Study On The Effect Of Salinity On Anaerobic Digestion Of Organic Components Of Food Waste

Anaerobic digestion is a common method for the treatment of food waste,which has the characteristics of organic waste treatment and energy regeneration at the same time.However,the salt content and organic composition of food waste are greatly affected by eating habits,sources of food waste and other factors,which lead to the instability of biogas production and quality during anaerobic digestion,affecting the subsequent utilization effect,and even endangering the normal operation of the anaerobic digestion system.However,most of the existing studies focus on the effect of salt concentration in digestate on biogas production,but do not consider whether the effect is consistent for different organic composition of food waste,so the result is not universal.In this study,through the batch anaerobic digestion experiment,the effect of salinity on the organic composition of food waste was explored.Then,through the semi-continuous anaerobic digestion experiment,the optimal salt concentration was found and the reaction kinetic model under the influence of salinity was constructed in order to study the inhibitory effect of increasing salt concentration in digestate on the anaerobic digestion process from the view of kinetics quantitatively.Finally,the prediction model of biogas production by anaerobic digestion of food waste was established to accurately predict the biogas production of food waste with different organic composition under the influence of salinity.Thus,it provides theoretical support for increasing the biogas yield of anaerobic digestion of food waste and maintaining the stable operation of the anaerobic digestion system.The main conclusions of the study are as follows:Firstly,the anaerobic digestion process of three kinds of organic composition of food waste,such as carbohydrate,protein and oil and fat,was studied by batch experiment.It was found that the addition of 2 to 4 g·L^-1 salt could increase the biogas and methane production of organic composition and the biogas production rate of organic composition except fat and oil.The addition of 12 g·L^-1 salt inhibits the biogas yield,methane yield and biogas production rate of organic composition.Based on the factorial analysis,the increase of carbohydrate fermentation concentration significantly enhanced the salt inhibition(p<0.05).This is because salt can inhibit methanogenesis and reduce the rate of organic acid consumption,while the increase of carbohydrate substrate concentration will increase the yield of organic acid.As a result,the p H value decreases greatly,the anaerobic digestion system finally collapses,and the salt inhibition is strengthened.The increase of protein substrate concentration significantly weakened the salt inhibition(p<0.05).The large amount of ammonia nitrogen released by protein degradation alleviates the decrease of p H.What's more,the increase of protein fermentation concentration may increase the activities of dehydrogenase,F₄₂₀ and neutral protease.At the same time,proline in protein plays a protective role in the growth of microorganisms under salt osmotic stress.Finally,all of them lead to the weakening of salt inhibition.The oil benefits from the slow degradation rate,and the increase of substrate concentration will not produce a large amount of organic acid in a short time,and acidification will not occur in the digestion system even at high salt addition and high fermentation concentration,so the increase of substrate concentration will not significantly affect the salt inhibition(p>0.05).However,the decreasing/increasing trend of p H and VOA/TIC in the digestion still indicates that there is a risk of acidification in the system.Secondly,a semi-continuous experiment was conducted to study the effect of salinity on the anaerobic digestion of food waste with different organic compositions under high organic load(OLR=2.0 g VS(L·d)^-1).The results showed that the optimum salt concentration range for food waste was 0.98-1.27 g·L^-1.And when the salt concentration is in this range,the increase of oil and fat content between 19%TS and 39%TS will not significantly affect the anaerobic digestion efficiency(85.2%-87.6%),and there is no acidification phenomenon.When the salt concentration exceeds this range,the VOA/TIC in the system will increase with the increase of salt concentration,and the system will eventually appear acidification phenomenon.At the same time,the production of biogas and methane will also be inhibited.In order to quantitatively explain the different inhibitory effect of salt concentration on food waste with different organic components,the reaction kinetic model was established according to the two-stage theory,and the salt inhibition parameter I was added.The simulation results show that the model has a good fitting effect on the methan production curve(91%R²>0.8),and the average prediction error of volatile organic acid concentration is only 11.5%.The fitting of I and salt concentration x showed that the fitting equations of food waste with low-carbohydrate-high-protein,medium-carbohydrate-medium-protein and high-carbohydrate-low-protein were I=20.682e^-2.414x(R²=0.965),I=19.289e^-2.575x(R²=0.964)and I=314.09e^-4.639x(R²=0.951)respectively.It can be seen that I of high-carbohydrate-low-protein food waste decreases most rapidly with the increase of x,indicating that it is more vulnerable to the increase of salt concentration.Finally,the paper modified the first-order biogas production model to eliminate the interference of organic matter accumulation on the prediction of biogas production in the process of semi-continuous anaerobic digestion and improve the fitting effect of biogas production curve.The modified model and BP neural network model were combined to form a mixed prediction model,and based on the food waste components and digestate parameters in the semi-continuous anaerobic digestion experiment,the biogas yield of food waste with different organic compositions under the influence of salt can be accurately predicted.The results show that,compared with the single BP neural network model,the prediction result of the hybrid prediction model is more accurate,and the average relative error of prediction was reduced from 5.8%to 2.9%.After the target parameters are replaced by biogas production with anaerobic digestion efficiency,the accuracy of the model prediction is further improved,and the average relative error is reduced to 2.1%.The accurate prediction of biogas production of food waste with different organic components under the influence of salinity can be achieved.The above results show that:(1)appropriate amount of salinity can promote the anaerobic digestion of food waste,and the salt concentration range is basically the same for different organic compositions.But at high salt concentration,the risk of system acidification can be reduced by reducing and increasing the proportion of carbohydrate and protein in the feed.(2)the modified first-order biogas production-BP neural network hybrid prediction model based on food waste compositions and digestate parameters is established to accurately predict the biogas production of food waste with different organic compositions under the influence of salinity and meet the application demand.