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Impact of food industrial waste on anaerobic co-digestion of sewagesludge and pig manure

M. Murto*, L. Bjornsson, B. Mattiasson

Department of Biotechnology, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

Received 11 April 2003; revised 19 October 2003; accepted 12 November 2003

Abstract

The performance of an anaerobic digestion process is much dependent on the type and the composition of the material to be digested. The

effects on the degradation process of co-digesting different types of waste were examined in two laboratory-scale studies. In the first

investigation, sewage sludge was co-digested with industrial waste from potato processing. The co-digestion resulted in a low buffered

system and when the fraction of starch-rich waste was increased, the result was a more sensitive process, with process overload occurring at a

lower organic loading rate (OLR). In the second investigation, pig manure, slaughterhouse waste, vegetable waste and various kinds of

industrial waste were digested. This resulted in a highly buffered system as the manure contributed to high amounts of ammonia. However, it

is important to note that ammonia might be toxic to the micro-organisms. Although the conversion of volatile fatty acids was incomplete the

processes worked well with high gas yields, 0.81.0 m3 kg21 VS.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Alkalinity; Anaerobic; Biogas; Co-digestion; Manure; Sewage sludge; Slaughterhouse waste; Vegetable waste

1. Introduction

The EU countries have agreed on a directive stating that

the amount of biodegradable organic waste that is deposited

in landfills should be decreased by 65% by July 2016

(Council Directive 1999/31/EC on the landfill of waste,

1999). The Swedish goal is stricter: no biodegradable waste

should be landfilled after 2005 (SFS, 2001) and a tax of 25

Euro per ton of biodegradable material deposited in landfills

was introduced in 2000. While most of the municipalities inSweden regard incineration as the main alternative, it is also

important to investigate and improve techniques for the

biological treatment of organic waste.

Anaerobic digestion has many environmental benefits

including the production of a renewable energy carrier, the

possibility of nutrient recycling and reduction of waste

volumes (Ghosh et al., 1975; Hawkes and Hawkes, 1987;

van Lier et al., 2001). Many kinds of organic waste have

been digested anaerobically in a successful way, such as

sewage sludge, industrial waste, slaughterhouse waste, fruit

and vegetable waste, manure and agricultural biomass.

The wastes have been treated both separately and in co-

digestion processes (Callaghan et al., 2002; Claassen et al.,

1999; Gunaseelan, 1997). Our knowledge about the

anaerobic digestion process is increasing. Nevertheless,

studies are needed to investigate the effects of variations in

the input to a digester, and how the waste composition

influences the overall stability of the process.

There is a long tradition of treating sewage sludge

anaerobically at wastewater treatment plants to reduce the

volume of sludge, but the process has not been focused onoptimal biogas production. Anaerobic digesters are often

very simple in construction and the process is poorly

monitored. As a result, they are often run at a low OLR to

avoid overload. In a society where landfilling of organic

waste is prohibited or limited it would be of interest to use

the already existing biogas plants for waste treatment.

Co-digestion of suitable organic waste with municipal

sludge would provide a means of using the extra capacity of

the anaerobic digesters.

The main steps in anaerobic digestion are hydrolysis,

acidogenesis, acetogenesis and methanogenesis (Gujer and

Zehnder, 1983). Protein- and carbohydrate-degrading bac-

teria grow rapidly, and these kinds of substrates are

rapidly fermented, with a retention time of less than a day

0301-4797/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jenvman.2003.11.001

Journal of Environmental Management 70 (2004) 101107www.elsevier.com/locate/jenvman

* Corresponding author. Tel.: 46-46-2228193; fax: 46-46-2224713.

E-mail address: [email protected] (M. Murto).
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(Bryant, 1979). If the substrate is easily hydrolysed, the last

degradation step is often rate limiting since methanogens

grow more slowly than the acidogens upstream in the

degradation chain. This can give rise to negative effects in

the case of organic overload or exposure to toxic

compounds that may induce a build-up of the metabolic

intermediates, mainly volatile fatty acids (VFAs) (Rozzi,

1991). The acid-consuming methanogenic species are more

inhibited by a decrease in pH than are the acid-producing

species (Anderson and Yang, 1992). This causes further

acid accumulation and eventually leads to process failure.

The resistance to a pH-change in the digester liquid

depends on the buffering capacity, which is mainly

comprised of the bicarbonate/carbon dioxide buffer

(Rozzi, 1991). If other ions are present they also

contribute to the alkalinity. For example, when proteinsare degraded, ammonium is released forming ammonium

bicarbonate, which results in additional buffering of the

digester liquid (Gallert et al., 1998; Nyns, 1986) and

thereby gives higher resistance to organic overload.

However, the anaerobic degradation process may be

inhibited by high amounts of ammonia (Hansen et al.,

1998). The toxicity is related to temperature and the

pH-dependent concentration of free ammonia (Gallert

et al., 1998). In unadapted cultures, a free ammonia level

of 0.15 g l21 can cause growth inhibition (Braun et al.,

1981). If the culture has undergone gradual adaptation, a

level of up to 1.1 g l21 free ammonia can be tolerated

and it has been reported that the aceticlastic methanogensare most sensitive to ammonia toxicity (Hansen et al.,

1998).

In co-digestion, it is important to consider the effects of

the different incoming waste streams. Better handling and

digestibility can be achieved by mixing solid waste with

diluted waste. Furthermore, successful mixing of different

wastes results in better digestion performance by improving

the content of the nutrients and even reduces the negative

effect of toxic compounds on the digestion process. Pig and

poultry manure have high amounts of ammonia (4 g l21 as

ammonia-N). These are preferably co-digested with waste

that has high carbon content to improve the C/N ratio.Sievers and Brune (1978) have reported that the C/N ratio

should be 16/1 for optimal operation.

The number of full-scale co-digestion plants is increasing

and there are many full-scale digesters in operation

co-digesting manure and industrial organic waste (Danish

Energy Agency, 1995; Hedegaard and Jaensch, 1999).

This paper reports on two investigations: co-digestion of

sewage sludge and potato processing industrial waste, and

co-digestion of manure, slaughterhouse and agricultural

waste, both performed in laboratory-scale reactors. The aim

was to investigate how the co-digestion of the different

kinds of waste affected the conditions in and performance of

the anaerobic digestion process.

2. Materials and methods

2.1. Co-digestion of sewage sludge and potato processing

industrial waste

The model for the first study was a full-scale anaerobic

digester co-digesting sludge from wastewater treatment

with starch-rich waste from a potato processing facility. The

full-scale plant consists of two serially connected meso-

philic reactors of 3500 m3 each. The substrate for this plant

has, on average, a total solids (TS) content of 3.4%, with

85% of this being volatile solids (VS). The main volumetric

contribution to the plant is excess sludge from municipal

wastewater treatment (64% of the volumetric flow rate).

However, in terms of organic material the main constituent

is starch-rich waste from a food industry facility (72% of the

VS). The average OLR is 1.4 kg VS m23 d21 and the

hydraulic retention time (HRT) is 20 days.

2.1.1. Reactor design

In the laboratory-scale study, the experimental set-up

consisted of a jacketed glass reactor (35 8C) with a volume

of 500 ml, sealed with a rubber stopper. A magnetic stirrer

was used for mixing. The mixed substrate was fed from a

cooled vessel (4 8C) once per day into the reactor. The

amount of gas was measured according to the water

displacement principle. Four reactor set-ups were used in

parallel.

2.1.2. Inoculum and feedstocksThe inoculum for the reactors was taken from the full-

scale anaerobic digester described above. The two sludge

fractions, primary sludge and excess activated sludge, and

the starch-rich food industrial waste were collected at

Table 1

Composition of feedstocks as a percentage of the volume and organic material fed to the four reactors co-digesting sewage sludge and potato processing

industrial waste

Reference and reactor 1 Reactor 2 Reactor 3

(% vol) (% VS) (% vol) (% VS) (% vol) (% VS)

Food industrial waste 36 72 44 80 49 84

Primary sludge 11 9 9 6 8 5Excess activated sludge 53 19 47 14 43 11

M. Murto et al. / Journal of Environmental Management 70 (2004) 101107102


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the full-scale biogas plant, dispersed with a blender, and

stored at 220 8C until use.

2.1.3. Experimental procedure

During a start-up period of 40 days the reference reactor

and reactors 1 3 were run at an OLR of 1.5 kg VS m23 d21

and with a substrate composition as in the full-scale

anaerobic digester (Table 1). The substrates for the referencereactor and reactor 1 had this composition during the whole

study. The reference reactor, used as a control, was

maintained at this OLR throughout the study to verify that

the substrate remained unchanged during storage. On day 40,

the composition of the substrates for reactors 2 and 3 was

changed, in that the fraction of starch-rich sludge was

increased (Table 1). The OLR was then increased stepwise in

the reactors 13 by decreasing the HRT until failure of the

process. The reactors were maintained at each OLR for a

minimum of three HRTs.

2.2. Co-digestion of manure, slaughterhouse

and agricultural waste

The model for the second investigation was a planned

full-scale co-digestion plant. The full-scale anaerobic

digester was intended to treat around 45,000 tonnes of

organic waste per year. The base fractions constituted of

pig manure (35,000 tonnes per year) and various

industrial waste (7000 tonnes), to which it was possible

to add two other waste fractions, slaughterhouse waste

(5000 tonnes) and restaurant, fruit and vegetable waste

(2000 tonnes), to obtain a more favourable carbon/nitro-

gen ratio in the feedstocks.

2.2.1. Reactor design

The experimental set-up consisted of a cooled

substrate vessel (4 8C) and a 3-litre jacketed glass reactor

(35 8C). An impeller (200 rpm) was used for mixing and

was turned off every second hour for 15 min. The

substrate was fed into the reactor once every 4 h with a

peristaltic pump. The produced gas was collected in agas-tight bag and the volume was measured with a wet-

type precision gas meter (Schlumberger, Karlsruhe,

Germany). Three reactor set-ups (reactors A C) were

used in parallel.

2.2.2. Inoculum and feedstocks

The inoculum was taken from a full-scale anaerobic

digester in Karpalund, Kristianstad, Sweden, where manure

and slaughterhouse waste are co-digested with small

amounts of household waste and industrial waste.

The waste fractions were collected from local

industries. The composition of the mixture of industrial

was te i s given i n Tabl e 2. The three differentcombinations of feedstocks used in the experiments are

given in T able 3. The different substrates were

homogenised, mixed and stored in bottles at 220 8C

until use. The substrate was thawed, and sanitised for 1 h

at 70 8C before use to mimic the procedure at full-scale

operation.

The characteristics of the separate waste fractions are

listed in Table 4, and those of the three different substrate

mixtures in Table 5.

2.2.3. Experimental procedure

During a start-up period of 30 days, the HRT was set

at 50 days and thereafter it was decreased to around 30

days, giving OLRs of 2.6, 3.1 and 3.7 kg VS m23 d21 for

reactors A, B and C, respectively. When the HRT was

decreased reactor C became unstable and foam was

Table 3

Compositions of feedstocks co-digested in the three reactors, AC

Reactor A (% vol) B (% vol) C (% vol)

Mixture of industrial waste 17 17 17

Pig manure 83 71 66Slaughterhouse wastea 12 12

Restaurant, fruit and vegetable waste 5

a 50% sludge, 25% rumen and intestinal contents and 25% manure.

Table 2

Composition of the mixture of industrial waste fed to reactors AC

Waste fraction (wt%)

Grease trap residues 87

Confectionary waste 7

Dairy product waste 2

Bakery waste 3

Fodder/mill waste 1

Table 4

Characteristics of the waste fractions

pH Total solids

(%)

Volatile solids

(% of TS)

Total nitrogen

(% of TS)

Total carbon

(% of TS)

Phosphorus

(% of TS)

C/N ratio

Mixture of industrial waste 5.4 19 93 0.4 20 0.9 49

Pig manure 7.2 9 76 7.4 40 2.1 5

Slaughterhouse waste 5.9 13 96 1.0 60 0.3 58Restaurant, fruit and vegetable waste 4.5 21 95 3.8 49 0.4 13

M. Murto et al. / Journal of Environmental Management 70 (2004) 101107 103


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produced. The foam caused clogging in the reactor,

which was then reconstructed, allowing a larger head-

space, and restarted. The OLR had to be decreased to2.6 kg VS m23 d-1 (HRT 36 days) before the foaming

stopped, and the reactor was operated at these conditions

throughout the remaining time of the study.

2.3. Analytical methods

The partial alkalinity (PA), total alkalinity (TA), VFA

concentrations measured by high performance liquid

chromatography (HPLC), TS, VS and gas composition

were measured by the methods described in (Bjornsson

et al., 2000). Samples were centrifuged (3,000 g) for

3 min and the supernatant was used for alkalinity and VFAmeasurements. The VFA samples were acidified and stored

at 220 8C. They were then filtered (0.45 mm Minisart,

Sartorius AG, Gottingen, Germany) before analysis. The

VFAs are given only as the total VFAs (TVFAs) expressed

as g acetic acid (HAc) l21.

A number of chemical characteristics of the feedstocks

were determined by AgroLab AB, Kristianstad, Sweden,

as follows: total nitrogen, (Swedish Standard SS

028101:1-92 mod), ammonium nitrogen (KLK nr7 1950

mod), total carbon (M-1011) and phosphorus (SS

028150, IC-AES).

3. Results and discussion

3.1. Co-digestion of sewage sludge and potato processing

industrial waste

The values of the parameters monitored in the four

reactors at different levels of OLR are given in Table 6.

The gas yields, 0.6 m3 kg21 VS, for the three reactors

(13) did not change upon increasing the OLR and were

independent of substrate composition during stable diges-

tion. The correlation between gas production rate and OLRwas linear and equal for all four reactors. The highest biogas

production rate was achieved in reactor 2, 1.2 l d21 at an

OLR of 4.2 kg VS m23 d21 (data not shown). Hawkes and

Hawkes (1987) reported a gas yield of 0.6 m3 kg21 VS from

the digestion of sewage sludge and Gunaseelan (1997) a

methane yield of 0.42 CH4 m3 kg21 VS for potato waste. In

the reference reactor, the gas composition was very uniform

at 67 ^ 2% methane and 30 ^ 2% carbon dioxide. For the

other reactors, the gas composition was in the same range

until the load was increased to around 4 kg VS m23 d21,

which resulted in a decrease in methane to 64 ^ 4%. No

H2S was present in the gas, probably because the chemicals

that are used at the wastewater treatment plant to precipitatephosphorus also react with the sulphide.

The pH values were 6.8 7.0 in all reactors during stable

operation. The PA values varied in the three different

reactors (13) depending on OLRs between 1.5 2.0 g

CaCO3 l21, 1.2 2.0 g CaCO3 l

21 and 1.02.0 g

CaCO3 l21, respectively. Jenkins et al. (1991) reported

that the PA should be above 1.2 g CaCO3 l21 for stable

operation. The maximum concentration of TVFAs was

Table 5

Characteristics of the feedstocks fed to reactors, A C, co-digesting

manure, slaughterhouse and agricultural waste

Reactor A B C

Total solids (%) 9.7 10.0 10.3

Volatile solids (% of TS) 78 81 82

Total nitrogen (% of TS) 6.3 5.4 5.2

NH4N (% of TS) 4.1 3.5 3.1

Total carbon (% of TS) 53 58 54

Phosphorous (% of TS) 1.9 1.7 1.6

C/N ratio 8 11 10

Table 6

Steady-state values for the measured parameters at different organic loading rate (OLR) and hydraulic retention time (HRT) in the four reactors co-digesting

sewage sludge and potato processing industrial waste

OLR (kg VS m23

d21

) HRT (d) Gas yield (m3

kg21

VS) pH PA (g CaCO3 l21

) TA (g CaCO3 l21

) TVFA (g HAc l21

)

Reference 1.6 ^ 0.2 19.7 ^ 1.8 0.6^ 0.1 7.2 ^ 0.1 1.93^ 0.04 2.34 ^ 0.05 0

Reactor 1 1.5 ^ 0.2 19.0 ^ 2.2 0.6^ 0.2 7.2 ^ 0.1 1.96^ 0.08 2.45 ^ 0.07 0

1.9^ 0.2 13.4 ^ 1.2 0.6 ^ 0.1 7.1 ^ 0.1 1.65^ 0.05 2.00 ^ 0.04 0

3.1^ 0.2 9.3 ^ 0.7 0.6 ^ 0.1 7.1 ^ 0.1 1.57^ 0.07 1.92 ^ 0.05 0.05 ^ 0.05

4.2^ 0.3 7.1 ^ 0.5 0.6 ^ 0.1 7.0 ^ 0.1 1.48^ 0.08 1.99 ^ 0.05 0.25 ^ 0.10

5.9^ 0.7 5.3 ^ 0.6 Process overload

Reactor 2 1.6 ^ 0.2 18.0 ^ 2.1 0.6 ^ 0.1 7.2 ^ 0.1 2.00^ 0.07 2.49 ^ 0.09 0

2.7^ 0.2 12.6 ^ 1.2 0.6 ^ 0.1 7.0 ^ 0.1 1.62^ 0.09 1.94 ^ 0.04 0

4.0^ 0.2 9.1 ^ 0.4 0.6 ^ 0.1 6.9 ^ 0.1 1.18^ 0.09 1.57 ^ 0.10 0.18 ^ 0.08


Reactor 3 1.5 ^ 0.3 20.2 ^ 3.6 0.6 ^ 0.2 7.2 ^ 0.1 1.99^ 0.06 2.47 ^ 0.07 0

3.9^ 0.4 10.4 ^ 0.6 0.6 ^ 0.1 6.8 ^ 0.1 1.05^ 0.09 1.51 ^ 0.08 0.22 ^ 0.11


The values are averages of 10 consecutive measurements.



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around 0.20 g HAc l21. The measured parameters reflected

the changing conditions in the reactors as the composition of

the substrate was changed or the OLR was increased. Even

small accumulations of VFAs in the reactors resulted in the

consumption of bicarbonate and, due to the low buffering

capacity, a decrease in pH. The concentration of VFAs has

been found to be a very good indicator of the metabolic

status of an anaerobic degradation process (Bjornsson et al.,

2000; Cobb and Hill, 1991). Thus in the case of an anaerobic

digestion system with low buffering capacity, PA and pH

measurements also could be as useful for process monitor-

ing. The high standard deviations in the concentrations of

VFAs may be due to inhomogeneous substrate and

variations in the feed rate caused by inlet clogging. The

reactors were fed once daily and the time at which samples

were collected, 10 14 h after feeding, might also have

caused variations.

The change in composition of the substrate fed to the

laboratory-scale reactors and the stepwise increase in the

OLR are shown in Fig. 1ad. The final increase in OLR

caused digester failure. The maximum OLR of the three

reactors before organic overload decreased as the proportion

of carbohydrate-rich sludge increased in the substrate.

Reactor 1, which had the lowest amount of starch-rich

sludge, could be run at an OLR of 4.2 kg VS m23 d21 with a

retention time of 7 days without decreased biogas yield. The

other two reactors, with higher amounts of starch-rich

sludge, could be run at an OLR of about 4.0 kg VS m23 d21

and HRT of 9.1 days (reactor 2) and at an OLR of

3.9 kg VS m

23

d

21

and HRT 10.4 days (reactor 3) withoutdecreased biogas yield. One cause of failure of the reactors

might be hydraulic overload causing a wash-out of the

micro-organisms. Another cause could be organic overload,

where the inhibition of the micro-organisms was caused by

the accumulation of VFAs, and due to the low buffering

capacity in the digester liquid pH decreased, causing further

inhibition. The full-scale digester used as a model for these

experiments is operated at around 1.4 kg VS m23 d21. This

shows that the full-scale plant has extra capacity, which

could be utilised. Provided that the laboratory-scale results

are applicable to the full-scale system, and that the system is

run either with the two reactors in parallel or with the

material completely mixed by recirculation, the presentorganic load could be increased at least threefold.

3.2. Co-digestion of manure, slaughterhouse

and agricultural waste

OLR, HRT and steady-state values of the measured

parameters are given in Table 7.

The gas yields were 0.8, 0.9 and 1.0 m3 kg21 VS for

reactors A, B and C, respectively. The theoretical gas yields

are 1.5, 0.9 and 0.8 m3 kg21 for pure substrates of fatty

acids, protein and starch, respectively (Hawkes and

Hawkes, 1987). Reported gas yield for pig manure is

0.4 m3 kg21 VS (Hashimoto, 1983). The methane contents

in the biogas produced from the three reactors were around

70%. Hydrogen sulphide was present in the biogas in each

of the reactors: to up to 2500 ppm in reactor A and up to

1500 ppm in reactors B and C. Hydrogen sulphide was

Fig. 1. Co-digestion of sewage sludge and potato processing industrial

waste. Influence of feed composition and organic loading rate (OLR) in the

reference reactor (a) and reactors 13 (b)(d).

Table 7

Steady-state values for the measured parameters in the three reactors co-digesting manure, slaughterhouse and agricultural waste

OLR

(kg VS m23 d21)

HRT

(d)

Gas yield

(m3 kg21 VS)

CH4(%)

pH PA

(g CaCO3 l21)

TA

(g CaCO3 l21)

TVFA

(g HAc l21)

Reactor A 2.6 ^ 0.1 30 ^ 2 0.8 ^ 0.1 70.5 ^ 1.1 7.9 ^ 0.1 14.9 ^ 0.6 19.8 ^ 0.4 1.13.4

Reactor B 3.1 ^ 0.2 28 ^ 2 0.9 ^ 0.1 69.3 ^ 0.6 7.9 ^ 0.1 14.2 ^ 0.3 17.8 ^ 0.3 0.52.6

Reactor C 2.6 ^ 0.3 36 ^ 6 1.0 ^ 0.1 68.2 ^ 1.1 7.9 ^ 0.1 13.8 ^ 0.5 17.2 ^ 0.3 0.21.3

The values are averages of 12 consecutive measurements.



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produced by the degradation of proteins. This may cause

microbial inhibition, but it is also problematic due to the

strong smell and must be removed from the biogas

(Chynoweth et al., 1999).

Manure with a low C/N ratio should be co-digested with

waste containing low levels of nitrogen to give a stable

process (Bryant, 1979). This has also been shown by

Callaghan et al. (2002) where the addition of fruit and

vegetable waste to cattle slurry increased the methane yield

in the process. However, when nitrogen-rich chicken

manure was added to cattle slurry the process performance

deteriorated, and this was assumed to be due to ammonia

inhibition. In the performed experiments organic waste

with high C/N ratio was mixed with pig manure to improve

the C/N ratio, but the ratios were still low (Table 5), for the

substrates fed to reactors AC. The values of the monitoredparameters in the three reactors were similar despite the fact

that the substrate compositions were different. The pH

values were stable at 7.9 in all three reactors. The PA values

were very high in each reactor, 13.814.9 g CaCO3 l21, and

the TA values were 17.219.8 g CaCO3 l21. The digested

sludge from the three reactors contained high levels of

ammonium, 4.04.5 g NH4N l21, which at pH 7.9 means

free ammonia values of 0.48 0.54 g l21 according to

Hansen et al. (1998). In this kind of process there is a

danger that ammonia may inhibit the process. The high

ammonia concentrations contributed to the high buffering

capacities. High levels of VFAs were accumulated, up to

3.4 g HAc l21, consisting mainly of acetic acid. The highamount of VFAs in spite of a relatively low OLR and high

HRT shows that the degradation was unbalanced but that the

high buffering capacity resulted in stable pH and high gas

yield. The dominating VFA was acetic acid, indicating that

the acetate-utilising methanogens were inhibited and this

effect was likely caused by ammonia (Hansen et al., 1998).

The amount of free ammonia was high enough to disturb the

sensitive aceticlastic methanogens (Braun et al., 1981).

4. Conclusions

The feedstocks were successfully co-digested anaero-

bically. In co-digestion it is important to consider the

effect that the composition of the incoming substrate will

have on the digester performance. The first system

studied was an example of an anaerobic process with low

buffering capacity, while the other system had very high

buffering capacity. The process treating the starch-rich

waste was sensitive to changes in the feed, e.g.

increasing the OLR or varying the composition. Due to

the low buffering capacity, the accumulation of VFAs

resulted in decreasing pH and finally led to digester

failure. When considering the full-scale anaerobic

digester (CSTR) it is of great importance that thecarbohydrate-rich sludge is co-digested with sludge

from the wastewater treatment plant due to the stabilising

effect of this sludge on pH and alkalinity.

In the other case, the system was not affected by the high

amounts of VFAs and even worked well with concentrations

of several g l21 due to the production of ammonia, which

kept the pH at neutral levels. It seemed likely, however, that

the VFA accumulation was caused by the toxic effect of

ammonia on acetate-degrading methanogens.

The cause of process imbalance and failure differed

depending on whether the process was a low or high

buffered system. In the first experiment wash-out and

pH/VFA inhibition caused digester failures, and in the

second experiment ammonia caused imbalance of the

degradation process.

In the low buffered system pH,PA andVFA measurements

were useful for process monitoring whereas in the highlybufferedsystemonly VFA measurementsindicated imbalance

in the degradation process. Thus, it is important to know how

the different waste fractions influence the digestion process so

that correct monitoring parameters are measured. Further-

more, it is important to perform laboratory-scale experiments

before running in large scale to obtain information about what

effects the mixing of different waste has on the co-digestion

process.

Acknowledgements

This work was supported by the Swedish InternationalDevelopment Cooperation Agency (Sida) and the Swedish

National Energy Administration (STEM). The collaboration

with Sysav AB and the assistance of the staff at Ellinge

wastewater treatment plant are gratefully acknowledged.

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