ARTICLE IN PRESS

doi:10.1016/j.biosystemseng.2006.06.006SE—Structures and Environment

Biosystems Engineering (2006) 95 (2), 245–254

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

Hamed M. El-Mashad1; Wilko K.P. van Loon2; Grietje Zeeman3; Gerard P.A. Bot2; Gatze Lettinga3

1Department of Agricultural Engineering, Faculty of Agriculture, Mansoura University, Mansoura, Egypt:e-mail: hamedel_mashad@mans.edu.eg

2Wageningen University, Systems and Control Group, PO Box 17, 6700 AA Wageningen, The Netherlands;e-mail of corresponding author: Wilko.vanLoon@wur.nl

3Wageningen University, Environmental Technology, PO Box 8129, 6700 EV Wageningen, The Netherlands;e-mail: Grietje.Zeeman@wur.nl

(Received 17 January 2005; accepted in revised form 21 June 2006; published online 28 August 2006)

The 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 filling time of 60 days. A relatively high methane production rate (MPR)and low stratification of intermediates occur with leachate recirculation. The leachate recirculation volumeflow and methane production rate are smaller at 40 1C than at 50 1C: 0�31 and 0�7 l [CH4] l�1 [reactor] day�1,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 0�2, 0�4 and 0�5 l [CH4] l�1

[reactor] day�1 for the reactor without inoculum, inoculum addition at the reactor bottom and inoculumaddition in different equal doses, respectively.r 2006 IAgrE. All rights reserved

Published by Elsevier Ltd

1. Introduction

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 difficult 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 solidsconcentrations has many advantages compared to theconventional slurry digestion systems at low solidsconcentrations (Ten Brummeler, 1993). High solidsdigestion limits the need for extensive mixing, additionof water, high-energy need for heating and also limitsthe need of effluent dewatering.

1537-5110/$32.00 245

In small traditional farms the cattle drop the manureon the stable floor. 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 &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 filling time. Earlierresults (El-Mashad et al., 2003) obtained from experi-ments during digestion of high solid cattle wastes [ca

25% total solids (TS)] using an accumulation systemwith addition of 10% V/V inoculum at the reactorbottom, showed a pronounced stratification of dissolvedchemical oxygen demand (CODdis) and VFA over thereactor height. The lowest concentrations of intermedi-ate compounds were found in the bottom layers where

r 2006 IAgrE. All rights reserved

Published by Elsevier Ltd

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H.M. EL-MASHAD ET AL.246

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 yardwastes 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 landfill co-disposal of municipalsolid waste, sewage sludge and marine sediment. Veekenand 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 flow prevents the irreversible acidificationof the methanogenic pockets (i.e. seeding material). Afterthe start up the methanogenic population will increaseand the leachate flow 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 acidification of theseeds, impeding the methanogenic activity. According toVieitez and Ghosh (1999) the inhibition of hydrolysisand acidification 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 effluent 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 & Hamelers, 2000).

From the literature mentioned above, the leachaterecirculation could be an option to improve the

TableSubstrate characteristics and average concentrations of different p

40 1C and 50 1C, R40AC and R50AC, respectively, with standard devi

and MPR, methane

Parameters Substrat

Volatile fatty acids, g [COD] kg�1 7 (2)CODdis, g kg

�1 27 (5)Kjeldahl nitrogen, g kg�1 5�8 (0�1Total ammonia, (g kg�1) 1�9 (0�4Accumulated CH4, l kg�1[manure] —MPR, l l�1 [reactor] day�1 —Hydrolysis, % —Acidogenesis, % —Methanogenesis, % —

performance of a stratified 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:

(1)

1aram

ation

pro

e

))

to study the effect of leachate recirculation on theperformance of the digestion of solid cattle manurein accumulation system at 40 and 50 1C; and

(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.

2. Material and methods

2.1. Substrate

Solid cattle waste, originating from fattening cows,was used in the present study. It consisted of faeces, urineand bedding material. The animals were fed concen-trated, 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.

2.2. Experimental arrangement

The effect of leachate recirculation and the effect ofinoculum addition modes were studied in two differentexperimental runs at 60 days filling time. For leachaterecirculation, one reactor was kept at 40 1C and another

eters at the end of the leachate recirculation experiment at

s are between the brackets; COD, chemical oxygen demand;

duction rate

R40AC R50AC

10 (7) 1�9 (2�5)39 (8) 30�5 (4)

6�8 (1�2) 7�3 (0�9)2�5 (0�4) 2�8 (0�4)18�7 39�2

0�3 (0�4) 0�7 (0�4)37 5826 4622 45

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Table 2

Substrate characteristics and average concentrations of different parameters at the end (i.e. after 60 days) of the fed batch digestion

of cow manure at different inoculation modes with standard deviations as shown between the brackets; COD, chemical oxygen

demand; and MPR, methane production rate

Parameters Substrate Inoculum additionin equal doses

Inoculum on thebottom

Without inoculum

Volatile fatty acids, g [COD] kg�1 1�8 (0�9) 7�0 (5�1) 6�9 (9�2) 10�9 (8�8)CODdis, g kg

�1 26�6 (1�0) 38�6 (5�8) 43�4 (9�8) 48�0 (9�4)Kjeldahl nitrogen, g kg�1 8�4 (1�1) 8�3 (0�4) 8�4 (0�6) 7�8 (0�7)Total ammonia, (g�kg�1) 1�2 (0�2) 3�4 (0�3) 3�6 (0�2) 3�5 (0�2)Accumulated CH4, l kg�1[manure] — 29�8 21�2 11�3MPR, l l�1 [reactor] day�1 — 0�5 (0�1) 0�4 (0�1) 0�2 (0�1)Hydrolysis, % — 51 43 33Acidogenesis, % — 38 28 28Methanogenesis, % — 35 25 13

ANAEROBIC DIGESTION OF SOLID CATTLE MANURE 247

at 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 filling 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 first 11 days.For the inoculum addition modes, three different

reactors were incubated at 50 1C and 60 days filling time.In the first 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 effluent of the reactor operated at 50 1C withleachate recirculation.

2.3. Experimental reactors and feed procedures

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 these

5 l bottles once per week 0�5 l manure was added.Sanders (2001) declared more detailed description of thelater reactor kind.

2.4. Sampling and analysis

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 first 11 days.

2.5. Calculations

Hydrolysis, acidogenesis, and methanogenesis werecalculated as described by Zeeman (1991). To quantifythe effect of recirculation and inoculum addition on theconcentration profiles of CODdis and VFA), a profileextent index parameter Ipe is defined as the difference inthe concentration of a particular intermediate betweenthe reactor top Ctop g kg�1 and the reactor bottomCbottom g kg�1 divided by the concentration of CODdis inthe influent denoted by Ddis g kg

�1

Ipe ¼Ctop � Cbottom

Ddis

(1)

To calculate the total energy input to a well-insulated10m3 AC system with an aspect ratio (height dividedby diameter) of 1�7, 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

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H.M. EL-MASHAD ET AL.248

on a constant temperature of both the ambient airand the feed of 25 1C. The net energy production ET,in J:

ET ¼ ðMCV Þ � ð1=ZÞ

�

Uð2Ac þ Asi þ AsgÞðTr � TamÞt

þP60

N¼1

UAsðTr � TamÞðt�N � 24� 3600Þ þm CpðTr � TamÞ

2664

3775

ð2Þ

where: M is the total methane production during thefilling time, in m3; CV is the caloric value of methane, avalue of 37MJm�3 [CH4] (Hill & Bolte, 2000) beingused; U is overall heat transfer coefficient between thereactor and the environment which was 0�3Wm�2K�1;Ac is cross-section area of the reactor, which was 2�9m2;Asi is reactor side area corresponding to the inoculumvolume, which was 2�1m2; Asg is reactor side areacorresponding to the gas volume, which was 0�7m2; As isreactor side area corresponding to the daily added feed,which was 0�3m2; Tr is reactor operation temperature inK; Tam is ambient temperature in K; t is reactor fillingtime in s; N is the day number during filling time; m isthe mass of daily feed, which was 150 kg; N is number ofdays before inserting a certain feed in day; Cp is specificheat of the manure, which was 4000 J kg�1K�1; and Z isheater efficiency, which was 70%.

0

0.4

0.8

1.2

1.6

0 10 20 30

T

MPR

, l l

−1 [

reac

tor]

day

−1

Fig. 1. Methane production rate (MPR) during the

3. Results and discussion

3.1. Leachate recirculation

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 adramatic decrease in the profile extent of bothVFA and CODdis, compared with earlier findings 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 becollected 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

40 50 60 70

ime, day

leachate recirculation at K, 40 1C and J, 50 1C

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0

2

4

6

8

10

12

Lea

chat

e, l

m−3

[re

acto

r] d

ay−1

0 10 20 30 40 50 60 70

Time, day

Fig. 2. Average leachate recirculation rate at J, 40 and K, 50 1C

0

5

10

15

20

25

30

35

40

45

50

0 20 40 60 80 100 120

% of reactor height

VFA

, g [

CO

D]

kg−1

; and

CO

Ddi

s g k

g−1

Fig. 3. Concentration profiles at the end of leachate recirculation experiments of: J, volatile fatty acids (VFA) at 40 1C; K, VFAat 50 1C; E,dissolved chemical oxygen demand (CODdis) at 40 1C; and B, CODdis at 50 1C

ANAEROBIC DIGESTION OF SOLID CATTLE MANURE 249

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H.M. EL-MASHAD ET AL.250

Fig. 2, it can be observed that the leachate amount in l

m�3[reactor]day�1 increases pronouncedly with time at50 1C. At 40 1C, some increase of leachate is shown after45 days of digestion. The amount of the leachate highlydepends on the accumulated amount of substrate, thetotal solids content and the field capacity. The gravita-tional drainage of leachate occurs when the moisturecontent of a waste is above the field capacity (Jang et al.,2002). For vegetable fruit and yard wastes digestion,Ten Brummeler (1993) revealed that TS concentration ina reactor must be low enough in order to have a certainamount of free liquid, which is available for recycling.He also mentioned that at initial TS of 30%, themaximum possible recycle flow is ca 0�3m3m�3 [reactor]day�1. According to Ten Brummeler (1993), leachaterecirculation is not possible during the digestion ofsubstrates having total solids exceeding 35% TS. This isdue to the lack of free moving water.

Three explanations may elucidate the positive effect ofthe leachate recirculation on the process performance.The first is that leachate brings the VFA from the toplayers (i.e. freshly added manure) to the bottom layers(i.e. methanogenic seeding material). Veeken andHamelers (2000) revealed this explanation during thesolid-state digestion of biowaste in batch reactors. Thesecond explanation is that leachate recirculation maycause some mixing of the substrate with inoculum (TenBrummeler, 1993). According to Torres-Castillo et al.(1995), the degree of substrate degradation is affected by

0

200

400

600

800

1000

1200

1400

0 10 20 30

T

VFA

, mg

[CO

D]

l−1 [

leac

hate

]

Fig. 4. Volatile fatty acids (VFA) concentratio

the distribution of recirculation leachate. It acts directlyon the contact between the substrate and microorgan-isms. The third explanation is that leachate recirculationmay contain some inoculum, which is mixed with thefreshly added manure. O’Keefe and Chynoweth (2000)revealed that leachate recirculation between a landfillcell and either anaerobic digester or ‘mature’ landfill cellwould provide inoculum to establish a balancedacidogenic and methanogenic population.

During the daily adding of manure air may infiltrateinto the system causing aerobic conditions, which resultsin an inhibition of the proces. However, the fillingsystem has been designed to minimise the ingoingairflow during filling (El-Mashad et al., 2003).

Figure 4 shows the VFA concentration of the leachateat both temperatures. The leachate from the reactoroperated at 50 1C has a very low and almost constantconcentration of VFA along the filling period. The VFAconcentrations in the leachate at 40 1C started at highlevel of ca 1�2 g [COD] l�1 and subsequently decreased.Such decrease of the VFA is reflected on the pronouncedmethane production rate (see Fig. 1).

Table 1 shows the average concentrations of differentparameters in the effluent and the percentages ofhydrolysis; acidogenesis and methanogenesis. Fromthis table it can be concluded that a much betterperformance is achieved at 50 1C as compared to40 1C. Besides the positive effect of the higherleachate recirculation rate and higher temperature on

40 50 60 70

ime, day

ns in the leachate at J, 40 1C and K, 50 1C

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ANAEROBIC DIGESTION OF SOLID CATTLE MANURE 251

methanogenesis, there is also a positive effect onhydrolysis and acidogenesis. The data presented inTable 1 show that methanogenesis is likely the ratelimiting step at 40 1C, while hydrolysis is the ratelimiting step at 50 1C. A non-acidified CODdis part isalmost equal for 40 and 50 1C. This can be considered asan inert CODdis (Zeeman, 1991). It can be noticed(Table 1) that acetate and propionate represent themajor VFA constituents at both temperatures. OtherVFAs are presented in very small portions.

3.2. Inoculum addition modes

Figure 5 shows MPR over the filling time for the threedifferent inoculum addition modes. The first is operatedwithout inoculum addition. The second reactor isoperated with adding an inoculum volume of 10%(V/V of the reactor volume) at the reactor bottom andthe third reactor is operated with adding the sameamount of inoculum (10% V/V) in equal doses with thefeed. As expected, the latter treatment does have thehighest MPR compared with the two others. Thisdefinitely is due to the presence of the methanogenicbacteria with the substrate along the reactor height.Without the addition of inoculum, methane productionoccurs at a very low rate. According to Hobson (1985),the faecal bacteria and bacteria picked up from theenvironment, provide the inoculum from which the

−0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30

T

MPR

, l l

−1[r

eact

or]

day−1

Fig. 5. Experimental methane production rate (MPR) at 50 1C forK, inoculum at the reactor b

active digester flora develops, but a large proportion ofthese bacteria does not have a major role in the digesterreactions. Unlike the experiments with leachate recircu-lation, no lag phase can be noticed (Fig. 5) even at theexperiment without adding inoculum. It can be recog-nised that the amount of active bacteria in the rawmanure changes from one batch to another and possiblydepends on the original ambient temperature. It shouldbe mentioned that the manure used in the leachaterecirculation was collected in February and for the otherinoculum addition modes in July, with average monthlyambient temperature of 4 and 17 1C, respectively.

Figure 6 shows the concentration of VFA and CODdis

along the reactor height at the end of the filling time.The reactor operated without inoculum addition has thehighest concentrations of both VFA and CODdis

compared with that operated at equal doses mode. Itshould be mentioned that Fig. 6 does not contain suchprofile for the bottom inocculum reactor as the samplingprocedures are slightly different from that for the othertwo reactors (see El-Mashad, 2003).

The data in Table 2 show significantly higherpercentages of hydrolysis; acedogenesis and methano-genesis occur in the distributed inocculum reactorcompared with the two others. From this table it canbe concluded that significant improved system perfor-mance (i.e. higher hydrolysis; acidogenesis and metha-nogenesis) can be realised after adding the inoculum indifferent doses. From the data presented in Table 2, it

40 50 60 70

ime, day

different inoculum addition modes: E, inoculum in equal doses;ottom; and J, no inoculum

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0

10

20

30

40

50

60

70

0 20 40 60 80 100 120

% of reactor height

VFA

, g [

CO

D]

kg−1

; an

d C

OD

dis g

kg−1

Fig. 6. Concentration profiles at the end of inoculum addition mode experiments at 50 1C: K, volatile fatty acids (VFA) at equaldose addition; J, VFA without addition; E, dissolved chemical oxygen demand (CODdis) at equal dose addition and B, CODdis at

50 1C without addition

H.M. EL-MASHAD ET AL.252

can be realised that methanogenesis is likely the ratelimiting step in the experimets without inoculum or evenadding inocuum at different doses. It can be noticed thatacetate and propionate represent the larger VFAsconstituents in all studied reactors. Other VFAs arepresent in very small portions.

3.3. Net energy production

Figure 7 shows the input energy, output energy andnet energy production from the simulated 10 m3 reactorsoperated with different inoculum addition modestogether with that operated with leachate recirculation(the present study) and without leachate recirculation(El-Mashad, 2003). The total energy input is thesummation of the total energy input during a fillingtime of 60 days. The output energy is calculated basedon the accumulated methane production during thefilling time. Higher specific net energy production can berealised at 50 1C in spite of higher energy losses to theenvironment. From the results presented in Fig. 7 andkeeping in mind the hygienic quality of the effluent, it is

clear that the operation at 50 1C is more attractivecompared with the operation at 40 1C. The experimentswith the inoculum addition modes were thereforecarried out at 50 1C only. From the data presented inFig. 7, it can be observed that the highest specific netenergy production can be achieved dividing the inocu-lum in different doses. Furthermore, specific net energyproduction with leachate recirculation is higher thanthat with dividing the inoculum in different doses. Thismay be attributed to the differences in composition ofthe substrates used in the different experiments(see Tables 1 and 2).

4. General discussion

The experiment with leachate recirculation, withlowest 10% of the reactor volume filled with inoculum,reveals a pronounced methane production comparedwith other experiments: methanogenesis is 45%. Lea-chate recirculation can therefore be considered asoperation strategy for improvement of the processperformance. As mentioned above, the amount of

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0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

Spec

ific

ene

rgy,

GJ

m−3

Leachate Leachate Noleachate

Noleachate

Inoculumequal dose

Noinoculum

50 °C 40 °C 50 °C 50 °C 50 °C 40 °C

Fig. 7. Specific energy input; output and net energy production from a 10 m3 accumulation system during a filling time of 60 days: ,input energy; , output energy; , net energy

Table 3

Calculated profile extent indices Ipe for volatile fatty acids

(VFA) and dissolved chemical oxygen demand (CODdis)

Reactor operation Ipe(VFA) Ipe(CODdis)

Inoculum addition in differentequal doses

0�44 0�65

Inoculum at the bottom 0�64 0�72No inoculum added 0�67 0�99Leachate recirculation at 40 1C 0�57 0�64Leachate recirculation at 50 1C 0�20 0�23

ANAEROBIC DIGESTION OF SOLID CATTLE MANURE 253

leachate depends on the characteristics of the substrate.In case of high solids manure, extra water can be added.Yet, such addition will affect the cost of the system, as alarger reactor volume is needed. However, such additionof extra water may help in the heating of the reactorcontent as a poor heat conductivity between layers existsat high solid concentrations, which decreases theefficiency of a heat exchanger installed at the reactorbottom (El-Mashad, 2003).Based on the experimental data, the calculated profile

extent indices Ipe for VFA and CODdis are shown inTable 3. The results show that the best operation occurswhen leachate recirculation is applied at 50 1C.Without adding inoculum a very poor system

performance is recognised when only 60 days digestiontime is applied, though the results show the possibility ofstarting up an accumulation system without inoculum atthermophilic conditions. To assure the presence of moreactive biomass in the manure, start up should bepreferably made in summer. However, such start up isnot recommended, as a long digestion time is required toachieve the same performance as compared to inocu-lated start up. The results of Zeeman (1991) indicatedthat the start up of a psychrophilic (p15 1C) accumula-tion system treating liquid manure without seed was notpossible within 5 months.The results of the experiments with inoculum addition

in different several equal doses with the influent showedbetter performance (i.e. higher MPR and lower Ipe)

compared with starting the system without inoculum oradding the inoculum at the reactor bottom. However, inpractice, the preservation of the activity of the inoculumtill the end of the filling time should be guaranteed.Moreover, a 10% extra reactor volume is required tostore the inoculum. The inoculum could be kept inanother batch digester provided that enough substrateis present.

In fact, dividing and addition of inoculum with thefeed could be considered as multiple batch reactors withdifferent digestion times: every added inoculum andmanure mixture represents one batch reactor. Toimprove the system performance, the inoculum percen-tage could be increased. However, the increase of theinoculum percentages increases the reactor volume. Indry anaerobic batch digestion of vegetable fruit andyard waste at 35 1C, Ten Brummeler (1993) found that

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H.M. EL-MASHAD ET AL.254

at an inoculum/substrate ratio of 0�55, a retention timeof 30 days is required with leachate recirculation. Moreresearch is required to study the effect of addingdifferent inoculum percentages with the feed on theaccumulation performance with and without leachaterecirculation.

The results of the present and earlier performedresearch (El-Mashad et al., 2003), illustrate that a fillingtime of 60 days is not enough for complete degradationof VFA especially in the reactor top. To improve thesystem performance, either increase of the filling time orthe use of a second reactor is possible. When using tworeactors the content of the reactor after filling time canbe kept for another 60 days of batch digestion. Anotheroption is to apply aerobic composting as a post-treatment. The last option could be attractive as thedigested manure from the accumulation system still hasa high moisture content, which causes high transportcosts. Aerobic composting could produce a stabilisedeasy-transferable useful end product (i.e. compost). TenBrummeler (1993) suggested this post-treatment step forthe effluent of the BIOCEL process after adding supportmaterial, such as wood chips and sawdust.

5. Conclusions

The effects of leachate recirculation and differentinoculum addition modes on the performance of theaccumulation system were studied for a filling time of 60days. The following conclusions can be drawn.

(1)

Leachate recirculation during fed batch digestion ofsolid manure improves the contact between biomassand substrate and therefore increases the systemperformance (i.e. methane production).

(2)

Increase of the temperature from 40 to 50 1Cincreases the leachate recirculation volume andmethane production.

(3)

An operation temperature of 50 1C can be recom-mended from the energetic and biotechnologicalpoint of view rather than the operation at 40 1C.

(4)

Addition of inoculum in different several equal doseswith the feed enhances the system performancecompared with adding the same amount of inoculumat the beginning of the filling time, at the reactorbottom.

Acknowledgements

This research was facilitated by a grant from theEgyptian ministry of high education to the first author.

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