Removal of Volatile Organic Compounds Emitted from a Manufacturer of Opto-electronic Apparatus by a Trickle-bed Air Biofilter

The goal of this study is to investigate the performance of a pilot-scale trickle-bed air biofilter (TBAB) for treating volatile organic compounds (VOCs) emitted from a manufacturer of optoelectronic apparatus. Different empty-bed residence times (EBRTs) of 90, 60, 30, 20 and 15 s were evaluated. Greater than 90% VOC removal efficiencies were achieved at EBRTs longer than 20 s. EBRTs of 20-30s are suggested as the applicable operating condition of a full-scale TBAB to reach relatively high VOC removal and minimize the capital cost of TBAB. During the experimental period, the pressure drops across the bed slowly increased from 29 to 882 Pa indicating that the effectiveness of TBAB could be maintained over 230 d of operation. The low temperature of surrounding air and the intermittent VOC feed have a negative effect on the TBAB performance. A comparative study between the TBAB and the activated carbon adsorber showed that the TBAB can be a cost-effective technology in the control of VOC emission from a manufacturer of optoelectronic apparatus. *Corresponding author Email: [email protected] INTRODUCTION The manufacture of opto-electronic apparatus is the most important industry in Taiwan. Due to the lack of a proper air pollution control device, a large amount of volatile organic compounds (VOCs) are released into the atmosphere during the manufacturing process every year. The commonly emitted VOCs are isopropyl alcohol (IPA), acetone, butyl acetate, 1,1,1trichloroethane, methyl ethyl ketone, toluene and xylene. The release of these VOCs into the atmosphere may have an adverse effect on air quality and thus endanger public health and welfare [1]. More stringent requirements for the removal of VOCs from waste gases in recent years necessitate the development of innovative, cost-effective treatment alternatives. Traditional technologies such as carbon adsorption, liquid scrubbing, condensation, thermal incineration, and catalytic incineration have been commonly used to remove VOC vapors from waste gases. However, these VOC control technologies may suffer from high operating costs and secondary waste stream issues [2]. The biofilter has been proven to be an effective process for treating low-strength (< 10 mg C m) and some medium-strength (100-1000 mg C m) VOCs from waste gases [3]. The system consists of a filter bed usually filled with natural organic media such as peat, compost, or leaves. The bed moisture is kept at a constant level by humidification of the influent air to maintain a biologically active layer surrounding the media, known as the “biofilm” [4]. VOC-containing air streams are transported to the air/biofilm interface, where they are absorbed into the biofilm and employed as energy sources by the microorganisms. The trickle-bed air biofilter (TBAB) is a type of biofilter process that employs synthetic, inorganic media and receives liquid nutrients through a spray nozzle on the top of the reactor. Due to better control of pressure drop across the bed, pH and nutrient feed, TBAB allows for more consistent operation and does not suffer the effects of aging as natural media do [5]. The TBAB has been successfully employed to treat many types of pure and mixed VOCs [5-10]. This research aims at extending the TBAB application in the control of VOC emissions from a manufacturer 204 J. Environ. Eng. Manage., 17(3), 203-209 (2007) of opto-electronic apparatus. Five various empty-bed residence times (EBRTs) were tested to establish the applicable operating conditions of a full-scale TBAB. A comparative study on the cost-effectiveness of TBAB and activated carbon adsorber was also given. MATERIALS AND METHODS 1. Experimental Setup The experimental setup of pilot-scale TBAB is shown in Fig. 1. The reactor was made of stainless steel and had a height of 100 cm and an internal diameter of 10.85 cm. A 10-cm headspace was designed for the waste gas inlet and for housing a nutrient spray nozzle, while a 10-cm bottom space was designed for the outlet of treated air and leachate. It was equipped with three sampling ports along its height located at the reactor inlet and outlet and middle height of the reactor. The diameter of sampling port was 1 cm. The reactor was packed at a height of 85 cm, corresponding to a packing volume of 7.85 L. The packing material consisted of coal particles with average pore volume of 0.036 cm g, surface area of 6.21 m g (measured by BET analyzer, ASAP 2000, Micromeritics, Atlanta, Georgia, USA), dry density of 0.57 g cm, particle volume of 6.28 cm and equivalent-volume diameter of 2 cm. The equivalent volume diameter is defined as the diameter of a sphere with the same volume as the coal particle. The choice of coal particles as packing material was due to the following advantages: (1) cheap, large available surface area for biofilm accumulation, (2) proven capacity to maintain moisture content, (3) no necessity of aseptic conditions, and (4) large capacity for buffering the fluctuation in VOC concentrations [11]. The void fraction before biofilm attachment was 44% of the packed volume. The temperature inside the reactor was not controlled throughout the study to simulate the performance of real-scale uncontrolled biofilter. In order to test the buffer capacity of VOCs on coal particles, an adsorption experiment was carried out before the TBAB start-up. The reactor was fed at 7.86 L min (EBRT = 60 s) using an air stream containing 300 ppmv IPA and acetone. The operating capacity of total hydrocarbon (THC) was equal to 17.0 mg g. The waste gas was emitted from a manufacturer of opto-electronic apparatus located in Hsinchu Science Park (Hsinchu, Taiwan). Waste gases were collected using a capturing hood. The major gas stream was delivered into a full-scale adsorption device packed with commercially available activated carbon at a flow rate of 150 m min, while the minor gas stream was mixed with a 2.4 mL min nutrient solution and delivered into the headspace of the reactor by a nozzle system. Regulation of the minor gas stream used a rotameter controlled EBRT. The nutrient solution, containing inorganic salts and trace metals vital Sampling port