Effect of Inoculum Addition Modes and Leachate Recirculation on Anaerobic Digestion of Solid Cattle Manure in an Accumulation System

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<ul><li><p>Biosystems Engineering (2006) 95 (2), 245254doi:10.1016/j.biosystemseng.2006.06.006</p><p>d</p><p>r</p><p>Aas,hoPZe</p><p>rm</p><p>concentrations has many advantages compared to theconventional slurry digestion systems at low solids</p><p>25% total solids (TS)] using an accumulation systemwith addition of 10% V/V inoculum at the reactor</p><p>ARTICLE IN PRESSdigestion limits the need for extensive mixing, additionof water, high-energy need for heating and also limitsthe need of efuent dewatering.</p><p>chemical oxygen demand (CODdis) and VFA over thereactor height. The lowest concentrations of intermedi-ate compounds were found in the bottom layers whereconcentrations (Ten Brummeler, 1993). High solids bottom, showed a pronounced stratication of dissolved</p><p>1537-5110/$32.00 245 r 2006 IAgrE. All rights reservedThe effect of both leachate recirculation (at 40 and 50 1C) and the mode of inoculum addition (at 50 1C) on theperformance of a non-mixed accumulation (i.e. fed batch) system treating solid cattle wastes was investigated,using laboratory scale reactors at a lling time of 60 days. A relatively high methane production rate (MPR)and low stratication of intermediates occur with leachate recirculation. The leachate recirculation volumeow and methane production rate are smaller at 40 1C than at 50 1C: 031 and 07 l [CH4] l1 [reactor] day1,respectively. The increased MPR at higher temperature is at one hand caused by the increase of microbialactivity, at the other hand by the lower viscosity causing the increased leachate recirculation volume. Dividingthe inoculum in equal doses and distributing them with the feed positively affects the system behaviour ascompared to adding the same inoculum amount at the reactor bottom at the start only. Without addition ofinoculum a very poor system performance was observed. The average MPR was 02, 04 and 05 l [CH4] l1[reactor] day1 for the reactor without inoculum, inoculum addition at the reactor bottom and inoculumaddition in different equal doses, respectively.r 2006 IAgrE. All rights reserved</p><p>Published by Elsevier Ltd</p><p>1. Introduction</p><p>The choice of an anaerobic treatment system stronglydepends on the substrate characteristics, the simplicityof the design and the operation (Lettinga, 2001) and oneconomical and technical aspects. Callaghan et al.(1999) mentioned that it is difcult to mix systems withtotal solid concentrations above 10% by conventionalmixing methods. As the total solid content of manuredepends on the bedding material (Hobson et al., 1981),the application of the digestion system depends onthe farm breeding system. Digestion at high solids</p><p>In small traditional farms the cattle drop the manureon the stable oor. The rather dry manure is usuallycollected once a day. This makes the accumulation(i.e. fed batch) system suitable for on farm applicationfor both storage and digestion of manure (Wellinger &amp;Kaufmann, 1982; Zeeman, 1991). According to Zeeman(1991) a stable digestion of liquid slurry in accumulationsystems is practically feasible, provided enough inocu-lum is present to prevent volatile fatty acids (VFA)accumulation at the end of the lling time. Earlierresults (El-Mashad et al., 2003) obtained from experi-ments during digestion of high solid cattle wastes [caSEStructures and Environment</p><p>Effect of Inoculum Addition Modes anDigestion of Solid Cattle Manu</p><p>Hamed M. El-Mashad1; Wilko K.P. van Loon2; G</p><p>1Department of Agricultural Engineering, Faculty ofe-mail: hamedel_m</p><p>2Wageningen University, Systems and Control Groupe-mail of corresponding aut</p><p>3Wageningen University, Environmental Technology,e-mail: Grietje.</p><p>(Received 17 January 2005; accepted in revised foLeachate Recirculation on Anaerobicre in an Accumulation System</p><p>ietje Zeeman3; Gerard P.A. Bot2; Gatze Lettinga3</p><p>griculture, Mansoura University, Mansoura, Egypt:had@mans.edu.egPO Box 17, 6700 AA Wageningen, The Netherlands;r: Wilko.vanLoon@wur.nlO Box 8129, 6700 EV Wageningen, The Netherlands;eman@wur.nl</p><p>21 June 2006; published online 28 August 2006)Published by Elsevier Ltd</p></li><li><p>the methanogenesis is the highest (Ten Brummeler,1993). So for improvement of the digestion in anaccumulation system, other operation strategies shouldbe applied.For the dry anaerobic digestion of vegetable and yard</p><p>wastes in a pilot batch reactor (BIOCEL), Ten Brum-meler (1993) showed a higher digestion rate with aleachate recirculation rate. According to Veeken andHamelers (2000), the transport of VFA from theacidogenic to the methanogenic pockets can take placeonly through the leachate. According to Chan et al.(2002) leachate recirculation was effective in enhancingthe degradation rate (i.e. reducing stabilisation time) andbiogas production from landll co-disposal of municipalsolid waste, sewage sludge and marine sediment. Veeken</p><p>performance of a stratied accumulation system seededwith inoculum at the reactor bottom. Another option toimprove such accumulation system performance couldbe the addition of the inoculum with the feed. So theobjectives of the present study are:</p><p>(1) to study the effect of leachate recirculation on theperformance of the digestion of solid cattle manurein accumulation system at 40 and 50 1C; and</p><p>(2) to study the effect of three different inoculumaddition modes on the process performance at50 1C: no inoculum addition, the addition of theinoculum in the reactor bottom, and the adding ofinoculum in equal doses with the feed.</p><p>and bedding material. The animals were fed concen-</p><p>ARTICLE IN PRESS</p><p>lepiationne pro</p><p>at</p><p>))14</p><p>H.M. EL-MASHAD ET AL.246and Hamelers (1999) mentioned that the performance ofdry batch digestion of biowaste can be improved byrecirculation of leachate. At the start-up of the reactor, alow leachate ow prevents the irreversible acidicationof the methanogenic pockets (i.e. seeding material). Afterthe start up the methanogenic population will increaseand the leachate ow can be increased thus preventinginhibition of hydrolysis in the acidogenic pockets (i.e.fresh biowaste). Veeken and Hamelers (1999) mentionedalso that leachate recirculation should be controlled: atoo large transport of VFA from biowaste to seed vialeachate can result in irreversible acidication of theseeds, impeding the methanogenic activity. According toVieitez and Ghosh (1999) the inhibition of hydrolysisand acidication during solid state digestion by accu-mulated VFA and lower pH can be alleviated byrecycling of the leachate through a separate methano-genic reactor and conveying methanogenic efuent to thesolid bed. Increasing the leachate recirculation rate andimproving the mixing of biowaste and seed result in ahigher biowaste conversion rate. This results in shortersolids retention times (Veeken &amp; Hamelers, 2000).From the literature mentioned above, the leachate</p><p>recirculation could be an option to improve the</p><p>TabSubstrate characteristics and average concentrations of different40 1C and 50 1C, R40AC and R50AC, respectively, with standard dev</p><p>and MPR, metha</p><p>Parameters Substr</p><p>Volatile fatty acids, g [COD] kg1 7 (2CODdis, g kg</p><p>1 27 (5Kjeldahl nitrogen, g kg1 58 (0Total ammonia, (g kg1) 19 (0Accumulated CH4, l kg</p><p>1[manure] MPR, l l1 [reactor] day1 Hydrolysis, % Acidogenesis, % Methanogenesis, % ) 25 (04) 28 (04)187 392</p><p>03 (04) 07 (04)37 5826 4622 45)10 (7) 19 (25)39 (8) 305 (4)68 (12) 73 (09)eeters at the end of the leachate recirculation experiment ats are between the brackets; COD, chemical oxygen demand;duction rate</p><p>R40AC R50AC1aramtrated, antibiotic-free diets. The manure was analysed forits composition then refrigerated (4 1C) over the experi-mental course. No pre-treatment was applied for themanure. The chemical characteristics of the substrateused in the experiments are mentioned in Tables 1 and 2.Organic matter contents are given in chemical oxygendemand (COD). This is the oxygen equivalent of theorganic mater that can be oxidised, using a strongchemical oxidising agent in an acidic medium.</p><p>2.2. Experimental arrangement</p><p>The effect of leachate recirculation and the effect ofinoculum addition modes were studied in two differentexperimental runs at 60 days lling time. For leachaterecirculation, one reactor was kept at 40 1C and another2. Material and methods</p><p>2.1. Substrate</p><p>Solid cattle waste, originating from fattening cows,was used in the present study. It consisted of faeces, urine</p></li><li><p>ARTICLE IN PRESS</p><p>lpavie</p><p>ANAEROBIC DIGESTION OF SOLID CATTLE MANURE 247at 50 1C. The reactors started with 10% V/V (of the totalvolume) digested manure taken from an accumulationsystem treating solid cattle waste (16% TS) at 40 and50 1C and at lling time of 60 days followed by another20 days without feeding. In the present study, theleachate was collected manually before the weeklyfeeding; then its volume was measured and mixed withthe new feed to assure an equivalent distribution of theleachate and the substrate. The leachate recirculationstarted after the rst 11 days.For the inoculum addition modes, three different</p><p>reactors were incubated at 50 1C and 60 days lling time.In the rst reactor no inoculum was added. The secondreactor was inoculated (10% V/V). This inoculum wasequally divided and added in different doses with thefeed. The third reactor is a multiple bottle reactorinoculated with 10% (V/V) at the reactor bottom. Theinoculum used in the three experiments was taken fromthe efuent of the reactor operated at 50 1C withleachate recirculation.TabSubstrate characteristics and average concentrations of differentof cow manure at different inoculation modes with standard de</p><p>demand; and MPR, m</p><p>Parameters Substrate</p><p>Volatile fatty acids, g [COD] kg1 18 (09)CODdis, g kg</p><p>1 266 (10)Kjeldahl nitrogen, g kg1 84 (11)Total ammonia, (gkg1) 12 (02)Accumulated CH4, l kg</p><p>1[manure] MPR, l l1 [reactor] day1 Hydrolysis, % Acidogenesis, % Methanogenesis, % 2.3. Experimental reactors and feed procedures</p><p>To study the effect of recirculation of leachate, thesame reactors and the gas measurement equipment usedin our previous study (El-Mashad et al., 2003) were alsoused in the current one. The reactor bottoms wereperforated to collect the leachate for the recirculation.For the reactors used for the inoculum addition modes,two reactor types were used. For the reactor withoutinoculum addition and the reactor with addition ofinoculum on different doses, the same 30 l reactors usedin our earlier study (El-Mashad et al., 2003) were alsoused. Once per week 3 l manure was added. For thereactor used with the inoculum at the bottom, amultiple-bottle (eight bottles) reactor was used. In these2.5. Calculations</p><p>Hydrolysis, acidogenesis, and methanogenesis werecalculated as described by Zeeman (1991). To quantify2.4. Sampling and analysis</p><p>At the end of the experiments samples were takenfrom different reactor heights. The analyses of TS;versatile solids (vs); total ammonium; Kjeldahl nitrogen;VFA; total and dissolved chemical oxygen demand(COD and CODdis; respectively) were carried out asdescribed by El-Mashad et al. (2003). In the leachaterecirculation experiments, the VFA of the leachate wasmeasured weekly after the rst 11 days.5 l bottles once per week 05 l manure was added.Sanders (2001) declared more detailed description of thelater reactor kind.</p><p>e 2rameters at the end (i.e. after 60 days) of the fed batch digestionations as shown between the brackets; COD, chemical oxygenthane production rate</p><p>Inoculum additionin equal doses</p><p>Inoculum on thebottom</p><p>Without inoculum</p><p>70 (51) 69 (92) 109 (88)386 (58) 434 (98) 480 (94)83 (04) 84 (06) 78 (07)34 (03) 36 (02) 35 (02)298 212 113</p><p>05 (01) 04 (01) 02 (01)51 43 3338 28 2835 25 13the effect of recirculation and inoculum addition on theconcentration proles of CODdis and VFA), a proleextent index parameter Ipe is dened as the difference inthe concentration of a particular intermediate betweenthe reactor top Ctop g kg</p><p>1 and the reactor bottomCbottom g kg</p><p>1 divided by the concentration of CODdis inthe inuent denoted by Ddis g kg</p><p>1</p><p>Ipe Ctop Cbottom</p><p>Ddis(1)</p><p>To calculate the total energy input to a well-insulated10m3 AC system with an aspect ratio (height dividedby diameter) of 17, a simple energy balance modelwas established [Eqn (2)]. In this simple model, thecalculation of heat losses to the environment andthe energy required for heating up of the feed is based</p></li><li><p>on a constant temperature of both the ambient airand the feed of 25 1C. The net energy production ET,in J:</p><p>ET MCV 1=Z</p><p>U2Ac Asi AsgTr Tamt</p><p> P60</p><p>N1UAsTr TamtN 24 3600 m CpTr Tam</p><p>2664</p><p>3775</p><p>2</p><p>where: M is the total methane production during thelling time, in m3; CV is the caloric value of methane, avalue of 37MJm3 [CH4] (Hill &amp; Bolte, 2000) beingused; U is overall heat transfer coefcient between thereactor and the environment which was 03Wm2K1;Ac is cross-section area of the reactor, which was 29m2;Asi is reactor side area corresponding to the inoculumvolume, which was 21m2; Asg is reactor side areacorresponding to the gas volume, which was 07m2; As isreactor side area corresponding to the daily added feed,which was 03m2; Tr is reactor operation temperature inK; Tam is ambient temperature in K; t is reactor llingtime in s; N is the day number during lling time; m is</p><p>3. Results and discussion</p><p>3.1. Leachate recirculation</p><p>Figure 1 shows the measured methane production rate(MPR) at both studied temperatures (40 and 50 1C). Forboth temperatures a lag phase of about 10 days can beobserved. Such long lag phase may be attributed to thelong digestion time (ca 80 days) in the previous run fromwhich inoculum was taken. As soon as the methane wasstarted at both temperatures, as expected, a noticeablehigher methane production rate is observed at 50 1C.This may be attributed to the higher hydrolysis rate and/or growth rate of methanogenic bacteria at 50 1Ccompared with that at 40 1C. However, the decayrate is also higher at the higher temperature. Anotherreason for such difference may be the larger amount ofleachate recirculation at 50 1C compared with that at40 1C (Fig. 2).As expected, the leachate recirculation caused a</p><p>dramatic decrease in the prole extent of bothVFA and CODdis, compared with earlier ndings incomparable experiments without leachate recirculation(El-Mashad et al., 2003), specially at 50 1C reactor(Fig. 3). At 50 1C, a higher leachate amount could be</p><p>ARTICLE IN PRESS</p><p>T</p><p>e</p><p>H.M. EL-MASHAD ET AL.248the mass of daily feed, which was 150 kg; N is number ofdays before inserting a certain feed in day; Cp is specicheat of the manure, which was 4000 J kg1K1; and Z isheater efciency, which was 70%.</p><p>0</p><p>0.4</p><p>0.8</p><p>1.2</p><p>1.6</p><p>0 10 20 30</p><p>MPR</p><p>, l l</p><p>1 </p><p>[reac</p><p>tor] d</p><p>ay</p><p>1</p><p>Fig. 1. Methane production rate (MPR) during thcollected compared with that at 40 1C (Fig. 2). Theincrease of the leachate amount at 50 1C could mainly beattributed to the lower viscosity at 50 1C. Also the higherdegradation rate 50 1C may affect the viscosity. From</p><p>40 50 60 70ime, day</p><p>leachate recirculation at K, 40 1C and J, 50 1C</p></li><li><p>ARTICLE IN PRESS</p><p>0</p><p>2</p><p>4</p><p>6</p><p>8</p><p>10</p><p>12</p><p>Leac</p><p>hate</p><p>, l m</p><p>3 </p><p>[reac</p><p>tor] d</p><p>ay</p><p>1</p><p>0 10 20 30 40 50 60 70Time, day</p><p>Fig. 2. Average leachate recirculation rate at J, 40 and K,...</p></li></ul>

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