Technology: Denitrifying Filters
Description
Nitrification is a process that converts ammonia to
nitrite, and then nitrite to nitrate. Nitrate and other
forms of nitrogen can have harmful effects on the
environment. Thus, standards require that the resulting
nitrate be converted to nitrogen gas. One treatment
option is denitrification. Denitrification is a process
wherein nitrate is converted to nitrogen gas. The
process reduces total nitrogen as well as removes
suspended solids from the effluent. A treatment system
that can be used for denitrifying wastewater effluent is
the denitrifying filter.
The denitrifiers use a readily biodegradable organic
compound (a carbon source). To sustain the growth
of denitrifiers, some organic material must be added
to the filter influent. Methanol is the carbon source
often selected because it can readily be degraded under
anoxic and aerobic conditions. Other carbon sources
(e.g., acetic acid) can also be used in denitrifying filter
systems.
Applicability
There are two main processes for denitrification
filters that are commercially available: downflow
denitrification filters and upflow continuous-backwash
filters. Downflow denitrification filters are comprised of
media and support gravel supported by an underdrain
and are operated in a regular filtration mode. Upflow
continuous-backwash filters, on the other hand, allow
TDS-WAS-01-2014
influent wastewater to flow upward through the filter,
against the movement of the sand bed. These systems
have been proven effective in treating wastewater to
meet low total nitrogen limits.
Performance
Denitrifying filters have been used for wastewater
treatment for many years. The first patent of combining
denitrification and solids removal was in the 1970s.
Since then, several companies have developed their own
denitrifying filters.
There are facilities that utilize denitrification filters not
only for the required total maximum daily load (TMDL),
but also as part of an advanced wastewater treatment
system. One such facility utilizes denitrification filters
in their treatment system, which enables them to
reuse treated wastewater to supplement wetlands and
recharge aquifers.
Cost
Capital costs of a denitrification filter system are based
on the application and overall effluent requirements
and location. Operational costs should also be
considered.
Costs will differ for new plants and retrofits. In addition
to the layout and design of the existing treatment
process, retrofit costs are more site-specific and can vary
based on size.
Advantages
• The technology has the ability to break down
nitrate.
• Denitrifying filters are ideal for upgrading
secondary treatment plants without having to
modify or alter the existing equipment or treatment
process.
• The filter is particularly a good choice when faced
with very low nitrate discharge limits.
• The system saves on process conversion expenses.
• Ease of expansion complements increased flow
capacity.
Disadvantages
• Denitrification takes time to set up as it is much
slower than nitrification.
• Incomplete denitrification process can result to
production of ammonium and nitrite.
Robert Domingo
Process
Source: Howard County, Maryland.
Note: In this document, “$” refers to US Dollars.
References:
Howard County, Maryland. Schematic of Denitrification Process. http://bit.ly/RDv60N.
United States Environmental Protection Agency (USEPA). 2007. Denitrifying Filters. Wastewater Management Fact Sheet. http://1.usa.gov/1i628BF.