vvEPA
                       United States
                       Environmental Protection
                       Agency
                       Office of Water
                       Washington, D.C.
EPA832-F-00-014
September 2000
Waste water
Technology  Fact Sheet
Trickling  Filters
DESCRIPTION

Trickling filters (TFs) are used to remove organic
matter from wastewater.  The TF is an aerobic
treatment  system that  utilizes  microorganisms
attached to a medium to remove organic matter
from wastewater. This type of system is common
to a number  of technologies  such  as  rotating
biological contactors and packed bed reactors (bio-
towers).    These  systems   are  known  as
attached-growth processes.  In contrast, systems in
which microorganisms are sustained in a liquid are
known as suspended-growth processes.

APPLICABILITY

TFs enable organic material in the wastewater to be
adsorbed  by  a  population  of  microorganisms
(aerobic, anaerobic, and facultative bacteria; fungi;
algae; and protozoa)  attached to the medium as a
biological film or slime layer (approximately 0.1 to
0.2 mm thick). As the wastewater flows over the
medium, microorganisms already  in the water
gradually attach themselves to the rock,  slag, or
plastic  surface and form a film.  The  organic
material is   then  degraded  by  the   aerobic
microorganisms in the outer part of the slime layer.

As the  layer thickens through microbial  growth,
oxygen cannot penetrate the medium face,  and
anaerobic  organisms  develop.  As the biological
film continues to grow, the microorganisms near
the surface lose their ability to cling to the medium,
and a portion of the slime layer falls off the filter.
This process is known as sloughing. The sloughed
solids are picked up by the underdrain system and
transported to a clarifier for removal from  the
wastewater.
                      ADVANTAGES AND DISADVANTAGES

                      Some advantages and disadvantages of TFs are
                      listed below.

                      Advantages

                          Simple, reliable, biological process.

                          Suitable in areas where large tracts of land
                          are not available for land intensive treatment
                          systems.

                          May  qualify  for  equivalent   secondary
                          discharge standards.

                      •    Effective in treating high concentrations of
                          organics depending on the type of medium
                          used.

                      •    Appropriate  for  small- to  medium-sized
                          communities.

                          Rapidly reduce soluble BOD5  in applied
                          wastewater.

                      •    Efficient nitrification units.

                      •    Durable process elements.

                      •    Low power requirements.

                      •    Moderate  level  of  skill  and  technical
                          expertise needed to manage and operate the
                          system.

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Disadvantages

•     Additional treatment may be needed to meet
      more stringent discharge standards.

      Possible accumulation of excess biomass that
      cannot retain an aerobic condition and can
      impair TF performance (maximum biomass
      thickness is controlled by hydraulic dosage
      rate, type of media, type of organic matter,
      temperature  and  nature  of the biological
      growth).

      Requires regular operator attention.

      Incidence of clogging is relatively high.

      Requires low  loadings  depending  on the
      medium.

•     Flexibility  and  control   are   limited  in
      comparison with activated-sludge processes.

      Vector and odor problems.

      Snail problems.

DESIGN CRITERIA

A TF consists of permeable medium made of a bed
of rock, slag, or plastic  over which wastewater is
distributed to trickle through, as shown in Figure 1.
Rock or slag beds can be up to 60.96 meters (200
feet) in diameter and  0.9-2.4 meters (3 to 8 feet)
deep with rock size varying from 2.5-10.2 cm (1 to
4 inches). Most rock media provide approximately
149 m2/m3 (15 sq ft/cu ft) of surface area and less
than 40 percent void  space. Packed plastic filters
(bio-towers), on the  other hand,  are  smaller in
diameter (6 to 12 meters (20 to 40 feet)) and range
in depth  from 4.3 to 12.2 meters (14 to 40  feet).
These filters look more like towers, with the media
in various configurations (e.g., vertical flow,  cross
flow,  or various random packings).  Research has
shown that cross-flow media may  offer better flow
distribution  than other  media,  especially at low
organic loads.   When comparing vertical media
with the  60 degree cross-flow media, the vertical
media provide  a nearly  equal  distribution of
wastewater minimizing potential plugging at higher
organic loads better than cross flow media.  The
plastic medium also required additional provisions,
including ultraviolet protective additives on the top
layer  of the plastic medium filter,  and  increased
plastic wall thickness for medium packs that are
installed in the lower section of the filter where
loads  increase.
Source: Metcalf & Eddy, Inc. andTchobonaglous, 1998.

  FIGURE 1 TYPICAL TRICKLING FILTER
The  design  of a  TF system  for wastewater  also
includes  a distribution system. Rotary  hydraulic
distribution is usually standard for this process, but
fixed nozzle distributors are also  being used in
square or rectangular reactors.   Overall, fixed
nozzle distributors are being  limited  to small
facilities  and  package  plants.   Recently some
distributors  have been equipped with motorized
units to control their speed.  Distributors can be set
up to be mechanically driven at all times or during
stalled conditions.

In addition,  a TF  has an underdrain system  that
collects the filtrate and solids, and also serves as a
source of air for the microorganisms on the filter.
The  treated  wastewater and solids are piped  to a

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settling tank  where the  solids  are  separated.
Usually, part of the liquid from the settling chamber
is recirculated to improve wetting and flushing of
the filter  medium,  optimizing  the  process and
increasing the removal rate.

It is essential that sufficient air be available for the
successful operation of the TF.  It has been found
that to supply air to the system, natural draft and
wind forces are usually sufficient if large enough
ventilation ports are provided at the bottom of the
filter and the medium has enough void area.

The following four basic categories of filter design
are based on the  organic  loading of the trickling
filter.

Low-rate filters

Low-rate filters are commonly used for loadings of
less than 40 kilograms five day biochemical oxygen
demand (BOD5)/100 meters cubed per day (25 Ib
BOD5/1000cu ft/day).  These systems have fewer
problems than other filters with regards to filter
flies, odors, and medium plugging because of the
lower loading rate.  Low-rate filters with a rock
medium range in depth from 0.9 to 2.4 meters (3-
8 ft.).  Most low-rate filters are circular with rotary
distributors, but some filters currently in use  are
rectangular.   Both  of  these  configurations  are
equipped with dosing syphons or periodic pumps to
provide a high  wetting rate  for short  intervals
between rest periods. A minimum wetting rate of
0.4 liters per  square meter-second (0.7  gal/sq
ft/min) is maintained to prevent the high rate plastic
filter  medium from  drying  out.   With a rock
medium, the  filters  tend  not to be  hydraulically
limited and have  application limits  ranging from
0.01 to 0.04 liters per square meter-second (0.02 to
0.06 gal/sq ft/min).

The sloughed solids from  a low-rate  filter  are
generally well-digested and as a result these filters
yield less solids than higher rate filters.  Secondary
quality effluent is readily achievable if the low-rate
trickling filter design incorporates filter media with
bioflocculation capabilities or  good  secondary
clarification.
Intermediate-rate filters

Intermediate rate filters can be loaded up to 64 kg
BOD5/100 m3-d (40 Ib BOD5/1000cu ft/day).  In
order to ensure good distribution  and thorough
blending of the  filter and secondary effluent, the
system  should  recirculate   the  trickling  filter
effluent. The biological solids that slough from  an
intermediate trickling filter are not as well digested
as those using a low-rate filter.

High-rate filters

High-rate  filters  are  generally  loaded  at the
maximum organic loading capabilities of the filter
and receive total BOD5 loading ranging from 64 to
160kgBOD5/100m3-d(40tol001b.BOD5/1000cu
ft/day).  Achieving a secondary quality effluent is
less likely for a  high-rate filter without a second-
stage process.  As a result, high-rate filters are often
used with combined processes.

Roughing Filters

Roughing filters  are designed to allow a significant
amount of soluble  BOD to  bleed through the
trickling filter. Filters of this type generally have a
design   load   ranging  from    160-480   kg
BOD5/100 m3-d (100 to 300  Ib.  BOD5/1000cu
ft/day).

PERFORMANCE

Recent  efforts have been made to combine fixed-
film reactors with  suspended growth processes to
efficiently  remove   organic   materials   from
wastewater.  For example, the combination  of a
trickling filter with an activated-sludge process has
allowed for the  elimination of shock loads to the
more sensitive activated  sludge while providing a
highly polished effluent that could not be achieved
by a trickling filter alone. Table 1 shows the BOD5
removal rates for the four filter types discussed.

Although the TF process is generally reliable, there
is still potential for operational problems.  Some of
the common problems are attributed to increased
growth  of biofilm, improper  design,  changing
wastewater characteristics, or equipment failure.
Some common  problems with TF function are

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discussed in  the  Operation  and  Maintenance
section.
  TABLE 1 BOD5 REMOVAL RATES FOR
         VARIOUS FILTER TYPES
Filter Type
Low Rate
Intermediate Rate
High Rate
Roughing Filter
BOD5 Removal (%)
80
50
65
40
-90
-70
-85
-65
 Source: Environmental Engineers Handbook, 1997.
OPERATION AND MAINTENANCE

Disagreeable Odors from Filter

Potential Cause: Excessive organic load causing
anaerobic decomposition in filter.
Remedy: Reduce loading; increase BOD removal
in  primary   settling  tanks;   enhance  aerobic
conditions in treatment units by adding chemical
oxidants, preaerating, recycling plant  effluent, or
increasing air to aerated grit chambers;  scrub off
gases; use plastic media instead of rock.

Potential Cause:  Inadequate ventilation.
Remedy:  Increase hydraulic loading to  wash out
excess biological growth; remove debris from filter
effluent channels, underdrains, and the top of filter
media; unclog vent pipes; reduce hydraulic loading
if underdrains are flooded; install fans to induce
draft  through filter;  check  for  filter  plugging
resulting from breakdown of the medium.

Ponding on Filter Media

Potential Cause: Excessive biological growth or
foreign matter in or on the filter.
Remedy:    Reduce   organic  loading;   increase
hydraulic  loading  to increase sloughing;  use
high-pressure stream of water to flush filter surface;
maintain 1 to 2 mg/L residual chlorine  on the filter
for several hours; flood filter for 24  hours; shut
down filter  to dry out  media;  replace  media if
necessary; remove debris.
Filter Flies (Psychoda)

Potential Cause Inadequate filter media moisture.
Remedy: Increase hydraulic loading; unplug spray
orifices or nozzles; use orifice opening at end of
rotating distributor arms to spray filter walls; flood
filter for several hours each week during fly season;
maintain 1-2 mg/L residual chlorine on the filter for
several hours.

Potential Cause:  Poor housekeeping.
Remedy: Mow area surrounding filter and remove
weeds and shrubs.

Icing

Potential Cause: Low temperature of wastewater.
Remedy: Decrease recirculation; use high-pressure
stream  of  water to  remove ice  from  orifices,
nozzles, and distributor arms; reduce number of
filters in service as long as effluent limits can still
be met; reduce  retention time in  pretreatment and
primary treatment units; construct windbreak or
covers.

Rotating Distributor Slows Down or Stops

Potential  Cause:     Insufficient  flow  to  turn
distributor.
Remedy:    Increase  hydraulic  loading;  close
reversing jets.

Potential  Cause:    Clogged  arms  or  orifices.
Remedy:  Flush out arms by opening end plates;
remove solids from influent wastewater; flush out
orifices.

Potential Cause:  Clogged distributor arm  vent
pipe.
Remedy:  Remove  material  from vent  pipe by
rodding or flushing; remove solids from influent
wastewater.

Potential  Cause:    Distributor  arms  not  level.
Remedy: Adjust guy wires at tie rods.

Potential Cause:   Distributor rods hitting media.
Remedy: Level media; remove some media.

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Rotary distributors  are very reliable and easy to
maintain.  A  clearance of 15.2-22.9 centimeters
(6-9 inches) is needed between the bottom of the
distributor arm and the top of the medium bed to
allow the wastewater from the nozzles to spread out
and  cover the bed uniformly.   This also helps
prevent ice from  accumulating during  freezing
weather.

Care should be taken to prevent leaks. Follow the
manufacturer's operation and maintenance (O&M)
instructions on pumps, bearings, and motors.  All
equipment must be  tested  and  calibrated as
recommended by the equipment manufacturer. A
routine O&M schedule should  be developed and
followed for any TF system.  It is critical that a TF
system be pilot tested prior to installation to ensure
that  it  will  meet  effluent   discharge  permit
requirements for that particular site.

Disagreeable Odors from Filter
     Excessive  organic  load  causing anaerobic
     decomposition  in  filter—Reduce  loading;
     increase BOD removal in primary  settling
     tanks;  enhance  aerobic  conditions  in
     treatment units by adding chemical oxidants,
     preaerating,  recycling  plant  effluent,  or
     increasing air to aerated grit chambers; scrub
     off-gases; use plastic media instead of rock

•    Inadequate ventilation—Increase hydraulic
     loading to wash out excess biological growth;
     remove debris from filter effluent channels,
     underdrains,  and the  top of filter media;
     unclog vent pipes; reduce hydraulic loading if
     underdrains are flooded; install fans to induce
     draft through filter; check for filter plugging
     resulting from breakdown of media.

Ponding on Filter Media

     Excessive    biological   growth—Reduce
     organic loading; increase hydraulic loading to
     increase sloughing; use high-pressure stream
     of  water  to flush  filter  surface  (recycled
     water); maintain  1  to  2 mg/L  residual
     chlorine on the filter for several hours; flood
     filter for 24 hours; shut down filter to dry out
     media; replace media if necessary; remove
     debris.
Filter Flies (Psychoda)

•    Inadequate   moisture   on   filter
     media—Increase hydraulic loading;  unplug
     spray orifices or nozzles; use orifice opening
     at end of rotating distributor arms to spray
     filter walls; flood filter for several hours each
     week during fly season; maintain 1 to 2 mg/L
     residual  chlorine  on the filter  for  several
     hours.

     Poor housekeeping—Mow area surrounding
     filter and remove weeds and shrubs.

Icing

•    Low temperature of wastewater—Decrease
     recirculation;  use high-pressure  stream of
     water to remove ice from orifices, nozzles,
     and distributor arms; reduce number of filters
     in service as long as effluent limits can still
     be met; reduce retention time in pretreatment
     and  primary  treatment  units;   construct
     windbreak or covers.

Rotating Distributor Slows Down or Stops

•    Insufficient flow to turn distributor—Increase
     hydraulic loading; close reversing jets.

•    Clogged arms or orifices—Flush out arms by
     opening  end plates; remove  solids  from
     influent wastewater; flush out orifices.

•    Clogged distributor arm vent pipe—Remove
     material  from vent  pipe by rodding or
     flushing;  remove  solids  from  influent
     wastewater.

•    Distributor arms not level—Adjust guy wires
     at tie rods.

•    Distributor rods hitting media—Level media;
     remove some media.

Rotary distributors are  very reliable and  easy to
maintain.  A clearance of 15 to 23 cm (6 to 9
inches) is  needed  between  the bottom  of the
distributor arm and the top of the media bed to
allow the wastewater from the nozzles to spread out

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and cover the bed uniformly. This also prevents ice
from accumulating during freezing weather.

Care should be taken to prevent leaks.  Follow the
manufacturer's operation and maintenance (O&M)
instructions on pumps, bearings, and motors.  All
equipment  must  be  tested  and  calibrated as
recommended by the equipment manufacturer. A
routine O&M  schedule should be developed and
followed for any TF system. It is critical that a TF
system be pilot tested prior to installation to ensure
that  it  will   meet  effluent  discharge   permit
requirements for that particular site.

COST

The cost for a TF system are summarized in Table
2.  These  costs include construction,  labor, total
O&M,  and materials  needed.  Since every  TF
system is unique to its site, the overall cost will be
site specific.
REFERENCES

Other Related Fact Sheets

Trickling Filter Nitrification
EPA832-F-00-015
September, 2000
Other EPA  Fact Sheets can  be found  at the
following web address:

http ://www. epa. gov/owmitnet/mtb fact, htm

1.     Liu and Liptak.   1997.  Environmental
      Engineering Handbook. 2d ed.  The CRC
      Press, LLC. Boca Raton Florida.

2.     Martin, Edward J. and Edward T. Martin.
      Technologies  for   Small  Water  and
      Wastewater Systems.   1991.  p. 122. New
      York, New York.

3.     Metcalf & Eddy, Inc. 1991. Wastewater
      Engineering:  Treatment,  Disposal,  and
      Reuse.   3d   ed.   The   McGraw-Hill
      Companies. New York, New York.

4.     Mulligan,  T. J. and O. K.  Scheible.  1990.
      Upgrading Small Community Wastewater
      Treatment  Systems  for   Nitrification.
      HydroQual, Inc. Mahwah, New Jersey.

5.     U.S.  EPA, 1991. Assessment of Single-
      Stage Trickling Filter Nitrification. EPA
      430/09-91-005, EPA  Office of Municipal
      Pollution Control. Washington, D.C.

6.     U.S.  EPA,   1993.   Manual:  Nitrogen
      Control.  EPA Office  of  Research and
      Development.  EPA/625/R-93/010.
      Cincinnati, Ohio. EPA Office  of Water.
      Washington, D.C.
                  TABLE 2 COST SUMMARY FOR A TRICKLING FILTER
  Wastewater   Construction Cost         Labor               O&M              Materials
  Flow (MGD)   (Millions of Dollars)  (Millions of Dollars)   (Millions of Dollars)   (Millions of Dollars)
1
10
50
100
0.76
6.34
25
63.40
0.05
0.23
0.5
1.0
0.063
0.15
0.70
1.3
0.009
0.05
0.1
0.2
 Source: Adapted from Martin and Martin, 1990.

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7.      Water Environment Federation  (WEF).
       1996. Operation of Municipal Wastewater
       Treatment Plants. Manual of Practice No.
       11.  5th  ed.  vol.  2.  WEF.  Alexandria,
       Virginia.

ADDITIONAL INFORMATION

Tony Post, Plant Manager
Central Wastewater Treatment
1020 Sargent Road
Dallas, TX 75203

Jim Medlock, Operations Supervisor
Littleton/Englewood Wastewater Treatment Plant
2900 South Platte River Drive
Englewood, CO 80110

National Small Flows Clearing House
at West Virginia University
P.O. Box 6064
Morgantown, WV 26506

The  mention  of  trade  names or  commercial
products  does  not  constitute  endorsement  or
recommendation for use by the U.S. Environmental
Protection Agency.

This fact sheet was developed in cooperation with
the National Small Flows Clearinghouse whose
services are greatly appreciated.
                                                         For more information contact:

                                                         Municipal Technology Branch
                                                         U.S. EPA
                                                         Mail Code 4204
                                                         1200 Pennsylvania Ave., NW
                                                         Washington, D.C., 20460
                                                          »MTB
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                                                          MUNICIPAL TECHNOLOGY BRANCH

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