08 Biostabilization-biodrying of municipal solid waste by inverting air-flow

Biostabilization–biodrying of municipal solid wasteby inverting air-flowMara Sugni a ,Enrico Calcaterra b ,Fabrizio Adania,*aDipartimento di Produzione Vegetale,sez.FCA,Via Celoria 2,20133Milano,ItalybEcodeco S.p.a.,Loc.Cassinazza di Baselica,27010Giussago,ItalyReceived 1August 2002;received in revised form 22November 2004;accepted 22November 2004Available online 20January 2005AbstractThe process of biodrying could be a good solution for municipal solid waste management,allowing the production of fuel with an interesting energy content.Previous work (Adani,F.,Baido,D.,Calcaterra,E.,Genevini,P.L.,2002.The influence of biomass tem-perature on biostabilization–biodrying of municipal solid waste.Bioresource Technology 83(3),173–179)has indicated that appro-priate management of the processing parameters (air-flow rate and biomass temperatures)could achieve biomass drying in very short times (8–9days).However,the data of that work also evidenced that if the conditions do not consider pile turning,and the air-flow is always from one direction,temperature gradients arise within the biomass,resulting in a lack of homogeneity in the moisture and energy content of the final product.Therefore,a new laboratory study was conducted on municipal solid waste biodrying–biostabilization in an effort to obtain homogeneous final products.Our proposal to solve this lack of homogeneity is to periodically invert the air-flow direction.Thus,in line with a previous study,two trials,A and B,were carried out,dividing the biomass into three layers to study temperature and moisture gradients throughout the process,and a third trial (C)simulating air-flow inversion at regular intervals was introduced.The results suggest that the daily inversion of air-flow eliminates marked tem-perature differences and leads to a homogeneous final product.Ó2005Elsevier Ltd.All rights reserved.Keywords:Calorific power;Biodrying;Municipal solid waste;Refuse-derived fuel;Inverted air-flow1.IntroductionIn an effort to solve problems connected with waste management,and in line with new European legislation (91/156/EEC;91/689/EEC;94/62/EU)(Adani et al.,2002),innovative technologies for waste treatment such as composting,biostabilization and biodrying have be-come of general interest.After composting or biostabili-zation the final product can be used for agricultural purposes,or the biostabilized material can be stored safely in a landfill (Wiemer and Kern,1996;Adaniet al.,1998).In the case of biodrying the refuse can be used as a source of energy,i.e.,fuel (Calcaterra et al.,2000;Adani et al.,2002).Although there are many stud-ies on composting (AA.VV.,1993)there is very little in the literature on biostabilization or,especially,on biodrying.Parameters like biomass temperature and humidity during aerobic processing were investigated in order to better understand their relative importance,but,again,most studies tend to address composting (Finstein and Hogan,1993).Recently,Adani et al.(2002)reported studies on the influence of biomass temperature and air-flow rate on the biostabilization–biodrying of municipal solid waste.The experimental results gave proof of the negative0960-8524/$-see front matter Ó2005Elsevier Ltd.All rights reserved.doi:10.1016/j.biortech.2004.11.016*Corresponding author.Tel.:+39050316546;fax:+39050316521.E-mail address:fabrizio.adani@unimi.it (F.Adani).Bioresource Technology 96(2005)1331–1337correlation between biodegradation and biodrying.In fact,if,during the process,the biomass was subjected to low air-flow rates biodegradation was favoured but water elimination was inadequate.Vice versa,if the air-flow rate is higher there is a more consistent water loss but the degradation rate is slower.Therefore,to achieve satisfactory water loss,and ensure high energy content in thefinal product(fuel),the biodrying process requires the maintenance of high air-flow rate combined with high temperature.During the process,temperature gra-dients were observed throughout the pile;also moisture gradients became evident but,whereas the temperature gradients disappeared in the course of the processing, the moisture gradients persisted to the end.This fact resulted in a lack of homogeneity in thefinal product (Adani et al.,2002).Thus,it was decided to investigate more fully these gradients.Our purpose was to avoid het-erogeneity in thefinal products.To do this we considered it necessary to eliminate temperature gradients in the pile.The aim of the present experimentation was to investigate,in the laboratory,a new procedure for bio-drying municipal solid waste(MSW),i.e.,inversion of air-flow,comparing it with what is commonly done in biodrying plants(Calcaterra et al.,2000)and what we had done in previous experimentation(Adani et al., 2002).The suggested technical solution involves varying the air-flow direction through the biomass.In fact,in biodrying processes,air is usually pumped through the biomass in just one direction,the water being removed in the form of vapour because of the high temperatures reached during biotic degradation.2.MethodsThe trials were performed using an adiabatic compo-sting reactor of148L capacity,like that detailed in pre-vious work(Adani et al.,2001).Two non-separated MSW samples were taken from an MSW treatment plant situated in Giussago(Pavia,Italy).One sample was part of a sampling used in previous experiments dried to a moisture content of50g kgÀ1wet weight (w.w.)and stored at4°C(Adani et al.,2002),the other was new and presented,being sampled at a diverse time, different characteristics from the previous ones(see after).Each sample consisted of about300kg of shred-ded material(B<50mm)collected from a starting amount of about1000kg of non-shredded MSW.The material was dried in an open area until its moisture content was below50g kgÀ1w.w.and then transferred to the laboratory and stored at4°C.The trials were car-ried out using about40kg of wet weight sample,deion-ised water being added to each sample to guarantee the original moisture content.Three biodrying processes were carried out:In the first(Trial A)and second(Trial B)trial,the objective was to identify the different temperature and moisture conditions in the vertical profile of the biomass(gradi-ent)during the processing considering the static condi-tions of the mass and one way air-flow,as in a full scale process(Calcaterra et al.,2000).These investiga-tions were undertaken by sub-dividing,vertically,the biomass in the reactor of each trial in three20-cm high layers by a thin meshed net.In the case of Trial A the material came from the same sample used in previous experiments(moisture: 419g kgÀ1w.w.;volatile solids(VS):654g kgÀ1total solids(TS);ash:346g kgÀ1TS)(Adani et al.,2002). The sample used in the Trial B had the following char-acteristics:moisture:247g kgÀ1w.w.;VS:776g kgÀ1 TS;ash:224g kgÀ1TS.In the third trial(Trial C),the same waste of Trial B was used but there was periodic inversion of the air-flow through the biomass.This was achieved by inverting, daily,the position of the top(layer A)and of the bottom (layer C)layers,so that air-flow passed one day through A–C layer,respectively and,in the following day through C–A layer,respectively.Obviously layer B did not ever change its position.All the trials were performed using a45°C set-point temperature to obtain rapid biodrying,as was explained in earlier work(Adani et al.,2002).The centre of the reactor(layer B)was the location of the set-point tem-perature that was maintained by manual control,using air-flow rate to cool the biomass(Adani et al.,2002). Temperature was measured daily for each of the three layers.On the contrary,air-flow rate and dynamic respi-ration index were measured for the entire bed of waste, being impossible to measure both respirometric activity and air-flow rate for each single layer.Analytical determinations were made during the bio-drying process according to a scheduled sampling plan (see Section3),on sub-samples of about300g from each layer.Bed layer sampling was performed according to the US EPA guidelines for compost materials sampling (The US Composting Council,1997)and the practical guides for analytical chemists to collect samples of solid waste material(Osberghaus and Helmers,1997).Before sampling,each layer in the bed was homogenized and several sub-samples,total weight300g,were taken at random points throughout each entire homogenised layer.The samples were air-dried at40°C and cut to particles smaller than1.0mm by a cutting mill(Retsch SM2000by Retsch,Haan,Germany).Three analytical samples were taken from each bulk sample and sub-jected to chemical analyses.Moisture,volatile solids and ash contents were analysed by standard methods (WHO,1978).During the processes complete mass balances for wet weight(w.w.),total solids(TS),volatile solids(VS)and water were obtained on the basis of ash preservation during the composting processes,using the wet weight1332M.Sugni et al./Bioresource Technology96(2005)1331–1337of the starting biomass(kg)and relative data(g kgÀ1) (Haug,1986a).A water balance,with particular atten-tion to water loss,was also determined experimentally by capturing water vapour from the outgoing air-flow by the appropriate water condenser box positioned at the bottom of the reactor.Moreover water losses were calculated using biomass temperature and air-flow rate data,assuming the relative inlet and outlet moisture content of air at100kPa kPaÀ1(Haug,1986b).The Dynamic Respiration Index(DRI),used as a measure of biological stability,was determined by the biomass oxygen uptake rate,as reported by Adani et al.(2001).Gross calorific values(H0)and energy con-tents(EC)were determined by ASTM methods(ASTM, 1996).Results were analyzed by ANOVA using the Tukey test(Natrella,1966)to compare means and to assess the significance of the difference between mean values. Since chemical analyses were performed on three analyt-ical samples withdrawn from the300g composite bulk sample,standard deviation values,calculated from the data for the three replicates,were estimates of the vari-ability due to both the waste bed homogeneity and the analytical method.Standard deviation for absolute val-ues were calculated as Std C=C(Std A/A),in which C represents the absolute value(kg),A the relative value (g kgÀ1),and Std C and Std A the corresponding stan-dard deviations.3.Results and discussion3.1.Experiment repeatabilityTrial A was conducted using the same waste used in our previous work(see Section2),the aim of which was to assess the effect of temperature on the biostabili-zation–biodrying process of MSW(Adani et al.,2002). As in the earlier work,we adopted a45°C set-point temperature,thus allowing us to compare the present re-sults with those of analogous experiments conducted the year before(Adani et al.,2002).The previous work had taken the entire waste bed into consideration,whereas in this work three layers were considered(see Section2). Thus,in order to compare the two experiments we cal-culated,starting from the data determined for each layer,the average data that represented all the waste bed.This was done for moisture loss,gross calorific value and energy contents,which well represent biodry-ing performance.The calculation was made by consider-ing results obtained for each layer expressed as relative data(g kgÀ1w.w.for moisture content,kJ kgÀ1TS for gross calorific value and kJ kgÀ1w.w.for energy con-tent)and,the absolute dry mass and wet mass contents (kg).Table1shows the results compared to those in our previous work(Trial C of the previous work)(Adani et al.,2002).The moisture loss in this experiment was about200g kgÀ1lower than in the previous work.This diversity could be explained by the different temperature trend observed for this trial with respect to the previous ones(compare Fig.1of the present work with Fig.1of the previous ones)(Adani et al.,2002),that led to vari-ability in drying performance.The fact that the temper-ature set points were difficult to maintain manually probably caused the different temperature trends ob-served.More interesting were the data concerning calo-rific power and energy content.First of all,the differences we recorded in the H0and EC content of the starting material in the two experiments(Table1) were far from negligible.In fact,both gross calorific power and energy content of the starting material of the present experiment were20%lower than in the pre-vious trial.In theory this should not have happened as the waste material was the same for both(optimal stor-age conditions:4°C and moisture content below 50g kg w.w.À1).Probably,the use of different labora-tory to make these determinations with respect to previ-ous work(Adani et al.,2001)gave under estimated results,revealing a scarce reproducibility of the method (ISO,1994).As consequence of that,also the H0and EC values determined forfinal products were under esti-mated with respect to the corresponding values in the previous work(Adani et al.,2002).Thus,in order to compare the drying efficiency and the effect of the biodrying processes on calorific values under these conditions,we considered relative para-meter variations before and after biodrying(see Var% in Table1).They were not similar.The EC value,that is influenced by both volatile solids degradation and water content,increased more in the previous work than in this one.This was due,above all,to the lesser drying than in the present experiment.Thus,with regard to experiment repeatability our re-sult was not very good.Nevertheless,it can be seen from the experimental data that the two processes occurred under different conditions.In fact,for the present work we calculated Table1Comparison between Trial A performed in the present work and the trial conducted in our previous work(Adani et al.,2002)Adani et al.(2002)a Var%This work b Var% Water losses(g kgÀ1initialcontent)667n.s.535n.s.H0start18,012014,5670H0end17,392À3.414,220À2.4 EC start10,856083300EC end14,056+29.4%982417.9 Both trials were conducted in the same conditionfixing a set-point temperature of45°C.a For more explanation,see text.b Value obtained as weighed means of the different layers.M.Sugni et al./Bioresource Technology96(2005)1331–13371333a mean temperature for all processes of46.4°C which was higher than that in the other work(41°C).This dif-ference caused lower drying as a consequence of the low-er air-flow rate(mean air-flow rate calculated for Trial C of previous work of25L kg VSÀ1and for Trial A of this work of21L kg VSÀ1)(Adani et al.,2002).In this case the experiments showed rapid adaptation to the condi-tions used(temperature vs air-flow rate),indicating that the ad hoc equipment set-up worked well.Nevertheless,there is room for improvement in the experimental design,mainly the need to introduce an automatic control for the set-point temperature.More-over,gross calorific value,should be determined taking into considering a standardised sample for which the H0 value is well known.This would exclude problems re-lated to trueness of the measure,that is the agreement between the data obtained by analysis performed on a standardized sample and true data reported(ISO, 1994),as happened in this work.3.2.Temperatures and dynamic respiration indexThe temperature trends during Trials A–C are shown in Fig.1.Throughout the process Trial A showed tem-perature gradients.This was more accentuated in the critical part of the process,i.e.,maximum microbial activity.It can be seen from Fig.1that in the critical part of the process the temperature gradients of the bio-mass reached up to30°C,the top being colder(30°C) than the bottom(60°C).As reported,the set-point tem-perature was maintained manually so the temperature tended to be irregular.As the process reached the end of the trial,the temperatures approached the ambient value and the gradient disappeared.A similar trend was observed in Trial B,though the sample tested was different from that used in Trial A. There was better set-point temperature regulation and the trend showed minorfluctuation,the gradients being more evident than in Trial A.These were of the same ex-tent as those observed for Trial A,confirming about a 30°C degree difference between the top and bottom of the reactor.We calculated mean temperatures of46, 41and31°C for bottom,middle and upper layers, respectively.Therefore,the two previously mentioned trials confirmed the presence of a temperature gradient in the reactor,as was measured in previous work(Adani et al.,2002).These temperature gradients are considered a cause of the heterogeneity in moisture and energy con-tent typical of static biodrying processes(Adani et al., 2002).In Trial C similar temperature gradients to Trial B were observed,but in this case,such as explained in Sec-tion2the top and bottom layers were exchanged daily in order to simulate air-flow inversion.It was observed that when the layer was on the top(alternatively layers A and C)it was always the coldest(about30°C during high microbial activity),but when it was put at the bot-tom its temperature increased(about60°C),giving a gradient between the top and bottom of30°C,as in the two previous trials for which no air-flow inversion was considered.Therefore,independently by the layer presented on the top or on the bottom of the waste bed,temperature trend on the entire bed was similar to that observed for Trial B(Fig.1).Using the temperature trend we calculated mean tem-peratures of37,40and35°C for layers A–C,respec-tively.This means that by inverting the air-flow the mean temperatures of the two opposite layers were very similar,and close to the middle layer that was similar to the temperature registered in the middle layer during Trial B.These data confirmed that periodic air-flow1334M.Sugni et al./Bioresource Technology96(2005)1331–1337inversions led to similar mean temperature trends within the biomass irrespective of position.All three processes ceased after about 200h (Trial A—233h,Trial B—214,Trial C—205),as indicated by the dynamic respiration index trends determined for each trial (Adani et al.,2002)(Fig.2),indicating the cessation of microbial activity.This was confirmed by the final temperatures that were found to be near room temperature (Fig.1).3.3.BiodegradationAs all the trials were focused on the biodrying process it was not,in theory,important to verify whether or not any decrease in organic matter occurred.However,in the present case the final product could be used as fuel if the original organic content could be maintained,thus giving a better product.Adani et al.(2002)showed that limiting the biodegradation saves calorific power,and the savings cannot be ignored.In Trial A,the loss of volatile solids (VS)was not evi-dent,in accordance with earlier results (Adani et al.,2002)being the differences registered,in the same range of the standard deviations (Table 2).Only the bottom layer can be assumed to have a certain degradation rate.On the contrary Trial B showed for all the layers a de-crease in VS content.This ranged from 160to 480g kg À1.The greatest decrease was in the bottom layer and was due to lower water loss (Adani et al.,2002)and so to the preservation of the microbial activity (Adani et al.,2002).The top layer showed an unex-pected degradation coefficient (higher than in the middle layer),thought drying performance was higher (see Sec-tion 3.4).Considering the data of the gross calorific value (H 0)(see Section 3.5)(Table 3),we concluded that the degradation that occurred should be lower than for the middle layer.Probably the heterogeneity of the waste did not allow an accurate detection of VS.Statistical differences were not observed between the before and after biodrying process for both the top and bottom layers in Trial C.On the contrary the mid-dle layer presented a significant reduction in VS content.Probably the exchange of the opposite layers (A and B)determined faster drying (see Section 3.4)than in the middle layers (layer B)allowing more intense and pro-longed degradation activity in the latter.If we compare Trials B and C,that used the same waste,higher VS deg-radation occurred for the solution that did not consider layer exchange.This could,above all,be due to the high degradation coefficient that occurred in the bottom layer,due to maintenance of optimal moisture content.The middle layers for both processes (Trials B and C)showed the same VS losses (0.9and 0.8g kg À1),as ex-pected.This suggested,indirectly,good performance of both the experimental equipment and the trials themselves.Table 2Volatile solid contents in the different layers at the beginning and at the end of the trials Sample Initial volatile solid content (kg)Final volatile solid content (kg)Trial AUpper layer 5.1±0.15a A 4.8±0.19b A Middle layer 5.5±0.15a A 5.6±0.20c A Bottom layer 5.4±0.15a B 4.2±0.27a A Trial BUpper layer 7.4±0.13a B 6.2±0.03b A Middle layer 7.6±0.13a B 6.7±0.02c A Bottom layer 7.6±0.13a B 3.9±0.36a A Trial C Layer A 7.7±0.5a A 7.4±0.7a A Layer B 7.8±0.5a B 7.0±0.1a A Layer C7.8±0.5a A7.9±0.35a AMeans followed by the same letter are not statistically different using Tukey test to compare means (p <0.05):small letters in the same column and capital letters on the same line.Table 3Calorific value (H 0)and energy content (EC)of biomass at the start and at the end of the process in the three trials SampleH 0(kJ kg À1)EC (kJ kg À1)Trial AInitial sample14,559±313b 8330±42a Upper layer (final)15,036±506b 11,344±381b Middle layer (final)14,898±707b 10,904±515b Bottom layer (final)12,663±313a 8384±205a Trial BInitial sample14,525±628c 11,021±114b Upper layer (final)16,715±464d 14,697±477c Middle layer (final)13,604±389b 11,515±326b Bottom layer (final)12,390±820a 10,297±678a Trial CInitial sample 14,525±628a 11,021±473a Layer A (final)16,279±427b 14,098±368b Layer B (final)16,224±4.2b 13,951±3.3b Layer C (final)16,045±611b13,893±66bMeans reported in the same column followed by the same letters are not statistically different using Tukey test to compare means (p <0.05).M.Sugni et al./Bioresource Technology 96(2005)1331–13371335The results confirmed that fast biodrying,that relates to layer position and air-flow rate,leads to a lower deg-radation coefficient(Adani et al.,2002).3.4.Water lossMoisture gradients occurred when no air-flow inver-sion was considered—Trials A and B—Fig.3(Adani et al.,2002).Unlike temperature,the water gradients re-mained through to thefinal samples.Consequently the moisture content in thefinal products for trials were dif-ferent in the three layers as follows:Trial A,top layer 205g kg w.w.À1,middle layer229g kg w.w.À1,bottom layer301g kg w.w.À1and for Trial B,top layer 120g kg w.w.À1,middle layer153g kg w.w.À1,bottom layer169g kg w.w.À1.In this way the moisture hetero-geneity in thefinal products was confirmed as high. The variation coefficient for moisture content in the three layers was calculated as20%and16%for Trials A and B,respectively.These results agree with what had been supposed about degradation,that it was im-proved by moisture maintenance.This is because the air coming into contact with the material takes the mois-ture from it,and as the air passes down through the pile it reaches saturation point.Thus,on reaching the bot-tom of the reactor the air can no longer absorb moisture because it is already overloaded;this also results in tem-perature gradients.In fact limited water removal means limited heat removal,and thus higher temperatures in the biomass(Haug,1986b).Therefore,at the end of the processing there is marked heterogeneity in the mois-ture content of the whole sample.On the contrary thefinal sample from Trial C was homogeneous in moisture content(top layer 134g kg w.w.À1,middle layer140g kg w.w.À1,bottom layer134g kg w.w.À1)(Fig.3)because the whole bio-mass had undergone a sequence of different conditions that had the effect of equalising the conditions at the dif-ferent heights in the reactor.Contrary to the other two trials we calculated a variation coefficient of moisture content in the three layers of only2%.For the degrada-tion coefficients it can be seen that there was good agree-ment for water content in the middle layer for both Trials B and C,confirming that the same process condi-tions(middle layer did not change position)gave similar results.On considering the three layers as a whole,the mean value of thefinal water content for Trial B was 147g kg w.w.À1while for Trial C it was136g kg w.w.À1. It is quite evident that theflow inversion trial led to lower water content(À80g kgÀ1with respect to TrialB)and higherfinal product homogeneity.3.5.Calorific values and energy contentThe calorific analysis confirmed what was observed in biodegradation and drying processes.In thefinal prod-uct of Trial A,the calorific value of the bottom layer was lower(about15–16%less)than in the other two lay-ers(Table3),a consequence of the more consistent bio-degradation processes occurred(Table2).The same observation was for Trial B,though the differences were more accentuated.With regard to energy content,the final products of Trials A and B showed a rise in energy content due to the decrease in water content,but in Trial B thefinal product in the bottom layer had values com-parable with those of the starting sample(Table3).This can be explained by a smaller decrease in moisture con-tent and higher organic matter degradation in the bot-tom layer(see Table2and H0values),both of which determine a lower energy content than in the higher lay-ers.This causes heterogeneity in thefinal product.In particular we calculated,with respect to H0and EC con-tent,a variation coefficient between different layers of 9.4%and15.6%,and15.6%and18.6%,respectively, for Trials A and B.Trial C showed a general increase in calorific values with respect to the initial sample (about11.5%).Such an increase was probably due to sampling problems and/or determination.The energy content of the three layers at the end of the process had similar values(about13,813kJ kg w.w.À1),indicat-ing that the drying process was more homogeneous. Contrarily to Trial B,where the same sample was used, we obtained afinal product with great homogeneity1336M.Sugni et al./Bioresource Technology96(2005)1331–1337regarding the different layers.We calculated for H0and EC variation coefficients of0.7%and of0.7%,respec-tively.This is quite different from the lack of homogene-ity in Trial B that had the same starting sample. Therefore,thefinal product of Trial C is the best one to use as fuel,and is evident proof of the benefits of air-flow inversion in composting aimed at energy production.4.ConclusionsOur previous work pointed out how biomass temper-ature affects biodrying processes.Furthermore,on that occasion we saw that temperature and moisture gradi-ents occurred throughout the biomass and we put it down to the one way air-flow approach that is com-monly used in full scale plants.In fact,we found that such unidirectionalflow causes lack of homogeneity in thefinal product,in relation to the parameters of mois-ture,calorific power and energy content.Thus,the pres-ent work confirmed the presence of biomass temperature gradients(Trials A and B)that resulted in a not very homogeneousfinal product.To overcome these problems,afirst step could be the elimination of such temperature gradients.We tested this in the present work by inverting,on a daily basis, the air-flow direction(Trial C).The results clearly indi-cate that this technique mitigates the gradients and leads to a more homogeneousfinal product.ReferencesAA.VV.,1993.In:Hoitink,H.A.J.,Keener,H.M.(Eds.),Science and Engineering of Composting:Design,Environmental,Microbiolog-ical and Utilization Aspects.The Ohio State University,Wooster, OH,p.728.Adani, F.,Tambone, F.,Genevini,P.L.,Calcaterra, E.,1998.Stabilization of municipal solid waste organic fraction:a labora-tory approach.Technology&Environment4,2–8.Adani,F.,Lozzi,P.,Genevini,P.L.,2001.Determination of biological stability by oxygen uptake on municipal solid waste and derived post Science&Utilization9(29),163–178. Adani, F.,Baido, D.,Calcaterra, E.,Genevini,P.L.,2002.The influence of biomass temperature on biostabilization–biodrying of municipal solid waste.Bioresource Technology83(3),173–179. ASTM,1996.Standard Test Method for Determining the Stability of Compost by Measuring Oxygen Consumption.American Society for Testing and Materials,D5975-96.Calcaterra,E.,Baldi,M.,Adani,F.,2000.An innovative technology for municipal solid waste energy recovery.In:C.I.P.A.—Centro di Ingegneria per la Protezione dellÕAmbiente(Eds.).CIPA,Milano, Italy,pp.123–135.Finstein,M.S.,Hogan,J.A.,1993.Integration of composting process microbiology,facility structure and decision-making.In:Hoitink,H.A.J.,Keener,H.M.(Eds.),Science and Engineering of Compo-sting:Design,Environmental,Microbiological and Utilization Aspects.The Ohio State University,Wooster,OH,pp.1–23. Haug,R.T.,posting process design criteria,part1—feed conditioning.Biocycle(October),38–43.Haug,R.T.,posting process design criteria,part3—aeration.Biocycle(October),53–57.ISO,1994.Accuracy(trueness and precision)of measurement methods and results—part2:basic method for the determination of repeatability and reproducibility of a standard measurement methods5725-2,p.42.Natrella,M.G.,1966.Experimental statistics.In:Besson,F.S.,Astin,A.V.(Eds.),National Bureau of Standards Handbook91(Chap-ters3–4).US Government Printing Office,Washington,DC(Std of 117kJ kg TSÀ1).Osberghaus,U.,Helmers,E.,1997.Waste.In:Stoeppler,M.(Ed.), Sampling and Sample Preparation.Springer,Berlin,pp.57–73. The US Composting Council,1997.Sample collection and prepara-tion.In:Leege,P.B.,Thompson,W.H.(Eds.),Test Methods for the Examination of Composting and Compost,first ed.The US Composting Council,Bethesda,MD,6-1–6-75.WHO,1978.Methods of analysis of sewage sludge soil wastes and compost.Du¨bendorf,Switzerland,WHO Publishing.Wiemer,K.,Kern,M.,1996.Mechanical–Biological Treatment of Residual Waste based on the Dry Stabilate Method.Abfall-Wirtschaft,M.I.C.Baeza-Verlag Publisher,Witzenhausen, Germany.M.Sugni et al./Bioresource Technology96(2005)1331–13371337。