Study On Mixed Anaerobic Fermentation Of Waste Chromium Scrap Collagen And Rice Straw

China's leather industry produces about 1 million tons of chrome-containing shavings every year.The treatment and disposal of these wastes has become the most headache of the leather industry.At present,the main treatment methods of chrome shavings are safe landfill and incineration,which not only causes waste of resources,but also may cause secondary pollution to the atmosphere,soil and water bodies.In addition,the common treatment method is to use chemical reagents to hydrolyze the chrome shavings and obtain additional products.However,these methods are still in the laboratory stage,and the additional products need to be further modified before they can be used.Therefore,recycling through clean production,such as the production of clean fuel biogas,opens up new ideas for the treatment of chrome shavings and improves the environmental benefits of the leather industry.The degradability?or availability?of chrome shavings is a key factor in assessing whether it can be used as a substrate for anaerobic fermentation.However,the presence of chromium in chrome shavings limits the feasibility of anaerobic fermentation,so it is necessary to dechromize the chrome shavings before anaerobic fermentation.In this study,the optimal conditions for CaO dosing amount,CaO reaction time,CaO reaction temperature,enzyme dosing amount,enzyme reaction time,and enzyme reaction temperature were determined by a single-factor experiment using alkaline-enzyme method to hydrolyze the chrome shavings.The results of single factor experiments were analyzed by orthogonal experiments and response surface optimization.Based on the optimal conditions obtained by optimizing the analysis,the collagen was extracted by alkaline-enzymatic hydrolysis of chrome shavings,and the obtained collagen solution was mixed with rice straw hydrolysate for anaerobic fermentation experiments.The effects of different carbon-nitrogen ratios and different chromium contents on the biogas production Characteristics of anaerobic fermentation were investigated,and the changes of pH,the concentration of VFAs,SCOD,ammonia nitrogen,and chromium concentration during the anaerobic fermentation process were analyzed.The microbial diversity of the fermentation hydrolysate was analyzed by high-throughput sequencing and 16rDNA gene detection.The following conclusions were reached:?1?The best hydrolysis conditions obtained by single factor experiments are:2%CaO,CaO reaction time of 3 h,CaO reaction temperature of 80?,enzyme dosage of 3%,enzyme reaction time of 1 h,and enzyme reaction temperature of 40?.The collagen extraction rate reached 92.98%,the dechromization rate reached 96.07%,and the chromium content in the hydrolysate was 3.686 mg/L.?2?The best scheme of L16?35?orthogonal test is A2B2C2E3F2G2,those were,2%CaO,CaO reaction time of 3 h,CaO reaction temperature of 80?,5%neutral protease,enzyme reaction time of 1 h,and enzyme reaction temperature of 40?.The optimal conditions for response surface analysis optimization were:CaO dosing amount of 2.1%,CaO reaction time of 3.3 h,CaO reaction temperature of 79.7?,enzyme dosing amount of 5.6%,enzyme reaction time of 1.2 h,and enzyme reaction temperature of 48.6?.After optimization,the collagen extraction rate reached 99.14%,and the dechromization rate reached 98.9%,which increased by 6.16%and 2.83%,respectively,than before optimization.?3?The optimal conditions for the hydrolysis of chrome shavings after optimization of response surface showed that the collagen extraction rate reached 98.97%,the dechromization rate reached 99.02%,and the chromium content in the hydrolysate was 1.029 mg/L.Collagen molecular weight is in the range of 14?97 kDa.?4?When the ratio of rice straw hydrolysate and collagen solution?C/N,VS?was 20:1,the total biogas production and cumulative methane production reached 739.5 mL/gVS and 438.4 mL/gVS,respectively,and the methane content reached 51.2%;when the chromium content in the fermentation system was 5 mg/L,the total gas production and cumulative methane production were 1531.0 mL/gVS and 761.2 mL/gVS respectively,and the methane content reached 49.7%.The optimal conditions for anaerobic fermentation of collagen solution and rice straw hydrolysate were:C/N was 20:1,and chromium content was 5 mg/L.The SCOD removal rate of the treatment group of C/N 20:1 was the best,being 54.54%.With the increase of chromium content,the increase degree of SCOD concentration decreased,the treatment group with chromium content of 5 mg/L had the highest degradation rate of SCOD.?5?The abundance of microbial composition in fermentation systems with different carbon-nitrogen ratios and different chromium contents was significantly different.Adding rice straw hydrolysate could increase microbial diversity during anaerobic fermentation.In the treatment group CN20₁,the main dominant bacteria was Sphaerochaeta,and in the treatment group Cr5,the main dominant bacteria was Tissierella.In this study,alkaline-enzyme was used to extract the collagen solution by hydrolyzing chrome shavings,and the anaerobic fermentation experiment was performed with rice straw hydrolyzate to provide data support and theoretical guidance for the anaerobic fermentation of chrome shavings.