Comparative Scale-Up and Cost Estimation of a Biological Trickling Filter and a Three-Phase Airlift Bioreactor for the Removal of Methylene Chloride from Polluted Air

Laboratory scale biological trickling filters and three-phase airlift bioreactors have been studied for the elimination of methylene chloride (or dichloromethane) vapors from waste air, and the results used herein for the design of small industrial-scale reactors. The conditions chosen for scale-up were an air flow rate of 100 m3 h-1, a methylene chloride inlet concentration of 2 g m-3, and a target removal of 99.5%. The scale-up procedure, design, and cost analysis are discussed. The full-scale biotrickling filter appears to be the most cost-effective reactor, with global costs of about $62 per 1,000 m3 treated. Treatment in the airlift reactor was estimated to be twice as expensive and catalytic oxidation 5 times as expensive. Biological waste air treatment offers economical alternatives to conventional techniques for waste air treatment. INTRODUCTION Biological treatment of contaminated air is a new and promising technique that utilizes mixed microbial populations to degrade gaseous chemicals. Gaseous pollutants or vapors are sorbed into an aqueous phase prior to biodegradation. Biological treatment techniques offer cost-effective treatment for large air streams containing low pollutant concentrations. These techniques excel in two main domains: removal of odoriferous compounds,1-3 and elimination of volatile organic chemicals, primarily solvents, from waste air.3-9 Inorganic wastes may also be efficiently removed.10-12 Recently, biological waste air treatment was also considered for waste air generated by soil vapor extraction.13 Biological waste air treatment is an environmentally friendly technique that represents a major development towards reducing industrial atmospheric pollution. Reactors for biological waste air treatment are distinguished by their operational mode, the existence of a free liquid phase, and their continuous phase reaction media (gas or liquid). Biofilters are reactors in which a humid, polluted air stream is passed through a porous packed bed on which pollutant-degrading microbial cultures are naturally immobilized. Biofilters present a tremendous potential for air treatment. They have been implemented at full-scale and have proven cost-effective operation,3,14-16 but are generally not appropriate for continuous treatment of high concentrations of acid-producing pollutants, such as chlorinated solvents. Bioscrubbers are reactors where a pollutant-containing waste air stream is contacted with a scrubbing solution. Here, either simultaneous or subsequent biodegradation of absorbed pollutants occurs as a result of the actions by growing microbes. Because of the continuous supply of fresh scrubbing solution, bioscrubbers generate small IMPLICATIONS Biological waste air treatment is a promising development in environmental protection. Unlike biofilters, which have been applied to a number of large-scale applications, no full-scale biotrickling filter or airlift reactor has been constructed yet for waste air treatment. This is particularly unfortunate because these reactors have shown remarkable performance at the bench scale. In this paper we detail the scaling-up of both a biotrickling filter and an airlift reactor based on three years’ operation at the bench scale. The scale-up procedure, treatment costs, and design presented here provide a general framework for the choice of cost-effective alternatives to conventional treatment techniques.