&EPA
United States
Environmental Protection
Agency
Wastewater Management Fact Sheet
Denitrifying Filters
INTRODUCTION
Discharge permits for treated wastewater from
publicly owned treatment works (POTWs) often
include effluent limitations for nutrients. Total
maximum daily loads (TMDLs) for nutrients are
being developed for many waterbodies
throughout the United States. TMDLs and other
water quality-drivers have resulted in POTWs
having to comply with more stringent effluent
limitations for parameters such as total nitrogen
(TN).
Untreated domestic wastewater contains
ammonia. Nitrification is a biological process
that converts ammonia to nitrite and nitrite to
nitrate. If standards require that the resulting
nitrate be removed, one treatment alternative is
the process of denitrification, in which nitrate is
reduced to nitrogen gas. One treatment system
used for denitrifying wastewater effluent is the
denitrifying filter. In addition to the reduction of
total nitrogen, this treatment process removes
suspended solids from the effluent.
NITRIFICATION/DENITRIFICATION
Nitrification is a microbial process by which
ammonia is sequentially oxidized to nitrite and
then to nitrate. The nitrification process is
accomplished primarily by two groups of
autotrophic nitrifying bacteria that can build
organic molecules by using energy obtained
from inorganic sources—in this case, ammonia
or nitrite.
In the first step of nitrification, ammonia-
oxidizing bacteria oxidize ammonia to nitrite
according to equation (1):
NH3 + O2 ~> NO2
2e
(1)
Nitrosomonas is the most frequently identified
genus associated with this step, although other
genera, including Nitrosococcus and
Nitrosospira, may be involved. The subgenera
Nitrosolobus and Nitrosovibrio
autotrophically oxidize ammonia.
can also
In the second step of the process, nitrite-
oxidizing bacteria oxidize nitrite to nitrate
according to equation (2):
NO2 + H2O -> NO3 + 2H+ +2e
(2)
Nitrobacter is the genus most frequently
associated with this second step, although other
genera, such as Nitrospina, Nitrococcus, and
Nitrospira, can also autotrophically oxidize
nitrite (U.S. EPA, Nitrification, August 2002).
Denitrification is the process by which nitrates
are reduced to gaseous nitrogen by facultative
anaerobes. Facultative anaerobes, such as fungi,
can flourish in anoxic conditions because they
break down oxygen containing compounds (e.g.,
NO3") to obtain oxygen. Once introduced into
the aquatic environment, nitrogen can exist in
several forms — dissolved nitrogen gas (N2),
ammonia (NFLt+ and NHa), nitrite (NO2"), nitrate
(NO3"), and organic nitrogen as proteinaceous
matter or in dissolved or paniculate phases. The
energy reactions are (Metcalf and Eddy, 1979):
6 NO3- + 2 CH3OH -» 6 NO2" + 2 CO2 + 4 H2O
(Step 1)
6 NO2' + 3 CH3OH -» 3 N2 + 3 CO2 + 3 H2O +6 Off
(Step 2)
Overall,
6 NO3- + 5 CH3OH
Off
5 CO2 + 3 N2 + 7 H2O + 6
The organisms carrying out this process are
called denitrifiers. In general, they are
heterotrophic bacteria that metabolize readily
biodegradable substrate under anoxic conditions
using nitrate as the electron acceptor. If oxygen
is available, these bacteria use it for metabolism
before they use the nitrate. Therefore, dissolved
oxygen concentrations must be minimized for
the denitrification process to function
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efficiently. Oxygen is typically minimized by
avoiding aeration of the wastewater and having
a high concentration of biochemical oxygen
demand (BOD) so that the microorganisms use
all the oxygen.
A readily biodegradable organic compound (a
carbon source) must be available for the
denitrifiers to use. Because the typical
denitrifying filter installation is downstream of
aerobic treatment, in which most of the organic
material is oxidized, some organic material must
be added to the filter influent to sustain the
growth of the denitrifiers. The carbon source
most often selected is methanol, which is readily
degraded under anoxic and aerobic conditions.
Other carbon sources, such as acetic acid, also
can be used in denitrifying filter systems.
DESIGN FEATURES
Filter Configurations
Denitrifying filters have been utilized for
wastewater treatment for a number of years. The
combination of denitrification and solids
removal was first patented in the 1970s. Since
that time, several companies have developed
their own denitrifying filters. In addition to
meeting TMDL requirements, facilities such as
the East Central Regional Water Reclamation
Facility in West Palm Beach, Florida, are
utilizing denitrification filters as part of an
advanced wastewater treatment system to enable
them to reuse treated wastewater to augment
wetlands and to recharge aquifers (Figures 1 and
2).
There are two main process configurations for
denitrification filters commercially available,
downflow and upflow continuous backwash
filters.
Downflow denitrification filters operate in a
conventional filtration mode and consist of
media and support gravel supported by an
underdrain. Manufacturers include Severn Trent
Services (Fort Washington, Pa.), maker of the
TETRA Denite system; F.B. Leopold Co. Inc.
(Zelienople, Pa.), maker of the elimi-NITE
system; and Siemens Water Technology Davco
Figures 1 and 2. Denitrifying filters at the
East Central Regional Water Reclamation
Facility, West Palm Beach, Florida
Products (Thomasville, Ga.),
Davco denitrification filter.
maker of the
Wastewater enters a downflow filter over weirs
along the length of the filter bed on both sides.
Filter effluent is conveyed from the bottom of
the filter over a control weir into a clear well.
Backwashing is required at regular intervals.
Backwashing typically involves air scouring and
backwashing with air and water. During the
process, nitrate is metabolized to nitrogen gas,
which becomes embedded in the filter media.
Nitrogen-release cycles are needed to remove
these nitrogen gas bubbles that accumulate. The
piping for the filter influent and backwash is
similar to that of conventional filters.
Upflow continuous-backwash filters differ in
that influent wastewater flows upward through
the filter, countercurrent to the movement of the
sand bed.
Wastewater enters the filter through the influent
pipe (where methanol can be added), and then is
transported downward through a supply pipe
and distributors (Figure 3). The water moves up
through the filter media and filtrate is
discharged from the upper portion of the filter.
The filter media travels slowly downward and is
drawn into an airlift pipe in the center of the
filter. Compressed air is introduced to the airlift,
drawing sand upward and scouring it. At the top
of the airlift, the media is returned to the filter
bed. Filtered water rises through a separator that
removes the light dirt particles by washing them
away and returns the large, heavy sand grains to
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©'
feed pipe
(3 distributors
(7) airlift pipeline
Figure 3. Astrasand upflow continuous-
backwash filter.
the top of the filter bed. The reject, or backwash,
water continuously exits near the top of the
filter. The reject-water weir is set at a lower
elevation than the effluent weir to allow clean
water to enter the washer and separator
continuously by differential head, eliminating
the need for typical backwash-supply pumps.
Manufacturers include Parkson Corp. (Fort
Lauderdale, Fla.), maker of the DynaSand filter,
and Paques bv (Balk, Netherlands), maker of the
Astrasand filter. Siemens Water Technologies
has a license agreement with Paques to supply
this filter in the United States and Canada.
Filter Design Characteristics
When designing a denitrification filter, there are
many considerations that should be taken into
account by wastewater professionals. Table 1
presents a brief overview of the systems offered
by different manufacturers (deBarbadillo et al.
2005). Major design considerations include 1) a
manufacturer's experience and 2) the system's
performance, which includes influent weir
configuration, types of filter media, underdrain,
process controls such as backwash and filter
control, and methanol feed control.
Filter Influent Weirs
Many downflow denitrification filters are
capable of being operated at variable levels and
may have a significant drop over the influent
weir. This drop can result in the entrainment of
dissolved oxygen (DO). The increase in DO
reduces the efficiency with which the filter
removes nitrate and increases methanol
consumption. In order to address this issue,
manufacturers have developed different designs
to mitigate the problem. The TETRA Denite
system has a patented curvilinear weir block to
encourage laminar flow down the wall to
minimize DO entrainment. The elimi-NITE
system can also be installed with a curved
stainless steel weir to solve this problem.
Additionally, the F.B. Leopold Company has
suggested that operating the system in a
constant-level mode would reduce the elevation
drop from the influent weir, thereby decreasing
the level of DO entrainment. Since influent in
upflow continuous-backwash filters is conveyed
to the feed radials within the filter bed through
submerged manifold piping, DO entrainment
over the influent weir is less an issue for those
filters utilizing this configuration.
Media
The preferred media for each filter manufacturer
is also presented in Table 1. The filter media in
the TETRA Denite system consists of a
monomedia granular sand with a two to three
millimeter effective size. Uniform and relatively
spherical media reportedly allow for more
rolling and contact with other media grains,
resulting in more effective backwash and
nitrogen-release cycles and, ultimately, lower
backwash water volume requirements. Davco
filters can be supplied with the same media.
Finer media are used with the DynaSand and
Astrasand filters that utilize the upflow
continuous-backwash filter design.
Underdrain
Early experience with downflow denitrification
filters suggested that nozzle underdrains were
prone to fouling and failure. To avoid these
problems, manufacturers have developed unique
block underdrains (Figure 4) (deBarbadillo et al.
2005). Severn Trent Services offers the TETRA
T-block underdrain, which is specifically
designed for bioreactor service and consists of
concrete-filled blocks enclosed in high-density
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TETRAD T-Block™ Leopold Universal®
TypeS®
Figure 4. Block Underdrain Systems.
US Filter Multiblock™
polyethylene (HDPE). F.B. Leopold developed
its Universal Type S underdrain, which consists
of HDPE blocks. Although existing Davco
filters were constructed with pipe lateral
underdrains, new installations will be supplied
with the Multiblock HDPE underdrain. Upflow
continuous-backwash filters do not require an
underdrain.
Nitrogen Release Cycle
During the denitrification reaction, nitrogen gas
accumulates in the media bed. Wastewater is
forced to flow around the gas and increases head
loss in the filter. The nitrogen release cycle
emits the nitrogen gas into the atmosphere. The
TETRA Denite system offers a control package,
known as SpeedBump, which pumps backwash
water up through the filter for 30 seconds to 2
minutes. The influent valve to the filter remains
open to minimize filter downtime. The elimi-
NITE and Davco systems offer nitrogen-release
cycles that fully close the influent valve, and the
additional time required for the nitrogen-release
cycle should be accounted for in the filter
design. Since the DynaSand and Astrasand
upflow systems operate in the same direction
that the nitrogen gas travels, and the gas also is
drawn into the airlift, a separate degassing cycle
is unnecessary.
Backwashing and Filter Controls
During operation of the denitrification filter,
solids removed from the wastewater accumulate
in the media. Additional solids from the growth
of denitrifying bacteria also build up in the filter
media. This increases the head loss in the filters.
To clean the media, backwashing cycles for the
downflow filters are initiated on the basis of
increased head loss through the filter or on a
timed basis. All three manufacturers of
downflow filters offer air scouring and air-water
backwash as part of the backwash cycle.
Integrated process control systems are offered for
the TETRA Denite, elimi-NITE, and Davco
filtration systems which control the backwashing,
air-scour, and nitrogen-release cycles.
The DynaSand and Astrasand systems operate
with a small continuous-backwash stream. A
process monitoring tool for the Astrasand filter,
the Astrameter system, is used to measure the
sand circulation rates at several locations
throughout the filter.
Questions remain regarding the bed turnover
rate (backwash frequency) and how it relates to
maintaining good solids removal while
supporting sufficient biomass for denitrification.
Available for use with the Astrasand filter, the
Astracontrol system was developed to maintain
biological activity within the filter under
varying conditions.
The control system continuously adjusts the
media movement and washing rate to maintain a
fixed volume of active biomass in the filter.
Studies performed by Siemens Water Systems
suggest that optimizing the backwash rate based
on hydraulic loads through automation of the
airlift provides excellent control of the process
(Freed and Pauwels). Parkson Corporation has
indicated that changing the bed turnover rate in
the DynaSand system might be necessary to
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Table 1. Comparison of De nitrification Filter Manufacturers and Equipment
Manufacturer/
filter
Flow regime
Undbrdrain
Air header
arrangement
Media
Nitrogen- release
cycle
Backwash
water and air
requirement
Influent weir type
Backwash flow
as percent of
forward flow
Patented
features
Severn Trent
Services/
TETRA* Denite'
Downflow
T-block; concrete-
filled, HDPEjacket
SS box header;
laterals beneath
underdrain
457 mm (18 in.)
graded gravel,
1.8 m (6 ft) of 6
x 9 mesh silica
sand, uniformity
coefficient 1,35,
0,8 minimum
sphericity-
Initiated by
headless or time-
controlled cycle;
Speed Bump
controls
244 L/min-m2
(6 gal/rnin.ft2);
1.5 mymin-m2
(5 scfm/ft2)
Curvilinear weir
block
<5; often 1 to 2
T block underdrain,
curvilinear weir
block, Speed
Bump, TetraPace,
TetraFlex
F, B. Leopold/
elimi-NITE
Downflow
Universal Type S
HOPE block
SS header across
filter; laterals
381mm (15 in.)
graded gravel,
1,8m (6 ft) of 6 x
12 mesh sand
Initiated by
headless or time-
controlled cycle
244 L/min-m2
(6 gal/'min-ft2);
1.5 m'/inm-m2
(5 scfm/ft2)
Curved stainless
steel weir
2
Universal
underdrain and
features
USFilter/Davco
Downflow
Pipe lateral; or
Multi block HOPE
block
SS air header; 50-
mm (2-in.) laterals
2 layers support
gravel,
1.8m (6 ft) of 6 x
9 mesh sand
Initiated by
headless or time-
controlled cycle
40? l/min-m2
(10 gal/min-ft3);
1.5 mVmin-m2
(5 scfm/ft2)
Varies
Not documented
None
Parkson/
DynaSand
Upflow
None required
Vertical air lift
1,35 to 1.45
mm subround
media or 1,55
to 1.65 mm
subangular
media with
uniformity
coefficient of 1,3
to 1.6; 2-m (6.6-
ft) bed depth
None required
Continuous
through air lift
and sand washer
Feed radials at
bottom of unit
3 to 5
None
Paques and
USFilter/
Astrasand
Upflow
None required
Vertical air lift
1.2 to 1.4 mm
sand,2-rn (6.6-ft)
bed depth
None required
Continuous
through air lift and
sand washer
Feed radials at
bottom of unit
3 to 12
None in
United States;
Astracontrol in
Europe
HOPE = h/jti-density potyethytene.
SS = Statutes*
meet a specific requirement. However, the
company has not seen a need to adjust it during
routine operation
Table 1 Courtesy of Christine deBarbadillo
Methanol Feed Control System
Methanol is usually dosed to the filter influent
before it is divided among the filter cells. In the
Denite system, methanol is dispensed on the
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basis of the filter influent flow rate and the
concentrations of nitrate in the influent and
effluent, as measured by an online nutrient
analyzer. The manufacturer guarantees no net
increase in total organic carbon across the filter
when this control system is used.
The other manufacturers suggest using the filter
influent flow rate and nitrate concentration to
determine the methanol dosage through a flow-
paced or feed-forward automatic control system.
Although a feed-forward control scheme can
reasonably match methanol dosing to actual
requirements, periods of slight overdosing and
the resulting increase in concentrations of
biochemical oxygen demand (BOD) in the filter
effluent might be difficult to avoid. In cases in
which effluent BOD and nitrate-nitrogen limits
are less stringent, the need for a high level of
methanol control is related to optimizing
chemical usage.
Costs
There are several factors that are related to a
denitrification filter system's capital costs.
Depending on the application and overall
effluent requirements, it might be desirable at
times to use a more conservative design for
filters in meeting the required limit. Alternately,
pilot testing can be conducted to verify the
design loadings. Another factor that may affect
the overall cost of the project includes whether
the influent and backwash piping and the valves
associated with downflow filters are installed
outdoors or housed in a building.
In addition to capital cost, operational costs are
also important. The energy costs associated with
backwashing, air-scour, and nitrogen-release
cycles must be considered, along with a proper
accounting of the frequency of these operations.
The cost of "retreatment" of spent backwash
water must also be included: Filters using only 2
percent of the forward flow for backwashing
have a lower cost for treatment than those that
consume greater amounts of backwash water.
Finally, the ability to optimize methanol
dosages can affect the operating cost
significantly. Some facilities have reduced their
chemical consumption as much as 30 percent
after implementing more efficient control
systems.
Costs will differ for new plants and retrofits.
Retrofit costs are more site-specific and vary
considerably for any given size category.
Retrofit costs are based on the same factors as
new plants, in addition to the layout and design
of the existing treatment processes. A case study
performed for the Maryland Department of the
Environment suggests costs in dollars per pound
of total nitrogen removed can range from $0.55
to $7.69. For these examples, this equates to a
cost of approximately $1.46 per gallon of
wastewater treated (Maryland Department of the
Environment, 2005).
ACKNOWLEDGMENTS
EPA acknowledges external peer reviewers
Alan Cooper, Christine deBarbadillo, and J.B.
Neethling for their assistance.
PRODUCT LITERATURE USED
Siemens. Product literature.
http://www. water. Siemens. com/en/Product_Line
s/Davco_Products/Davco_Products/Pages/da
vco_denitrification_filter_product_page.aspx
http://www.water.si emens.com/en/Product_
Lines/Davco_Products/Davco_Products/Pages
/davco_astrasand_product_page.aspx
Severn Trent Services literature
http://www.severntrentservices.com/
LiteratureDownloads/Documents/
650-0001.pdf
F.B. Leopold Company literature
http://www.fbleopold.com/wastewater/
denitrification/deni trification.htm
Parkson Corporation
http://www.parkson.com/Content.aspx7ntopicid
=139&parent=process&processID=73
REFERENCES
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Metcalf&Eddy. 1979. Wastewater
Engineering, Treatment, Disposal, Reuse.
New York: McGraw-Hill.
Metcalf & Eddy. 2003. Wastewater
Engineering, Treatment and Reuse, 4th ed.
New York: McGraw-Hill.
U.S. Environmental Protection Agency, August,
2002, Nitrification.
http://www.epa.gov/safewater/disinfection/tcr/
pdfs/whi tepaper_tcr_nitrification.pdf
deBarbadillo, C., R. Rectanus, S. Lambert., D.
Parker, J. Wells, and R. Willet. June 2005.
Evaluating Denitrification Filters. Water
Environment & Technology.
Denitrifying Filters Case Studies: Maryland
Department of the Environment,
http://www.mde.state.md.us/assets/
document/BRP%20Gannett%20Fleming-
GMB%20presentation.pdf
vvEPA
United States
Environmental Protection
Agency
EPA 832-F-07-014
Office of Water
September 2007
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