Regeneration of a Compost Biofilter Degrading High Loads of Ammonia by Addition of Gaseous Methanol

Abstract

The long-term stability of a biofilter loaded with waste gases containing NH₃ concentrations larger than 100 ppmv was studied in a laboratory-scale compost reactor. At an empty bed residence time (τ) of 21 sec, elimination capacities of more than 300 g NH₃/m³/day were obtained at elimination efficiencies up to 87%. Because of absorption and nitrification, almost 80% of the NH₃-N eliminated from the waste gas could be recovered in the compost as NH₄+-N or NO₂ ⁻/NO₃ ⁻-N. The high elimination capacities could be maintained as long as the NH₄+/NO_x concentration in the carrier material was less than 4 g NH₄+/NO_x ⁻-N/kg wet compost. Above this critical value, osmotic effects inhibited the nitrifying activity, and the elimination capacity for NH₃ decreased. To restore the biofilter performance, a carbon source (methanol) was added to reduce NH₄+/NO_x ⁻ accumulated in the compost. Results indicate that methylotrophic microorganisms did convert NH₄+/NO_x ⁻ into biomass, as long as the NO₃ ⁻ content in the compost was larger than 0.1 g NO₃ ⁻-N/kg compost. Removal efficiencies of CH₃OH of more than 90% were obtained at volumetric loads up to 11,000 g CH₃OH/m³/day. It is shown that addition of CH₃OH is a suitable technique for regenerating the compost material from osmotic inhibition as a result of high NH₃ loading. The biofilter was operated for 4 months with alternating loading of NH₃ and CH₃OH.