Biofiltration of odorous gas emissions

Sammanfattning: Biofiltration has shown its potential as an interesting treatment alternative for contaminated gas streams. Unlike conventional technologies, such as adsorption, scrubbers, and incineration, biofiltration offers effective pollution control at relatively low capital and operating costs, and without the generation of secondary pollution that may require subsequent treatment. The disadvantages of biofiltration have been the large space requirements and frequent media replacements as a result of deterioration or ageing. Extensive biofilter research and development have taken place over the past 20 years internationally, in particular laboratory experiments that address the removal of single pollutants at fairly high concentrations under constant operating conditions. In field applications, such conditions are highly unusual and the feasibility of treating complex mixtures at very low concentrations relevant to many odorous gas emissions has not received much attention. The overall objective of this thesis was to reduce the knowledge gap between laboratory studies and field conditions on the topic of biofiltration for odorous gas emissions. Various operational and process related problems, such as fluctuating flows, temperatures, and pollutant concentrations, that affected the biofilter performance by creating suboptimal living conditions for the microbes were identified. A newly designed compact pilot-scale biofilter was used in three different applications with odour problems, namely restaurant, pulp mill and wastewater pumping station. The gas streams were complex mixtures with chemically diverse contaminants whose concentrations varied significantly with time. Aldehydes were the dominant compounds in restaurant emissions, while reduced sulphur compounds, primarily dimethyl sulphide, dominated the pulp mill and wastewater emissions. Overall, very low concentrations of individual compounds were found (ppb-level), and very low or no removal of the targeted compounds was achieved in the biofilter. Limitations of the biomass density in the filter media is a plausible explanation since pollutant concentrations at the ppb-level may have been too low to build up and support the bacteria. Due to the low solubility of many identified compounds, a mass transfer limitation may also have occurred due to the prevailing short residence times. Drying of the filter medium was partly a problem, pointing to the need for an improved humidification system or using a trickling filter design. In a case study at a wastewater treatment plant, a method to evaluate odour problems was developed involving local observers in an odour panel together with operational data and weather observations. Working with an odour panel proved useful in several ways; they took an active interest in and increased their knowledge of the complexity of odour problems. However, relating the panel reports to specific events at the treatment plant proved difficult, and the reports were not always consistent with current wind directions.