Ammoniacal nitrogen (ammonia nitrogen)

Ammoniacal nitrogen in the effluent context is the nitrogen present as ammonia and ammonium (NH₃ + NH₄⁺), expressed as N. It is a standard effluent discharge parameter, distinct from the air-pollutant and fertiliser senses of the term, with an effluent limit of 50 mg/L for inland surface water, public sewers and marine coastal discharge. It is the dissolved-nitrogen counterpart that, together with organic nitrogen, makes up the TKN, and the source of the toxic free-ammonia fraction.

Ammoniacal nitrogen in a discharge causes two problems: it contributes to eutrophication (nutrient over-enrichment and algal blooms) in the receiving water, and through its free-ammonia fraction it is directly toxic to fish. It also exerts an oxygen demand as it is nitrified in the receiving water. This is why it is capped relatively tightly at 50 mg/L across the main discharge modes.

For recyclers, ammoniacal nitrogen is the defining effluent parameter of the CBG/biogas sector. Anaerobic digestion converts the nitrogen in protein-rich feedstock into ammoniacal nitrogen, so digestate liquor is characteristically high in it — often well above the 50 mg/L discharge limit, requiring removal before any water discharge. The same value-versus-pollutant duality applies as with TKN: it is fertiliser nitrogen when the digestate is used on land, but a regulated pollutant when released to water.

The practical management mirrors the related nitrogen parameters. For fertigation/land use, the ammoniacal nitrogen is the digestate's principal fertiliser value, applied at agronomic rates. For water discharge, it must be reduced to 50 mg/L by ammonia stripping (with possible recovery as ammonium sulphate), nitrification-denitrification, or other treatment. A CBG operator's whole nitrogen strategy — digestate as product versus digestate liquor as effluent — turns on how this parameter is handled, making it one of the most important numbers in the sector's environmental and commercial planning.