Nitrogen Elimination from Digester Supernatant with Magnesium-Ammonium-Phosphate Precipitation

Ammonium nitrogen was eliminated from digester supernatant (0.6–0.7 g NH4-N/I) by magnesium-ammonium-phosphate (MAP) precipitation in laboratory and pilot-scale tests (three 0.5 m3 reactors in series). MAP precipitation was carried out by adding phosphoric acid and magnesium oxide in a Mg: P: N ratio of 1.3: 1: 1. Prior to MAP precipitation, the remaining suspended solids in the digester supernatant were eliminated by flocculation. Phosphoric acid was added to the inlet of the first reactor, where the CO2 produced was stripped. Magnesium oxide was added to the second reactor where 70% of the ammonium was eliminated. In the third reactor, pH was adjusted to 9.0. 85–90% of the ammonium was eliminated at a hydraulic load of 0.5 m3/d. The MAP slurry (12–14 g TS/l) was directly dewatered with a decanter centrifuge to 50% dry solids.