Iron-containing carriers stimulate nitrogen conversions in one-stage reactors treating N-rich digester supernatant

Abstract

Iron incorporation in biomass carriers (0%, 1%, 5%, and 10% of carrier weight) at nitrogen loading rates (NLR) from 0.08 to 0.30 g/(g MLVSS·d) was tested during the treatment of nitrogen-rich digester supernatant in one-stage reactors, in which hybrid biomass (suspended/attached) was inhabited by ammonia-oxidizing bacteria, Anammox bacteria, and denitrifiers. The Anammox process dominated at the highest NLR and increased the volumetric nitrogen removal rate to about 500 g/(m3·d). Total microbial composition was most strongly affected by NLR, however, Anammox bacteria abundance was also affected by the iron content. These bacteria predominated in reactors with 5%-iron carriers, which ensured the highest nitrogen removal efficiency (up to 75%) and kinetics: the average ammonium removal rate was at least 40% higher than in the reactor without iron. The presence of 10%-iron carriers in the reactor inhibited the ammonium oxidation rate; an increase in ammonium removal rate with increasing initial ammonium concentration was the lowest. The contribution of Anammox to nitrogen removal was demonstrated by the simultaneous increase in biomass heme c content to about 0.1 µmol/mg protein along with increasing iron content to 5% and NLR. The incorporation of iron into the carrier structure allowed for a long-term release of mostly ferrous iron to the supernatant, thus accelerating its treatment.