Biofiltration of hexane, enhancement of performance by nitrogen supplementation

Abstract

A study was conducted to investigate the improvement of hexane biofiltration through nutrient supplementation. Two supplementary nutrient addition protocols, low and high frequency additions, were applied to biofilters containing pearlite, peat and their 50:50% (v/v) mixture. In high frequency nutrient addition, 1 kg of nitrogen was added to 1 m3 of biofilter medium every week. In low frequency nutrient supplementation, the addition was reduced to once every 30 days. The biofilters were inoculated with a mixed culture containing Pseudomonas corrugata, which was identified as the dominant species capable of utilizing hexane as the sole carbon source.

Biodegradation of hexane was found to strongly depend on nitrogen concentration in the solid media, such that media formulation and particle size had little effect on biofiltration rates. Hexane removal rate 50 g/m3.h at 100ppm and a flow rate of 20 m/h during low nutrient supplementation for all media formulations. For high nutrient supplementation, the Hexane removal rate was up to 67 and 120 g/m3.h at hexane concentrations between 560-680 ppm and gas flow rates of 20 and 100 m/h, respectively. The elimination capacity for hexane was 75 g/m3.h

Pressure drop ranges during high frequency addition of nutrients were 20 and 60 Pa/m to 230 and 2900 Pa/m for pearlite and the mixture, respectively at gas flow rate of 20 m/h. The pressure drop range during low frequency addition of nutrients was lower (10-250 Pa/m) for all media. In both cases, the pearlite exhibited the lowest pressure drop. The lime was effective in maintaining the pH value above 5 for more than 50 and 240 days of high and low nutrient addition, respectively. A long term biofiltration showed that the performance factor, a measure of the amount of hexane removed per unit volume, energy and time was higher for the pearlite averaging 5 g/m3.J.h than that of the peat and the mixture which was less than 1 g/m3.J.h.

The hexane removal followed zero order kinetics. Mathematical models described by Ottengraf are effective in describing biofiltration kinetics. However, apparent reaction rate constants changed with nitrogen concentration in the solid media.