47    ENHANCEMENT OF BIOFILTRATION PROCESS FOR
AIR PURIFICATION
Brian C. Shade, Research Assistant
Walter J. Maier, Professor
Civil and Mineral Engineering Department
University of Minnesota
Minneapolis, Minnesota 55455
INTRODUCTION
Biofiltration is a biologically mediated process for purifying air contaminated with organic vapors.
Early applications by sanitary engineers focused on filtering odorous gases through soil beds to
eliminate odors from septic tanks.4 Microbes were found to be the purifying agents. U.S. Utility
Patent 2,793,096 was granted to R.D. Pomeroy in 1957 for a soil bed which enhanced the "deodorizing of gas streams by the use of microbial growth." This patent expired in 1974, but design patents are
currently held by others for design modification, such as composition of packing materials, configuration of packed beds, and gas delivery systems. Several investigators have studied the unique properties of soils in related applications. Rapid disappearance of cyanide in aerobic soils is an example.
Dioxin and other halogenated hydrocarbons have been found to oxidize at slower rates in air and
water than in soil.
As regards industrial applications for controlling emission of volatile organics, West Germany and
the Netherlands are leaders in the field with more than 500 plants in operation. Biofiltration is
currently viewed as BACT (best available control technology) in West Germany and the Netherlands.9
However, there is growing interest in biofiltration for industrial applications in the United
States.2,5,6>7 A recently published survey3 lists ethanol and butylaldehyde removal from dryer air at
Monsanto Chemical Company, removal of pharmaceutical odors at Upjohn Company, and hydrocarbon vapors removal from fuel storage tanks at Esso of Canada. This diversity of applications suggests
that there are numerous potential industrial applications.
The major features of biofiltration have been described in the literature. Gas flow through the filter
(packed-bed) is characterized as plug flow with minimal back mixing.13 At the mass loadings of air
commonly applied to biofilters, air flow is in the turbulent flow regime and mass transport rates in the
air phase are correspondingly rapid. However, surface transfer of organic vapors from the air phase
to the stationary packing phase is dependent on the physical-chemical properties of the packing
surfaces, their moisture content, and the physical chemical properties of the chemical, namely, its
affinity for adsorption on surfaces, its solubility in water, and its volatility. At high hydrocarbon
concentrations, removal rates are reported to be zero order." Kampbell reported increasing rates of
removal with increasing hydrocarbon concentrations8 which implies that removal follows first order
kinetics. A number of investigators have suggested that the rate limiting step may be either the rate of
reaction within the biofilm, or the rate of diffusion in the biofilm.15,16 Zero order kinetics imply that
100 percent removal is possible for any biodegradable compound.14 However, it is unlikely that zero
order kinetics will prevail at very low concentrations. Studies of removal rates at low concentrations
are needed. Also a more precise description of the interrelated effects of transport, adsorption, and
microbial kinetics are needed.
Two important questions that have not been described adequately in the literature concern the
effectiveness of biofilters under variable loading conditions and the stability of biofilters operating at
high removal efficiencies to achieve essentially complete removal of chemicals at low concentrations.
The answer to these questions are closely tied to the dynamics of cell mass production and accumulation of active cells throughout the reactor.
This paper describes the initial results of a systematic pilot plant study to examine the effects of
different air flow rates and mass loadings of chemical under constant and variable loading conditions.
The objective is to generate process data at precisely defined operating conditions. The paper also
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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