Biofiltration of Cyclic Air Emissions of [H9251]-Pinene at Low and High Frequencies

Abstract

Biofiltration of periodically fluctuating concentrations of an ␣-pinene-laden waste gas was investigated to treat both high-frequency and low-frequency fluctuations. The effects of periodic concentration fluctuations on biofilter performance were measured. Controlled variables of periodic operation included cycle period and amplitude. The cycle period ranged from 10 min to 6 days, with the inlet ␣-pinene concentration fluctuating between 0 and 100 parts per million volume. At high-frequency concentration cycling (i.e., on the order of minutes), both cyclic and constant concentration biofilters maintained similar long-term performance with an average removal efficiency of 77% at an averaged loading rate of 29 g ␣-pinene/m 3 bed/hr. A first approximation suggests kinetics that are time-independent, indicating that steady-state data can be used to predict transient behavior at this time scale. Cyclic biofilter operation with a cycle period of 24 hr (with equal on/off time) was achievable for biofilters without a significant loss in performance. At longer time scales, cyclic biofilter performance decreased at the restart of the ON cycle. The recovery time to previous levels of performance increased with increasing cycle period; the recovery time was less than 1 hr for a cycle period of 24 hr and between 6 and 8 hr for a cycle period of 6 days. INTRODUCTIONIn treatment processes such as biofiltration, the feeds to the biofilter are byproducts of other processes and, therefore, one would generally not have very good control over it as a process input. Because changes in inlet concentration of pollutants in a waste gas may affect the operation and performance of a biofilter treating such streams, it is important to consider these types of concentration fluctuations. Fluctuations in air emissions or waste gases can be the result of seasonal changes in operation, daily variations in operating conditions, hourly fluctuations, or variations every minute caused by process conditions and operation. In the forest products industry, air emissions from some pulping operations (e.g., brownstock washers) and other manufacturing processes (e.g., press vent operation in the manufacture of particleboard or other laminated wood products) are cyclic in nature and, as such, the pollutant concentrations vary with time. 1,2 This is because of the cyclic nature of the process or unit operation producing the emission. It is difficult, if not impossible, to keep constant the amounts of pollutants in the off gases, and it is therefore impractical to control the concentration of contaminants in the air emission.Fluctuating concentrations caused by process operations (e.g., a press vent or brownstock washer operation) may have a significant impact on biofiltration because the residence time, , in biofilters (i.e., on the order of seconds to several minutes) is often on the same time scale as the concentration fluctuations. In the literature, one finds a broad range of residence times, ranging from seconds 3 to several minutes. 4,5 This...

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