United States
                       Environmental Protection
                       Office of Water
                       Washington, D.C.
September 2000
Waste water
Technology Fact Sheet
Trickling  Filter  Nitrification

Nitrogen is one of the principal nutrients found in
wastewater.  Discharges containing nitrogen can
severely  damage  a  water  resource  and  it's
associated ecosystem.   As  a  result,  several
chemical,  physical and biological processes have
been used to promote the  removal of nitrogen.
Nitrification and denitrification are two suggested
processes that significantly reduce nitrogen levels in
wastewater. This fact sheet will primarily focus on
the  nitrification process using a trickling  filter
system.   TFs are designed as aerobic attached
growth reactors and have been proven to be suitable
for the removal of ammonia nitrogen.

Nitrogen Content in Wastewater

Nitrogen exists in many forms in the environment
and can enter aquatic systems from either natural or
human-generated sources.  Some of the primary
direct sources or transport mechanisms of nitrogen
from sewage include:

•    Untreated sewage—direct discharge.

•    Publically owned treatment works (POTW)
     effluent—direct discharge, land application.

     POTW waste solids—direct discharge, land

•    Septic   tanks    and    leaching
     fields—groundwater movement.

Untreated  sewage  flowing  into  a  municipal
wastewater facility has total nitrogen concentrations
ranging from  20 to 85 mg/L.  The nitrogen in
                      domestic  sewage  is  approximately  60 percent
                      ammonia nitrogen, 40 percent organic nitrogen, and
                      small quantities of nitrates.

                      Treated domestic  sewage  has varying levels  of
                      nitrogen, depending  on the method of treatment
                      used. Most treatment plants decrease the level of
                      total nitrogen via cell synthesis and solids removal.
                      However, unless  there  is  a  specific treatment
                      provision for nitrification, most ammonia nitrogen
                      passes through the system and is discharged as part
                      of the plant effluent.

                      The presence of ammonia-nitrogen in discharges
                      from wastewater facilities can result in ammonia
                      toxicity to aquatic  life, additional oxygen demand
                      on receiving waters, adverse public health effects,
                      and decreased suitability for reuse.

                      Biological Nitrification

                      Nitrification is a process  carried out by a series of
                      bacterial populations that  sequentially oxidize
                      ammonium to nitrate with intermediate formation
                      of nitrite  carried out  by nitrosomonas  and
                      nitrobacter.   These  organisms are  considered
                      autotrophic because they obtain  energy from the
                      oxidation of inorganic nitrogen compounds.  The
                      two steps in  the  nitrification process and  their
                      equations are as follows:

                      1) Ammonia is  oxidized  to nitrite  (NO2") by
                      Nitrosomonas bacteria.
                        2 NH4+
3 O9 •  2 NO,

2) The nitrite  is converted to nitrate (NO3") by
Nitrobacter bacteria.

           2NO2- +  O2  •  2NO3-

Once the nitrate is  formed, the wastewater can
either flow to a clarifier or continue on through a
denitrification process  to  reduce the nitrate  to
nitrogen gas  that is released into the atmosphere.
The process is dependent on the desired percent of
nitrification.   Since  complete nitrification is  a
sequential reaction treatment process, systems must
be designed to provide an environment suitable for
the growth of both groups of nitrifying bacteria.
These two reactions essentially supply the energy
needed by nitrifying bacteria for growth.

There are several major factors that influence the
kinetics of nitrification.  These are organic loading,
hydraulic loading,   temperature,  pH,  dissolved
oxygen concentration, and filter media..

1.     Organic  loading:  The  efficiency  of the
       nitrification  process  is  affected  by the
       organic  loadings.      Although  the
       heterotrophic biomass is not essential for
       nitrifier   attachment,  the   heterotrophs
       (organisms that use organic carbon for the
       formation of cell tissue) form biogrowth to
       which   the   nitrifiers   adhere.   The
       heterotrophic  bacteria grow much  faster
       than nitriifers at high BOD concentrations.
       As a result, the nitrifiers can be over grown
       by  heterotrophic bacteria  and eventually
       cause the nitrification process to cease.  In
       order to achieve a high level of nitrification
       efficiency, the  organic loadings  listed  in
       Table 1  should be maintained.

2.     Hydraulic loading: Wastewater  is normally
       introduced at the top of the attached growth
       reactor  and  trickles  down  through   a
       medium.    The  value  chosen  for the
       minimum hydraulic loading should ensure
       complete media wetting under  all influent
       conditions. Hydraulic and organic loading
       are not independent parameters because the
       wastewater concentration entering the plant
       cannot be controlled. The total hydraulic
       flow to the filter can be controlled to some
       extent  by recirculation  of  the  treated
       effluent.   Recirculation also increases the
       instantaneous flow at points in the filter and
       reduces the resistance to mass transfer.
       This also  increases the apparent substrate
       concentration and the growth and removal
       rate.     The   third   major   benefit   of
       recirculation in nitrifying trickling filters is
       the  reduction  of  the  influent   BOD
       concentration  which  makes the nitrifiers
       more competitive.   This in turn increases
       the nitrification efficiency and increases the
       dissolved oxygen concentration.

  TF Media
 Loading Rate Ib
BOD/1, 000 ft3/d(g

Tower TF
6-3 (96-48)
Source :  Metcalf & Eddy, Inc. with permission from The
McGraw-Hill Companies, 1991.

3.      Temperature:  The nitrification process is
       very dependent on temperature and occurs
       over a range of approximately 4» to 45* C
       (39* to 113 •  F). Quantifying the effects of
       temperature on the nitrification process has
       been very difficult and as a result the effects
       are  variable.  Higher nitrification rates are
       expected to be more affected by temperature
       than lower rates of nitrification. Figure 1
       shows   how   temperature   can   effect
       nitrification rates in a TF system.

4.      pH:  According  to  EPA  findings  (EPA,
       1993), pH levels in the more acidic range
       have been reported to decrease the rate of
       ammonium   oxidation.     As  a  result,
       nitrification rates may drop significantly as
       pH  is lowered below neutral range.   For

 performance stability it is best to maintain a
 pH between 6.5  and 8.0.   The effect of
 lower pH conditions, if anticipated, should
 not  be ignored when sizing nitrification
 reactors,  even though acclimation may
 decrease the effect of pH on the nitrification
         Mkfemd J
           s    io   is   ao   as
                    TamparafijrB, "C
Source: Parker etal.,  1990.


5.     Dissolved Oxygen (DO):  The concentration
      of dissolved  oxygen  affects  the  rate  of
      nitrifier  growth   and  nitrification   in
      biological  waste treatment systems.   The
      DO value  at which nitrification is limited
      can be 0.5  to 2.5 mg/L in either suspended
      or attached growth systems under steady
      state conditions  depending on the degree of
      mass-transport or diffusional resistance and
      the solids  retention time.  The  maximum
      nitrifying growth rate is  reached at a DO
      concentration of 2 to 2.5 mg/L. However, it
      is not necessary to grow at the maximum
      growth rate to get  effective nitrification if
      there is adequate contact time in the system.
      As a result there is a broad range of DO
      values where DO  becomes rate limiting.
      The DO value might be at 2.5 in a high rate
      activated  sludge   process  because  the
      bacteria have  little time  to accomplish
                                                        nitrification   while   very   effective
                                                        nitrification can be achieved in an aeration
                                                        ditch where  the hydraulic retention time is
                                                        24 hours.  A high solids retention time may
                                                        be required to ensure complete nitrification
                                                        at low DO concentrations and for conditions
                                                        where  diffusional resistance is significant.
                                                        Under transient conditions of organic shock
                                                        loading,  diffusional   resistance  and
                                                        heterotrophic/nitrifier  competition  can
                                                        increase the limiting DO value  significantly.
                                                        As a result, nitrite conversion to nitrate can
                                                        become  the  rate  limiting step   in  the
                                                        nitrification process.  The intrinsic growth
                                                        rate  of nitrosomonas is not limited at DO
                                                        concentrations  above 1.0 mg/L,  but DO
                                                        concentrations greater than 2.0  mg/L may
                                                        be required in practice. Figure 2 illustrates
                                                        how  the  BOD5  surface  loading can
                                                             the  percent  of   ammonium
                                                I  «
                                                        BOD g Surfaco Loading, W1.000 id fW
                                                     S      1.0     2,0     3.0     4,0
                                                                       Stockton Plan!.
                                                                       RusfcMwSifSI) _
                                                           BGDgSurfew Loading,
                                            Source: Parker & Richards, 1986.

                                              FIGURE 2 EFFECT OF BOD5 SURFACE
                                                   LOADING ON NITRIFICATION
                                               EFFICIENCY OF ROCK AND PLASTIC
                                                    MEDIA TRICKLING FILTERS
                                                   Filter Media: The greater the surface area of
                                                   plastic media, the greater the ability of the
                                                   TF  to  accomplish nitrification at higher
                                                   volumetric loadings relative to rock media

       filters.  Filter media provide more area for
       bacteria growth and therefore provide more
       bacteria "workmen."  Plastic filter  media
       also provide better gas transfer due to the
       greater draft and higher void fraction, and
       less plugging. One of the greatest benefits
       of plastic filter media is that they are light
       and can be constructed to greater depths.
       This increases the  hydraulic load capacity
       and improves mass transfer. Rock  filters,
       on  the  other  hand,  often  have  poor
       ventilation, particularly when water and air
       temperatures are similar or identical.  Figure
       3 evaluates how different filter media can
       affect the nitrification process.
           iOOs Surface Loading, lb/i,000 sq W
Two-Stage Nitrification—Allentown,

A treatment facility  in Allentown, Pennsylvania,
was  required to meet effluent ammonia  nitrogen
limits of 3 mg/L in the warmer months and 9 mg/L
during colder months.  This facility was designed
for an average flow of 17,280 m3/d (40 MOD) with
an effluent BOD5 limit of 30 mg/L.

The various unit processes in this facility included
screening,  grit  removal,   primary  clarification,
first-stage   TF,  intermediate  clarification,
second-stage TF, final clarification,  and  chlorine

The  first  stage had  four plastic  media TFs in
parallel, while the  second stage had a single large
rock filter. A recycle ratio of 0.2:1 was practiced
only on the second-stage TF. Temperatures during
the warmer months ranged between 17° C and 19°
C and during the colder months temperatures varied
from 11° to 16° C.

The  BOD5 volumetric loading in  the first stage
during the study period was high,  averaging  330
g/m3/d  (66  lb/1,000  ft3/d),  with  an  equivalent
NH4-N loading  of 33,5 g/m3/d (6.7  lb/1,000 ft3/d).
The    average   first-stage   effluent   BOD5
concentrations during warmer and colder periods
were 50 and 73  mg/L, respectively, with associated
NH4-N levels of 10.0 and 11.4 mg/L, respectively.

The BOD5 loading in the second stage averaged
42.5 g/m2/d  (8.5  lb/1,000  ft2/d).   The  average
monthly  effluent  BOD5  concentration  was
consistent throughout the  study  year,  ranging
between 6 and 18 mg/L. The effluent NH4-N level
averaged 4.7 mg/L during the warmer months and
5.9 mg/L during the colder months.  This plant was
able to consistently meet its effluent BOD5 standard
and ammonia-nitrogen limits throughout the study.

Nitrification process reliability is directly related to
carbonaceous BOD (CBOD) loading.  Low levels
of  organics  in  the  influent   to   two-stage,
attached-growth reactors can potentially eliminate
the need for intermediate solid-liquid separation
between the stages.  Short-circuiting is less of a
concern because clogging of voids in the media is
also reduced.

In the absence of significant CBOD5 loadings (e.g.,
in the second stage of a two-stage system), the rate
of  nitrification  in attached-growth  reactors is
proportional to the concentration of both ammonia
nitrogen and DO concentrations in the liquid phase.
The reported effect of temperature is varied for TFs
operating at low CBOD5 levels  by factors such as
oxygen availability, influent and effluent ammonia
nitrogen  concentration,  and  hydraulic  loading

Different   media   require  different  minimum
hydraulic loadings to ensure complete wetting of
the  TF surface.  In addition, cross-flow media offer
greater oxygen transfer  efficiency   and  higher
specific surface area than vertical -flow media.


Some  advantages  and  disadvantages of TFs are
listed below:


      Simple, reliable process.

      Suitable in areas where large tracts of land
      are not available for a treatment system.

•     May   qualify  for  equivalent  secondary
      discharge standards.

•     Effective  in treating high concentrations of
      organics depending  on the type of media
      used, and flow configuration.

      Appropriate   for  small-  to medium-sized

•     High degree of performance reliability at low
      or stable loadings.

•     Ability to  handle and recover from  shock

•     Durability of process elements.

•    Low power requirements.

•    Requires only a moderate level of skill and
     technical expertise to manage and operate the

•    Reduction   of   ammonia-nitrogen
     concentrations in the wastewater.


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

•    Regular operator attention needed.

•    Relatively high incidence of clogging.

•    Relatively low  organic  loadings  required
     depending on the media.

•    Limited flexibility and control in comparison
     with activated-sludge processes.

•    Potential for vector and odor problems.

•    Autotrophic bacteria (nitrifiers) are sensitive
     to changes in the waste  stream (e.g. pH ,
     temperature, and organics).

•    Autotrophic  bacteria  (nitrifiers)  are  more
     sensitive to "shock loads" than other bacteria.

     Predation  (i.e.  fly larvae,  worms,  snails)
     decreases  the nitrifying  capacity  of the


The  two  general  types   of  TF  nitrification
configurations  are single-stage  and  two-  (or
separate) stage.

     Single-stage:   Carbon   oxidation   and
     nitrification take place in a single TF unit.

•    Two-stage: Reduction of CBOD5 occurs  in
     the first treatment stage; nitrification occurs
     in the second stage.
Numerous types and combinations of treatment
units are in use, depending on permit requirements,
site conditions,  historical development,  designer
experience, and feed concentrations. In general, a
single-stage TF removes organic carbon or CBOD5
in the upper  portion  of the unit and  provides
bacteria for nitrification in the lower portion.

There  are several  factors  that  do promote  a
significant amount of nitrification in a TF system.
In general, TF are designed with at least a minimum
effluent recycle capability to  maintain  a stable
hydraulic  loading  during  seasonal variations.   In
order   to  increase  nitrification  efficiency,
recirculation and forced air ventilation should be
practiced.  One way of  ensuring  this is to use
ventilation fans.  Both of these actions increase the
DO concentration in the bulk liquid and ultimately
performance improvement has been achieved.

The value of the hydraulic loadings and organic
loadings is also critical to nitrification efficiency.
The value selected for minimum hydraulic loading
should ensure complete media wetting under all
influent conditions.  The value is dependent on the
media  employed in  the filter.  Typical minimum
hydraulic loading values range from 1 to 3  m3/m2/hr
(0.41-1.22 gpm/sq.ft).    Additional factors that
influence   nitrification  efficiency  include  the
specific hydraulic pattern of the TF media and the
retention time of the wastewater within the plastic
media.     Plastic  media  with   crossflow
characteristics,  when compared to  vertical flow
media, increase the hydraulic retention  time  or
contact time between the biofilm and influent and
provide superior oxygen transfer.  The rock media
typically used in Tfs are about 2.5 to 10 cm (1 to 4
inches) in diameter with a recirculation ratio of 1:1.

As mentioned before, pH  conditions in TF liquids
below  certain  critical  levels   can  affect the
nitrification performance. Normally, significant pH
effects can be avoided by ensuring that the effluent
alkalinity  is equal to or greater than 50  mg/L  as
CaCO3.   For  design purposes and performance
stability, it is best to maintain  pH at 6.5 to 8.0. The
importance of DO concentration can often mask the
effects of pH  and temperature on nitrification  in
TFs,  particularly   at  high  carbonaceous feed

The  importance of the DO concentration in the
operation  of all  TFs  highlights  the need for
sufficient ventilation. If enough passageways are
provided, the differences in the air and wastewater
temperatures and humidity differences between the
ambient air and the  air  in the TF provide a draft.
This mechanism may provide the necessary aeration
requirements on occasion,  but not consistently.
Historically engineers have selected an appropriate
BOD 5 surface loading as a function of temperature
to  design  TFs  for nitrification  of municipal
wastewater at high CBOD5.

With regards to the feed concentrations, the number
of operating TFs designed to achieve nitrification of
municipal wastewater containing a high  CBOD5
concentration of primary treated  wastewater is
limited.  There are as  of  1991,  10  plants that
achieve CBOD5 removal and nitrification in single
trickling filter units known as combined or single-
stage units.  The aforementioned recommended
values for  pH, temperature,  hydraulic loading,
effluent alkalinity,  and depths of rock media can be
applied to systems handling high CBOD5 loads.

Table 2 demonstrates some of the  design criteria
recommended   for  trickling   filters  handling
wastewater  with  low   carbonaceous   feed
           LOW CBOD5 SYSTEM
 Design Criteria
Low CBOD5 Feed
 Wastewater flow characteristics m3/d (MGD)
 raw wastewater average flow
 total secondary effluent average
   21,055 (5.5)
 Actual Secondary Effluent Concentrations, mg/L
 Soluble COD
 Nitrogen available for nitrification
 Alkalinity as CaCO3
 Trickling filter Reactor Effluent Characteristics, mg/L
 Soluble COD
 Ammonia Nitrogen
 Design Conditions/Assumptions
 Reactor temperature, • C
 Reactor pH range
 Air flow rate (at average
 secondary loading) kg O2
 supplied/kg 02 required
Source: U.S. EPA, 1993.

A  degree of  ammonium oxidation  has  been
achieved for many years in low or standard rate
rock media trickling filters. In order for these filters
to complete  nitrification (90  percent ammonium
removal) the  organic volumetric loading rate must
be limited to approximately 80 grams BOD5/m3/d (5
lb/1000 ft3/d).

The performance  of  the nitrification  process
depends on many factors, including availability of
oxygen (i.e., adequate ventilation), level of CBOD,
ammonia  nitrogen concentration, media type and
configuration, hydraulics of the TF,  temperature,
and pH.
Single-Stage Nitrification

To achieve adequate nitrification in a single-stage
TF, the organic volumetric loading rate must be
limited to the approximate ranges shown in Table 1.
Filters with a plastic media have greater surface
contact area (approximately 80 percent) per unit
volume than rock or slag, and achieve the same
degree of nitrification with higher organic loadings.
Plastic media also  provide  better ventilation  and
improved oxygen transfer.


Although  TFs are generally reliable, operating
problems can be caused by increased growth  of
biofilm  due to  high organic loads,  changes  in
wastewater  characteristics,  improper design,  or
equipment failure.  If nitrification is not achieved,
steps  should be taken to determine the probable

cause(s). The first step is to sample and analyze the
TF influent wastewater for an appropriate level  of
pH, temperature, soluble BOD,  dissolved oxygen
(DO), and proper organic and hydraulic loading.
The  soluble BOD  concentration must be low  in
order for autotrophic bacteria to compete with the
heterotrophic bacteria.  The  second step involves
checking the TF influent DO to ensure that the
autotrophic bacteria are able to derive oxygen from
that  source.  They can also obtain  oxygen via
oxygen transfer within the filter media. Excessive
biological growth can minimize oxygen transfer and
may also promote ponding on the filter media. The
final step involves checking to ensure that the TF is
receiving influent wastewater and recirculation  at
the proper organic and hydraulic loading.

More information  on operating and  maintaining
trickling filters (TF) can be obtained from the U.S.
EPA Wastewater Technology Fact sheet, Trickling
Filters, EPA 832-F-99-078.


Typical costs for a TF  system are summarized  in
Table 3.  The costs associated with operating and
maintaining a TF Nitrification system are expected
to be higher due to increased system size and the
additional maintenance required to   support the
media.  Nitrification is considered very site specific
and as a result it is hard to determine a "general"
cost.  For example, two identical systems in two
parts of the United States (e.g.  Florida and New
England) will require  different tank  volumes  to
nitrify due to temperature differences.
Flow (MGD)

Other Fact Sheets

Trickling Filters
EPA 832-F-00-014
September, 2000

Other EPA  Fact Sheets  can be  found at the
following web address:

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

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

3.     Parker, D.S., M.P., Lutz,  and A.M., Pratt.
       1990.  New Trickling Filter Applications in
      the U.S.A. Water Sci. Tech.  22(1/2):215.

4.     Parker,  D.S.   and  T. Richards.  1986.
      Nitrification  in Trickling  Filters.  JWPCF

5.     U.S.    EPA,   1991.  Assessment   of
      Single-Stage Trickling Filter Nitrification.
      EPA Office of Municipal Pollution Control.
      Washington, D.C. EPA 430/9-91-005.

6.     U.S. EPA, 1993. Manual: Nitrogen Control.
      EPA Office of Research and Development.
      Cincinnati, Ohio.  EPA Office  of Water.
      Washington, D.C. EPA/625/R-93/010.

7.     Water  Environment Federation (WEF).
       1996.  Operation of Municipal Wastewater
      Treatment Plants. Manual of Practice No.
       11.  5th  ed. vol.  2.  WEF. Alexandria,
 Source: Adapted from Martin and Martin, 1990.
 Note: Costs are in millions of dollars.

8.      Water Environment Federation (WEF) and
       American  Society  of  Civil  Engineers
       (ASCE).  1998.  Design  of  Municipal
       Wastewater Treatment Plants. Manual of
       Practice No.  8. vol. 2. WEF. Alexandria,


Danbury Wastewater Treatment Plant
Public Works Department
Danbury, CT 06810

Hudson Wastewater Treatment Facility
1 Municipal Drive
Hudson, MA 01749

John Mainini, Director
Milford STP
P.O. Box 644
Milford, MA 01757

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
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