Denitrification is removal of Nitrates from water or waste water. There are two basic approaches for Denitrification.
- Denitrification of waste water stream
- Denitrification of Potable water
Nitrate is a stable and highly soluble ion with a low potential for precipitation or adsorption. These properties make it difficult to remove from water using treatment processes such as filtration or activated carbon adsorption. As a result, more complex treatment processes must be considered.
For waste water streams biological treatment is suitable which combines with BOD removal.
For potable water systems processes such as ion exchange, reverse osmosis, and electrodialysis or Engineered Biological Treatment systems are employed depending upon Nitrate content & System Flow. These processes physically/chemically or biologically remove nitrate from drinking water.
Untreated domestic wastewater contains ammonia. Nitrification is a biological process that converts ammonia to nitrite and then 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.
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 below equation:
NH3 + O2 → NO2– + 3H++ 2e–
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 can also autotrophically oxidize ammonia.
In the second step of the process, nitrite-oxidizing bacteria oxidize nitrite to nitrate according to below equation:
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.
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 (NH4+ and NH3), nitrite (NO2-), nitrate (NO3-), and organic nitrogen as proteinaceous matter or in dissolved or particulate phases. The energy reactions can be depicted as below:
6 NO3- + 2 CH3OH → 6 NO2- + 2 CO2 + 4 H2O (Step 1)
6 NO2- + 3 CH3OH → 3 N2 + 3 CO2 + 3 H2O +6 OH- (Step 2)
6 NO3- + 5 CH3OH → 5 CO2 + 3 N2 + 7 H2O + 6 OH-
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 properly. Oxygen is typically is minimized by avoiding aeration of the waste water and having a high concentration of BOD so that microorganisms use all the oxygen.
In waste water treatment facilities the BOD of waste water is reduced in aeration tank & nitrification of Ammonia to nitrates is done. Sufficient oxygen is made available so that all Ammoniacal Nitrogen is converted to Nitrates. Then the waste water is recirculated back to an anoxic zone (for creating condition of High availability of BOD) before aeration zone. The anoxic zone is not aerated to restrict supply of oxygen.
Ion Exchange for Nitrate Removal:
In the ion exchange system, nitrate ions bind to ion exchange resin and, in the process, displace chloride ions. The resin is contained within a pressure vessel and is periodically regenerated with a concentrated salt solution. The function is similar to Water softeners which remove Hardness from water in exchange for sodium.
The frequency of regeneration depends upon the raw water quality. Special performance nitrate selective ion exchange resins are used to treat the water. The resins used to treat water are approved under the American National Standards Institute/National Sanitation Foundation (ANSI/NSF) standards for contact with potable water. If an ion exchange column is not regenerated frequently enough, the concentration of nitrate could spike to levels well above 10 mg/L, which is a public health concern.
- Ease of operation; relatively reliable
- Lower initial cost
- Effective; More widely used than other forms of treatment
- Suitable for small installations
- Requires frequent monitoring for nitrate removal
- Requires storing large volumes of salt
- Resins are susceptible to organic fouling
- Potential for “dumping” of the nitrate from the column resulting in periodic high concentrations of nitrate in the finished water.
- Changes in finished water pH, so requires pH correction
- Spent brine disposal can be difficult.
Reverse Osmosis for Nitrate Removal:
Reverse Osmosis (RO) is a physical process in which contaminants are removed by applying direct pressure to raw water through a semi-permeable membrane allowing water to pass through while retaining most of the dissolved minerals. Reverse Osmosis has advantage of removing excessive TDS along with Nitrates making even sea water potable.
- Produces high quality water with significant nitrate removal
- Proven technology with calculated risks
- Less chemical consumption compare to ion exchange.
- Expensive to install and maintain
- Skilled manpower may be needed to operate
- Disposal of concentrate and pretreatment waste will be a challenge
- Membranes are prone to fouling
- Frequent membrane monitoring and maintenance is required
Electrodialysis for Nitrate Removal:
In the electrodialysis (ED) process, ions migrate through ion-selective semi permeable membrane as a result of electrically charged membrane surfaces. A positive electrode (cathode) and a negative electrode (anode) are used to charge the membrane surfaces and attract oppositely charged ions. As a result of this process, ions such as nitrate are removed from the raw water. In electrodialysis reversal (EDR), the charge on the membranes is periodically reversed to minimize scale development. An electrodialysis reversal package plant is depicted below.
- Can operate without fouling, scaling, or chemical addition
- Low pressure requirements
- Typically quieter than RO
- Higher membrane life expectancy
- Pretreatment required for high levels of Fe, Mn, H2S, chlorine, or hardness
- Concentrate may require special disposal
- Very high Capital cost
- Skilled manpower required
- High level of automation required.
- Suitable for low flow applications only.
Biological Treatment (Engineered) for Nitrate Removal:
Biological denitrification is a process through which bacteria convert nitrate to nitrogen gas under anoxic (oxygen free) conditions. The nitrogen gas and bacteria are then removed from the water before entering the distribution system. Methanol is normally used as carbon source to facilitate the biological denitrification process. The major advantage of this system is it does not alter the composition of the water except removing the Nitrates.
- No concentrated salt brine or nitrate for disposal
- No change in composition of treated water.
- Extensive piloting may be required.
- Several weeks required from start-up to stable operation for new systems.
- Post treatment filtration and disinfection required.
- Process is temperature sensitive
- Taste and odor problems may require additional treatment
- Post pH correction might be required.
- Costly continuous addition of Carbon source like Methanol.
- Media may wash out the bacterial growth during backwash period.
- may require post filtration for Suspended Solids removal.