ENGINEERING MODEL FOR FIXED-FILM BIOSCRUBBERS By Hanneke F. Ockeloen/ Thomas J. Overcamp,:Z and C. P. L. Grady Jr. 3

TLDR: In this article, an engineering simulation model of a fixed-film bioscrubber was used to investigate the applicability, removal efficiency, operational parameters, and design requirements for gaseous waste streams.

Abstract: The three basic types of biological treatment systems for the control of volatile organic compounds in air streams are the following: biofilters, in which microorganisms grow on a medium, such as soil, compost, peat, or mixtures of these materials with wood chips or polystyrene particles; suspended-growth bioscrubbers, in which microorganisms are suspended in a liquid; and fixed-film bioscrubbers, in which microorganisms are attached to a packing material. Design and application of biological treatment methods for air pollution control are difficult because only limited experimental data and few theoretical models are available. This paper utilizes an engineering simulation model of a fixed-film bioscrubber to investigate the applicability, removal efficiency, operational parameters, and design requirements for gaseous waste streams. Model results indicate that the removal efficiencies can be increased by increasing the column height, decreasing the superficial gas velocity or the superficial liquid velocity, or by treating the liquid prior to recirculation to the absorber. High removal efficiencies can be obtained for compounds with relatively low values of the Henry's Law coefficient with either cocurrent or countercurrent operation. However, as the Henry's Law coefficient increases, the removal efficiency decreases and high removal efficiencies can be obtained only with cocurrent flow. Cocurrent operation is usually more efficient because stripping does not occur at the top of the column. Physical, chemical, and biological scrubbing methods can be used to control pollutants in waste gases. Physical absorp­ tion transfers the waste components of the gas stream into a liquid. Prior to reuse, the liquid must be regenerated or treated to remove the contaminant. One approach is to use chemical reagents to neutralize acidic or alkaline pollutants or to oxidize pollutants. Alternatively, biological processes often can be used to mineralize organic compounds, allowing the liquid to be recirculated. Such biological treatment may be more eco­ nomical than conventional physical or chemical treatment. Despite successes of biological treatment systems in a num­ ber of air-pollution control applications, only limited experi­ mental data and few theoretical models are available, making design difficult. This paper presents an engineering model for fixed-film bioscrubbers to predict the removal efficiency, op­ erational parameters, and design requirements for various gas­ eous pollutants. The bioscrubber modeled in this paper is a combination of a packed adsorption tower in chemical engi­ neering and a high-rate trickling filter or roughing filter used in wastewater treatment. Generally, there is direct recirculation of the liquid from the bottom to the top of the tower. Simu­ lations were conducted to determine the important factors gov­ erning bioscrubber performance. The results offer guidance for the future development of this technology. BACKGROUND In aerobic biological treatment of waste gases, the pollutants are degraded by appropriate microorganisms to form carbon dioxide, water, and cell material. There are three basic types of biological treatment systems: biofilters, suspended-growth bioscrubbers, and fixed-film bioscrubbers. In a biofilter, the polluted gas passes through a moist, biologically active ma­ terial, such as soil, compost, peat, or mixtures of these materials