Category / type: biological nutrient removal (BNR).
Application: medium, large and municipal wastewater treatment, industrial water process.
Size: from 3 up to 20 m3.
Flow: from 1,0 up to 100 m3 per day.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (grp).
Focus: high efficiency treatment.
Biological nitrification is the second of two processes involved in the biological removal of nitrogen, in which nitrate is converted into nitrogen gas (stripping). It follows the nitrification process, where ammonium is converted into nitrate by autotrophic bacteria.
The activated sludge process, can be extended with the nitrification process.
The process is based on nitrogen cycle: it represents one of the most important nutrient cycles found in terrestrial ecosystems.
Carbon and nitrogen are the major life sources but their inlet in water bodies through wastewater discharge, if not regulated, contributes to environmental quality problems. All of the pollution sources (i.e. municipal, industrial, and agricultural) must be managed in order to reduce carbon and nitrogen concentrations within a certain level to improve the quality of the environment. A nitrification and denitrification steps are needed to remove nitrogen.
The nitrification step is a biological process by which ammonia is converted for first to nitrite and then to nitrate. Nitrification can be achieved in any aerobic-biological process at low organic loadings and where suitable environmental conditions are provided. Nitrifying bacteria have a slower growth than the heterotrophic bacteria, which comprises the greater proportion of the biomass in both fixed film and suspended growth systems.
The denitrification step is a biological process through which nitrate is converted to nitrogen and other gaseous-end products. The requirements for the denitrification process are th presence of nitrates, an organic carbon source, and an anaerobic environment.
Since the bacteria are facultative, low rate of dissolved oxygen is maintained (ideal condition is 0.5 mg/l). Around values of 2 mg/l denitrification process ends.
Denitrification can be associated with all the activated sludge process as extended aeration, MBR, MBBR, etc.
How It Works
Biological conversion of ammonia to nitrogen gas is a two-steps process.
Ammonia must first be oxidized to nitrate; nitrate is then reduced to nitrogen gas. These reactions require different environments and are often carried out in separate areas in the wastewater treatment system.
Denitrification requires organic substrate for the reduction of nitrate into nitrogen gas. The first distinction is whether the source of the organic substrate is internal (present in the wastewater) or external (added to the wastewater). The second distinction is between pre-denitrification (pre-D) systems, where the denitrification reactor precedes the nitrification reactor, and post-denitrification (post-D) systems.
The first step in the process (conversion of ammonia to nitrite and then to nitrate) is called nitrification; the second step of the process, (nitrate to nitrogen gas) is referred to as denitrification.
The necessary conditions for denitrification developing in an activated sludge process are presence of a facultative bacterial mass (capable of using oxygen and nitrate or nitrite), presence of nitrate, absence of dissolved oxygen in the mixed liquor (anoxic environment), suitable environmental conditions for bacterial growth and presence of an electron donor: (i.e. organic matter).
There are many design methods based on models or empirical data.
Net volume is calculated by considering denitrification rate.
In an activated sludge plan, denitrification chamber volume is normally around 25%-40% of the total volume. It is not recommended to exceed 50%.
The kinetics of the denitrification process are determined for both pre-D and post-D anoxic reactors. The minimum anoxic sludge mass fraction is designed to maximize nitrate removal with the easily biodegradable material present in the influent. It depends on pH, nitrogen concentration (TKN) according Monod kinetic and temperature.
Any wastewater treatment unit that is going to remove nitrogen by the nitrification/denitrification process must be designed to provide both aerobic and anaerobic areas, to allow the occurrence of both nitrification and denitrification processes.
There are many nitrogen removal technologies. Recirculation flow rate, for example, is an important design step for entire purifying process.
Operation and maintenance
Denitrification excess sludge volume calculation is similar to oxidation basin for BOD or COD reduction.
Since bacteria are facultative, low concentration of dissolved oxygen must be maintained in denitrification chamber in order to allow an anoxic reaction. Otherwise there is a worsening in purification efficiency.
Each gram of D.O. is like to 0.35 grams of N-NO3 in denitrification reaction.
Oxygen coming from turbulence flow, weirs, pumps and groundwater should be avoided as much as possible.
Where local regulation imposes limits on nitrogen discharge, denitrification process is required, especially in industry, agribusiness, canning industry or domestic wastewater.
Nitrogen and phosphorus are essential elements for biological reactions (nutrients) but they causes, if present in large quantities, serious environmental problems (sea, lakes, rivers and aquifers). For this reason, currently law pays particular attention to the eutrophication of water receiving bodies.
Features and benefits
A biological wastewater treatment plant always removes a certain amount of nitrogen from the wastewater thanks to the growth of the biomass. When the nitrogen-carbon ratio in a wastewater is too high, the incorporation of nitrogen in biomass is often not sufficient to respect the discharge limits. By expanding a biological wastewater treatment plant with a nitrification-denitrification process the excess of nitrogen is removed biologically.
Low temperatures and incorrect pH values cause slow nitrifier growth rate.
Nitrification often take place in aeration basin (especially in extended aeration plants) and requires high level of molecular oxygen in the wastewater solution
called Dissolved Oxygen (D.O.).
On the contrary, denitrification requires a dedicated volume in which anoxic conditions and mixing process are met.
To combine an activated sludge wastewater treatment plant with a nitrification-denitrification process, an additional non-aerated reactor is commonly added to the wastewater treatment plant. This reactor could be installed before (pre-denitrification) or after (post-denitrification) the aeration basin.
In the pre-D reactor, which is the more common choice, raw wastewater, a mixture of recirculated sludge and mixed liquor feeds the process.
Post-D requires an external organic substance source (methanol, acetic acid, etc).
Combinations of pre- and post denitrification are also quite common.
In a batch reactor (SBR) the denitrification process is implemented by adding a non-aerated step in the purification process.
- high quality effluent;
- pH adjustment (nitrification-denitrification).
- advanced treatment;
- controls, analysis needed.