Study On The Efficiency Of Hydrothermal Carbonization-anaerobic Digestion Combined Treatment Of Food Waste

Hydrothermal carbonization(HTC)technology has the characteristics of converting wet biomass into solid fuel or carbon materials,which can be used to treat solid waste with high moisture content.As a type of typical wet solid waste,food waste(FW)is suitable for treating by HTC.Studies have found that the liquid product of HTC(HTC liquid)has the potential for methane production and can be subjected to anaerobic digestion(AD)treatment for energy recovery.In addition,adding the solid product of HTC(hydrochar)to the AD system can improve the methane production performance.However,above-mentioned researches have mostly focused on sludge,manure,garden waste and straw.Few studies have used FW as the object of such HTC-AD combined treatment.Whether this method can be used in the FW treatment and improve its energy output efficiency are worthy of exploration.In view of the fact that the reaction conditions significantly affect the distribution and characteristic of the product,we controled the temperature and reaction time to treated FW by HTC at 3 different process severities:mild(180°C,1 h),intermediate(220°C,2 h)and severe(260°C,4 h),to explore the influence of process severity on the distribution of HTC three-phase products and the characteristics of HTC liquid and hydrochar.Then,AD experiments were respectively carried out with FW and above obtained HTC liquid to clarify the methane production performance of HTC liquid.In addition,we estimated the energy utilization efficiency of HTC-AD combined treatment and separate AD treatment of FW to explore the effect of combined treatment on its improvement.Finally,taking hydrochar,activated hydrochar and granular activated carbon(as control)as substrate to AD system to clarify their biodegradability.In addition,3 types of above carbon materials were added to the AD system of FW to analyze their effect on methane production performance.The following conclusions were mainly obtained.The process severity had a significant influence on the three-phase distribution for products obtained from FW treated by HTC.With the increase of process severity,the yield of hydrochar increased initially and then decreased,the yield of HTC liquid decreased and then stabilized,while the yield of gas product(HTC gas)kept increasing.Overall,the HTC products mainly existed in the form of hydrochar(40.4%～52.1%)and HTC liquid(38.3%～57.5%).In addition to product distribution,the process severitiy also significantly affected the characteristics of HTC liquid and hydrochar.With the increase of the process severity,the p H value of HTC liquid gradually increased from 3.35 to 4.61,while the conductivity showed a trend of decreasing and then increasing with a range from 3.7 ms/cm to 5.0 ms/cm.High concentrations of COD and s COD were detected in all HTC liquid with ranges from 32879.7 mg/L to 105330.8 mg/L and 31740.0 mg/L to104596.7 mg/L,respectively,which were both decreasing significantly with the increase of the process severity.The gas chromatography-mass spectrometry analysis found that the HTC liquid contained a variety of ketones,aldehydes and VFAs,which had high biotransformation potential.The pores and microspheres structures of hydrochar were clearer under environmental scanning electron microscope with the increase of process severity.HTC treatment contributes to the enrichment of carbon element in hydrochar,and the enrichment degree increases significantly with the increase of process severity.In addition,due to the lower ratios of H/C and O/C,the similar calorific value to lignite from23.9 MJ/kg to 28.3 MJ/kg,and the high ignition temperature from 267.5°C to 302.3°C,hydrochars had the potential for energy applications as a safe and clean fuel.HTC liquid obtained by FW had the potential of producing methane,but its accumulative methane yield was significantly affected by the HTC process severity.With the increase of process severity,the accumulative methane yields gradually decreased,which were 207.5±11.1 m L/g COD,154.0±10.2 m L/g COD and 87.7±9.9 m L/g COD,all lower than that of FW(291.1±7.9 m L/g COD).Taking the unit mass of FW(wet weight)as the measurement standard,the combustion heat of methane generated by the AD of HTC liquid and hydrochar obtained by HTC under the corresponding conditions were3003.1 k J/kg,3380.0 k J/kg and 3193.1k J/kg respectively,which were all higher than the combustion heat of methane generated by separate AD of FW(2911.5 k J/kg).The HTC-AD combined treatment for FW had a certain effect on improving its energy utilization efficiency,and the improvement was 3.1%(180°C-1 h)and 16.1%(220°C-2 h)and 9.7%(260°C-4 h).There were significant differences in the degradability of hydrochar,activated hydrochar and granular activated carbon(GAC).Hydrochar had high degradability and its AD accumulative methane yield was 125.2±19.5 m L/g.GAC was not degradable and had no significant impact on the methane production performance of the inoculum,while activated hydrochar significantly inhibited the activity of the inoculum.The accumulative methane yields of FW-AD with above carbon materials adding were 413.9±7.9 m L/g VS(FW control group),428.3±23.0 m L/g VS(hydrochar adding group),88.3±8.6 m L/g VS(activated hydrochar adding group)and 390.2±21.9 m L/g VS(GAC adding group).It was indicated that the addition of hydrochar and GAC had no significant effect on it,while hydrochar can significantly increased the maximum methane production rate(R_m)of the system.The Logistic equation was used to fit the accumulative methane yield of hydrochar adding group,the obtained R_m was 22.4 m L/(g VS·d),which was increased by38.9%compared with that of FW control group.Microbial community structure also showed that hydrochar adding can increased the abundance of microorganism in the system and improved the methane production rate,the addition of GAC had no significant impact on it.However,the addition of activated hydrochar significantly inhibited the performance of FW-AD because that the uneluted Cl~-on the carbon material entered the system and inhibited activity of microorganism,and the microorganism enrichment was also significantly inhibited.