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Leopold elimi-NITE Denitrification System
Process Description
The process of biological denitrification can be used to remove nitrogen from wastewater when the nitrogen is predominantly in the form of nitrate. Nitrogen is present in raw wastewater primarily in organic and ammonium-nitrogen
form and must be converted (nitrified) to an oxidized form (nitrate) before biological denitrification can take place.
Therefore, the upstream treatment process must oxidize most of the ammonia to nitrite and then to nitrate form prior
to discharging to the denitrification unit. A supplemental carbon source is needed because the preceding carbon
oxidation and nitrification step has removed nearly all of the degradable carbonaceous material from the wastewater.
Oxygen will inhibit the activity of denitrifying enzymes. In order for denitrification to take place, the dissolved oxygen
level of the nitrified influent should be as low as possible entering the denitrification unit. Denitrification is dependent
upon optimum chemical and microbiological conditions that are not unique to particular systems. These conditions
can occur in suspended-growth systems, such as a mixed tank, as well as attached-growth systems, such as a packed
bed. The packed bed comes in many forms with differing types of media and modes of operation.
Backwash
Waste
Carbon
Storage
Nitrified
Influent
Gravity Downflow
Packed Bed
Sand Filter
Air Scour
Backwash
Clearwell
Effluent
The Attached-Growth
Microbiological Process
The gravity, downflow, packed-bed denitrification system is an attached-growth, microbiological process.
Physically, it is identical to a deep-bed downflow sand
filter. Denitrifying microorganisms attach to the filter
media, which provides the support system for their
growth. A carbon source such as methanol, acetic acid,
molasses, etc. is added upstream of the packed-bed
filter and a nitrified influent is filtered through the
media. The packed-bed filter system is well-suited for
denitrification because it provides the necessary hydraulic detention time for the biological reaction to take
place. The filter media is composed of a coarse, hard,
predominantly siliceous material. This media can filter
out solids and serve as a support system for the denitrifying microorganisms. The downflow packed-bed
system eliminates the requirement for downstream filtration or clarification required of other denitrification
systems.
Releasing the Nitrogen
As denitrification occurs, nitrogen gas accumulates
in the filter media, which increases the headloss over
the headloss due to the accumulation of solids. The
nitrogen gas bubbles are periodically released from
the media by taking the filter offline and applying
backwash water for a few minutes. This process is
called the nitrogen release cycle or filter bumping. The
frequency of the nitrogen release cycle is a function
of both nitrate removal and a minimum acceptable
time between cycles, typically not less than one hour.
Usually a filter needs to be bumped once every four to
eight hours, again depending on the nitrogen-loading
rate. The bumps are usually set on a time basis. After
a bump the headloss in the filter is reduced or recovered. However, when the liquid level
in the filter
reaches a
designated
high level,
signifying that
the bumps are not
effective in reducing
headloss, a full backwash
is performed on the filter.
Cleaning the Filter Media
As with a conventional gravity filter, suspended solids
gradually accumulate in the filter. Part of the nitrogen
removal will occur because nitrogen in the suspended
solids is removed. Physical removal mechanisms of
suspended solids/nitrogen may include straining, sedimentation, and particle interaction-charge attraction.
Chemical removal mechanisms for nitrogen may include
adsorption, coagulation-flocculation, and biological
activity. As with all filters no one mechanism can
account for the total effect. The suspended solids
in the filter are removed by backwashing using an
air/water wash in a step that also dislodges excess
biomass. Therefore, it is not desirable to completely
clean the filter media during backwash.
After backwashing a loss of denitrification capability
may be observed due to the loss of accumulated biomass. A cleaner filter placed into operation may require
some time to reestablish more biomass and may experience some decreased performance when first placed
back into service. Therefore, several small filters produce
a better-blended effluent than a few large filters. If the
accumulation of suspended solids were allowed to
continue without backwashing, the filter would become
clogged and the frequency of gas bumping would have
to be increased. Under normal conditions, headloss will
require that the filter be automatically backwashed
every one to five days, which is comparable to the backwashing frequency for a filter used only for suspended
solids removal.
Design Considerations
The design of denitrification filters must consider solids
removal, denitrification kinetics, and the limitations on
the frequency of the nitrogen release cycle. The process
hydraulic loading rate is generally 1 to 2 gpm/sf of filter
area. Another hydraulic consideration is the empty bed
contact time. Typically this is designed to be 30 minutes
or longer. Denitrification rates can also be expressed in
terms of nitrate removal rates per unit of filter surface
area and unit of filter volume. The first approach uses
mass of nitrogen removed per unit area of the filter surface, typically 0.5 pounds of nitrate removed per square
foot of filter area per day or less. The second approach
uses the mass of nitrogen removed per unit volume of
the filter in 1,000 cubic feet and typically is 70 pounds
of nitrate removed per 1,000 cubic feet per day or less.
Filter run times vary from 24 hours to more than 120
hours. The run time is dependent upon both solids
removal and the growth of the microorganisms.
Carbon Source, Dosage, and Control
The most common carbon substrate used in biological
denitrification is methanol because of its availability, low
cost, favorable sludge production, low volatile organic
compound emissions potential, and lack of nitrogen and
phosphorous. The typical design methanol dosage is 3
pounds of methanol per pound of nitrate removed. The
methanol can be manually set or paced on influent flow
and influent nitrate concentration. Manually pacing the
methanol feed has the disadvantage that it does not
take into account variations in flow and nitrate concentration. Therefore there will be times when there is not
enough methanol and times that there is too much
methanol. Not enough methanol would starve the biology and nitrate removal would decrease. Too much
methanol would increase the biochemical oxygen
demand (BOD) in the effluent.­­­­­­
The next possible step in control is to
pace the methanol by influent flow
control. This has the advantage of
matching the methanol feed by flow
but has the same disadvantage as
the manual control when the nitrate
concentration varies causing overand under-feed.
With the advent of reliable nitrate analyzers, standard
control methods can be devised to pace the methanol
by both flow and nitrate concentration. Influent flow
can be used as long as the flow takes into account any
streams added or subtracted through the filters such as
backwash water. The nitrate concentration can be measured in the influent and combined with the flow, which
is called feed-forward control or open loop. This type
of arrangement has the advantage of matching the
methanol feed rate to both influent flow and influent
nitrate concentration or the total influent nitrate mass.
Or, the nitrate concentration can be measured in the
effluent and combined with the flow rate, which is
called feedback or closed loop. Compound loops can be
used that combine both the influent and the effluent
nitrate concentrations. The influent nitrate concentration
is used to provide a gross methanol feed signal, and the
effluent concentration is used to trim the methanol feed.
Flow Patterns in a Typical Denitrification Filter
The following describes the flow patterns in a typical
denitrification filter: The flow enters the tertiary filtration
system via the influent channel. Here the flow is uniformly split into each filter. The flow proceeds from
the influent channel through gated openings in the
wall that feed the washwater troughs. The flow split
occurs due to setting the weir on the washwater
troughs at the same elevation for all of the filters.
The flow proceeds over the washwater trough weir
and into the filter cell. The flow must pass through
the deep-bed, coarse sand media, the reverse-graded
gravel, and into the Leopold® Universal® dual parallel
lateral underdrain. Orifices in the Leopold® Universal®
dual parallel lateral underdrain allow the flow to drain
to a flume where it is accumulated from each lateral.
The flow then proceeds through the valved filter
effluent pipe
and into the
clearwell. Here
the filtered flow
accumulates until
it crests the effluent
channel weir where it
then proceeds out of the
tertiary system.
Filter Control
The least expensive control system for the filters is to
use a split-flow, variable-level mode determined by the
elevation of the effluent piping or effluent weir and
the solids built-up in the media. The influent is split
between the filters using the same elevation of the
weirs on the washwater troughs. The effluent discharge
piping into the clearwell or a clearwell weir is set so
that the coarse sand media is covered by the liquid
level in the filter cell. As solids accumulate in the filter
cell, the liquid level increases. Periodically the filter is
bumped to remove gas. The filter is isolated and only
the backwash pumps are operated for five minutes or
less. At a designated level the filter is put through a
full backwash. Backwash water and the backwash
pumps in the clearwell along with pressurized air are
used to reverse the flow in the filter cell to remove the
accumulated solids and some of the biomass. The
backwash water is removed from the filter cell via
the washwater trough and sent to the mudwell.
Mudwell pumps remove the accumulated solids
and backwash water.
Another method to control the filters is the split-flow,
constant-level mode. The influent is split as above
using the weir on the washwater troughs but the level
in the filter is maintained by modulating the effluent
valve. A level transmitter measures the level in the filter
continuously and a controller modulates the effluent
valve to provide sufficient headloss to maintain a prescribed level in the filter. The advantage of using this
control system is that the influent flow does not drop
from the influent weir away from the wye wall to the
water surface thereby not entraining more air and
increasing the dissolved oxygen content.
Water Bumping Air-Bound Filters
A typical backwash procedure consists of isolating the
filter, air wash at 5 to 6 scfm/sf for about 1 to 2 minutes,
air/water wash at 5 to 6 scfm/sf air and 6 to 8 gpm/sf
water for about 10 to 15 minutes, and a final water
wash of 6 to 8 gpm/sf for 5 minutes. The backwash
sequence and flow rate can be operator-varied.
In certain cases, gases such as high levels of nitrogen
or dissolved oxygen build up in the filter and may
cause air binding. In this case the filters are water-only
“bumped.” The bump consists of isolating the filters
from the influent flow, closing the effluent valve,
starting the backwash pump, opening the backwash
valve, opening the waste valve (optional if the water
depth stays below the effluent launder), and backwashing the filter for approximately 2 to 5 minutes. This
reversal of flow allows the built-up gases to escape
the filter. The filter is then put back on-line. The
bumps can be programmed to occur either on time
or on level and are site specific.
The typical backwash consists of the following sequence:
• Influent and effluent valves are closed
• Waste valve is opened
• Blower is started
• Air isolation valve is opened, vent valve is closed,
and air-only wash for at least 1 minute
Reference: United States Environmental Protection Agency, Nitrogen Control
Manual, EPA/625/R-93/010, Washington, D.C., September 1993.
• Backwash pump is started
• Backwash isolation valve is opened and air/water
backwash for at least 10 to 15 minutes
• Air isolation valve is closed, vent valve is opened,
and the blower is stopped
• Water-only backwash continues for at least 5
minutes to purge air from the filter
• Backwash isolation valve is closed and the backwash
pump is stopped
• Waste valve is closed
• Influent and effluent valves are opened
Leopold is a brand of Xylem. For the latest version of this document and
more information about Leopold products visit www.fbleopold.com
© 2012 Xylem, Inc.
Xylem, Inc.
227 South Division Street
Zelienople, PA 16063
Telephone: (724) 452-6300
Fax: (724) 452-1377
www.xyleminc.com
LB005-1239 • Leopold® elimi-NITE® Denitrification System Process Description • 03/2012 • US
Backwash Sequence