Denitrification is often required due to health concerns, for example the United States Environmental Protection Agency (US EPA) has set a maximum contaminant level of 10 mg/l nitrate in drinking water to prevent methemoglobinemia (blue-baby syndrome) and nitrates have been linked to carcinogenesis and birth defects.
Denitrification is the conversion of nitrate (N03) to nitrogen gas (N2). Heterotrophic bacteria use the nitrate as an oxygen source under anoxic conditions to break down organic substances.
Nitrates + Organics + Heterotrophic Bacteria = Nitrogen Gas & Oxygen & Alkalinity
An anoxic zone is established which is an unaerated basin where the dissolved oxygen levels are kept below 1 mg/l and must be as close, without reaching 0 mg/ as possible. A target operating point would be 0.2 to 0.5 mg/l.
The heterotrophic bacteria obtain their oxygen using the following sequence: free and dissolved oxygen, nitrate, and then sulfate. If your zone has no free or dissolved oxygen, the “bugs” will have to obtain their oxygen source by breaking down the nitrates that are returned to the anoxic zone in the activated sludge.
As the bugs use the nitrate as an oxygen source to break down the carbon, their source of food, nitrogen gas will be released to the atmosphere. The mixed liquor suspended solids concentration must be kept in balance. The pH of the anoxic zone should be close to neutral (7.0) and never drop below 6.5
There must be a carbon source. Typically 2.72 mg BOD5 are required per mg of NO3-N removed. This is a particularly important consideration where the biochemical oxygen demand (BOD) is low compared to the ammonia levels, for example in commercial applications such as offices and schools or where a residential subdivision has a slow build-up of BOD load as houses are occupied.
Using the raw influent as the carbon source through recirculating the nitrified effluent to an anoxic zone prior to the aeration stage or to an adequately designed septic tank (sufficient hydraulic retention time for denitrification and settlement must be provided) is common. Recirculation is typically suitable where influent levels of total kjeldahl nitrogen (TKN) are <50 mg/l and the nitrate standard is not more stringent than 10 mg/l requiring a recirculation rate in the order of four times the influent flow rate.
Where insufficient BOD is available a carbon dosing system may be required. Sufficient flow equalization is required to reduce the peak flows prevalent in residential and decentralized systems. A benefit of recirculation is the recovery of alkalinity where approximately half the alkalinity lost during nitrification is recovered which may remove the need for chemical dosing systems in low alkalinity wastewaters.
Other denitrification methods include cycling the aeration system on and off allowing the whole aeration basin to be used intermittently as an anoxic zone, such as in a sequencing batch reactor (SBR), and using a post anoxic tank after aeration where a supplemental carbon source such as Micro C, methanol or ethanol is added. It takes on the order of 2.0 – 2.5 parts methanol for every part nitrate that is denitrified.
Bugs + Carbon + Nitrate (CO3) = Nitrogen Gas (N2) + Oxygen (O2) + 3.6 parts Alkalinity