Research On The Emission Pattern And Purification Process Of Exhaust Gas From Thermophilic Aerobic Fermentation Of Dead Pigs

As an important approach to dispose organic waste resource,the thermophilic aerobic fermentation can make dead pigs recycled and utilized properly.However,a large amount of odor gas was always generated in this process concomitantly,which caused a threat for both air environment and human health as a kind of air comtamination material.In this research,a three-day experiment for dead pig was designed and carried out,where 1.2kg chopped body tissue sample of dead pig was taken as the raw material,0.218kg crushed maize stovers and 12g BM strains was taken as auxiliary material.The raw material,auxiliary materials and strains were mixed well and put together into a 10L fermenter equipped with a stirring device to start fermentating with such an experimental condition that the temperature of interlayer water bath during fermentation was kept at 60℃and the ventilation rate was retained to 8L/(L·min).To determine the primary odorants and the release regularity of odor pollutants in the process of thermophilic aerobic fermentation in a 72-hour fermentation period,toally 14 times of gas sampling from tail gas tank would be conducted,with three samples collected each time and 500ml of fermentation gas was collected for each sample acting.During the first 6 hours,the sampling interval was set as3 hours,but it changed to 6 hours from 6th hour to 72th hour.The organic odor components and their concentration of sampling gas were analized and the concentration of ammonia was determined.This work can explore the types and the emitting regularity of odor gas which produced from thermophilic aerobic fermentation for dead pig,and provide a basis for controlling the concentration of odor gas and decreasing the air contamination during nonhazardous dead pig processing.Then,an experiments on the purification of odor pollutants was carried out to investigate the effect of nozzle type,plant additive type,plant additive concentration,packing layer height and ratio of liquid flow to gas flow on the purification effect of the counter-circulation scrubber,while the optimal operating parameters of the counter-circulation scrubber were obtained through orthogonal tests.The results and conclusions of this study were as follows:(1)The results showed that 36 odor pollutants in total were detected during the thermophilic aerobic fermentation of dead pigs 18 of which could be accurately detected qualitatively and quantitatively-including 3 sulfur compounds(methanethiol,dimethyl sulfide,dimethyl disulfide),1 alkane compound(n-hexane),12 aromatic hydrocarbon(benzene,toluene,ethyl benzene,p-xylene,o-xylene,2-ethyltoluene,3-ethyltoluene,4-ethyltoluene,1,2,4-trimethylbenzene,1,3,5-trimethylbenzene,cumene,styrene),1 phenolic compound(p-cresol),and 1 inorganic compound(ammonia).According to odor pollutant standards and odor activity value of odor pollutants,3-ethyltoluene,4-ethyltoluene,dimethyl disulfide,dimethyl sulfide,ammonia,p-cresol and methanethiol were the main odor pollutants in the exhaust gas of thermophilic aerobic fermentation of dead pigs.The highest concentrations of these 7 main odor pollutants respectively reached up to 0.241,0.350,0.247,0.280,69.06,0.041 and 0.314mg/m~3,correspondingly,the ratio of which to olfactory thresholds were 2.7457、8.6347、29.3256、36.9815、66.6691、173.3145 and374.77.(2)Based on the regularity that OAV of the main odor pollutants changed with the changing period of fermentation and the results of correlation analysis among the main odor pollutants,it could be found that the components of the primary odorants were different in different fermentation periods.methanethiol,dimethyl sulfide,dimethyl disulfide,ammonia,p-cresol,3-ethyl toluene and 4-ethyl toluene were the primary odorants in the fermentation periods of 0-12h,methanethiol,dimethyl sulfide,dimethyl disulfide,ammonia and p-cresol,were the primary odorants in the fermentation periods of 12-36h,methanethiol,dimethyl sulfide,dimethyl disulfide and ammonia were the primary odorants in the fermentation periods of 36-72h;Meanwhile,the odor intensity would changd a lot with different fermentatuion stages:it increased slowly in the stage of first 3 hours,but there would have a significant drop when it reached the point of the 6th hour,then went up again from 6th hour to 18th hour and got up to the the peak in the 18th hour,and turned to went down until kept stable to the end.(3)The process parameters of the scrubber were optimized by purifying the exhaust gas produced in the fermentation process through a counter-circulation scrubber tower,with tea extract concentration,packing layer height and liquid flow to gas flow ratio as the test factors and malodor gas removal rate as the test index.The results of the orthogonal test showed that the influence of each factor on the removal rate of odor pollutants was in the following order:liquid flow to gas flow ratio,tea extract concentration,and packing layer height.The optimal process parameters were:tea extract concentration of 0.5g/L,packing layer height of 25cm,and liquid flow to gas flow ratio of 1.5.A validation test was conducted under the optimal process conditions,and the results showed that the average total removal rate of the main odor pollutants in the exhaust gas from the scrubber tower under these conditions reached 98.49%.(4)The emission concentrations of the main odor pollutants in the purified exhaust gas were compared with the emission standard values of odor pollutants.The results show that the emission concentration of dimethyl sulfide meets the secondary emission standard value of odor pollutants,and the emission concentrations of methyl mercaptan,dimethyl disulfide and ammonia can meet the primary emission standard value.