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

Biomass and Bioenergy 27 (2002) 71–77

Continuous co-digestion of cattle slurry with fruit and vegetable wastes and chicken manure F.J. Callaghana , D.A.J. Wasea , K. Thayanithya , C.F. Forsterb; ∗ a School

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

b School

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 e5ect 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 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 digester c 2002 Elsevier Science Ltd. All rights reserved. performance. This appeared to be caused by ammonia inhibition.
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 industries; for example, agriculture, food processing and drink manufacture; and their quantities are appreciable. 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: +44-121414-3675. E-mail address: [email protected] (C.F. Forster).

Agriculture and the food processing industry also generate a signiFcant amount of waste. In addition, domestic waste must be considered. In the UK, the solid household wastes generated in 1995=96 were some 24 × 106 wet tonnes and it has been estimated that between 20% and 45% of this type of waste is organic in nature [2]. Over the years, an array of ideas for the utilisation of these wastes have been put forward. These have ranged from the chemical hydrolysis of the cellulose 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

c 2002 Elsevier Science Ltd. All rights reserved. 0961-9534/02/$ - see front matter  PII: S 0 9 6 1 - 9 5 3 4 ( 0 1 ) 0 0 0 5 7 - 5

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[4] materials. However, anaerobic digestion of organic wastes to produce energy in the form of biogas is, arguably, the most likely option to be of commercial interest, provided that the economics were favourable. A recent review, however, has demonstrated that the use of anaerobic digestion for the treatment of the organic fraction of municipal solid waste would reduce the emission of carbon dioxide [5]. Therefore, in the light of the emission reductions agreed at the Kyoto Summit, environmental considerations may be of greater signiFcance than economics. Anaerobic digestion of cattle slurry (CS) has been assessed over the last 25 –30 years and is now an established waste management technique in the UK [6] and there are 18 installations around the UK successfully processing CS. Fruit and vegetable waste (FVW) has also been evaluated as a digester feed-stock by a number of workers [7,8] with a methane production of 0:37 m3 kg−1 VS being reported [7]. However, it has been suggested that the nitrogen and phosphorus in FVW can be low and this is one reason why it has also been used in co-digestions with other wastes, for example, chicken manure (CM) [9]. Indeed, it has been suggested that CM is best treated with other wastes because of its high nitrogen content [10]. The wide range of waste solids=slurries which would be amenable to anaerobic biodegradation is such that a series of centralised digestion centres receiving a variety of these wastes might realistically be considered. Co-digestion as a process has been examined for a number of waste combinations [11,12] and the concept of a centralised facility, which co-digested a base material, for example, CS, together with a number of waste products, is not a new idea [1,3]. What is not clear is whether some wastes would have adverse e5ects when added to a stable digester or were used in conjunction with another waste. Also, it is not clear how well a digestion system would operate under non-steady-state conditions, which is what would be likely to happen with a commercial centralised facility. Previously, a series of batch (1 l) co-digestions were used as screening trials to determine which wastes could best be used with CS. These showed that 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 compared in this paper. 2. Experimental 2.1. Waste sources The FVW was collected from a group of student vegetarians. Each item of waste was weighed before being placed in the bin so that the overall composition was known. The bin was emptied once a week and the contents macerated (Magimix SA, Montceau en ◦ Bourgougne, France) and stored at −10 C. During the pilot-plant operation, a quantity suIcient for 1 week’s operation was thawed at the beginning of each week. Immediately before use it was diluted to 10% total solids (w=v) to aid mixing. Its characteristics are described in Tables 1 and 2. The CS was obtained from a local farm. After collection, long (¿ 50 mm) straw was removed and the residue was macerated (Magimix SA, Montceau en ◦ Bourgougne, France) and stored at 4 C. Its characteristics are described in Table 2. Table 1 Composition of the FVW Waste fraction

Percentage (w=w, wet weight)

Banana skins Broccoli stalks Brussels sprouts Grapefruit pieces Grapefruit skins Kiwi fruit skins Orange skins Potato skins Rice

7.5 5.7 17.0 7.5 7.5 13.2 13.3 24.5 3.9

Table 2 Characteristics of the feed solids as sampled

pH Total solids (g l−1 ) Volatile solids (g l−1 ) Ammoniacal-nitrogen (mg kg−1 )

Cattle slurry

Chicken manure

FVW

7.8 100 –137 70 –107 1040 –1925

7.3 300 – 450 150 –220 7000 –12,800

4.2 167 156 ¡ 10

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

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 digestion as it is diIcult to mix systems with solids levels of above 10% by conventional methods. However, as it is envisaged that co-digestion would involve adding slurried CM as only a fraction of the total feed to a digester, the other fraction being CS at 8–10% total solids, slurried CM with solids levels greater than 10% could be used without pushing the overall feed solids concentration over 10%. The manure was, therefore, diluted with water to 15% total solids (w=v). Because of the variability in the composition of the wastes, particularly the CS and the CM, the solids’ concentrations of each daily feed were measured to ascertain the 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 cylinder (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 base of the tank and epoxy-painted mild steel end plates (12 mm). PTFE O-rings (QVF) and silicone sealant were used to e5ect a gas and water-tight seal. Wastes were added and withdrawn through 50 mm ABS ball valves (Capper PC, Birmingham). The working volume of the digester was 18 l with a headspace volume of 3:2 l. The biogas was collected by the downward displacement of acidiFed water (0:05 M H2 SO4 ) and its volume was measured at STP. The temperature ◦ ◦ was maintained at 35 C (±0:5 C) by an external water jacket. Initially, two digesters were operated with a feedstock of CS (7.6% volatile solids), a loading rate of

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3:62 kg VS m−3 d −1 and a hydraulic retention time of 21 days. The choice of this value for the retention time was based on the results reported for the digestion of vegetable wastes [15]. This start-up phase lasted for 4 months. The operational regimes for the subsequent co-digestion trials are given in Table 3. The ratios used, which were quite arbitrary, were based on wet weights. The trials were not run for the 3– 4 hydraulic retention times needed for steady state. Rather, they were run for 28 days.

2.3. Analytical methods Total and volatile solids and pH were measured by the techniques described in standard methods [16]. Ammoniacal nitrogen (NH3 plus NH+ 4 ) was measured with a speciFc ion electrode (Hach Model HH=45400-00, Camlab Ltd.). The free ammonia concentrations (i.e. unionised NH3 ) are a function of the total ammoniacal-nitrogen concentration, the pH and the dissociation constant and formulae for the calculation of free ammonia concentrations are available in the literature [17,18]. In this study, they were calculated using the formula provided by Abeling [17]. Alkalinity was measured by titration to pH 4.5 with 0:05 M H2 SO4 . Methane and carbon dioxide concentrations in the biogas were measured with a Pye Unicam series 104 gas chromatograph Ftted with a Porapak Q packed column (3 mm ID and mesh size 80 –100) and a thermal conductivity detector (TCD). Helium was used as a carrier gas, at a Pow rate of 40 ml min−1 . Volatile fatty acids (VFA) were measured by the distillation method followed by titration with 0:1 M NaOH with a phenolphthalein indicator. All statistical analyses were done with the Analysis ToolPak in Microsoft Excel 97.

Table 3 Organic loading rates (OLR) for the di5erent feed regimes CS:FVW (wet weight)

OLR (kg VS m−3 d −1 )

CS:CM (wet weight)

OLR (kg VS m−3 d −1 )

100 : 0 80 : 20 70 : 30 60 : 40 50 : 50

3:62 ± 0:15 4:22 ± 0:10 4:52 ± 0:11 5:22 ± 0:10 5:01 ± 0:07

100 : 0 70 : 30 50 : 50 25 : 75 10 : 90

3:19 ± 0:14 3:83 ± 0:19 3:97 ± 0:26 4:44 ± 0:21 4:75 ± 0:42

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

3. Results and discussion

0.5

METHANE YIELD (m3 kg-1 VS added)

0.14

0.12

0.1

0.08

0.06

0.04

0.02

0 0

2

4

6

8

TIME (weeks)

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

0.45

METHANE YIELD (m3kg-1 VS added)

After start-up was achieved with CS as the feed, a co-digestion was started with a feed of CS and CM. The component ratio was 70% CS : 30% CM (Table 3). Two digesters were used so that a comparison could be made between the duplicated systems. The performance of the two digesters was very comparable, as can be seen from Fig. 1 which shows the methane yields during this phase. This comparability led to the decision to operate the digesters as separate systems during the remainder of the study. As can be seen from Table 3, the organic loading rate (OLR) altered as di5erent proportions of co-digestate were used in the feed. The methane yields (m3 CH4 kg−1 VS added) achieved with the di5erent feeds varied as the OLR increased (Fig. 2). The data in Fig. 2 are shown as mean values which have standard deviations of, typically, 5 –7%. The regression equations, which are clearly di5erent, have correlation coeIcients which are signiFcant at

0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0

1

2

3

4

5

6

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

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

the 90% level, showing that there is a clear di5erence between the CM and the FVW-based digestions. The latter gave increased yields as the OLR increased. The former gave the opposite, implying that as the proportion of CM was increased, corresponding to the increased OLR, some inhibition occurred. As is shown by a review of the production of methane from biomass [19] the methane yields from fruit and vegetable residues which have been reported previously, are variable, depending on the carbohydrate:lipid:protein balance in the waste. The reported range is from 0.11 to 0:42 m3 kg−1 VS added. Most of these results come from the digestion of single wastes. However, Viswanath et al. [7] have reported data for the digestion of a mixture of fruit wastes at an OLR of 3:8 kg VS m3 d −1 and a retention time of 20 days, conditions very similar to those in this current study. The methane yield they obtained was 0:37 m3 kg−1 VS added. The results presented in Fig. 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

VOLATILE SOLIDS REDUCTION (%)

60

50

40

30

20

10

0 0

20

40

60

80

100

CATTLE SLURRY IN FEED (% w/w)

Fig. 3. E5ect of adding CM (- - - - - -) and fruit and vegetable waste (——) on the volatile solids reduction (standard deviations based on n = 6).

of poultry manure alone and showed that, at the 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 converted to methane decreased as the organic loading was increased. The suggestion that inhibition is occurring is supported by the data in Fig. 3, which shows that the mean reduction in VS altered as the amount of CS in the feed was reduced. These mean values have a typical standard deviation of 8%. For the co-digestion of FVW and CS, with the exception of the 70% CS mixture, there was no signiFcant change in the mean reduction in VS (ANOVA, p ¿ 0:05). The mixtures containing CM showed a di5erent pattern of behaviour. There was no signiFcant di5erence between the reductions in VS in mixtures containing 10%, 25% and 50% CS. However, the 70% and 100% CS mixtures gave reductions in VS which were signiFcantly higher (ANOVA, p ¡ 0:05). A comparison of the two 50% mixtures also showed a clear di5erence between the behaviour of the two co-digestates (ANOVA, p ¡ 0:05).

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Overall, the anaerobic digestion process can be inhibited at low pH values. The inhibition of acetate and propionate degradation by propionate (substrate inhibition) is also a recognised phenomenon [22,23]. The concentrations of total VFAs produced by the digesters are given in Table 4. They show that the lowest proportions of co-digestate, 30% CM and 20% FVW, caused the VFA concentrations to increase only slightly compared with the mono-digestion of CS. The higher proportions produced signiFcantly higher concentrations of VFAs. However, as individual acid concentrations were not measured, it is not possible to judge whether substrate or product inhibition was occurring. The pH of the CM-based digestions did not show any appreciable variation, staying in the range 7.8–8.0. The digestions based on FVW did show a slight variation, with the pH decreasing from a value of 7.7 when the CS was being digested alone to one of 7.2 with 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 values for this [24,25]. ¡ 0:4 0:4–0:8 ¿ 0:8

digester should be stable; some instability will occur; signiFcant instability:

When CM was being added to the feed, the VFA:alkalinity ratio did not rise above the critical value of 0.4, although when 50% or more was used, the ratio did start to approach this value. The FVW-based digestions also produced increases in the VFA:alkalinity ratio as the proportion of FVW was increased and with proportions of 30% or more, the ratio was in the 0.4 – 0.8 range, implying that despite the 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 balanced digestion of FVW, the alkalinity should not be less than 1500 mg l−1 and that the VFA:alkalinity ratio should be less than 0.7 [20]. Throughout the study using FSW, the alkalinity was ¿ 10; 000 mg l−1 . Free (unionised) ammonia can also a5ect digester stability, although knowledge of how ammonia toxicity occurs is limited [18]. Work with pure cultures has suggested that ammonia can act in two possible ways,

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

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

Volatile fatty acids (mg l−1 )

%

OLR

0 30 50 75 90

3.19 3.83 3.97 4.44 4.75

2192 ± 342 2723 ± 380 7990 ± 625 9272 ± 154 6369 ± 598

by inhibiting the enzyme which synthesizes methane or by di5using into the cells and causing a proton imbalance [18]. Webb and Hawkes [20] have suggested that a concentration of 138 mg l−1 will cause inhibition and de Baere et al. [27] have quoted the inhibitory range as being 80 –100 mg l−1 . Working with acetoclastic methanogens, Poggi-Varaldo et al. [28] have demonstrated that their growth rates are very sensitive to the concentrations of free ammonia below about 100 mg l−1 . When the CS was digested alone, the free ammonia concentrations were between 40 and 85 mg l−1 . The concentrations of free ammonia which were measured when co-digestion was taking place, again showed a signiFcant difference between the two systems. When FVW was used, the free ammonia concentrations were less than 100 mg l−1 , suggesting that free ammonia was not involved in causing instability in the digesters. When CM was present in the feed, the concentrations of free ammonia were always ¿ 100 mg l−1 , implying that this was the cause of the inhibition.

4. Conclusions When fruit and vegetable waste was co-digested with cattle slurry with the feed containing 30% or more FVW, high concentrations of volatile fatty acids were produced. Despite this, mixtures of CS and FVW, with proportions of FVW of up to 50% in the feed, gave a good co-digestation in terms of methane yield, but the VS reduction did decrease slightly. Chicken manure was not as successful as a co-digestate. As the amount of CM in the feed and

FVW

Volatile fatty acids (mg l−1 )

%

OLR

0 20 30 40 50

3.62 4.22 4.52 5.22 5.01

2202 ± 357 2752 ± 229 7458 ± 1118 5320 ± 813 7994 ± 913

the organic loading was increased, the VS reduction deteriorated and the methane yield decreased. This appeared to be due to the concentrations of free ammonia present in the liquors. Acknowledgements This work was supported by the Biotechnology and Biological Science Research Council and their Fnancial support is gratefully acknowledged. References [1] Dagnall S. UK strategy for centralised anaerobic digestion. Bioresource Technology 1995;52:275–80. [2] Ahring BK, Johansen K. Anaerobic digestion of source-sorted household solid waste together with manure and organic industrial waste. Proceedings of the International Symposium on Anaerobic Digestion of Solid Waste, April 14 –17, Venice, Italy, 1992. p. 203–8. [3] Forster CF, Jones JC. The Bioplex concept. In: Birch GG, Parker KJ, Worgan JT, editors. Food from waste. London: Applied Science Publishers Ltd., 1976, p. 278–89. [4] Sharma N. Recycling of organic wastes through earthworms: an alternative source of organic fertiliser for crop growth in India. Energy Conserv Management 1994;35:25–50. [5] Mata-Alvarez J, MacUe S, LlabrUes P. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresource Technology 2000;74:3–16. [6] Wase DAJ, Thayanithy K. Biogas production. In: Dewi IA, editor. Pollution control in livestock production systems. UK: CAB International Ltd., 1992. [7] Viswanath P, Devi SS, Nand K. Anaerobic digestion of fruit and vegetable processing wastes for biogas production. Bioresource Technology 1992;40:43–8. [8] Viturtia AM, Mata-Alvarez J, Cecchi F, Fazzini G. Two-phase anaerobic digestion of a mixture of fruit and vegetable wastes. Biological Wastes 1989;29:189–99.

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