1J U N E 2 0 0 5
W ith the development of total
maximum daily loads for nutri-
ent discharges, many wastewater
treatment plants (WWTPs) face
stringent nutrient limits. Deep-bed denitrifica-
tion filters that remove nitrogen and solids are
a proven technology for treating wastewater to
meet low total nitrogen (TN) limits. A variety of
denitrification filters are available, offering differ-
ent features and levels of experience, and WWTP
managers may have a hard time selecting the sys-
tem that best meets their needs. To simplify the
evaluation process, the following comparison of
denitrification filter equipment and performance
is intended to highlight some of the similarities
and differences among the systems.
Filter ConfigurationsA deep-bed filter capable of concurrent de-
nitrification and solids removal was first pat-
Evaluating Denitrification Filters
Before selecting a
denitrification filter,
designers should
compare all
available systems
and assess their
performance to date
Christine deBarbadillo,
Robert Rectanus, Shannon Lambert,
David Parker, Jeff Wells, and
Robert Willet
US
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©2005 Water Environment Federation. All Rights Reserved. For website posting only. Bulk printing prohibited.
2 W E & T
Downflow denitrifi-cation filters at the city of Largo, Fla.
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ented in 1973. Dravo Corp. — now known as
Dravo Lime Co. (Pittsburgh) — and later Tetra
Technologies (The Woodlands, Texas) pioneered
the development of denitrification filter technol-
ogy. Following the expiration of the original pro-
cess patents, several other filter manufacturers
have developed a denitrification option for their
equipment. Moreover, some utilities in the United
States and Europe have retrofitted existing filters
to achieve denitrification.
Two main denitrification filter configurations
— downflow filters and upflow continuous-
backwash filters — are available. With both
configurations, methanol (or another readily
biodegradable carbon source) is added to waste-
water ahead of the filter to enable denitrifying
bacteria to grow.
Downflow denitrification filters operate in
a conventional filtration mode and consist of
gravel and sand supported by an underdrain (see
photo, above). 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 USFilter Davco Products (Thomasville, Ga.),
maker of the Davco denitrification filter.
Wastewater enters a downflow filter over
weirs located 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
clearwell. Filters must be taken out of service at
regular intervals for a short backwashing cycle
consisting of air scouring and backwashing with
air and water. Nitrogen-release cycles are needed
to remove nitrogen gas bubbles that are pro-
duced during denitrification and accumulate in
the media. To avoid sending slugs of spent back-
wash water to a plant’s headworks, a mudwell
normally is provided for equalization. The piping
for the filter influent and backwash is similar to
that of conventional filters and can be housed in
an indoor pipe gallery or installed outdoors.
Upflow continuous-backwash filters differ in
that influent wastewater flows upward through
the filter countercurrent to the movement of the
sand bed. The filters are supplied as modular
units installed in steel tanks or in multiple cells in
concrete basins (see photo, p. 26). Manufacturers
include Parkson Corp. (Fort Lauderdale, Fla.),
maker of the DynaSand filter, and Paques bv
(Balk, Netherlands), maker of the Astrasand filter.
Since October 2003, USFilter Davco Products has
had a license agreement with Paques to supply
this filter in the United States and Canada.
Wastewater enters an upflow continuous-back-
wash filter at the top and is conveyed downward
through the feed pipe and distributed to the filter
bed through feed radials (see Figure 1, below).
3J U N E 2 0 0 5
After traveling upward
through the media, ef-
fluent wastewater is
removed at the top of
the filter. Sand media
slowly travel down-
ward and are drawn
into an airlift pipe in
the center of the filter.
Compressed air is in-
troduced to the airlift,
drawing sand upward
and scouring it. At the
top of the airlift, the
media are returned
to the filter bed. Filtered water rises through a
separator that removes lighter dirt particles by
washing them away and returns the large, heavy
sand grains to 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 enable
clean water to enter the washer and separator
continuously by differential head, eliminating
the need for typical backwash-supply pumps.
Individual filter cells are not taken out of service
for backwashing, enabling a relatively simple pip-
ing and valve arrangement.
Filter ManufacturersWhen designing a denitrification filter, one
must examine differences in equipment and ex-
perience offered by the manufacturers (see Table
1, p. 27). Major design considerations include a
manufacturer’s experience, system performance,
and such factors as design loading rates, influent
weir, media, underdrain, process control, and
methanol feed control.
With more than 25 years of full-scale experi-
ence, the TETRA® Denite® system has been
installed in greater numbers than any other
denitrification filter. Leopold has manufactured
conventional water and wastewater filters for
several years and recently began offering the
elimi-NITE filter, an adaptation of its conventional
filter, for tertiary denitrification applications.
USFilter Davco has offered denitrification filters
for about 15 years, mostly at small installations
with capacities less than 3800 m3/d (1 mgd).
Although use of DynaSand filters is not wide-
spread, several installations capable of denitri-
fication have been constructed in the United
States and Puerto Rico within the past 15 years.
Five Astrasand installations in Europe currently
are operated for denitrification, with the first
one commissioned in 1999. The first Astrasand
unit in the United States is under construction
at the U.S. Army’s Aberdeen Proving Ground in
Maryland.
All the filter manufacturers indicate that
their equipment can reduce concentrations of
nitrate–nitrogen and nitrite–nitrogen in effluent
to less than 1 mg/L. The TETRA Denite system
is guaranteed to meet this target, and its perfor-
mance is well documented, with full-scale data
from multiple facilities. Recent full-scale data
also show that the system can achieve an efflu-
ent nitrate–nitrogen concentration of less than
0.5 mg/L under cold weather conditions. Davco
filters have been installed at some small WWTPs
in Florida that have met an effluent limit of
3 mg/L of TN. Although the elimi-NITE system has
limited full-scale experience in the denitrifica-
tion mode, data collected at two installations in
North Carolina show that concentrations of less
than 1 mg/L nitrate–nitrogen can be met at low
loading rates.
Full-scale data from several DynaSand facilities
in Puerto Rico show that the filters can achieve
concentrations of less than 1 mg/L nitrate–
nitrogen and nitrite–nitrogen. This information
is supplemented with data from pilot test-
ing conducted in 1989 by the University of
Concrete installa-tion of Astrasand® filter modules.
Figure 1. Schematic of Dynasand® Upflow Continuous Backwash Filter
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4 W E & T
Florida (Gainesville) and testing completed in 2005 at the
Hagerstown (Md.) WWTP. Extensive full-scale data from
the De Groot Lucht Sewage Treatment Plant (Vlaardingen,
Netherlands) show that the Astrasand filters reduced aver-
age influent nitrate–nitrogen concentrations of 18 mg/L to
about 2 mg/L at cold wastewater temperatures.
All the filters can achieve effluent concentrations of 5
mg/L or less of total suspended solids (TSS). In addition,
the TETRA® Denite® system has received conditional
acceptance from the California Department of Health
Services, under its so-called Title 22 reuse regulations, for
use as a standard filtration system and as a denitrifica-
tion filter. The elimi-NITE, Davco, and DynaSand systems
have received conditional acceptance for filtration only
Table 1. Comparison of Denitrification Filter Manufacturers and Equipment
Manufacturer/filter
Severn Trent Services/
TETRA® Denite®
F. B. Leopold/ elimi-NITE
USFilter/Davco Parkson/DynaSand
Paques and USFilter/Astrasand
Flow regime Downflow Downflow Downflow Upflow Upflow
Underdrain T-block; concrete-filled, HDPE jacket
Universal Type S
HDPE blockPipe lateral; or
Multiblock HDPE block
None required None required
Air header arrangement
SS box header; laterals beneath
underdrain
SS header across filter; laterals
SS air header; 50-mm (2-in.) laterals
Vertical air lift Vertical air lift
Media 457 mm (18 in.) graded gravel,
1.8 m (6 ft) of 6 × 9 mesh silica sand, uniformity coefficient 1.35,
0.8 minimum sphericity
381 mm (15 in.) graded gravel,
1.8 m (6 ft) of 6 × 12 mesh sand
2 layers support gravel,
1.8 m (6 ft) of 6 × 9 mesh sand
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
1.2 to 1.4 mm sand, 2-m (6.6-ft)
bed depth
Nitrogen-release cycle
Initiated by headloss or time-controlled cycle;
Speed Bump controls
Initiated by headloss or time-controlled cycle
Initiated by headloss or time-controlled cycle
None required None required
Backwash water and air requirement
244 L/min·m2 (6 gal/min·ft2);1.5 m3/min·m2
(5 scfm/ft2)
244 L/min·m2 (6 gal/min·ft2);1.5 m3/min·m2
(5 scfm/ft2)
407 L/min·m2 (10 gal/min·ft2);1.5 m3/min·m2
(5 scfm/ft2)
Continuous through air lift
and sand washer
Continuous through air lift and
sand washer
Influent weir type Curvilinear weir block
Curved stainless steel weir
Varies Feed radials at bottom of unit
Feed radials at bottom of unit
Backwash flow as percent of forward flow
<5; often 1 to 2 2 Not documented 3 to 5 3 to 12
Patented features
T block underdrain, curvilinear weir block, Speed
Bump, TetraPace, TetraFlex
Universal
underdrain and features
None None None in United States; Astracontrol in
Europe
HDPE = high-density polyethylene.
SS = Stainless steel
5J U N E 2 0 0 5
and have not been tested in denitrification mode. The
Astrasand filter has not undergone testing to receive
conditional acceptance by the Department of Health
Services.
Although tertiary denitrification filters historically have
been used for nitrogen removal only, the need to meet
stringent effluent total phosphorus (TP) limits at the same
time recently has become an issue in some locations. For
example, the State of Maryland plans to require WWTPs to
upgrade to meet effluent TN and TP limits of 3 and 0.3 mg/
L, respectively, to reduce nutrient loads to the Chesapeake
Bay. Although many plants in Florida and elsewhere meet
moderate TP limits of about 1 mg/L while complying with
stringent TN limits, few facilities have experience operat-
ing denitrification filters to produce low effluent TN and
TP concentrations simultaneously. As the TP limit decreas-
es, phosphorus limitations on the denitrification process
become a concern: Enough phosphorus must be present
to meet the requirements for growth of the denitrifying
bacteria, but the phosphorus concentration must remain
low enough to meet the effluent limit reliably.
Several WWTPs using TETRA ® Denite ® f i l -
ters have achieved low ef f luent phosphorus
concentrations while producing eff luent con-
centrations of less than 1 mg/L nitrate–nitrogen and nitrite–
nitrogen. A key component of the pilot test of the
DynaSand filters at the Hagerstown WWTP involved add-
ing chemicals to the denitrification filter influent to aid in
phosphorus removal. The pilot results show that adding
ferric chloride enabled the system to reduce influent TP
levels of approximately 1 mg/L to less than 0.3 mg/L in the
effluent while also reducing concentrations of nitrate–ni-
trogen and nitrite–nitrogen from approximately 7 to less
than 1 mg/L. Automatic chemical-dosing control and on-
line nutrient monitoring are important components when
operating in this mode.
Design Loading RatesAs reported in the literature, design procedures for
denitrification filters have focused on hydraulic and mass
loading guidelines. Based on early data, design curves
were developed to compare nitrate-removal efficiency
versus empty-bed detention time for downflow denitrifi-
cation filters, and these findings have been included in
subsequent textbook publications.
Designers historically have depended on filter manu-
facturers to provide design guidance. However, differing
levels of operating experience among manufacturers and
the wide range of design loading rates cited in the litera-
ture often prompt questions regarding appropriate loading
rates for denitrification. The industry would benefit over-
all by establishing better design criteria for achieving low
nitrate–nitrogen concentrations, particularly in temperate
and cold climates. A summary of design loading rates from
different sources is given in Table 2 (see p. 6).
Filter Influent WeirsDownflow denitrification filters are operated at a vari-
able level and generally have a significant drop over the
influent weir, resulting in entrainment of dissolved oxygen
(DO). The increase in DO reduces the efficiency with
which the filter removes nitrate and increases methanol
consumption. To mitigate this problem, the TETRA Denite
system is provided with a patented curvilinear weir block
to encourage laminar flow down the wall to minimize DO
entrainment. The elimi-NITE system can be installed with
a curved stainless steel weir. Leopold also 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. Because 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 of an issue for those filters.
MediaTable 1 (p. 27) lists the preferred media of each filter
manufacturer. The 6 × 9 mesh media used in the Tetra Denite
system meet relatively stringent standards for uniformity
and sphericity. The uniform and relatively spherical media
reportedly allow for more rolling and contact with other
media grains, resulting in more effective backwash and ni-
trogen-release cycles and, ultimately, lower backwash water
volume requirements. Davco filters can be supplied with
the same media. The elimi-NITE filter typically is supplied
with 6 × 12 mesh media, but the 6 × 9 mesh media can be
provided if desired. Finer media are used with the DynaSand
and Astrasand filters.
UnderdrainEarly experience with downflow denitrification filters
suggested that nozzle underdrains were prone to foul-
ing and failure. To avoid this issue, the manufacturers
have developed unique block underdrains (see Figure
2). Severn Trent Services offers the TETRA® T-block un-
derdrain, which is specifically designed for bioreactor
service and consists of concrete-filled blocks enclosed
in high-density polyethylene (HDPE). Leopold developed
its Universal Type S underdrain, which consists of HDPE
blocks. Although existing Davco filters were constructed
with pipe lateral underdrains, new installations would be
supplied with the Multiblock HDPE underdrain. Experience
is limited with the underdrains used with the elimi-NITE
and Davco systems, and it is unclear whether fouling will
be a significant issue over the long term when operating
in the denitrification mode at moderate to high loading
rates. Upflow continuous-backwash filters do not require
an underdrain.
Nitrogen-Release CycleThe TETRA Denite system offers a patented nitrogen-
release cycle control package, known as SpeedBump,
6 W E & T
that pumps backwash water up through the filter for 30
seconds to 2 minutes. The influent valve to the filter re-
mains 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. Because the DynaSand and Astrasand
upflow systems operate in the same direction that the ni-
trogen gas travels, and the gas also is drawn into the airlift,
a separate degassing cycle is unnecessary.
Backwashing and Filter ControlsDuring operation of the denitrification filter, solids
removed from the wastewater accumulate in the media,
and additional solids are formed from the growth of de-
nitrifying bacteria. To clean the media, backwashing cycles
for the downflow filters are initiated based on 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.
The TETRA® Denite®, elimi-NITE, and Davco filtration
systems offer integrated control packages for backwash-
ing, air-scour, and nitrogen-release cycles. For installations
in which only partial denitrification is required, the TETRA
Denite system is available with another patented control
system that allows some filter cells to be operated for full
denitrification, while others are operated in parallel for
TSS removal only at a higher hydraulic throughput.
The DynaSand and Astrasand systems operate with a
small continuous-backwash stream. Media bed turnover
rates historically have ranged from 305 to 457 mm/h (12 to
18 in./h) or four to six bed turnovers per day for conven-
Table 2. Summary of Design Guidance for Denitrification Filters
Source
Hydraulic loading rate [L/min·m2 (gal/
min·ft2)]
Volumetric mass loading rate [kg NO3-N per m3/d
(lb NO3-N per ft3/d)] Other information
Manual: Nitrogen Control (U.S. Environmental Protection Agency, 1993)
41 to 82 (1 to 2), 30 minutes empty bed contact time
0.29 to 1.6 (0.018 to 0.1)
Design curves from Savage, E.S. (1983), “Biological Denitrification Deep Bed Filters,” presented at the Filtech Conference, Filtration Society, London, England.
1.33 (0.083) [referenced Tetra data]
Hydraulic loading rate versus effluent nitrate–nitrogen concentration (referenced Tetra data)
Biological and Chemical Systems for Nutrient Removal, Special Publication (Water Environment Federation, 1998)
0.24 to 3.2 (0.015 to 0.2) depending on tem-perature
Design curves from Savage, 1983
Wastewater Engineering, Treatment and Reuse (Metcalf & Eddy, 2003)
41 to 82 (1 to 2) at 20oC
1.4 to 1.8 (0.087 to 0.112) at 20oC 20 to 30 minute empty bed
contact time20 to 61 (0.5 to 1.5) at 10oC
0.8 to 1.2 (0.05 to 0.075) at 10oC
Severn Trent Services TETRA®Denite®
<123 (<3) at aver-age flow; <308 (<7.5) peak hydrau-lic with one cell out of service
Determined using pro-cess model
In-house kinetic model, extensive full-scale data
F.B. Leopold 41 to 82 (1 to 2) 1.12 (0.07) Full-scale data (North Carolina)
USFilter/Davco 82 (2) Not available Full-scale data (Florida)
Parkson Up to 183 (4.5) 0.24 to 1.9 (0.015 to 0.12)
Full-scale and pilot data (Puerto Rico, Maryland)
Paques/USFilter 168 (4.1) 2.08 (0.13) Full-scale data (Netherlands), dry weather flow only
7J U N E 2 0 0 5
tional filtration. Recent pilot testing of the DynaSand filter
indicated that bed turnover rates of 203 to 254 mm/h (8
to 10 in./h) were effective for denitrification. As 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 condi-
tions. The control system continuously adjusts the media
movement and washing rate to maintain a fixed volume
of active biomass in the filter. Parkson has indicated that
it may be necessary to change the bed turnover rate in
the DynaSand system to meet a specific requirement.
However, the company has not seen a need to adjust it
during routine operation.
Methanol Control SystemMethanol normally is dosed to the filter influent before
it is divided among filter cells. A patented methanol-con-
trol system named TetraPace is available for use with the
TETRA® Denite® system. TetraPace dispenses methanol
based on the filter influent flow rate and the concentra-
tions of nitrate in the influent and effluent, as measured
by an online nutrient analyzer. When this control system
is used with the TETRA Denite system, the manufacturer
guarantees no net increase in total organic carbon across
the filter.
The other manufacturers suggest using the filter influ-
ent flow rate and nitrate concentration to determine the
methanol dosage via either a flow-paced or feed-forward
automatic control system. Although a feed-forward control
scheme can reasonably match methanol dosing to actual
requirements, it may be difficult to avoid periods of slight
overdosing and the resulting increase in concentrations of
biochemical oxygen demand (BOD) in the filter effluent.
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. However,
tighter measures for controlling methanol, such as that
offered by the TetraPace system, can provide significant
advantages to a facility that has a stringent BOD limit of
5 mg/L or lower and must achieve low (less than 1 mg/L)
concentrations of nitrate–nitrogen and nitrite–nitrogen.
Cost and Bidding ConsiderationsWhen selecting a denitrification filter, wastewater
professionals should consider several factors related to a
system’s capital costs. The most critical factor involves the
overall area called for in the design. Depending on the appli-
cation and overall effluent requirements, it may be desirable
at times to use a more conservative design for filters with
less full-scale operating experience in meeting the required
limit. Alternately, pilot testing can be conducted to verify
the design loadings. Because they need backwash pumps
and a clearwell and mudwell, downflow filters may have a
higher structural cost than upflow continuous backwash
filters. However, since relatively small modules are used for
upflow continuous backwash filters, a greater number of
smaller filter cells normally is required to provide the same
surface area; this, too, can affect a structure’s cost. Whether
the influent and backwash piping and the valves associated
with downflow filters are installed outdoors or housed in
a building will affect the cost of a full installation. The 6 × 9
mesh media with a uniformity coefficient of approximately
1.35 and minimum sphericity of 0.8 tend to be more costly
than other media because only a limited number of sources
of media meet this specification. However, these media may
offer some operating cost savings by allowing more efficient
backwashing.
In addition to capital cost, it is important to consider
operating costs. The energy costs associated with back-
washing, air-scour, and nitrogen-release cycles must be
included, along with a proper accounting of the frequency
of these operations. The cost of “retreatment” of spent
backwash water also must be included: Filters using only 2%
of the forward flow for backwashing will have a lower cost
Figure 2. Block Underdrains
TETRA® T-Block™
8 W E & T
for treatment compared to
those that consume great-
er amounts of backwash
water. Finally, the ability
to optimize methanol dos-
ages can affect the operat-
ing cost significantly. Some
facilities have reduced their
chemical consumption as
much as 30% after imple-
menting more efficient con-
trol systems.
The significant differ-
ences in denitrification
experience, patent consid-
erations, and competitive
bidding laws can compli-
cate a municipal facility’s
efforts to procure a denitri-
fication filter. The features
associated with each type
of filter and each manufac-
turer should be evaluated
carefully before design and
procurement to identify the
advantages and disadvan-
tages of each. The differ-
ences between downflow
and upflow continuous-backwash filters are significant enough that the
type of filter should be selected before design. Procurement methods
that have been used recently for denitrification filter projects include
sole source, base bid, prequalification, and conventional bidding.
Christine deBarbadillo, P.E., is a process engineer in the
Charlotte, N.C., office of Black & Veatch Corp. (Overland Park, Kan.).
Robert Rectanus, P.E., is a senior project manager in the company’s
Gaithersburg, Md., office; Shannon Lambert, P.E., is a project
PAR
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In an upflow continuous-backwash filter, influent flows upward through the filter countercurrent to the movement of the sand bed.
About Severn Trent ServicesSevern Trent Services (www.severntrentservices.com), based in Fort Washington, Pa., is a leading supplier of water and wastewater treatment solutions. The company’s broad range of products and services is con-centrated around disinfection, instrumentation, and filtration technologies, pipeline analysis, rehabilitation and repair services, contract operating services and state-of-the-art residential metering products and services. Our international management services business provides support in all aspects of water and wastewater utility development and transformation. Severn Trent Services is a member of the Severn Trent Plc (London: SVT.L) group of companies. An international environmental services leader, Severn Trent is a FTSE 100 company.
For additional information about the TETRA® Denite® system email Ken Wineberg [email protected] or call 1.412.788.8300.
manager in the Nashville, Tenn., office; David
Parker, P.E., is a project manager in the Charlotte
office; Jeff Wells, P.E., is a project manager in the
Greenville, S.C. office; and Robert Willet, P.E., is a
project manager in the Raleigh, N.C., office.
Reprinted with permission from Water Environment & Technology, June 2005, by The Reprint Dept., 1-800-259-0470; #9856-0705