United States
     Environmental Protection
                               Wastewater Management Fact Sheet
                               Denitrifying Filters
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

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 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
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
                                                 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)

          6 NO3- + 5 CH3OH
                                                                     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

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.

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
There are two main process configurations for
denitrification filters  commercially  available,
downflow  and  upflow continuous backwash

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

    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.

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.

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

        TETRAD T-Block™        Leopold Universal®
  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

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

Table 1. Comparison of De nitrification Filter Manufacturers and Equipment
Flow regime
Air header
Nitrogen- release
water and air
Influent weir type
Backwash flow
as percent of
forward flow
Severn Trent
TETRA* Denite'
T-block; concrete-
filled, HDPEjacket
SS box header;
laterals beneath
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
Initiated by
headless or time-
controlled cycle;
Speed Bump
244 L/min-m2
(6 gal/rnin.ft2);
1.5 mymin-m2
(5 scfm/ft2)
Curvilinear weir
<5; often 1 to 2
T block underdrain,
curvilinear weir
block, Speed
Bump, TetraPace,
F, B. Leopold/
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
underdrain and
Pipe lateral; or
Multi block HOPE
SS air header; 50-
mm (2-in.) laterals
2 layers support
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)
Not documented
None required
Vertical air lift
1,35 to 1.45
mm subround
media or 1,55
to 1.65 mm
media with
coefficient of 1,3
to 1.6; 2-m (6.6-
ft) bed depth
None required
through air lift
and sand washer
Feed radials at
bottom of unit
3 to 5
Paques and
None required
Vertical air lift
1.2 to 1.4 mm
sand,2-rn (6.6-ft)
bed depth
None required
through air lift and
sand washer
Feed radials at
bottom of unit
3 to 12
None in
United States;
Astracontrol in
  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

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.

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

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).

EPA  acknowledges external peer  reviewers
Alan Cooper, Christine deBarbadillo,  and J.B.
Neethling for their assistance.

Siemens. Product literature.

http://www. water. Siemens. com/en/Product_Line

http://www.water.si emens.com/en/Product_

Severn Trent  Services literature


F.B. Leopold Company literature

  denitrification/deni trification.htm

Parkson Corporation



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.
  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,
                                                         United States
                                                         Environmental Protection
                                                         EPA 832-F-07-014
                                                         Office of Water
                                                         September 2007