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Trickling Filters

Vocabulary

Biomass A mass or clump of organic material consisting of living organisms feeding on the wastes

in wastewater, dead organisms and other debris.

Colloids Very small, finely divided solids (particles that do not dissolve) that remain dispersed in a

liquid for a long time due to their small size and electrical charge. When most of the

particles in the water have a negative electrical charge, they tend to repel each other. This

repulsion prevents the particles from clumping together, becoming heavier, and settling

out.

Distributor The rotating mechanism that distributes the wastewater evenly over the surface of a

trickling filter or other process unit.

Fixed Spray Nozzle Cone-shaped spray nozzle used to distribute wastewater over the filter media, similar to a

lawn sprinkling system. A deflector or steel ball is mounted within the cone to spread the

flow of wastewater through the cone, thus causing a spraying action.

Microorganisms Very small organisms that can be seen only through a microscope. Some microorganisms

use the wastes in wastewater for food and thus remove or alter much of the undesirable

matter.

Nitrification An aerobic process in which bacteria change the ammonia and organic nitrogen in

wastewater into oxidized nitrogen (usually nitrate). The second-stage BOD is sometimes

referred to as the “nitrogenous BOD” (first-stage BOD is called “carbonaceous BOD”).

Ponding A condition occurring on trickling filters when the hollow spaces (voids) become plugged

to the extent that water passage through the filter is inadequate. Ponding may be the result

of excessive slime growths, trash, or media breakdown.

Recirculation The return of part of the effluent from a treatment process to the incoming flow.

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Shock Load – The arrival at a plant of a waste which is toxic to organisms in sufficient

quantity or strength to cause operating problems. Possible problems include odors and

sloughing off of the growth or slime on the trickling filter media. Organic or hydraulic

overloads also can cause a shock load.

Trickling Filter Media – Rocks or other durable materials that make up the body of the

filter. Synthetic (manufactured) media have been used successfully.

Zoogleal Film – A complex population of organisms that form a “slime growth” on the

trickling filter media and break down the organic matter in wastewater. These slimes

consist of living organisms feeding on the wastes in wastewater, dead organisms, silt and

other debris. “Slime growth” is a more common term.

Purpose of Trickling Filter

To remove dissolved and suspended colloidal organic waste from wastewater.

Process Description

A typical trickling filter is a large diameter round tank containing a media on which

organism can grow for the removal of dissolved and suspended organic waste.

Wastewater is “trickled” over the top and is treated as the water makes its way

downward. The zoogleal film on the trickling filter media uses the organic matter in the

water for cellular reproduction. This process reduces the strength of the wastewater by

reducing BOD.

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Component Purpose

Inlet pipe Transports wastewater to trickling filter

Distributor base Support rotating distributor arm

Distributor bearings Allows distributor arm to rotate

Distributor arm Distributes wastewater evenly over media

Outlet orifice Controls flow to filter media and are adjustable

Speed retarder orifice Controls speed of distributor arm

Splash plate Assist in even wastewater distribution over media

Arm dump gate Drains distributor arm (used for flushing and fly control)

Filter media Provides large surface area for biological slime growth

Support grill Supports media and separates it from underdrain system

Underdrain system Collects treated wastewater from under filter / allows air flow

Underdrain channel Transports wastewater to outlet box

Outlet box Collects filter effluent

Outlet pipe Transports filter effluent to next process

Retaining wall Holds filter media in place

Ventilation port Allows air circulation through media

Stay rod Supports distributor arm

Turnbuckle Allows adjustment and leveling of distributor arm

Center well Provides higher water head to ensure equal flow to all orifices

Splitter box Divides flow between recirculation and secondary clarifier

Recirculation pump Returns flow to trickling filter for further treatment

Types of Trickling Filters

Standard Rate Filter

Hydraulic loading = 25 – 100 gal/day/ft2

Organic Loading = 5 – 25 lb BOD/day/1,000 ft

3

Produce effluent BOD of 20 – 25 mg/L

High Rate Filter

Hydraulic loading = 100 – 2,000 gal/day/ft2

Organic Loading = 25 - 300 lb BOD/day/1,000 ft

3

Produce effluent BOD of 20 – 50 mg/L

Roughing Filter

Hydraulic loading = 100 – 2,000 gal/day/ft2

Organic Loading = 100 - 300 lb BOD/day/1,000 ft

3

Removes 50% - 70% BOD

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Controlling Process

Under normal conditions, operate the filter at a constant hydraulic load by

adjusting the recirculation rate. This will help to maintain adequate DO

throughout the filter while preventing hydraulic shock, which could cause

unwanted sloughing. Recirculation is used to accomplish three things:

1. Dilute incoming BOD load

2. Increase DO

3. Maintain hydraulic load on filter

Possible Problems and Solutions

Ponding

1. Flush the area of ponding with high pressure water or flood by stopping

distributor arm over troubled area

2. Remove leaves or debris with rake

3. Dose filter with 5 mg/L clorine solution for several hours

4. Flood filter for 24 hours then carefully release to avoid permit violation

5. Shut off flow for several hours to dry slime growth and remove with rake

Odors

1. Do all possible to maintain aerobic conditions in collection system

2. Pre-chlorinate or aerate if necessary

3. Ensure adequate ventilation in filter

4. Increase recirculation rate to increase DO

Filter Flies

1. Increase recirculation rate to flush fly larvae

2. Keep orifice openings clear

3. Apply approved insecticides

4. Flood filter for 24 hours weekly (prevents completion of life cycle)

5. Dose filter with 1 mg/L chlorine for a few hours weekly

6. Minimize habitat with proper landscape maintenance

Sloughing

1. Increase recirculation rate weekly to cause a regular controlled slough

2. Reduce distribution arm speed during this weekly flush period

Poor Quality Effluent

1. Check influent BOD to determine if filter is overloaded

High Plant Flow

1. Increase number of filters in operation

2. Reduce or stop filter recycle

3. Operate filters in parallel rather than series if possible