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

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 mu mol/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.

Automated summary

Iron incorporation in biomass carriers at different percentages significantly affected the nitrogen removal efficiency and the abundance of Anammox bacteria. The addition of 5% iron carriers resulted in the highest nitrogen removal efficiency (up to 75%) and increased ammonium removal rate. However, the presence of 10% iron carriers inhibited ammonium oxidation rate. The long-term release of ferrous iron from the carrier structure accelerated the treatment of the digester supernatant.