Simultaneous nitrification-denitrification
Encyclopedia
Simultaneous nitrification-denitrification (SNdN) is a wastewater
treatment process. Microbial simultaneous nitrification
-denitrification
is the conversion of the ammonium ion to nitrogen
gas in a single bioreactor
. The process is dependent on floc characteristics, reaction kinetics, mass loading of readily biodegradable chemical oxygen demand
, rbCOD, and the dissolved oxygen
, DO, concentration
, obtain carbon
from CO2
, and use oxygen
as the electron acceptor
. They are termed autotrophic because of their carbon source and termed aerobes because of their aerobic environment. The heterotrophic organisms are responsible for denitrification or the oxidation of nitrate
, NO3-, to nitrogen gas, N2. They use carbon from complex organic compounds, prefer low to zero dissolved oxygen, and use nitrate as the electron acceptor.
by adding oxygen in one location in the basin. Next to the injection point, a high DO concentration is maintained allowing for nitrification and oxidation of other organic compound
s. The oxygen is the electron acceptor and is depleted. The DO level in localized environments decreases with increasing distance from the injection point. In these low DO locations, the heterotrophic bacteria complete the nitrogen removal. The Orbal process is a technology in practice today using this method. Two is to produce an oxygen gradient within the bio floc. The DO concentration remains high in the outside rings of the floc where nitrification occurs but low in the inner rings of the floc where denitrification occurs. This method is dependent on the floc size and characteristics; however controlling flocs is not well understood and is an active field of study
Typically, SNdN has slower ammonia and nitrate utilization rates as compared to separate basin designs because only a fraction of the total biomass is participating in either the nitrification or the denitrification steps. The SNdN limitation due to partial active biomass has led to research in novel bacteria and system designs. Huang achieved significant ammonia removal in an attached growth process with ciliated columns packed with granular sulfur
where the denitrifying bacteria used the sulfur as the electron donor
and nitrate as the electron acceptor. Another well established pathway is via autotrophic denitrifying bacteria in the process termed the Anammox process. It is typically used for high ammonia strength wastewater.
Wastewater
Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...
treatment process. Microbial simultaneous nitrification
Nitrification
Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. Degradation of ammonia to nitrite is usually the rate limiting step of nitrification. Nitrification is an important step in the nitrogen cycle in soil...
-denitrification
Denitrification
Denitrification is a microbially facilitated process of nitrate reduction that may ultimately produce molecular nitrogen through a series of intermediate gaseous nitrogen oxide products....
is the conversion of the ammonium ion to nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
gas in a single bioreactor
Bioreactor
A bioreactor may refer to any manufactured or engineered device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substances derived from such organisms. This...
. The process is dependent on floc characteristics, reaction kinetics, mass loading of readily biodegradable chemical oxygen demand
Chemical oxygen demand
In environmental chemistry, the chemical oxygen demand test is commonly used to indirectly measure the amount of organic compounds in water. Most applications of COD determine the amount of organic pollutants found in surface water or wastewater, making COD a useful measure of water quality...
, rbCOD, and the dissolved oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, DO, concentration
Microbiology
The oxidation of the ammonium to nitrogen gas has been achieved with attached growth and suspended growth wastewater treatment processes. The most common bacteria responsible for the two step conversion are the autotrophic organisms, Nitrosomonas and Nitrobacter, and many different heterotrophs. The former obtain energy from the oxidation of ammoniaAmmonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, obtain carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
from CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, and use oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
as the electron acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
. They are termed autotrophic because of their carbon source and termed aerobes because of their aerobic environment. The heterotrophic organisms are responsible for denitrification or the oxidation of nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
, NO3-, to nitrogen gas, N2. They use carbon from complex organic compounds, prefer low to zero dissolved oxygen, and use nitrate as the electron acceptor.
Systems Design
The most common design uses two different basins: one catering to the autotrophic bacteria and the second to the heterotrophic bacteria. However, SNdN caters to both in one basin with strict control of the DO. This has been done in two common approaches. One is to develop an oxygen gradientDiffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
by adding oxygen in one location in the basin. Next to the injection point, a high DO concentration is maintained allowing for nitrification and oxidation of other organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s. The oxygen is the electron acceptor and is depleted. The DO level in localized environments decreases with increasing distance from the injection point. In these low DO locations, the heterotrophic bacteria complete the nitrogen removal. The Orbal process is a technology in practice today using this method. Two is to produce an oxygen gradient within the bio floc. The DO concentration remains high in the outside rings of the floc where nitrification occurs but low in the inner rings of the floc where denitrification occurs. This method is dependent on the floc size and characteristics; however controlling flocs is not well understood and is an active field of study
Typically, SNdN has slower ammonia and nitrate utilization rates as compared to separate basin designs because only a fraction of the total biomass is participating in either the nitrification or the denitrification steps. The SNdN limitation due to partial active biomass has led to research in novel bacteria and system designs. Huang achieved significant ammonia removal in an attached growth process with ciliated columns packed with granular sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
where the denitrifying bacteria used the sulfur as the electron donor
Electron donor
An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....
and nitrate as the electron acceptor. Another well established pathway is via autotrophic denitrifying bacteria in the process termed the Anammox process. It is typically used for high ammonia strength wastewater.