vvEPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA832-F-00-015
September 2000
Waste water
Technology Fact Sheet
Trickling Filter Nitrification
DESCRIPTION
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
application.
• 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,
4H+
2H2O
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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.
TABLE 1 TYPICAL LOADING RATES
FOR SINGLE-STAGE NITRIFICATION
TF Media
Nitrification
Loading Rate Ib
BOD/1, 000 ft3/d(g
BOD/m3/d)
Rock
Plastic
Tower TF
75-85
85-95
75-85
85-95
10-6(160-96)
6-3 (96-48)
181-12(288-192)
12-6(192-96)
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
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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
rate.
3JS
3.0
ZS,
ii
I
2.0
1,5
0.6
Lima
Mkfemd J
Zurich
s io is ao as
TamparafijrB, "C
Source: Parker etal., 1990.
FIGURE 1 EFFECTS OF TEMPERATURE
ON NITRIFICATION RATES IN TRICKLING
FILTERS
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
influence
removal.
the percent of ammonium
100
ao
I
I «
g
E
BOD g Surfaco Loading, W1.000 id fW
S 1.0 2,0 3.0 4,0
Stockton Plan!.
RusfcMwSifSI) _
10
15
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
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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.
100
80
80
iOOs Surface Loading, lb/i,000 sq W
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Two-Stage Nitrification—Allentown,
Pennsylvania
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
disinfection.
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
conditions.
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.
ADVANTAGES AND DISADVANTAGES
Some advantages and disadvantages of TFs are
listed below:
Advantages
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
communities.
• High degree of performance reliability at low
or stable loadings.
• Ability to handle and recover from shock
loads.
• Durability of process elements.
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• Low power requirements.
• Requires only a moderate level of skill and
technical expertise to manage and operate the
system.
• Reduction of ammonia-nitrogen
concentrations in the wastewater.
Disadvantages
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
system.
DESIGN CRITERIA
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
concentrations.
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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
concentrations.
TABLE 2 DESIGN INFORMATION FOR
LOW CBOD5 SYSTEM
Design Criteria
Low CBOD5 Feed
Concentration
System
Wastewater flow characteristics m3/d (MGD)
raw wastewater average flow
18,925(5.0)
total secondary effluent average
flow
21,055 (5.5)
Actual Secondary Effluent Concentrations, mg/L
Soluble COD
27
Nitrogen available for nitrification
21
Alkalinity as CaCO3
120
Trickling filter Reactor Effluent Characteristics, mg/L
Soluble COD
20
Ammonia Nitrogen
1.5
Design Conditions/Assumptions
Reactor temperature, • C
15
Reactor pH range
7.0-7.6
Air flow rate (at average
secondary loading) kg O2
supplied/kg 02 required
50
Source: U.S. EPA, 1993.
PERFORMANCE
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.
OPERATION AND MAINTENANCE
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
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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.
COSTS
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.
TABLES COST SUMMARY FORA
TRICKLING FILTER
Wastewater
Flow (MGD)
1
10
100
Construction
Cost
0.76
6.34
63.40
Labor
0.05
0.23
1.01
O&M
0.63
0.36
1.3
Materials
0.011
0.004
0.20
REFERENCES
Other Fact Sheets
Trickling Filters
EPA 832-F-00-014
September, 2000
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm
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
58:896.
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,
Virginia.
Source: Adapted from Martin and Martin, 1990.
Note: Costs are in millions of dollars.
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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,
Virginia.
ADDITIONAL INFORMATION
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
sMTB
Excelence fh tompfance through optfhial tethnltal solutfons
MUNICIPAL TECHNOLOGY BRANCH
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