Effects of nitrogen and oxygen on biofilter performance

Three laboratory-scale biofilters packed with inert material were used to study the nitrogen and oxygen requirements for biofiltration of methanol. Mixtures of methanol with inorganic nitrogen (NH3 or NO3) at nitrogen-to-carbon (N:C) ratios ranging from 0.015 to 0.4 were employed to reveal nitrogen effects on biofiltration. In the oxygen study, mixtures of air and oxygen at different oxygen contents were used. At low nitrogen levels, the removal rate increased with increasing N:C ratio for both NH3 and NO3. However, at high concentrations, NH3 had an inhibitory effect on biodegradation while the removal rate reached a plateau at high NO3 concentrations. Biofiltration with 63% oxygen in the inlet gas stream increased the maximum removal rate from 120 to 145 g/m3/hr after 3 days in comparison with biofiltration with air. However, a further increase in oxygen content up to 80% did not lead to a further improvement in biofilter performance, suggesting that both oxygen and biofilm thickness can be the relevant factors limiting biofilter performance and creating the plateau in removal rates at high loadings.