Free ammonia (free ammonia)

Free ammonia is the un-ionised form of ammonia (NH₃) dissolved in water, as opposed to the ionised ammonium (NH₄⁺). The two exist in a pH- and temperature-dependent equilibrium: at higher pH and temperature, more of the ammonia is in the free (NH₃) form. This distinction is critical because free ammonia is far more toxic to fish and aquatic life than ammonium — it crosses biological membranes easily and is lethal to fish at low concentrations. The effluent limit for free ammonia is a tight 5.0 mg/L for inland surface water and marine coastal discharge.

The toxicity mechanism is direct: free ammonia damages gill tissue and disrupts fish metabolism, causing kills well below the levels at which the same total ammonia would be harmful if it were all in the ammonium form. Because the free fraction rises with pH, an effluent with acceptable total ammonia can become acutely toxic simply because its pH is high — making pH control part of ammonia management.

For recyclers, free ammonia is again a CBG/biogas concern, intimately linked to ammoniacal nitrogen and TKN. Digestate liquor from nitrogen-rich feedstock is ammonia-laden, and at the alkaline pH typical of digestate, a significant fraction is present as toxic free ammonia. The same chemistry that causes ammonia inhibition inside the digester (free ammonia stalling methanogenesis above roughly 1,500-3,000 mg/L) makes the discharged liquor toxic to aquatic life if released untreated.

The practical implication is that free ammonia must be controlled before discharge to a water body, and managed even for land application. Removal options include ammonia stripping (raising pH and air-stripping the NH₃, which can recover it as ammonium sulphate fertiliser), nitrification, or dilution within agronomic limits for fertigation. For CBG operators, the existing glossary's entries on ammonia inhibition and ammonia evaporation connect directly here — controlling free ammonia protects both the digester's biology and the receiving water, and recovered ammonia can become a saleable product.