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Denitrification Filters Wastewater Management Fact Sheet Denitrifying Filters INTRODUCTION Nitrosolobus and Nitrosovibrio can also autotrophically oxidize ammonia. Discharge permits for treated wastewater from publicly owned treatment works (POTWs) often In the second step of the process, nitrite- include effluent limitations for nutrients. Total oxidizing bacteria oxidize nitrite to nitrate maximum daily loads (TMDLs) for nutrients are according to equation (2): being developed for many waterbodies – – + – throughout the United States. TMDLs and other NO2 + H2O → NO3 + 2H +2e (2) water quality-drivers have resulted in POTWs Nitrobacter is the genus most frequently having to comply with more stringent effluent associated with this second step, although other limitations for parameters such as total nitrogen genera, such as Nitrospina, Nitrococcus, and (TN). Nitrospira, can also autotrophically oxidize Untreated domestic wastewater contains nitrite (U.S. EPA, Nitrification, August 2002). ammonia. Nitrification is a biological process Denitrification is the process by which nitrates that converts ammonia to nitrite and nitrite to are reduced to gaseous nitrogen by facultative nitrate. If standards require that the resulting anaerobes. Facultative anaerobes, such as fungi, nitrate be removed, one treatment alternative is can flourish in anoxic conditions because they the process of denitrification, in which nitrate is break down oxygen containing compounds (e.g., - reduced to nitrogen gas. One treatment system NO3 ) to obtain oxygen. Once introduced into used for denitrifying wastewater effluent is the the aquatic environment, nitrogen can exist in denitrifying filter. In addition to the reduction of several forms—dissolved nitrogen gas (N2), + - total nitrogen, this treatment process removes ammonia (NH4 and NH3), nitrite (NO2 ), nitrate - suspended solids from the effluent. (NO3 ), and organic nitrogen as proteinaceous matter or in dissolved or particulate phases. The NITRIFICATION/DENITRIFICATION energy reactions are (Metcalf and Eddy, 1979): - - Nitrification is a microbial process by which 6 NO3 + 2 CH3OH Æ 6 NO2 + 2 CO2 + 4 H2O ammonia is sequentially oxidized to nitrite and (Step 1) then to nitrate. The nitrification process is - - 6 NO2 + 3 CH3OH Æ 3 N2 + 3 CO2 + 3 H2O +6 OH accomplished primarily by two groups of (Step 2) autotrophic nitrifying bacteria that can build organic molecules by using energy obtained Overall, from inorganic sources––in this case, ammonia - 6 NO3 + 5 CH3OH Æ 5 CO2 + 3 N2 + 7 H2O + 6 or nitrite. OH- In the first step of nitrification, ammonia- The organisms carrying out this process are oxidizing bacteria oxidize ammonia to nitrite called denitrifiers. In general, they are according to equation (1): heterotrophic bacteria that metabolize readily – + – NH3 + O2 Æ NO2 + 3H + 2e (1) biodegradable substrate under anoxic conditions using nitrate as the electron acceptor. If oxygen Nitrosomonas is the most frequently identified is available, these bacteria use it for metabolism genus associated with this step, although other before they use the nitrate. Therefore, dissolved genera, including Nitrosococcus and oxygen concentrations must be minimized for Nitrosospira, may be involved. The subgenera the denitrification process to function 1 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 Figures 1 and 2. Denitrifying filters at the East Central Regional Water Reclamation growth of the denitrifiers. The carbon source Facility, West Palm Beach, Florida most often selected is methanol, which is readily degraded under anoxic and aerobic conditions. Products (Thomasville, Ga.), maker of the Other carbon sources, such as acetic acid, also Davco denitrification filter. can be used in denitrifying filter systems. Wastewater enters a downflow filter over weirs DESIGN FEATURES along the length of the filter bed on both sides. Filter effluent is conveyed from the bottom of Filter Configurations the filter over a control weir into a clear well. Denitrifying filters have been utilized for Backwashing is required at regular intervals. wastewater treatment for a number of years. The Backwashing typically involves air scouring and combination of denitrification and solids backwashing with air and water. During the removal was first patented in the 1970s. Since process, nitrate is metabolized to nitrogen gas, that time, several companies have developed which becomes embedded in the filter media. their own denitrifying filters. In addition to Nitrogen-release cycles are needed to remove meeting TMDL requirements, facilities such as these nitrogen gas bubbles that accumulate. The the East Central Regional Water Reclamation piping for the filter influent and backwash is Facility in West Palm Beach, Florida, are similar to that of conventional filters. utilizing denitrification filters as part of an advanced wastewater treatment system to enable Upflow continuous-backwash filters differ in them to reuse treated wastewater to augment that influent wastewater flows upward through wetlands and to recharge aquifers (Figures 1 and the filter, countercurrent to the movement of the 2). sand bed. There are two main process configurations for Wastewater enters the filter through the influent denitrification filters commercially available, pipe (where methanol can be added), and then is downflow and upflow continuous backwash transported downward through a supply pipe filters. and distributors (Figure 3). The water moves up through the filter media and filtrate is Downflow denitrification filters operate in a discharged from the upper portion of the filter. conventional filtration mode and consist of The filter media travels slowly downward and is media and support gravel supported by an drawn into an airlift pipe in the center of the underdrain. Manufacturers include Severn Trent filter. Compressed air is introduced to the airlift, Services (Fort Washington, Pa.), maker of the drawing sand upward and scouring it. At the top TETRA Denite system; F.B. Leopold Co. Inc. of the airlift, the media is returned to the filter (Zelienople, Pa.), maker of the elimi-NITE bed. Filtered water rises through a separator that system; and Siemens Water Technology Davco removes the light dirt particles by washing them away and returns the large, heavy sand grains to 2 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 Figure 3. Astrasand upflow continuous- drop from the influent weir, thereby decreasing backwash filter. the level of DO entrainment. Since influent in upflow continuous-backwash filters is conveyed the top of the filter bed. The reject, or backwash, to the feed radials within the filter bed through water continuously exits near the top of the submerged manifold piping, DO entrainment filter. The reject-water weir is set at a lower over the influent weir is less an issue for those elevation than the effluent weir to allow clean filters utilizing this configuration. water to enter the washer and separator continuously by differential head, eliminating Media the need for typical backwash-supply pumps. The preferred media for each filter manufacturer is also presented in Table 1. The filter media in Manufacturers include Parkson Corp. (Fort the TETRA Denite system consists of a Lauderdale, Fla.), maker of the DynaSand filter, monomedia granular sand with a two to three and Paques bv (Balk, Netherlands), maker of the millimeter effective size. Uniform and relatively Astrasand filter. Siemens Water Technologies spherical media reportedly allow for more has a license agreement with Paques to supply rolling and contact with other media grains, this filter in the United States and Canada. resulting in more effective backwash and nitrogen-release cycles and, ultimately, lower Filter Design Characteristics backwash water volume requirements. Davco When designing a denitrification filter, there are filters can be supplied with the same media. many considerations that should be taken into Finer media are used with the DynaSand and account by wastewater professionals. Table 1 Astrasand filters that utilize the upflow presents a brief overview of the systems offered continuous-backwash filter design. by different manufacturers (deBarbadillo et al. 2005). Major design considerations include 1) a Underdrain manufacturer’s experience and 2) the system’s Early experience with downflow denitrification performance, which includes influent weir filters suggested that nozzle underdrains
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