| The vermicomposting is widely acknowledged as a useful biological approach to dispose large-scale organic solid waste such as livestock manure,plant residue,kitchen waste and municipal sludge.During the vermicomposting,the earthworm mature,and the vermicompost that is recognized as the best organic fertilizer can be produced.At the end of vermicomposting,the live earthworm is needed to be separated as soon as possible for the demand of market and further process.However,with the enlargement of vermiculture scale in recent years,rapid and high-efficient earthworm separation and harvest gradually became a crucial problem at the end of vermicomposting.Nowadays,most of earthworm separation job is still done manually,resulting in the low separation efficiency,low separation speed and high expense.To replace labor,save energy and improve the earthworm separation and harvest efficiency,this study aims at developing a rapid,simple,high-efficient,low-energy-consumption earthworm separation technology that can be used in practical field of the large-scale vermicomposting factory.Base on the situation of domestic vermicomposting mode in recent years,firstly,the vermicomposting digestion experiment of cow manure was conducted.The chemical,physical,mechanical and motion characteristics were tested.Secondly,different separation method including low electric method,light method and screening method were compared to evaluate the separation effect.After that,the earthworm-vermicompost rolling separator(EVRS)was developed to separate the earthworm based on the two rapid step separation technology.The separation mechanism was revealed from three aspects:kinetic analysis,micro contacting surface analysis and DEM simulation.Finally,with regard to all the material,EVRS optimal parameters,the EVRS was tested in practical vermicomposting factory.The main works and results were as follows,(1)The field vermicomposting experiment was conducted for 60 days.Before and after vermicomposting,four results(chemical elements,industrial and fiber components,Thermo-gravimetric analysis and Fourier infrared spectroscopic analysis)were tested to explore the change of material in element nutrition,metal,stability and functional group.The result indicated that the TN,TP,TK decreased lightly while AN,AP,AK increased;pH tended to be neutral(7.47);Organic mass and volatile matter decreased;The material such as lignin,cellulose,and hemicellulose,which contained composite structure of aliphatic compounds broken down and degrade into smaller and stable particle.The Cu,Cr,Pb,Cd,and Hg decreased for the earthworm activity.Compared with domestic standard,the vermicompost was a high-value fertilizer and effective substrate.(2)The physical and motion properties are key parameters for the vermicomposting mechanization,calculation and simulation.In this study,the basic material properties of vermicompost and earthworm were tested.To obtain the key mechanized parameters of flow ability,DEM calibration method was carried out with moisture content changing from25%?65%.Plackett-Burman(PB)design was conducted to select the impacted physical parameter factors that have significant influence on pile angle.Then,the steep climbing test was conducted to shorten the parameter ranges rapidly within the maximum and minimum value of PB design.Finally,Box-Benhnken experiment was carried out to build the response surface methodology(RSM)model.The results indicated that three key factors were vermicompost-vermicompost rolling friction coefficient,vermicompost-steel rolling friction coefficient and surface bonding energy(P<0.05).With the increase of material moisture content(MC)from 25%to 65%,the vermicompost rolling friction coefficient and vermicompost-steel rolling friction coefficient decrease.The surface energy that represented the surface adhesion effect,was the most impacted factor in three significant factors(P<0.01),which changed from0.179 to 0.345 J/m~2 with the increase of MC.Few rise of moisture content can improve the flow ability of vermicompost significantly under 50%MC condition.When the moisture content was over 50%,the surface energy and the cohesion force were increased sharply,which would restrict the flow movement of vermicompost to some extent.(3)To investigate stimulation effect of environmental factors in earthworm and applied these factors in separator,two factors(Micro current electric field and Light)are quantitively analyzed.The different types of electric field,strength,worm stocking density,and time gap were conducted in experiment.The different LED light spectral wavelength(white,green,blue,yellow,red),light strength,and natural light were carried out in light expelling experiment.The result indicated that the best separation efficiency could be obtained at the condition of pulsed direct electric field with electric strength in 25m A,electric time of 10s with blank gap in 30s.The earthworm separation rate reached 85.5%and earthworm survival rate was over 95%.The light experiment result indicated that when the light intensity was less than<10 lx:almost no reaction in earthworm;from 10?30 lx:weak photophobic reaction in earthworm;from 30?210lx:photophobic reaction got increased gradually;>210 lx,photophobic reaction reached the maximum.The white light and direct sunlight had the most significant influence on earthworm and the disappearing time was 6.5 and 5 min,respectively.And the thickness of the layer without earthworm reached 15?20mm.With the light intensity increasing,the disappearing time of earthworm dropped logarithmically in all light except the red light.The red light exerted the weakest impact on earthworm in all light spectral wavelength treatments.(4)The rolling screen method was investigated for separating earthworm from the vermicompost.Based on the screening theory,the Earthworm-vermicompost rolling separator(EVRS)was developed.Four screen types were compared,including 304 stainless steel punching plate,304 stainless steel mesh,galvanized-iron mesh,high density polypropylene(HDPP),and polyethylene iron mesh.The result indicated that the HDPP screen exhibited the best flexibility and friction ability.The screening experiments were conducted to evaluate the separation effect under different treatments of vermicompost MC(25%to 65%)and mesh hole size(3mm-12mm).The result indicated that the optimal size was 6mm with earthworm and vermicompost separation rate of 11%and 54%,respectively.The vermicompost could be screened out when the MC of vermicompost was within 35%to 55%.The high MC of vermicompost(65%)leaded to the phenomenon of adhesion,material aggregation and screen mesh blinding,resulting in the low screening rate of both earthworm and vermicompost in all test group.The screening equipment is more suitable with regard to the comprehensive comparison between mechanized environment factors method and rolling screen method.(5)Two rapid steps and continuous earthworm separation technology was developed based on the EVRS.First,relatively small vermicompost granules that is smaller than the mesh size were separated out.To ensure the small vermicompost could be screened as much as possible and earthworm as less as possible,the Central Combination Design(CCD)experiment was conducted in the framework of RSM model.Second,the big vermicompost bulks and mature earthworm were separated centrifugally at the conical separator.Two factor experiment(MC of vermicompost and conical separator types)was carried out.The result showed that the optimal parameters of EVRS were rolling speed of 33 rpm,screening length of 1290mm,genetrix angle of 28°,and MC of vermicompost at 45%.The average earthworm and vermicompost separation rate were 84.38%and 39.52%,respectively.The survival rate of earthworm reached 96.25%with electric consumption in 0.0025KWh within 41.55s.The separation efficiency of EVRS was stable in treating capacity of 15kg/min vermicomposting mixture even under different worm stocking density(No significance,P>0.05).(6)To reveal the earthworm separation mechanism,explain the separation phenomenon and provide effective harvest data,three major studies were conducted based on the developed earthworm separator EVRS,which comprises:earthworm separation motion kinetics analysis,micro interface contact mechanism analysis,and key separation process DEM simulation.The result indicated that the critical cause of earthworm separation is the friction force at contact interface.The vermicompost and earthworm friction effect was originated from the coupling effect of the rough contact surface shearing force and liquid membrane adhesion.The earthworm could form stable liquid membrane and cover all the body surface,resulting in the liquid bridge when contacting with the plane.After centrifugal separation at conical separator,the final separated X velocity of earthworm is higher,whereas the Y and Z velocities is less than that of vermicompost.The earthworms are thrown and dropped to a certain area called the earthworm harvest position,where the center of the harvest position is located at X displacement of 310 mm and Y-displacement of 160 mm on the ground under plane coordinates.The finding of this study can provide practical reference for improvement of earthworm harvest efficiency and shorten the recycling period of vermicomposting products in practical field.(7)With regard to the actual situation of vermicomposting factory,before the separation,it is necessary to lower the MC of vermicompost for obtaining the suitable operating condition of EVRS(45%).A mixing method of dry and wet material was developed for rapidly lowering the MC of vermicompost in practical field based on the porous medium theory of water penetration.The mixing experiment of dry and wet vermicompost material result indicated that the method was effective.To reduce the material dropping disordered and possible blend phenomenon,the harvest experiment was conducted after the pretreatment of vermicompost moisture content.The result indicated that when the incline angle of conveyor was 30°with speed of 50mm/s,the earthworm harvest rate was 77.50%with nearly no vermicompost.The deviation error between validation experiment was 8.34%.To evaluate the real efficiency of EVRS in practical field,based on all the optimal parameters in this study,field experiment was carried out at large-scale vermicomposting factory.The result indicated that the earthworm harvest efficiency is 5.70%with pretreatment of vermicompost.The capacity of mixture separation for one EVRS is 18 kg/min,which is rapid and high efficient in vermicomposting process.Compared with the traditional labor separation in one day(65kg/day),the EVRS could separate nearly 222.48kg per day within effective working time in 6h.The technology developed in this study is simple,low energy consumption,high efficiency and applicable in practical field.Furthermore,the recycling period of organic waste can be shortened for accelerating the harvest speed of vermicomposting product,which is of great significance for the development of vermicomposting in the long run. |
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