternating exposure provides contact with pollutants and oxygen.

RBC Components

Major components can include:

• shaft;
• media discs;
• drive system;
• bearings;
• tank or basin;
• cover or enclosure where provided.

RBC Biofilm

As with trickling filters, RBC treatment depends on biological growth attached to media surfaces.

Biofilm condition is influenced by:

• organic loading;
• oxygen availability;
• temperature;
• rotation;
• wastewater characteristics.

RBC Staging

RBC systems commonly use several stages in series.

Earlier stages generally receive higher organic loading than later stages.

Why Staging Matters

As wastewater moves through successive stages:

• organic concentration usually decreases;
• biofilm conditions change;
• later stages may support different biological activity.

Nitrification in RBC Systems

Later RBC stages can support nitrification when:

• organic loading is sufficiently reduced;
• oxygen is available;
• temperature and pH are suitable;
• toxic conditions are absent.

RBC Rotation

Correct rotation is critical because it supports:

• oxygen exposure;
• wastewater contact;
• biofilm control.

Loss of Rotation

If an RBC stops rotating, treatment can deteriorate quickly.

Possible causes include:

• drive failure;
• bearing failure;
• shaft problems;
• electrical failure;
• mechanical overload.

Uneven Biofilm Growth

Uneven growth can indicate:

• uneven wastewater loading;
• rotation problems;
• mechanical misalignment;
• process imbalance.

Excessive Biofilm Thickness

Very heavy biofilm can increase:

• shaft loading;
• drive load;
• sloughing;
• mechanical stress.

RBC Sloughing

Biofilm naturally detaches from RBC media as it grows and ages.

Sloughed solids must be removed downstream, commonly in secondary clarification.

Secondary Clarification After RBCs

Secondary clarifiers separate biological solids from the treated wastewater.

Operators should monitor:

• effluent solids;
• sludge blanket;
• hydraulic loading;
• solids withdrawal.

Organic Loading and RBC Performance

High organic loading can produce heavy biofilm and increase oxygen demand.

Sudden loading changes can disturb process stability.

Hydraulic Loading and RBC Performance

High flow can reduce contact time and increase hydraulic stress.

Wet-weather conditions can therefore affect attached-growth performance even when influent concentration becomes diluted.

Temperature

Biological reaction rates generally decrease as wastewater temperature decreases.

Cold conditions can reduce:

• organic removal rates;
• nitrification rates;
• overall biological activity.

pH

Biological treatment depends on suitable pH conditions.

Nitrification can be particularly sensitive to unfavorable pH.

Toxic or Inhibitory Loads

Attached-growth microorganisms can be affected by:

• industrial chemicals;
• extreme pH;
• toxic compounds;
• other inhibitory wastes.

Attached-Growth Processes Can Recover Gradually

Biofilm can provide some process stability, but severe toxic or hydraulic disturbances can still damage treatment performance.

Influent BOD

BOD trends help operators understand organic loading to trickling filters and RBCs.

Mass loading can be calculated as:

BOD Load, lb/day = Flow, MGD × BOD, mg/L × 8.34

Loading Example

An RBC receives 0.9 MGD at 170 mg/L BOD.

BOD Load = 0.9 × 170 × 8.34

BOD Load = 1,276.4 lb/day

Removal Efficiency

A simplified relationship is:

Removal Efficiency, % = (Influent - Effluent) ÷ Influent × 100

Removal Example

Influent BOD to a treatment process is 160 mg/L and effluent BOD is 32 mg/L.

Removal Efficiency = (160 - 32) ÷ 160 × 100

Removal Efficiency = 80%

Percent Removal Is Not Enough

Operators should also evaluate:

• final concentration;
• flow;
• mass loading;
• downstream process performance.

Process Monitoring for Trickling Filters

Useful observations can include:

• influent flow;
• recirculation flow;
• distributor rotation;
• media wetting;
• ponding;
• odors;
• filter flies;
• secondary clarifier performance.

Process Monitoring for RBCs

Useful observations can include:

• shaft rotation;
• drive load;
• bearing condition;
• biofilm appearance;
• stage-to-stage changes;
• effluent quality;
• secondary clarifier performance.

Example: Trickling Filter BOD Removal Declines

Review:

• influent BOD loading;
• hydraulic loading;
• recirculation;
• distribution;
• ventilation;
• ponding;
• temperature.

Example: Ponding Develops

Review:

• media blockage;
• biofilm thickness;
• underdrain condition;
• hydraulic loading;
• distribution pattern.

Example: Trickling Filter Odor Increases

Possible causes include:

• poor ventilation;
• septic wastewater;
• ponding;
• overloading.

Example: Distributor Arms Rotate Unevenly

Check:

• nozzles;
• arm blockage;
• bearings;
• flow distribution;
• mechanical drive where present.

Example: Secondary Clarifier TSS Increases After Heavy Sloughing

The biological filter may be sending more detached solids to the clarifier.

Review:

• hydraulic loading;
• biofilm condition;
• clarifier solids handling;
• effluent TSS.

Example: RBC Stops Rotating

Investigate immediately.

Check:

• motor and drive;
• bearings;
• shaft condition;
• electrical supply;
• mechanical overload.

Example: First RBC Stage Has Very Heavy Growth

This can indicate high organic loading to the first stage.

Review influent BOD load and compare stage-to-stage conditions.

Example: Nitrification Declines in Later RBC Stages

Review:

• temperature;
• pH;
• organic loading;
• oxygen exposure;
• toxic conditions;
• rotation.

Example: Biofilm Sloughing Increases After a Loading Change

Sudden hydraulic or organic changes can disturb mature biofilm.

Monitor downstream clarification and final effluent closely.

Mechanical Maintenance

Trickling filter equipment can include:

• distributor bearings;
• pumps;
• nozzles;
• recirculation equipment;
• underdrains.

RBC equipment can include:

• shafts;
• bearings;
• motors;
• gear reducers;
• chains or drives.

Mechanical Problems Affect Biology

A failed distributor or stopped RBC can reduce wastewater contact and oxygen transfer even when the biological organisms themselves were initially healthy.

Routine Trickling Filter Inspection

1. Review influent and recirculation flow.
2. Observe distributor rotation.
3. Check media wetting.
4. Look for ponding.
5. Check odors and flies.
6. Inspect underdrain flow.
7. Review downstream clarifier performance.

Routine RBC Inspection

1. Observe rotation.
2. Listen for abnormal mechanical noise.
3. Inspect shaft and bearings.
4. Observe biofilm condition.
5. Compare stages.
6. Review influent loading.
7. Review secondary clarifier performance.

Common Attached-Growth Treatment Mistakes

• Confusing attached-growth treatment with suspended-growth activated sludge.
• Ignoring uneven trickling filter distribution.
• Failing to include recirculation flow when evaluating hydraulic loading.
• Ignoring ponding or blocked underdrains.
• Assuming biofilm sloughing is always abnormal.
• Ignoring secondary clarifier loading after heavy sloughing.
• Operating RBCs without monitoring shaft, bearing, and drive condition.
• Ignoring temperature and pH effects on nitrification.
• Looking only at concentration while ignoring mass loading.
• Trying to correct mechanical failure only through process adjustments.

A Practical Trickling Filter Review

1. Review influent flow and BOD.
2. Review recirculation flow.
3. Calculate total hydraulic flow.
4. Observe wastewater distribution.
5. Inspect media condition.
6. Check for ponding.
7. Review ventilation.
8. Review secondary clarification.
9. Compare influent and effluent quality.

A Practical RBC Review

1. Review influent flow and organic load.
2. Verify RBC rotation.
3. Inspect drive and bearings.
4. Observe biofilm condition.
5. Compare biological growth between stages.
6. Review temperature and pH.
7. Review downstream clarification.
8. Compare influent and effluent quality.

A Practical Sloughing Review

1. Determine whether loading changed.
2. Review hydraulic flow.
3. Review organic load.
4. Inspect biofilm condition.
5. Check equipment operation.
6. Monitor secondary clarifier blanket.
7. Monitor effluent TSS.

What to Remember for the Exam

• Trickling filters and RBCs are attached-growth biological treatment processes.
• Attached-growth microorganisms develop as biofilm on treatment media.
• Trickling filters distribute wastewater over media while air supplies oxygen to the biofilm.
• Good trickling filter operation requires even wastewater distribution and adequate ventilation.
• Blocked distributor nozzles can create dry or overloaded areas.
• Underdrains collect treated flow and support ventilation.
• Hydraulic loading equals flow divided by filter surface area.
• Organic loading can be calculated as MGD × mg/L × 8.34.
• Hydraulic loading and organic loading are different concepts.
• Recirculation can improve media wetting, distribution, and process stability.
• Total trickling filter flow includes influent plus recirculation flow.
• Sloughing is the detachment of biofilm from media and is a normal part of attached-growth treatment.
• Secondary clarification removes sloughed biological solids.
• Ponding can result from clogged media, excessive growth, hydraulic overload, or poor drainage.
• RBC discs rotate through wastewater and air, providing pollutant contact and oxygen exposure.
• RBC systems commonly use multiple stages in series.
• Later RBC stages can support nitrification under suitable conditions.
• Loss of RBC rotation can rapidly reduce treatment performance.
• Temperature, pH, hydraulic loading, organic loading, and toxic wastes can affect attached-growth treatment.
• Good attached-growth operation requires process monitoring, solids separation, and reliable mechanical equipment.