Analysis Of Thermal Hydrolysis Process And Anaerobic Digestion Characteristics Of Mixture Of Grease And Sludge

With the acceleration of urbanization,sewage production has increased with each passing year,and wasted activated sludge?WAS?has also increased significantly as a by-product of biological treatment of sewage.The WAS contains a large number of toxic and harmful substances such as pathogenic bacteria,heavy metals and organic pollutants,whose safe treatment is essential for the ecological environment and human health.Anaerobic digestion?AD?is considered to be an economical and environmentally friendly WAS treatment technology,but there are problems such as low biogas yield and large digester volume about conventional AD.AD of high-solid sludge?solid content greater than 10%?can effectively increase the organic load and digestion efficiency of the digester,reduce the volume of equipment,and combine with thermal hydrolysis pretreatment?THP?to accelerate sludge hydrolysis and increase methane production and achieve efficient and low-cost treatment of sludge.However,there exist air in the sludge particles when the solid content of WAS increases to more than 15%,meanwhile the thermal conductivity and THP efficiency will decrease,thus affecting the energy balance of THP-AD process of high-solid sludge.Kitchen grease is the organic waste generated by kitchen waste.It has high organic content and high methanogenic potential,which is an ideal substrate for AD.At the same time,the grease is also a good heat transfer medium,and its thermal conductivity is much larger than that of air.Besides,convection heat transfer of grease under high temperature can promote heat transfer further.If the grease is mixed with high-solid sludge for THP,the heat transfer performance can be accelerated by the good thermal conductivity of grease,thereby improving the THP efficiency.In view of the problem of low heat transfer performance of high-solids sludge,a suitable amount of kitchen grease was added to the sludge with solid content of more than 15%to coordinate THP.First,the point whether grease can promote THP was discussed.Then,the optimum grease ratio and THP time was determined by AD test of WAS,grease and the mixture of WAS and grease.Finally,In view of the problem of the inhibition from high grease concentration,the AD test of grease and mixture of WAS and grease under different condition was discussed.The main conclusions are as follows:?1?The THP effect of grease promoting WAS was discussed.The results showed that adding kitchen grease could promote the hydrolysis of insoluble organic matter of WAS.When THP temperature was 165°C,the THP time was 90 min,and the mass ratio of grease to sludge was 0.4:1,the insoluble organic matter was fully hydrolyzed.Kinetic analysis indicated that the addition of grease could increase the THP rate of organic matter,and the hydrolysis rate of organic matter increased by 23.30%of 0.2 g _Grease/g_WAS,43.63%of 0.4 g_Grease/g_WAS and 62.98%of 0.6 g_Grease/g_WAS,respectively.The analysis of fatty acid composition of grease showed that THP could change the ratio of fatty acids of grease.The proportion of palmitic acid and stearic acid of grease increased while oleic acid and linoleic acid decreased after THP.When the THP temperature was 165°C,the increase of THP time had little effect on the change of fatty acids proportion.?2?The AD test of WAS,grease and the mixture of WAS and grease before and after THP was carried.The results showed that co-digestion of grease and WAS could significantly increase the methane yields and rate,and reduce the pH value and free ammonia concentration in the AD system,thereby reducing the risk of ammonia inhibition.but the lag phase lengthened after dosing grease.In order to avoid the long lag phase of AD,the THP time was 90 min and the mass ratio of grease to WAS was 0.2:1was considered as the optimum condition.The AD test of the grease before and after THP indicated that THP did not affect the methane yield of grease obviously.The methane yield of grease before and after THP was between 631⁷56 mL?gVS^-1.?3?To avoid the inhibition from high grease concentration and determine the suppression threshold of grease,the AD batch test of grease and the mixture of WAS and grease under different grease concentration and temperature was carried.The methane yield and stability of AD system was analyzed,and the results showed that when the grease concentration was less than 10 g·L^-1,the AD system of all samples were stable,whose methane yield were high.When the grease concentration reached 20 g·L^-1and above,the stability and methane yield of all AD system were significantly reduced.So the grease concentration was advised to less than 10 g·L^-1 during AD process of grease and the mixture of grease and WAS.The AD of grease and co-digestion of the mixture of WAS and grease were compared,and the results showed that co-digestion could increase the methane yield of AD significantly.The mesophilic AD system and thermophilic AD system of all samples was compared,and the results showed that when the grease concentration reached 20 g·L^-1 and above,the methane yield and stability of thermophilic AD system was higher than that of mesophilic AD system.