The Degradation Of Allyl Isothiocyanate And Its Impact On Methane Production From Anaerobic Co-digestion Of Kitchen Waste And Waste Activated Sludge

Due to economic development,food waste and activated sludge have become the two major solid wastes in the world,Efficient treatment means not only achieve environmental friendliness,but also may enable mankind to obtain additional benefits.Anaerobic co-digestion of food waste and activated sludge is undoubtedly one of such treatment means and has been widely used in practice.However,anaerobic co-digestion is a complex process dominated by a variety of microorganisms.The toxic and harmful substances in food waste and sludge will affect this process,resulting in them can not be greatly transformed into available energy for people.Allyl isothiocyanate（AITC）is the main active ingredient in cruciferous vegetables and pungent spices.It exists in a large number of kitchen waste.At the same time,AITC,as a natural antibiotic,inhibits the activity of a variety of bacteria.Its fate and influence in anaerobic co-digestion have never been recorded.Therefore,the purpose of this study is to clear the degradation of AITC and its effect on the production of hydrogen and methane in the process of anaerobic co-digestion.Firstly,the degradation of AITC in the co-digestion of kitchen waste and sludge was studied.The results showed that the initial concentration of AITC in the reactor was 40mg/L.after 10 days,the concentration of AITC in the reactor was less than 1mg/L,and the average degradation rate of AITC was 4mg/L.HPLC-MS/MS analysis showed that AITC was first degraded into allyl thiocyanate（ATC）and cyclopropyl isothiocyanate（CPITC）in anaerobic co-digestion system,and the final products of degradation were carbon dioxide,ammonia and hydrogen sulfide,and AITC could also be used as substrate to produce CH₄.Secondly,the influence of the existence of AITC on the process of anaerobic co digestion was studied.With the increase of AITC from 0 to 60mg/L,the maximum methane production decreased from 285.1 to 35.8mL/g VS,the methane production rate decreased from 15.3 to 1.8mL/1.8 mL/（g VS·d）,and the optimal digestion time increased from 7.2 to 13.7d.when anaerobic co digestion was 96h,COD mass balance analysis was carried out for AITC reactors with different concentrations.The results showed that with the increase of AITC concentration,VSS The ratio of methane and"others"to total COD showed a downward trend,while the ratio of protein,sugar,hydrogen and volatile fatty acids to total COD showed an upward trend.Finally,by measuring the changes of SCOD,VSS,soluble protein and soluble carbohydrate in the reactor,to analyze the effect of AITC concentration on sludge dissolution stage.The results show that the existence of AITC promotes the dissolution of sludge cells.The results of flow cytometry also confirmed this.With the increase of AITC from 0 to 40mg/L,the fluorescence percentage of viable cells decreased significantly from 89.0%to 78.4%.At the same time,the fluorescence percentage of early apoptotic cells and necrotic（or late apoptotic）cells increased from 2.48%and 2.40%to 9.89%and 9.69%respectively.All the facts show that the solubilization is promoted due to the existence of AITC.The effects of AITC on hydrolysis,acidification and Methanation in the process of anaerobic co-digestion were analyzed.The results showed that the presence of AITC inhibited the microbial activities involved in these three stages,and the inhibition order was acetyltrophic methanogenesis>hydrogen trophic methanogenesis>hydrolysis>acid production.The existence of AITC can change the physical and chemical properties of cell membrane,inhibit cell proliferation and energy metabolism,and lead to cell death.This is the fundamental reason why AITC inhibits hydrolysis,acidogenesis and methanogenesis,especially acetyltrophic methanogenesis.The presence of AITC promotes the number of Bacillus capable of degrading polycyclic aromatic hydrocarbons and total petroleum hydrocarbon pollutants,but leads to the reduction of the number of anaerobic bacteria involved in hydrolysis,acidogenesis and methanogenesis,such as Clostridium,Proteobacteria and methanogens.Therefore,this study is helpful to understand the fate of AITC in anaerobic co digestion and guide the anaerobic co digestion system containing AITC.