Experimental Study On Defoaming Performance And Mechanism In Anaerobic Digestion Of Food Waste

Foaming is a common technical bottleneck in anaerobic digestion plants,which can lead to serious operational,economic and environmental problems.Chemical defoaming,that is adding defoamer,is a widely used defoaming method in biogas field at present.However,the complexity of foam makes defoamers often be selected by experience,and the effect is uneven.In order to develop targeted defoaming strategies,clarify its defoaming mechanism,and ensure the stable operation of the reactor,this study first improved the foaming potential experiment,and determined the foaming potential measurement method suitable for the anaerobic digestion system of organic waste,so that its characterization parameters（foaming trend and foam stability）can accurately evaluate the foaming phenomenon,and then accurately evaluate the defoaming efficiency of defoamers.Secondly,the defoamer was preliminarily screened based on the improved foaming potential experiment;The effect of defoamer on the gas production performance and biological toxicity of anaerobic digestion reactor was explored in the sequencing batch experiment.Finally,the long-term effectiveness of the previously optimized defoamer was verified in the semi-continuous experiment,and its defoaming mechanism was revealed.The main results of this study are as follows:（1）A method for determining the foaming potential of organic waste anaerobic digestion system based on the addition of effervescent tablets was developed.The effervescent tablets were used as the foaming agent,the maximum height of the foam in the foaming potential was used as the foaming trend,and the ratio of the foam height to the maximum height after 1 hour was used as the foam stability.The correlation R²between the foaming trend and the actual foam volume,surface tension and viscosity of the reactor is 0.91,0.88 and 0.90.（2）In the improved foaming potential experiment,the defoaming effects of high-carbon alcohol,silicone,polyether,polyether modified polysiloxane,corn oil,biogas residue biochar,sodium oleate and other defoamers were tested,and the effects of adding methods and doses on the defoaming effect were explored.It was found that the addition of defoamer at the bottom was the best,and the application doses of 0.1%and 0.5%V/V_samplewere appropriate.Organosilicone,polyether,and corn oil had good defoaming effects,and the foam inhibition amount was greater than 100 m L,and the foam reduction rates more than 90%,40%,and 40%.High-carbon alcohol defoamer has poor foam inhibition effect and foam breaking effect;Sodium oleate defoamer can achieve better antifoaming effect in the system medium without foam or weak foaming ability,but it is not recommended for the medium system with large foaming potential.Biochar defoamer prepared from biogas residue has little effect on foaming inhibition,but has a promoting effect on foam rupture,which can be recommended as an auxiliary filler for foam control after foaming.（3）In the sequencing batch experiment,the biological toxicity of 0.1%and 0.5%V/V _sampleof the above optimized silicone,polyether and corn oil defoamers was further analyzed,and it was found that the gas production was not inhibited after the addition of silicone and corn oil defoamers.The inhibition rates of polyether defoamer at two concentrations were 18.4%and 22.6%.It has biological toxicity and is not suitable for anaerobic digestion system of food waste.（4）In the semi-continuous experiment,the long-term defoaming effect of the optimized silicone and corn oil defoamer was analyzed.It was found that corn oil defoamer showed good defoaming characteristics in the short term,and the foam reduction rate was 48.7%,but the effect became worse after long-term use,and the foam reduction rate decreased to 17.9%.Organosilicon defoamer effectively control the foam at the dose of 0.5%V/V _sample,and the foam reduction rate was always 91.3%–98.4%in the eighty days defoaming experiment.（5）By analyzing the defoaming mechanism of various defoamers,it can be seen that the defoamer achieves foam rupture by changing the local surface tension of the foam liquid film,resulting in stress imbalance.The key points that play a major role are the hydrophobic and hydrophilic groups and hydrophobic particles in the defoamer.A better defoamer must have the following characteristics:insoluble in the medium system and good stability,low surface tension of defoamer molecules and easy dispersion and spreading in the medium,easy adsorption of defoamer molecules on the foam liquid membrane,and hydrophobic particles in the defoamer molecules.