Study On Removal SO₂ From Downstream Gas By Adsorption Method Based On Fe-rich Biochar And Biotrickling Filter

Sludge drying was a common pretreatment process for municipal sludge treatment and disposal.In this paper,sulfur dioxide（SO₂）,the removal of the main sulfur-containing substance in the downstream gas of sludge dewatering process（i.e.SO2）,was studied via adsorption using biochar and biotrickling filter.Based on previous studies,the Fe-rich sludge-based biochar（referred to as Fe-rich biochar）as the environment functional material was prepared from pyrolysis of the deep-dewatered sludge cake using Fenton’s reagent combined with red mud（referred to as Fenton-RM）as dewatering co-conditioner.The effect of pyrolysis temperature on the phase composition and content of iron in Fe-rich biochar,and the performance and mechanism of removing SO₂ using Fe-rich biochar as adsorbent were studied.Furthermore,an attempt was made to isolate high-activity desulphurization bacteria from municipal sludge and to set up a laboratory-scale biotrickling filter with the acquired bacteria.The influence mechanism of different process parameters on the removal of SO₂ by biotrickling filter was studied.Finally,a comparative study was made between the two desulfurization processes of adsorption using Fe-rich biochar and biodegradation by biotrickling filter.The main research contents and the corresponding achievements are presented as follows:1.Study on the preparation of Fe-rich biochar and its adsorption of SO₂The deep-dewatered sludge cake from Fenton-RM was used as raw material.The effect of pyrolysis temperature（400,500,600,700,800,900℃）on the phase composition and content of each Fe-containing substance in Fe-rich biochar was investigated.Firstly,it was found that the total iron content of Fe-rich biochar prepared at different pyrolysis temperatures was similar,about 150 mg/g.At 400℃,Fe-containing phases in Fe-rich biochar were mainly Fe₂O₃ and amorphous phases,reaching 50 mg/g and 64 mg/g,respectively.With the increase of pyrolysis temperature,Fe₂O₃ in Fe-rich biochar was gradually reduced to low-valent iron,and its content decreases continuously from 33 mg/g at 700℃to 4 mg/g at 800℃,and finally to 2mg/g at 900℃.At the same time,Fe₃O₄,Fe Al₂O₄ and Fe⁰ contents in Fe-rich biochar increased continuously.At 800℃,the contents of Fe₃O₄,Fe Al₂O₄ and Fe⁰ were 18 mg/g,31 mg/g and 57 mg/g,respectively.The adsorption of SO₂ in the concentration range of 100-500 ppm with Fe-rich biochar prepared at various pyrolysis temperatures was investigated.It was found that with the increase of pyrolysis temperature during the preparation of Fe-rich biochar,the SO₂ breakthrough time was firstly shortened and then prolonged.The adsorption capacity of SO₂ with Fe-rich biochar prepared at various pyrolysis temperatures was investigated.It was found that the adsorption capacity of Fe-rich biochar prepared at400℃was the highest,which was 0.33 mg/g.The Fe-rich biochar prepared at 400℃was used as experimental material to explore the effects of different environmental conditions on the adsorption performance of SO₂.It was found that the increase of inlet SO₂ concentration and flow rate will shorten the breakthrough time of SO₂.The presence of O₂ and water vapor in the gas phase composition can significantly improve the adsorption performance of 400℃.But when gas moisture content reach more than20%,it will destroy its pore structure of the adsorbent and decline the performance.The adsorption mechanism with Fe-rich biochar prepared at 400℃exhibited a combination of physical adsorption and chemical reaction and the large amount of Fe₂O₃ in the biochar can efficiently catalyze the oxidize SO₂ to form SO₄^2-in the oxygen-rich humid environment,thus improving chemisorption effect.2.Study on desulfurization microorganism culture and enrichment and the process optimization of removal of SO₂ by biotrickling filterThiobacillus ferrooxidans（T.f）,a desulfurization strain,was isolated from waste activated sludge and activated sludge in municipal sewage treatment plant.It was found that high activity and concentrated T.f bacteria were quickly isolated from waste activated sludge in liquid selective medium for 7 days and was purified by continuous enrichment.The two-step immobilization was used to form biofilm on the surface of the shale ceramsite as biofilm packing successfully,which exhibited good bio-activity and stability.The effect of the operating process parameters of the biotrickling filter on the desulfurization efficiency was investigated.When the concentration of SO₂ changed from 100 to 800 ppm,the desulfurization efficiency basically maintained at about 97%,but it dropped exponentially to 75%as the empty bed retention time decreased from150 s to 30 s.The desulfurization efficiency rapidly decreased to 92%when nutrient solution spray density was less than 0.36 m³/（m²·h）but remained close to 97%when it was at the range of 0.36-1.08 m³/（m²·h）.When Fe²⁺concentration in solution,as the key to desulfurization reaction,decreased from 0.1 mol/L to 0.02 mol/L,the desulfurization efficiency rapidly fall off to less than 84%.Finally the biotrickling filter was combined with the adsorption device packing Fe-rich biochar in order,and the change of desulfurization efficiency during the continuous operation of the combined process of biotrickling and biochar adsorption was studied.The desulfurization efficiency was close to 100%attributed to the combined process of biotrickling and biochar adsorption in the short term.However,with the increase of operating days,after Fe-rich biochar reached adsorption saturation,the whole desulfurization efficiency declined,along with the accumulation of SO₄^2-oxidized from SO₂ in the nutrient solution that inhibited the desulfurization process.The nutrient solution needs to be updated regularly to maintain a stable desulfurization efficiency.