Microsoft Word - 16zarra.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 68, 2018 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Selena Sironi, Laura Capelli Copyright © 2018, AIDIC Servizi S.r.l. ISBN 978-88-95608-65-5; ISSN 2283-9216 Odour Treatment Method of Wastewater Treatment Plant Based on Biological Oxidation Process Juan Wanga,b*, Jifeng Chenb, Yiren Wangc, Ye Wanga, Hongling Yana, Zhi Yana,d aHenan Rock & Minerals Testing Center, Zhengzhou 450000, China bSchool of Life Sciences, Zhengzhou University, Zhengzhou 450001, China cHubei Xingfa Chemicals Group Co., Ltd. Yichang 443711,China dHenan Engineering Research Center of Minerals Processing and Biological Mineral Processing, Zhengzhou 450012, China wangjuanlanbing@126.com With the continuous increase of urban domestic sewage and production wastewater discharge, the odour treatment has been enhanced in the sewage treatment process. At present, the odour problem of wastewater treatment plant should be treated jointly by various technologies. In this paper, the biological process and oxidation process were mainly selected to study the odour treatment in wastewater treatment plants, and two sets of pilot equipment in biological process and oxidation process respectively were established for pilot test. The test results showed that the biological process has a very good effect on treating the odour in the sewage tanks, and the treatment effect of N2 and H2S is relatively stable; the “alkali absorption + oxidation” method produces a stable effect on treating the odour gas produced by other adjusting tanks. At pH of 7.5-9, the potential of 700-850, and the liquid-gas ratio between 4.7L/m3-6.0L/m3, the best treatment effect can be achieved on the H2S. 1. Introduction As China’s economic development speeds up and the industrialization process deepens continuously, the discharge of urban domestic sewage and production wastewater has been increasing. The discharge of urban sewage has a great impact on the lives of the general public and industrial production (Roman et al., 2015). In the city, odour is usually generated during the sewage treatment process, and no matter whether the odour is harmful or not, it is unbearable to the public. Thus, it has become a key issue in urban environmental protection on how to control odours and conduct effective governance (Antonopoulou et al., 2014). Now, China has enhanced the treatment of odours in the sewage treatment process, but the single treatment technology has been unable to effectively deal with the odour problem in the wastewater treatment plants. The combination of various technologies has become an important development direction of future odour treatment (Ghoreishi and Haghighi, 2003). At present, a large number of scholars at home and abroad have conducted targeted research on the odour treatment and formed a series of research results. Some scholars proposed the use of chemical washing method, biological filtration method and other methods to deal with odour (Cowger and Labbe, 1965; Esplugas et al., 2004). Some scholars have conducted detailed research on the equipment and process of odour treatment (Ioannou- Ttofa et al., 2017; Silva et al., 2013); and some other scholars have studied the feasibility of some odour treatment projects (Beltrán et al., 2001; Moreira et al., 2015). This paper is mainly based on the biological oxidation process to study the odour treatment method of wastewater treatment plant, which has strong practical value (Rudina et al., 2018). 2. Related theory 2.1 Odorous substances The production of odour is mainly related to the process and the system operation of sewage treatment. Sewage contains a large number of anaerobic organisms, which produce odours in the process of consuming DOI: 10.3303/CET1868077 Please cite this article as: Wang J., Chen J., Wang Y., Wang Y., Yan H., Yan Z., 2018, Odour treatment method of wastewater treatment plant based on biological oxidation process, Chemical Engineering Transactions, 68, 457-462 DOI: 10.3303/CET1868077 457 organic matter (Holman and Wareham, 2003). The components of the odour are mainly composed of organic molecules and inorganic molecules, and the main inorganic gases are hydrogen sulphide (H2S) and ammonia. Organic odours are often the activity result of the living organisms, which decompose organic matter to form a foul odour composed of various organic gases (Chuaha et al., 2018). The common odorous sulfur-containing compounds are shown in Table 1. Table 1: Identification of sulfur-containing odour compounds in sewage treatment facilities Molecular formula Odor characteristics Critical value Molecular weight CH2=CH-CH2-SH Strong garlic taste 0.00004 75.31 CH3- (CH2)3-CH2-SH Rotten taste 0.0006 107.64 C6H5CH2-SH Intensely unpleasant 0.00019 1276.58 CH3-CH=CH-CH2-SH The stink of the weasel 0.000032 93.47 CH3-S-CH3 Rotten vegetable taste 0.0005 64.18 CH3-CH2-SH3 The rotten taste of cabbage 0.00024 60.83 The odour is the sensation caused mainly by stimulating the taste organs in the nostrils. The common odour gas in sewage is mainly H2S hydrogen gas formed by bacteria reducing sulphur under anaerobic conditions (Lee, 2018). SO4 2− + 𝑂𝑟𝑔𝑎𝑛𝑖𝑐 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 → 𝑆2− + 𝐻2𝑂 + 𝐶𝑂2 (1) 𝑆2− + 2𝐻+ → 𝐻2𝑆 (2) With the pH of 9, over 99% sulfide will be dissolved in the water, and the sulfur will exist in the non-odour HS form. With the pH over 8, the hydrogen sulfide gas won’t be released. With the pH below 8, it will be released from the sewage. With the pH over 9, ammonia gas will be released (Lee and Ahn, 2010). 2.2 Oxidation treatment In the oxidation treatment of odours, various hypo-chlorites with strong oxidation properties are mainly used as oxidants (Benner et al., 2013). The cyano group is not easily decomposed, so it is usually accelerated by the strong oxidation method. The basic ion reaction formula is as follows: Local oxidation: CN− + HOCI → CNCI + 𝑂𝐻− (3) CNCI + 2𝑂𝐻− → 𝐶𝑁𝑂− + 𝐶𝐼− + 𝐻2𝑂 (4) Complete oxidation: 2𝐶𝑁𝑂− + 3𝑂𝐶𝐼− + 𝐻2𝑂 → 2𝐶𝑂2 + 𝑁2 + 3𝐶𝐼− + 2𝑂𝐻− (5) Reaction in formula (3) can occur instantaneously at any pH (Peter and Von, 2007). In order to convert cyanogen chloride (CNCI) into cyanate according to reaction (4) in time, the pH should be over 10.5, and then the reaction (4) can be completed in a few minutes. Reaction in formula (5) is mainly the oxidative decomposition of cyanate to nitrogen and carbon dioxide (Muñoz et al., 2013). 3. Odour treatment based on biological oxidation process 3.1 Biological process-based odour treatment The odour treatment based on biological process is: organic pollutants are first contacted with water and dissolved in water; due to the differential concentration, the organic matter is further diffused from the liquid film into the biofilm after being dissolved in water, and then is absorbed by the microbes in the biofilm; afterwards, the microorganism through the metabolism can eventually turn the foul smell in the waste gas into the energy and carbon source, and decompose the organic matter into water and carbon dioxide; thus, the odour treatment is finally fulfilled. The whole process is shown in Fig.1. Table 2 lists the instruments and materials required for biological process-based odour treatment. 458 First-order biological reaction pool Two stage biological reaction pool Induced draft fan Exhaust cylinder Odor gas in aerobic pond Figure 1: Biological process flow chart Table 2: Experimental instruments and equipment Number Device name Specifications Texture of material 1 First-order biological reaction pool 1m*1.5m*1.5m Plexiglass 2 Two stage biological reaction pool 1m*1.5m*1.5m Plexiglass 3 Induced draft fan 15KW Glass fiber reinforced Plastics 4 Flowmeter 100m3/h 5 air sampler TH-110F Carbon steel 6 Bullous absorption tube Glass 7 Ultraviolet spectrophotometer UV743-GD 8 Analytical balance FA2008 9 Portable acidity meter PHB-2 10 Electromagnetic air compressor ACO-310 11 Glass rotor flowmeter LZB-5 Glass 12 Gas flowmeter LZB-3WB 13 Water circulating pump HJ-984 Carbon steel Table 3 lists the related data recording and processing of biological process-based odour treatment. Table 3: Data analysis of experimental results in biological system Serial number Flow (m3/h) Monitoring index Import Export Removal rate (%) 1 80 N2 (mg/m3) 10.52 0.64 93.75% H2S (mg/m3) 18.94 0.18 98.47% 2 80 N2 (mg/m3) 15.32 0.46 97.08% H2S (mg/m3) 19.07 0.17 98.97% 3 80 N2 (mg/m3) 12.39 0.52 96.37% H2S (mg/m3) 13.16 0.47 95.79% 4 80 H2S (mg/m3) 19.32 0.39 99.15% 5 60 H2S (mg/m3) 15.74 0.58 97.15% 6 60 H2S (mg/m3) 14.98 0.74 95.96% 7 60 H2S (mg/m3) 17.57 0.38 98.43% 8 60 H2S (mg/m3) 18.31 0.23 97.45% 9 60 H2S (mg/m3) 16.93 0.29 98.05% 10 60 H2S (mg/m3) 16.85 0.41 98.49% 11 60 H2S (mg/m3) 18.02 0.32 97.34% 12 60 H2S (mg/m3) 18.45 0.36 98.57% 13 40 H2S (mg/m3) 16.82 0.41 95.75% 14 40 H2S (mg/m3) 17.43 0.38 96.37% 15 40 H2S (mg/m3) 19.35 0.26 98.52% 16 40 H2S (mg/m3) 18.54 0.37 97.09% 459 The experimental results in the table indicate that the biological process has a very good effect on the odour treatment in the sewage tank, and the treatment effects of N2 and H2S are relatively stable. Among them, the average efficiency for treating N2 is 95.38%, with the highest up to 97.08%; the average efficiency of H2S is 98.01%, with the highest up to 99.15%. 3.2 Oxidation process-based odour treatment The oxidation process mainly uses the strong oxidizing property of hypochlorite to oxidize the organic matter, thereby achieving effective removal of the odour. In the wastewater treatment plant, with the high odour concentration, the concentration of the sodium hypochlorite solution is about 50-500 ppm, and the reaction is expressed as: NaOCI + 𝐻2𝑂 → 𝐻𝑂𝐶𝐼 + 𝑁𝑎𝑂𝐻 (6) The oxidation process mainly uses the strong oxidant to make gas-liquid contact with the odour generated by the sewage treatment, and oxidizes the odour component in the gas, thereby eliminating the odour generated in the sewage treatment. Some odorous substances such as organic sulfur compounds and oxygen- containing hydrocarbons etc. can be treated by the oxidation method. The specific process of alkali absorption + oxidation method is shown in Fig.2. Table 4 lists the data recording and processing of the oxidation process-based odour treatment. Table 4: Data analysis of pilot test results of "alkali absorption + oxidation" system Serial number Monitoring index Import Export Removal rate (%) 1 N2 (mg/m3) 4.25 0.34 96.38 H2S (mg/m3) 2.56 0.29 87.25 2 N2 (mg/m3) 4.56 0.84 81.93 H2S (mg/m3) 2.27 0.41 83.46 3 N2 (mg/m3) 5.03 0.57 90.04 H2S (mg/m3) 2.97 0.68 80.24 4 N2 (mg/m3) 4.47 0.41 89.56 H2S (mg/m3) 2.38 0.27 94.28 5 N2 (mg/m3) 2.96 0.41 92.63 H2S (mg/m3) 4.89 0.37 85.19 6 N2 (mg/m3) 4.74 0.37 91.27 H2S (mg/m3) 2.89 0.71 79.82 7 N2 (mg/m3) 4.62 0.32 85.76 H2S (mg/m3) 2.95 0.27 85.39 8 H2S (mg/m3) 2.74 0.31 79.32 9 H2S (mg/m3) 2.84 0.25 84.76 10 H2S (mg/m3) 2.69 0.37 85.27 11 H2S (mg/m3) 2.76 0.29 90.01 12 H2S (mg/m3) 2.19 0.51 89.53 13 H2S (mg/m3) 2.06 0.36 85.78 14 H2S (mg/m3) 2.84 0.26 87.51 15 H2S (mg/m3) 2.63 0.37 88.63 16 H2S (mg/m3) 2.59 0.43 89.27 Table 5: The relationship between the removal rate and the control parameters PH value Removal rate (%) Potential (mV) Removal rate (%) Circulatory volume (m3/h) Liquid to gas ratio (L/m3) Removal rate (%) 7 89.31 400 10.94 11 3.8 10.28 7.5 87.62 500 20.63 12 4.1 20.74 8 85.37 600 50.38 13 4.2 50.49 8.5 81.94 650 70.04 14 4.5 75.34 9 75.32 700 78.92 15 4.9 82.51 9.5 71.64 750 83.64 16 5.2 83.64 10 51.09 800 85.39 17 5.6 85.03 10.5 20.18 850 87.62 18 6.2 87.38 11 10.69 900 89.93 19 6.5 90.29 460 The experimental data indicate that the “alkali absorption + oxidation” method has a stable effect on treating the odorous gas produced by other adjusting tanks. The average efficiency of N2 treatment is 89.24%, with the highest up to 96.38%; the average efficiency of H2S treatment is 88.97%, with the highest up to 94.28%. The odor of other units in sewage treatment First grade alkali washing tower Two grade alkali washing tower Three stage oxidation tower Four grade water washing tower Exhaust cylinder Induced draft fan Sodium hypochlorite storage tank Alkali tank Figure 2: Process flow of alkali absorption+oxidation Figure 3: The change of hydrogen sulfide removal rate with pH value It can be seen from Fig. 3 that as the pH value increases, the removal rate of H2S decreases continuously. When the pH value is over 9.5, the removal effect is greatly reduced; when the pH value is below 7.5, despite the higher removal rate, the outlet gas will have some chlorine smell, so, in the range [7.5, 9] of pH, the optimal treatment effect can be achieved. Figure 4: The change of hydrogen sulfide removal Figure 5: The change of hydrogen sulfide removal rate rate with potential with the ratio of liquid to gas It can be seen from Fig. 4 that as the potential increases, the removal rate of H2S also increases. When the potential is below 650, the removal effect is greatly reduced; when the potential is over 850, despite the higher removal rate, the outlet gas will have some chlorine smell, so in the range [700, 850] of potential, the optimal treatment effect can be achieved. It can be seen from Fig. 5 that as the liquid-gas ratio continues to increase, the removal rate of H2S will also increase. When the liquid-gas ratio is less than 4.7L/m3, the removal effect is greatly reduced; when the liquid- gas ratio is higher than 6.0L/m3, the removal effect is not significantly improved, so at the liquid-gas ratio of [4.7, 6.0], the best treatment effect can be achieved. 4. Conclusions (1) The biological process has a very good effect on the odour treatment in the sewage tank, and the treatment effects of N2 and H2S are relatively stable. Among them, the average efficiency for treating N2 is 461 95.38%, with the highest up to 97.08%; the average efficiency of H2S is 98.01%, with the highest up to 99.15%. (2) The “alkali absorption + oxidation” method has a stable effect on treating the odorous gas produced by other adjusting tanks. The average efficiency of N2 treatment is 89.24%, with the highest up to 96.38%; the average efficiency of H2S treatment is 88.97%, with the highest up to 94.28%. At pH of 7.5-9, the potential of 700-850, and the liquid-gas ratio between 4.7L/m3-6.0L/m3, the best treatment effect can be achieved on the H2S. 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