Study On Enhancing Biohythane Production From Tofu(Soybean) Processing Residue Via Pretreatment And Anaerobic Digestion

Biofuel generation from biomass has received more attention as an alternative,renewable,sustainable,and clean energy source that replaces fossil fuel,alleviating energy demand and environmental concerns.Among biomass,tofu processing residue（TPR）,which is a by-product of the tofu and soymilk production industry,is rich in carbohydrates（50-60%）,proteins（20-30%）,and fats（10-20%）,as well as contain high moisture content（≥85%）,posing it as a suitable substrate for biofuels production through anaerobic digestion（AD）technology.TPR deteriorates rapidly and is thus mainly disposed of in landfills,causing environmental concerns.Therefore,how to efficiently treat TPR is a big concern for soybean processing plants.AD is the most sustainable and cost-effective method to treat organic wastes along with energy recovery as well as it reduces the emission of greenhouse gases,generated during the self-decomposition of biowastes.In this research,energy recovery from TPR was improved by the production of biohythane from one-stage and two-stage AD through controlling the operational conditions,pretreating TPR using ultrasonic pretreatment,dilute sulfuric acid（H₂SO₄）pretreatment,and the simultaneous combination of ultrasonic and dilute H₂SO₄ pretreatments,as well as adding molybdate(Mo O₄^2-)and ferric chloride（Fe Cl₃）.The main results are as follows:1)Two-stage AD of TPR was investigated considering the impacts of operational conditions on microbial community diversity and biohythane production.The results showed that the optimal conditions were dark fermentation（DF）operated at the substrate to inoculum ratio（SIR）of 8 and 37°C for 36 h,followed by methanogenic fermentation（MF）performed at the SIR of 1 and 37°C for 13 d,which produced 324.4 ml/g-VS_fedof biohythane along with103 mmol/L acetic acid and 38 mmol/L propionic acid.Two-stage AD improved specific energy recovery by 41.5%compared to one-stage AD,producing a biogas yield of 189.6 ml/g-VS_fed.SIR and temperature affected microbial community diversity of DF system,where high SIR of 8 speciated hydrogen producers such as Mobilitalea and Clostridium sensu stricto 1 at thermophilic and mesophilic temperatures,respectively,whereas low SIR of 0.5 stimulated methane generation by the speciation of Methanoculleus thermophilus.Likewise,hydrogenotrophic methanogens（Methanomassiliicoccus）enriched MF reactors operated at low SIR.Overall,this study demonstrated two-stage AD as an efficient technology for producing clean energy and value-added products using TPR.2)TPR has received more attention as a source of bioenergy.However,their low solubility has hindered biohythane generation.Consequently,the ultrasonic and H₂SO₄pretreatments were combined and compared for the first time to improve the hydrolysis of organic matter and carbohydrates as well as to increase free amino nitrogen generation from TPR.Besides,the impact of pretreatments on biohythane generation via one-stage AD was also investigated.Under the optimal conditions of 7.54%substrate level,8%H₂SO₄concentration,80°C,and 50 min,the coincident ultrasonic-H₂SO₄ pretreatment enriched the contents of soluble chemical oxygen demand,reducing sugar,and free amino nitrogen to49675 mg/L,26 g/L,and 1721 mg/L,respectively,greater than individual pretreatments.Also,Biohythane yield increased by 4.20-12.58%over control(389.39±23.8 ml/g-VS_fed).Furthermore,the highest hydrogen yields of 42.5±2.08 and 28.1±1.07 ml/g-VS_fed and the sulfate removal efficiencies of 93 and 92%were achieved with ultrasonic-H₂SO₄ and H₂SO₄pretreatments,respectively,indicating the enhancement of acidogenic and sulfidogenic activity.3)Biohythane production through one-stage AD of sulfate-rich hydrolyzed TPR has been hampered by high H₂S production.Herein,two-stage AD was investigated with the addition of Mo O₄^2-（0.24-3.63 g/L）and Fe Cl₃（0.025-5.4 g/L）to the DF stage to improve biohythane production.DF supplemented with 1.21 g/L Mo O₄^2-increased hydrogen yield by14.6%compared to the control(68.39 ml/g-VS_fed),while Fe Cl₃ had no effective effect.Furthermore,the maximum methane yields of MF were 524.75 ml/g-VS_fed with 3.63 g/L Mo O₄^2-and 521.60 ml/g-VS_fedwith 0.6 g/L Fe Cl₃ compared to 466.07 ml/g-VS_fed of the control.The maximum yields of biohythane and energy were 796.7 ml/g-VS_fedand 21.80MJ/kg-VS_fedwith 0.6 g/L Fe Cl₃ when the sulfate removal efficiency was 66.71%,and H₂S content was limited at 0.08%.Therefore,adding 0.6 g/L Fe Cl₃ is the most beneficial in improving energy recovery and sulfate removal with low H₂S content.