In this study, three chemical scrubbing processes were investigated to test their performances for removing odorous compounds in waste gases emitted from feed-drying operations. A gas chromatography with a flame ionization detector (GC/FID) and gas detector tubes were used for the identification of odorous species and their concentrations in the waste gases. Results indicate that toluene, ethyl benzene, xylenes, and styrene could be detected while their concentrations were all below the detection limits. However, trimethylamine, amine, mercaptans, ammonia, hydrogen sulfide, and acetic acid could be detected with the detector tubes. A two-stage scrubbing system with a gas flow rate of 10 L/min was used to test its performance for the odor removal. Results demonstrate that an ¡§acidic chlorination-alkaline sulfite scrubbing¡¨ process was effective. The pertinent operating conditions were found to be: (1) a pH of 6.4-6.6 and total residual chlorine of 55-85 mg/L for the acidic-chlorination solution, and (2) a pH of 8.0-8.5 for the alkaline sulfite solution with a sulfite concentration of 10,000 mg/L (as SO2). By the conditions, ammonia, trimethylamine, and amine in the odorous gas could be completely removed. A sensory measurement indicates that the odor concentration could be reduced from 17,378 to 232 after scrubbing. Cost estimation indicates that it required a chemical cost of NT$ 5.74 for removing the odors from 1,000 m3 of the gas. This study has successfully developed an economical and effective chemical scrubbing technology for the removal of odorous gas emitted from feed-drying operations. It could be applied to the treatment of odorous gases emitted from the manufacture of feeds, seafood, and other related operations.

Abstract This study armed to develop a chemical scrubbing process to treat an emitted odorous gas stream from a corn-germ oil-pressing machine. Analytical results demonstrated that the gas contains a total VOC (volatile organic compound) concentration of 323 to 2,100 ppm calibrated as methane. Major VOC ingredients are aldehydes, organic acids, and mercaptans. Ozone, sodium hypochlorite (NaOCl), and hydrogen peroxide (H2O2) were used as oxidants for VOCs other than aldehydes and sodium bisulfite (NaHSO3) was used as a reductant for aldehydes. Experimental setup used in this study consisted of a set of four-stage-in-series scrubbers for VOC oxidation and a set of 18-L bubble column for aldehyde reduction. Each stage of the scrubber was constructed from a 25-cm x 140-cm (OD x H) plastic column packed with 2¡¨-diameter plastic packings to a height of 90 cm. Experiment results showed that suitable conditions for the VOC and odor removal in the oxidation tower are: (1) QL/QG (flowrate ratio of circulating liquid and gas) = 0.0024-0.0035 m3 liquid/m3 gas, (2) dose rate of NaOCl solution (10 % effective chlorine) = 0.0125-0.015 L/m3 gas, (3) pH of the circulating liquid = 7.5-8.0, and (4) supplemental water flowrate = 2.4-5.0 L/m3 gas. Those for the reduction of the residual aldehydes and other odorous compounds are: (1) concentration of NaHSO3 scrubbering solution = 2,156 mg/L as SO2 and (2) pH of the NaHSO3 solution = 5.0-6.0. Under the conditions, experimental data demonstrated that the process could reduce H2S from 0-5.5 to 0 ppm, NH3 from 0.1-0.4 to 0-0.1 ppm, phenols from 0-2.0 to 0 ppm, acids from 15-25 to 0 ppm, acetaldehyde from 10-34 to 0-10 ppm, amines from 0.1 to 0.1 ppm, and mercaptans from 0.5-10.5 ppm to 0 ppm. Overall VOC removal efficacy was 85-100 %. Effluent gas from the oxidation stage had a chlorine smell, while that from the reduction one was odorless. Based on the results from the pilot study, a full-scale plant with a waste gas flow rate 105 m3/min was proposed. It was estimated that the full-scale plant has an equipment cost of US$ 45,000, and an operating cost of around US$ 110/(8-hr day) or US$ 0.75/(1,000 m3 waste gas).

Fumes emitted from coffee bean roasters contain various chemicals such as aldehydes, pyridine, volatile fatty acids, and ketones. These chemicals have high odor intensities and are sensitive to human breathing organs and eyes. In Taiwan, most of these fumes are not well controlled before venting to the atmosphere. This research attempted to eliminate these odorous and offensive chemicals by chemical scrubbing methods. Sodium hypochlorite oxidation followed by hydrogen peroxide reduction (hypochlorite-peroxide) was investigated first. After that, ozone oxidation followed by hydrogen peroxide reduction (ozone-peroxide) was also tried. In both methods, hydrogen peroxide acted as a reducing agent for the elimination of either residual chlorine or ozone emitted from the oxidation reaction. Experimental results indicated that both methods were effective. An oxidative scrubbing liquid with an initial effective chlorine content of 200 mg/L and a pH of 12, and an reducing scrubbing liquid with an initial hydrogen peroxide concentration of 1% and a pH of 12 could remove an average of 85% of total hydrogen carbons (THC) in the fume gas from a coffee bean roaster. With a dose of 10 ppm ozone in the fume gas to the oxidative scrubbing liquid (water only) followed by an reducing scrubbing liquid with an initial hydrogen peroxide concentration of 0.3% and a pH of 12, an average removal of 80% of the THC could be removed. An odor intensity (expressed by the dilution to threshold ratio) of 9,772 in the original fume could be reduced to as low as 31 by both methods. As indicated by the data from GC/MS (gas chromatography-mass spectrometry) analysis for the hypochlorite-peroxide method, acrolein, vinyl acetate, 2-butanone, and acetone in the roaster fume could almost completely be eliminated. Traces of chlorinated hydrocarbons as potential secondary pollutants were detected in the treated gases from the hypochlorite-peroxide scrubbing system. While by the ozone-peroxide one, satisfactory results were also obtained with no chlorinated hydrocarbons emitted. An economical analysis shows that the ozone-peroxide approach is a practical one for actual control applications.

Organic solvents in varnish can easily cause volatile organic compounds (VOCs) and odorous problems in manufacturing plants of enamel insulated wire. In general, the related process exhaust gases are treated by catalytic incinerators. However, the slight odors in the incinerated exhausts may induce uncomfortable feels to the inhabitants in the vicinity of the plants. Main components of odors are reported to be xylenes and to a lesser extent ketones. This study intended to oxidize the odorous compounds by chemical scrubbing method with either sodium hypochlorite or ozone as an oxidant and hydrogen peroxide as a reducing agent for the elimination of residual chlorine or ozone emitted from the oxidation scrubber. An additional study was the oxidation of aqueous mixed xylenes by sodium hypochlorite. The first part of the study was the chemical oxidation of aqueous xylenes by sodium hypochlorite. Results indicated that more than 95 % of 20 mg/L xylenes could be converted to some oxygenated hydrocarbons with an initial effective chlorine concentration of 180 mg/L at pH 6.5 over a prolonged reaction time of over 120 min. With pH < 3 and a reaction time of 120 min, xylenes could be oxidized to benzene carboxylic acid that precipitated as organic crystalline solids to the reactor bottom. The second part was chemical scrubbing of the odorous gases emitted from the catalytic incinerator. Operation conditions were an effective chlorine concentration of 1,500 mg/L and pH 6.5 for the oxidative scrubbing liquor, and a hydrogen peroxide concentration of 700 mg/L and pH > 12 for the reductive one. Results indicated that on an average, around 59 % of the influent xylenes could be removed, and the scrubbed gas was nearly odorless. 65.0 - 98.5 % of the influent non-aromatic compounds could be removed with trace amounts of chlorinated compounds in the gas were detected. The third one was the treatment of the odorous gases by using ozone as an oxidant and followed by activated carbon adsorption. 18 - 34 mg/m3 of ozone was added to the test gas with initial xylene concentrations of 25 - 55 mg/m3. Results indicated that only around 35 % of the added xylene was removed with ozone contact times of 0.15 - 0.6 min. Although 99.9 % of the residual ozone and odors were removed by the activated carbon, a longer operation time should be tested to verify the performance. Trace amounts of ring cleavage precursors of 2.4-dimethylphenol and ring cleavage products were found in the ozonized gas.

With the increases of environmental awareness and attention by the government and audiences, people begin to prosecute the pollution makers instead of tolerance. This study tried to control an air pollution source from a wallpaper plant for the purpose of attenuating odor prosecutes by residents near the plant. This study investigated chemical scrubbing, ozonation followed by GAC (Granular Activated Carbon) adsorption and biotrickling filtration techniques as control methods for eliminating odorous compounds from a waste gas stream emitted from the wallpaper plant. Scrubbing test results indicate that with an oxidative solution of 800 mg/L residual chlorine at pH 7.0 and a reductive solution of 0.2% hydrogen peroxide at pH = 12, around 90% of the VOCs (volatile organic compounds) in the tested gas could be removed. Odor intensities could be reduced from 2,317 (expressed as dilutions to threshold) to 130. Results also indicate that around 85 % of the carbonyl compounds, 41% of the alkenes and 22% of the alkanes in the waste gas could be eliminated. No removal of aromatic and halo-hydrocarbons was observed. It requires around NT$70 for chemicals to treat 1000 m3 of the gas. Ozonation-GAC adsorption tests indicate that with an ozone dosage of 5 ppm in the waste gas and a gas-ozone contact time of 12 s followed by GAC adsorption, around 95% of the VOCs in the tested gas could be removed. Odor intensities were reduced from 2,317 to <55. Around 94 % of the carbonyl compounds, 79% of the alkanes, 76% of halo-hydrocarbons, 41% of the alkenes and 20% of aromatics in the waste gas could be removed. Around NT$ 15,700 for ozone and GAC is required to treat 1000m3 of the gas. Results from biotrickling filtration tests indicate that with the supplementations of nitrogen and phosphorous nutrients to biofilms attached to wood-chip packings, around 83% of the influent VOCs could be biodegraded. Odor intensities could be reduced from 1,737 to <55. Around 61 % of the carbonyl compounds, 67% of the alkanes, 18% of halo-hydrocarbons and 33% of aromatics in the waste gas could be removed. Around NT$ 13 is required to treat 1000m3 of the gas. For an actual application, it is suggested that a process that a combination of chemical scrubbing and ozonation-GAC adsorption may be a proper solution to the control of the odorous emission from the plant. Biotrickling filtration can be best after verifying the performances of the technique by a pilot test.

Hot-melted asphalt (HMA) plants use sized gravels, asphalt and/or recycled asphalt as raw materials. In the plant, the materials are heated to certain preset temperatures and blended at fixed ratios at around 170oC to prepare the required HMA for road paving. In the asphalt-melting, hot-blending and dumping operations, fumes and particulates emit from the process equipments. The emitted gases contain various volatile organic compounds (VOCs) and poly aromatic hydrocarbons (PAHs) which are harmful to the health of the plant workers and nearby residents. Complaints from the residents also come with the fume and odorous emissions. In this study, an oxidation-reduction-in-series scrubbing process was tested to remove odorous compounds in waste gases emitted from HMA plants. Waste gas samples for test were collected from the vent hole of an oven which contains a heated sample of asphalt or recycled asphalt concrete. Sodium hypochlorite solution was used to scrub and oxidize the compounds and hydrogen peroxide to reduce the chlorine emitted from the oxidative scrubber. A gas chromatography with a mass spectrophotometric detector (GC-MSD) was used for the identification of the odorous species and their concentrations in the waste gases. Sensory tests were also used to determine the odor removal efficiency. GC-MSD examination results indicates that alkanes, arenes, alkenes, halides, esters, and carbonyl compounds were detected in the test gas. Scrubbing test results indicate that with oxidative solution of 50-60 mg/L residual chlorine at pH 7.0-7.5 and reductive solution of 35 mg/L hydrogen peroxide at pH >12, over 90% of the VOCs in the tested gas could be removed. Odor intensities could be reduced from 3,090 (expressed as dilutions to threshold) to 73. Pungent asphalt odor in the test gas was turned into slight sulfur smell after the scrubbing. For removing the odors from 500 Nm3/min of the flue gas vented from a HMA plant, an analysis indicates the required total cost for chemicals (sodium hypochlorite solution, hydrogen peroxide and sodium hydroxide) added to the scrubbers was around 2,800 NT$/day (US$ 95/day) for a daily operation time of 10 hours. The cost is far lower than that by the traditional thermal incineration one (25,000 NT$/day or US$ 850/day) or by the regenerative thermal oxidation (RTO) one (14,300 NT$/day or US$ 485/day). This study has successfully developed an economical and effective chemical scrubbing technology for the removal of odorous compounds in gases emitted from HMA plants.