continuous co-digestion of cattle slurry with fruit and vegetable wastes and chicken manure

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Biomass and Bioenergy 27 (2002) 71–77 Continuous co-digestion of cattle slurry with fruit and vegetable wastes and chicken manure F.J. Callaghan a , D.A.J. Wase a , K. Thayanithy a , C.F. Forster b; a School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK b School of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 27 March 2000; received in revised form 17 August 2001; accepted 12 September 2001 Abstract Anaerobic digestion is a well established process for treating many types of organic waste, both solid and liquid. As such, the digestion of cattle slurries and of a range of agricultural wastes has been evaluated and has been successful. Previous batch studies have shown that based on volatile solids (VS) reduction, total methane production and methane yield, co-digestions of cattle slurry (CS) with fruit and vegetable wastes (FVW) and with chicken manure (CM) were among the more promising combinations. A continuously stirred tank reactor (18 litres) was used as a mesophilic (35 C) anaerobic reactor to examine the eect of adding the FVW and CM to a system which was digesting CS. The retention time was kept at 21 days and the loading rate maintained in the range 3.19 –5:01 kg VS m 3 d 1 . Increasing the proportion of FVW from 20% to 50% improved the methane yield from 0.23 to 0:45 m 3 CH4 kg 1 VS added, and caused the VS reduction to decrease slightly. Increasing the proportion of chicken manure in the feed caused a steady deterioration in both the criteria for judging digester performance. This appeared to be caused by ammonia inhibition. c 2002 Elsevier Science Ltd. All rights reserved. Keywords: Solid wastes; Fruit and vegetable wastes; Chicken manure; Anaerobic digestion; Co-digestion; Performance; Inhibition; Cattle slurry 1. Introduction Organic wastes are produced by a range of indus- tries; for example, agriculture, food processing and drink manufacture; and their quantities are apprecia- ble. Dagnall [1] has reported that the waste produced by the UK livestock industry (cattle, pigs and poultry) amounts to about 34,000 tonnes of dry solids per day. Corresponding author. Tel.: +44-121-414-5069; fax: +44-121- 414-3675. E-mail address: [email protected] (C.F. Forster). Agriculture and the food processing industry also gen- erate a signicant amount of waste. In addition, do- mestic waste must be considered. In the UK, the solid household wastes generated in 1995= 96 were some 24 × 10 6 wet tonnes and it has been estimated that be- tween 20% and 45% of this type of waste is organic in nature [2]. Over the years, an array of ideas for the utilisa- tion of these wastes have been put forward. These have ranged from the chemical hydrolysis of the cel- lulose in refuse to provide a fermentation feed-stock for the manufacture of single cell protein [3] to the use of earthworms for the recycling of organic wastes 0961-9534/02/$ - see front matter c 2002 Elsevier Science Ltd. All rights reserved. PII:S0961-9534(01)00057-5

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Biomass and Bioenergy 27 (2002) 71–77

Continuous co-digestion of cattle slurry with fruit andvegetable wastes and chicken manure

F.J. Callaghana, D.A.J. Wasea, K. Thayanithya, C.F. Forsterb; ∗

aSchool of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UKbSchool of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Received 27 March 2000; received in revised form 17 August 2001; accepted 12 September 2001

Abstract

Anaerobic digestion is a well established process for treating many types of organic waste, both solid and liquid. As such,the digestion of cattle slurries and of a range of agricultural wastes has been evaluated and has been successful. Previous batchstudies have shown that based on volatile solids (VS) reduction, total methane production and methane yield, co-digestionsof cattle slurry (CS) with fruit and vegetable wastes (FVW) and with chicken manure (CM) were among the more promisingcombinations. A continuously stirred tank reactor (18 litres) was used as a mesophilic (35

◦C) anaerobic reactor to examine

the e5ect of adding the FVW and CM to a system which was digesting CS. The retention time was kept at 21 days andthe loading rate maintained in the range 3.19–5:01 kg VS m−3 d−1. Increasing the proportion of FVW from 20% to 50%improved the methane yield from 0.23 to 0:45 m3 CH4 kg−1 VS added, and caused the VS reduction to decrease slightly.Increasing the proportion of chicken manure in the feed caused a steady deterioration in both the criteria for judging digesterperformance. This appeared to be caused by ammonia inhibition. c© 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Solid wastes; Fruit and vegetable wastes; Chicken manure; Anaerobic digestion; Co-digestion; Performance;Inhibition; Cattle slurry

1. Introduction

Organic wastes are produced by a range of indus-tries; for example, agriculture, food processing anddrink manufacture; and their quantities are apprecia-ble. Dagnall [1] has reported that the waste producedby the UK livestock industry (cattle, pigs and poultry)amounts to about 34,000 tonnes of dry solids per day.

∗ Corresponding author. Tel.: +44-121-414-5069; fax: +44-121-414-3675.

E-mail address: [email protected] (C.F. Forster).

Agriculture and the food processing industry also gen-erate a signiFcant amount of waste. In addition, do-mestic waste must be considered. In the UK, the solidhousehold wastes generated in 1995=96 were some24×106 wet tonnes and it has been estimated that be-tween 20% and 45% of this type of waste is organicin nature [2].Over the years, an array of ideas for the utilisa-

tion of these wastes have been put forward. Thesehave ranged from the chemical hydrolysis of the cel-lulose in refuse to provide a fermentation feed-stockfor the manufacture of single cell protein [3] to theuse of earthworms for the recycling of organic wastes

0961-9534/02/$ - see front matter c© 2002 Elsevier Science Ltd. All rights reserved.PII: S0961 -9534(01)00057 -5

72 F.J. Callaghan et al. / Biomass and Bioenergy 27 (2002) 71–77

[4] materials. However, anaerobic digestion of organicwastes to produce energy in the form of biogas is,arguably, the most likely option to be of commercialinterest, provided that the economics were favourable.A recent review, however, has demonstrated that theuse of anaerobic digestion for the treatment of theorganic fraction of municipal solid waste would re-duce the emission of carbon dioxide [5]. Therefore,in the light of the emission reductions agreed at theKyoto Summit, environmental considerations may beof greater signiFcance than economics.Anaerobic digestion of cattle slurry (CS) has been

assessed over the last 25–30 years and is now an es-tablished waste management technique in the UK [6]and there are 18 installations around the UK success-fully processing CS. Fruit and vegetable waste (FVW)has also been evaluated as a digester feed-stockby a number of workers [7,8] with a methane pro-duction of 0:37 m3 kg−1 VS being reported [7].However, it has been suggested that the nitrogenand phosphorus in FVW can be low and this is onereason why it has also been used in co-digestionswith other wastes, for example, chicken manure (CM)[9]. Indeed, it has been suggested that CM is besttreated with other wastes because of its high nitrogencontent [10].The wide range of waste solids=slurries which

would be amenable to anaerobic biodegradation issuch that a series of centralised digestion centres re-ceiving a variety of these wastes might realistically beconsidered. Co-digestion as a process has been exam-ined for a number of waste combinations [11,12] andthe concept of a centralised facility, which co-digesteda base material, for example, CS, together with anumber of waste products, is not a new idea [1,3].What is not clear is whether some wastes would haveadverse e5ects when added to a stable digester orwere used in conjunction with another waste. Also,it is not clear how well a digestion system wouldoperate under non-steady-state conditions, which iswhat would be likely to happen with a commercialcentralised facility.Previously, a series of batch (1 l) co-digestions

were used as screening trials to determine whichwastes could best be used with CS. These showedthat CM, Fsh o5al and FVW were the most promising[13]. The results of an evaluation of the bench-scale(18 l) co-digestion of CS with FVW and CS with

CM using non-steady-state conditions are comparedin this paper.

2. Experimental

2.1. Waste sources

The FVW was collected from a group of studentvegetarians. Each item of waste was weighed beforebeing placed in the bin so that the overall compositionwas known. The bin was emptied once a week andthe contents macerated (Magimix SA, Montceau enBourgougne, France) and stored at −10◦C. Duringthe pilot-plant operation, a quantity suIcient for 1week’s operation was thawed at the beginning of eachweek. Immediately before use it was diluted to 10%total solids (w=v) to aid mixing. Its characteristics aredescribed in Tables 1 and 2.The CS was obtained from a local farm. After col-

lection, long (¿ 50 mm) straw was removed and theresidue was macerated (Magimix SA, Montceau enBourgougne, France) and stored at 4◦C. Its character-istics are described in Table 2.

Table 1Composition of the FVW

Waste fraction Percentage (w=w, wet weight)

Banana skins 7.5Broccoli stalks 5.7Brussels sprouts 17.0Grapefruit pieces 7.5Grapefruit skins 7.5Kiwi fruit skins 13.2Orange skins 13.3Potato skins 24.5Rice 3.9

Table 2Characteristics of the feed solids as sampled

Cattle slurry Chicken manure FVW

pH 7.8 7.3 4.2Total solids (g l−1) 100–137 300–450 167Volatile solids (g l−1) 70–107 150–220 156Ammoniacal-nitrogen 1040–1925 7000–12,800 ¡ 10(mg kg−1)

F.J. Callaghan et al. / Biomass and Bioenergy 27 (2002) 71–77 73

The CM was from laying hens and had a total solids(droppings, feathers, broken eggs) content of 27.2%(Table 2), which would make it unsuitable for diges-tion as it is diIcult to mix systems with solids levelsof above 10% by conventional methods. However, asit is envisaged that co-digestion would involve addingslurried CM as only a fraction of the total feed to adigester, the other fraction being CS at 8–10% totalsolids, slurried CMwith solids levels greater than 10%could be used without pushing the overall feed solidsconcentration over 10%. The manure was, therefore,diluted with water to 15% total solids (w=v). Becauseof the variability in the composition of the wastes,particularly the CS and the CM, the solids’ concen-trations of each daily feed were measured to ascertainthe exact amount being added to the digester.

2.2. Digesters

The digester has been described previously [14].Essentially, it was constructed from a QVF glass cylin-der (300 mm×300 mm ID; wall thickness 10 mm)Ftted with baMes, a six-bladed pitch-blade impeller(150 mm diameter) mounted 75 mm above the baseof the tank and epoxy-painted mild steel end plates(12 mm). PTFE O-rings (QVF) and silicone sealantwere used to e5ect a gas and water-tight seal. Wasteswere added and withdrawn through 50 mm ABS ballvalves (Capper PC, Birmingham). The working vol-ume of the digester was 18 l with a headspace volumeof 3:2 l. The biogas was collected by the downwarddisplacement of acidiFed water (0:05 M H2SO4) andits volume was measured at STP. The temperaturewas maintained at 35◦C (±0:5◦C) by an externalwater jacket.Initially, two digesters were operated with a feed-

stock of CS (7.6% volatile solids), a loading rate of

3:62 kg VS m−3 d−1 and a hydraulic retention time of21 days. The choice of this value for the retention timewas based on the results reported for the digestion ofvegetable wastes [15]. This start-up phase lasted for4 months. The operational regimes for the subsequentco-digestion trials are given in Table 3. The ratiosused, which were quite arbitrary, were based on wetweights. The trials were not run for the 3–4 hydraulicretention times needed for steady state. Rather, theywere run for 28 days.

2.3. Analytical methods

Total and volatile solids and pH were measuredby the techniques described in standard methods[16]. Ammoniacal nitrogen (NH3 plus NH+

4 ) wasmeasured with a speciFc ion electrode (Hach ModelHH=45400-00, Camlab Ltd.). The free ammonia con-centrations (i.e. unionised NH3) are a function of thetotal ammoniacal-nitrogen concentration, the pH andthe dissociation constant and formulae for the calcu-lation of free ammonia concentrations are availablein the literature [17,18]. In this study, they were cal-culated using the formula provided by Abeling [17].Alkalinity was measured by titration to pH 4.5 with0:05 M H2SO4. Methane and carbon dioxide con-centrations in the biogas were measured with a PyeUnicam series 104 gas chromatograph Ftted with aPorapak Q packed column (3 mm ID and mesh size80–100) and a thermal conductivity detector (TCD).Helium was used as a carrier gas, at a Pow rate of40 ml min−1. Volatile fatty acids (VFA) were mea-sured by the distillation method followed by titrationwith 0:1 M NaOH with a phenolphthalein indicator.All statistical analyses were done with the Analysis

ToolPak in Microsoft Excel 97.

Table 3Organic loading rates (OLR) for the di5erent feed regimes

CS:FVW (wet weight) OLR (kg VS m−3d−1) CS:CM (wet weight) OLR (kg VS m−3d−1)

100 : 0 3:62± 0:15 100 : 0 3:19± 0:1480 : 20 4:22± 0:10 70 : 30 3:83± 0:1970 : 30 4:52± 0:11 50 : 50 3:97± 0:2660 : 40 5:22± 0:10 25 : 75 4:44± 0:2150 : 50 5:01± 0:07 10 : 90 4:75± 0:42

74 F.J. Callaghan et al. / Biomass and Bioenergy 27 (2002) 71–77

3. Results and discussion

After start-up was achieved with CS as the feed,a co-digestion was started with a feed of CS andCM. The component ratio was 70% CS : 30% CM(Table 3). Two digesters were used so that a compar-ison could be made between the duplicated systems.The performance of the two digesters was very com-parable, as can be seen from Fig. 1 which shows themethane yields during this phase. This comparabilityled to the decision to operate the digesters as separatesystems during the remainder of the study.As can be seen from Table 3, the organic load-

ing rate (OLR) altered as di5erent proportions ofco-digestate were used in the feed. The methaneyields (m3 CH4 kg

−1 VS added) achieved with thedi5erent feeds varied as the OLR increased (Fig. 2).The data in Fig. 2 are shown as mean values whichhave standard deviations of, typically, 5–7%. Theregression equations, which are clearly di5erent,have correlation coeIcients which are signiFcant at

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 2 4 6 8

TIME (weeks)

ME

TH

AN

E Y

IELD

(m

3kg

-1 V

S a

dded

)

Fig. 1. A comparison of the methane yields obtained by the twodigesters treating a feed of CS (70%) and CM (30%).

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 1 2 3 4 5 6

ORGANIC LOADING RATE (kg VSm-3d-1)M

ET

HA

NE

YIE

LD (

m3 kg

-1 V

S a

dded

)

Fig. 2. The e5ect of organic loading rate on the methane yield ofco-digestions showing the measured mean data points, the standarddeviations (n=6) and the regression lines for CM (- - - - - ) andfruit and vegetable waste (——).

the 90% level, showing that there is a clear di5erencebetween the CM and the FVW-based digestions. Thelatter gave increased yields as the OLR increased.The former gave the opposite, implying that as theproportion of CM was increased, corresponding tothe increased OLR, some inhibition occurred. As isshown by a review of the production of methanefrom biomass [19] the methane yields from fruitand vegetable residues which have been reportedpreviously, are variable, depending on the carbohy-drate:lipid:protein balance in the waste. The reportedrange is from 0.11 to 0:42 m3 kg−1 VS added. Mostof these results come from the digestion of singlewastes. However, Viswanath et al. [7] have reporteddata for the digestion of a mixture of fruit wastes atan OLR of 3:8 kg VS m3 d−1 and a retention timeof 20 days, conditions very similar to those in thiscurrent study. The methane yield they obtained was0:37 m3 kg−1 VS added. The results presented inFig. 2 for the FVW co-digestions are, therefore,comparable with these earlier results.The data for the CM co-digestions may also be

compared with earlier work. Webb and Hawkes [20]examined two organic loading rates for the digestion

F.J. Callaghan et al. / Biomass and Bioenergy 27 (2002) 71–77 75

0

10

20

30

40

50

60

0 20 40 60 80 100

CATTLE SLURRY IN FEED (% w/w)

VO

LAT

ILE

SO

LID

S R

ED

UC

TIO

N (

%)

Fig. 3. E5ect of adding CM (- - - - - -) and fruit and vegetablewaste (——) on the volatile solids reduction (standard deviationsbased on n=6).

of poultry manure alone and showed that, atthe higher rate, the speciFc gas yield was lower(0:245 m3 biogas kg−1 VS added compared to 0.372).Bujoczek et al. [21] have also reported that, with CM,the eIciency with which organic matter was con-verted to methane decreased as the organic loadingwas increased.The suggestion that inhibition is occurring is sup-

ported by the data in Fig. 3, which shows that the meanreduction in VS altered as the amount of CS in the feedwas reduced. These mean values have a typical stan-dard deviation of 8%. For the co-digestion of FVWand CS, with the exception of the 70% CS mixture,there was no signiFcant change in the mean reductionin VS (ANOVA, p¿ 0:05). The mixtures containingCM showed a di5erent pattern of behaviour. Therewas no signiFcant di5erence between the reductionsin VS in mixtures containing 10%, 25% and 50% CS.However, the 70% and 100% CS mixtures gave reduc-tions in VS which were signiFcantly higher (ANOVA,p¡ 0:05). A comparison of the two 50% mixturesalso showed a clear di5erence between the behaviourof the two co-digestates (ANOVA, p¡ 0:05).

Overall, the anaerobic digestion process can be in-hibited at low pH values. The inhibition of acetate andpropionate degradation by propionate (substrate inhi-bition) is also a recognised phenomenon [22,23]. Theconcentrations of total VFAs produced by the digestersare given in Table 4. They show that the lowest propor-tions of co-digestate, 30% CM and 20% FVW, causedthe VFA concentrations to increase only slightlycompared with the mono-digestion of CS. The higherproportions produced signiFcantly higher concentra-tions of VFAs. However, as individual acid concen-trations were not measured, it is not possible to judgewhether substrate or product inhibition was occurring.The pH of the CM-based digestions did not show anyappreciable variation, staying in the range 7.8–8.0.The digestions based on FVW did show a slight vari-ation, with the pH decreasing from a value of 7.7when the CS was being digested alone to one of 7.2with the 50 : 50 feedstock. This implies that “souring”of the digesters was not occurring.One of the criteria for judging digester stability is

the VFA:alkalinity ratio. There are three critical valuesfor this [24,25].

¡ 0:4 digester should be stable;0:4–0:8 some instability will occur;¿ 0:8 signiFcant instability:

When CM was being added to the feed, theVFA:alkalinity ratio did not rise above the criti-cal value of 0.4, although when 50% or more wasused, the ratio did start to approach this value. TheFVW-based digestions also produced increases in theVFA:alkalinity ratio as the proportion of FVW wasincreased and with proportions of 30% or more, theratio was in the 0.4–0.8 range, implying that despitethe results for the methane yield and VS reduction,there was the potential for instability. Generally,FVW is thought of as being highly degradable [19],but it is essential that there is an adequate alkalinity[26]. The work by Lane suggested that, for a balanceddigestion of FVW, the alkalinity should not be lessthan 1500 mg l−1 and that the VFA:alkalinity ratioshould be less than 0.7 [20]. Throughout the studyusing FSW, the alkalinity was ¿ 10; 000 mg l−1.Free (unionised) ammonia can also a5ect digester

stability, although knowledge of how ammonia toxic-ity occurs is limited [18]. Work with pure cultures hassuggested that ammonia can act in two possible ways,

76 F.J. Callaghan et al. / Biomass and Bioenergy 27 (2002) 71–77

Table 4Volatile fatty acids generated during the di5erent co-digestions

Chicken manure Volatile fatty acids FVW Volatile fatty acids(mg l−1) (mg l−1)

% OLR % OLR

0 3.19 2192± 342 0 3.62 2202± 35730 3.83 2723± 380 20 4.22 2752± 22950 3.97 7990± 625 30 4.52 7458± 111875 4.44 9272± 154 40 5.22 5320± 81390 4.75 6369± 598 50 5.01 7994± 913

by inhibiting the enzyme which synthesizes methaneor by di5using into the cells and causing a protonimbalance [18].Webb and Hawkes [20] have suggested that

a concentration of 138 mg l−1 will cause inhibi-tion and de Baere et al. [27] have quoted the in-hibitory range as being 80–100 mg l−1. Workingwith acetoclastic methanogens, Poggi-Varaldo etal. [28] have demonstrated that their growth ratesare very sensitive to the concentrations of freeammonia below about 100 mg l−1. When the CS wasdigested alone, the free ammonia concentrations werebetween 40 and 85 mg l−1. The concentrations of freeammonia which were measured when co-digestionwas taking place, again showed a signiFcant dif-ference between the two systems. When FVW wasused, the free ammonia concentrations were less than100 mg l−1, suggesting that free ammonia was notinvolved in causing instability in the digesters. WhenCM was present in the feed, the concentrations offree ammonia were always ¿ 100 mg l−1, implyingthat this was the cause of the inhibition.

4. Conclusions

When fruit and vegetable waste was co-digestedwith cattle slurry with the feed containing 30% ormoreFVW, high concentrations of volatile fatty acids wereproduced. Despite this, mixtures of CS and FVW, withproportions of FVW of up to 50% in the feed, gavea good co-digestation in terms of methane yield, butthe VS reduction did decrease slightly.Chicken manure was not as successful as a

co-digestate. As the amount of CM in the feed and

the organic loading was increased, the VS reduc-tion deteriorated and the methane yield decreased.This appeared to be due to the concentrations of freeammonia present in the liquors.

Acknowledgements

This work was supported by the Biotechnology andBiological Science Research Council and their Fnan-cial support is gratefully acknowledged.

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