attempted by Kostenberg and Marchain (1993). The aim of their study was to evaluate the potential of the di- gested material as a growth medium for horticulture after thermophilic anaerobic digestion. These authors reported some problems in the experiments due to the high level of solids and to the high percentage of fiber. In spite of the experimental problems, biogas produc- tion with a composition of 70% CH4 and relatively sta- ble volatile fatty acids (VFA) concentrations was achieved.

All of the studies mentioned above are reported with regards to coffee waste, but most instant coffee substitutes are produced from a blend of barley, rye, malted barley, chicory and coffee. Due to the different raw matter used to produce the different substitutes, the waste composition changes sequentially, making it important to evaluate their individual performances as far as the anaerobic digestion process is concerned. Therefore, the aim of this work is to study the anaer- obic biomethanation process of five wastes from in- stant coffee substitute production under mesophilic conditions, co-digested with the excess of activated sludge from a wastewater treatment plant located in the same factory.

2. Materials and methods

2.1. Waste source

Five ‘‘coffee’’ wastes from instant coffee substitute production were obtained from the Nestle ́ factory in Avanca, Portugal. About 40 ton/d (dry matter between 13% and 22%) of waste are, on average, produced in this factory. A wastewater treatment plant is installed in the same factory, producing an excess of activated sludge of about 3.9 ton/d with a dry matter content of 22%. Table 1 shows the composition of the 5 wastes, from W1 to W5. All of the different wastes presented pH values be- tween 4.5 and 5.0, and the fiber content may be up to 45% (dry weight).

The characterization of total solids (TS), volatile sol- ids (VS) and chemical oxygen demand (COD) of each waste, and of the diluted sludge used in the assays (S), is presented in Table 2.

Table 1
Composition of the insoluble matter of the five wastes studied

Table 2
Characterization of each type of waste in TS, VS and COD

L. Neves et al. / Waste Management 26 (2006) 176–181 177

Waste #

W1
W2
W3
W4
W5
S 7±1 6±1 6±1

2.2. Inoculum

The granular sludge was collected from an UASB (upflow anaerobic sludge blanket) reactor treating a brewery effluent located in Oporto, Portugal. The pro- duction of methane due to the residual substrate present in the inoculum was 20 ± 1 ml CH4/g VSsludge. The quantification of the residual methane production was performed using a pressure transducer technique (Coll- eran et al., 1992). The test involves monitoring of the pressure increase developed in sealed vials without sub- strate. Strict anaerobic conditions were maintained, using an anaerobic basal medium composed of cys- teine-HCl (0.5 g/l), NaHCO3 (3 g/l), with the pH ad- justed to 7.0–7.2. Rezasurin was added as an indicator of redox potential. The hand held pressure transducer

TS
(g/kg waste)

VS
(g/kg waste)

COD
(g/kg waste)

131±4 217±5 214±2 144±8 139±11

127±4 215±5 208±2 141±8 136±11

111±4 208±9 123±1 130±6 109±9

Table 3
Methane yield, % of methanation, % reduction of TS and VS of the different coffee wastes in the batch assays

Assay #

SW1 SW2 SW3 SW4 SW5

Methane yield Methanation (m3 CH4(STP)/ (%)
kg VSinitial)
0.24 76

0.28 85 0.02 10 0.25 75 0.25 89

Reduction Reduction

Waste # Coffee Barley (%) (%)

W1 0 40 W2 45 32 W30100

4. W4  20 45
5. W5  20 45

Rye Malted barley (%) (%)

5 30 0 0 00 0 0 0 0

Chicory (%)

25 23 0 35 35

of TS (%)

of VS (%)

73 78 67 80 31 40 50 79 54 75

ml CH4

178 L. Neves et al. / Waste Management 26 (2006) 176–181

used was capable of measuring a pressure increase or de- crease of two atmospheres (0 to ±202.6 kPa) over a range of 􏰀200 to +200 mV. The sensing element was connected to a digital panel module and the device was powered by a 9.0 V DC transformer. The tests for the quantification of residual methane were performed in 25-ml vials, in triplicate. The volume of methane pro- duced was corrected to the standard temperature and pressure conditions (STP).

2.3. Batch experiments

2.3.1. Methane production assays

The methane production assays were performed in 160-ml vials, in duplicate. A constant ratio of 7g TScoffee waste/g TSsludge was kept in the assays, which re- flect the relative daily production of the two waste streams. In each assay, the ratio substrate/inoculum was kept constant at 2.3 g TSsubstrate/g TSinoculum. The pH was corrected to 7, and 0.75 g NaHCO3/g TS was added to give suitable alkalinity. The vials were then incubated at 37°C under stirring conditions (150rpm) and the pressure increase was monitored

using the above mentioned pressure transducer device. At regular time intervals, the vials were depressurised and the biogas composition was analyzed for CH4 and CO2 content. The batch assays had a total solid content in the range 6–9%. The volume of methane produced was corrected to standard temperature and pressure conditions. The results from the biomethana- tion process were expressed in terms of methane yield (m3 CH4/kg VSinitial) and in terms of percent methana- tion that corresponds to the percentage of methane produced relative to the biochemical methane potential (350 l CH4/kg COD).

2.3.2. Liquid composition assays

Parallel assays, with 500-ml working volume, were set up to assess the liquid composition in terms of sol- uble COD and VFA (acetate, propionate, iso-butyrate, n-butyrate and valerate).

2.4. Analytical methods

COD, TS and VS were determined according to Standard Methods (APHA, AWWA, WPCF, 1989).

COD (mg /l)

COD (mg /l) COD (mg /l)

COD (mg /l) COD (mg /l)

Methane and carbon dioxide content of the biogas was measured by gas chromatography using a Porapack Q (180 to 100 Mesh) column, with He as the carrier gas at 30 ml/min and a thermal conductivity detector. Tem- peratures of the detector, injector and oven were 110, 110 and 35 °C, respectively. VFA were determined by high-performance liquid chromatography using a chrompack column (300 · 6.5 mm) and a mobile phase of sulphuric acid 5 mM at 0.7 ml/min. The column was set at 60 °C and the detection was by spectropho- tometry at 220 nm.

3. Results and discussion

Fig. 1 shows the methane production curves obtained for the different assays.

Table 3 shows the methane yield, the percentage of methanation, and the reduction of TS and VS obtained in each assay, after the correction of the methane pro- duction due to the residual substrate present in the inoc- ulum (blank assays).

Among the different wastes, the assay SW2 showed the highest methane yield, 0.28 m3/kg VSinitial, which agrees with the higher VS reduction (80%) and the higher initial COD content of this waste. This assay also achieved 85% of the theoretical methane production, although it took 144 d to attain the ‘‘plateau’’. In the as- says SW1, SW4 and SW5, similar methane yields were obtained (0.24–0.25 m3/kg VSinitial), the VS reduction was in the range 75–79% and the percentage of metha- nation in the range 75–89%. The assay SW1 was faster than the others, since it stabilised after about 50 d, whereas the other assays needed about 100 d.

The methane yield achieved in assay SW3 was very poor (0.02 m3 CH4/kg VS), which corresponded to only 11% of the theoretical methane production. This is not surprising because carbohydrates from barley are about 69% composed by fiber (http://www.nutritiondata.com/ facts-001-02s04dq.html), being about 6% indigestible fi- ber (Potter and Hotchkiss, 1995). In this assay, the low- est values of TS and VS reduction were obtained.

Fig. 2 shows the evolution of methane, VFA and sol- uble COD, all expressed as COD.

L. Neves et al. / Waste Management 26 (2006) 176–181 179

COD (mg/l)

COD (mg/l) COD (mg/l)

COD (mg/l) COD (mg/l)

180 L. Neves et al. / Waste Management 26 (2006) 176–181

The assay that reached the higher concentration in VFA was SW2 (29 g/l) and this value did not seem to in- hibit the subsequent methanogenic process. The poor methane yield of 11% obtained in assay SW3 was likely due to the presence of products from the hydrolysis of complex heterocyclic compounds rather than to the lev- els of VFA which peaked at 22 g/l, a value lower than in assay SW2. All of the other assays achieved VFA con- centrations of around 13–15 g/l. The final pH in all the assays was in the range of 7.3–7.8 indicating that irre- versible acidification did not occur. At the end of the as- says, the VFA concentration was very low (almost near zero in some of the assays), except for SW3 that was still at 20 g/l, about 41 d after beginning the test.

Fig. 3 shows the cumulative methane as COD, hydro- lysed COD and acidified COD for all the assays. From this figure, the relative kinetics of hydrolysis, acidificat- ion and methanation can be assessed.

In general, it is accepted that hydrolysis of particu- late organic matter is the rate-limiting step in the anaerobic digestion of particulate substrates. However, in the present work this did not occur, since the curve of cumulative hydrolysed COD increased at a higher rate than the corresponding cumulative methane pro- duction curve.

For all of the wastes, 84–97% of the initial COD was hydrolysed, but the percentage of methanation was lower, in the range 75–89%, with the exception of SW3 where only 10% of methanation was observed.

Although the rate of hydrolysis is a function of pH, temperature, concentration of hydrolytic bacteria, and type of particulate organic matter (Pavlostathis and Gir- ald-Gomez, 1991), how the physicochemical properties of particulate organic substrates quantitatively affect the rate of hydrolysis (Veeken and Hamelers, 1999) is not well understood. In this study, all of the parameters mentioned above were the same in all of the assays, ex- cept the physicochemical properties of the organic waste. The hydrolysis rate constant for each assay was determined, assuming first order kinetics (Table 4).

Fig. 4 shows a negative correlation between the hydrolysis rate constant and the methane yield for all the assays.

This indicates that when hydrolysis was faster, the methane yield was lower, likely because the faster hydro- lysis induced a more important accumulation of inter- mediates potentially toxic to the methanogenic

Table 4
Hydrolysis constant rates (assuming first order kinetics)

population. Veeken and Hamelers (1999), when study- ing the anaerobic biodegradability of six components of biowaste containing lignocellulosic material, found that grass was less biodegradable (􏰂47%) than leaves (􏰂35%), although having a higher hydrolysis rate con- stant under mesophilic conditions. According to Tong et al. (1990), biodegradability depends on the structure of the lignocellulosic complex. Cellulose is readily degradable but becomes less degradable or even refrac- tory when incorporated in a lignocellulosic complex. Moreover, Azhar and Stuckey (1994) studied the influ- ence of chemical structure of instant coffee wastes on anaerobic catabolism and found that the individual chemical structure of compounds greatly influences and determines the rate and mechanisms of methano- genic degradation.

4. Conclusions

When studying five coffee wastes from the production of instant coffee substitutes, methane yields in the range of 0.24–0.28 m3/kg VSinitial were obtained with the exception of a barley-rich waste (SW3) that achieved only 0.02 m3 CH4/kg VSinitial. Four of the five wastes (SW1, SW2, SW4, SW5) also presented a high reduction of TS (50–73%) and VS (75–80%), as well as 75–89% of the theoretical methane potential (350 l/kg COD re- moved). Hydrolysis constant rates in the range of 0.035–0.063 d􏰀1 were obtained.

In the authorsÕ point of view, these wastes (SW1, SW2, SW4 and SW5) should be treated by anaerobic co-digestion rather than landfilled.

The SW3 waste achieved a methanation of 10% and reduction of TS and VS of 31% and 40%, respectively. However, this waste presented the highest hydrolysis rate constant (0.084 d􏰀1), indicating that hydrolysis was not, in this case, the rate limiting step in the anaer- obic digestion process. This was evidenced by plotting the hydrolysis rate constants and the methane yields that were inversely correlated, suggesting that intermediates

Assay #

SW1 SW2 SW3 SW4 SW5

Hydrolysis rate constant (d􏰀1)

0.063 0.035 0.084 0.040 0.036

k (d-1)