Practical experience with an industrial biofilter treating solvent vapour loads of varying magnitude and composition

A biofilter was installed to reduce levels of solvent vapours in air exhausted from a rotogravure printing press. This air was first humidified and then passed through two filter beds in series which contained a mixture of mushroom compost, activated carbon, lime and polystyrene beads. The printing press employed different inks for which the solvents included ethanol, ethyl acetate, n-propyl acetate, acetone, methyl ethyl ketone and toluene. The amount and composition of the solvent vapours in the exhaust air depended on the printing tasks undertaken. Breaks in the supply of solvent to the filter occured during job changeovers and press shut down. The performance of the biofilter under such varying operating conditions was studied. Maintaining moisture in the bed proved critical. Overall solvent removal efficiency was affected by the nature of the solvent mixture, the relative solubility of individual solvents and their relative biodegradabilities. No significant lag period was evident in biofilter performance when sudden changes in solvent vapour composition occurred. It was inferred that the activated carbon's adsorptive properties were at least partly responsible for this. Inferences drawn from observations during plant operation were used to gain insights into the relative importance of various physical, chemical and biological processes.