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Combustible gas production (methane) and biodegradation of solid and liquid mixtures of meat industry wastes
Applied Energy 87 (2010) 1729–1735
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Combustible gas production (methane) and biodegradation of solid and liquid mixtures of meat industry wastes A. Marcos a,*, A. Al-Kassir a, A.A. Mohamad b, F. Cuadros a, F. López-Rodríguez a a b
School of Engineering, University of Extremadura, Avda. De Elvá, s/n, 06071, Badajoz, Spain Department of Mechanical and Manufacturing Engineering, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada T2N 1N4
a r t i c l e
i n f o
Article history: Received 20 April 2009 Received in revised form 10 September 2009 Accepted 24 September 2009 Available online 2 December 2009 Keywords: Decontamination Meat industry Anaerobic biodigestion
a b s t r a c t This work is devoted to determine the optimal operational conditions on the methane production as well as on the biodegradation obtained from the anaerobic codigestión of solid (fat, intestines, rumen, bowels, whiskers, etc.) and liquid (blood, washing water, manure, etc.) wastes of meat industry, particularly the ones rising from the municipal slaughterhouse of Badajoz (Spain). The experiments were performed using a 2 l capacity discontinuous digester at 38 °C. The loading rate were 0.5, 1, 2, 3, and 4.5 g COD for wastewater (washing water and blood; Mixture 1), and 0.5, 1, 2, 3, and 4 g COD for the co-digestion of a mixture of 97% liquid effluent and 3% solid wastes v/v (Mixture 2) which represents the annual mean composition of the waste generated by the slaughterhouse. The maximal biodegradation rates obtained were: Mixture 1, 56.9% for a COD load of 1 g; and Mixture 2, 19.1% for a COD load of 2 g. For both mixtures, the greatest methane production was for the maximum COD load (4.5 g for Mixture 1, and 4 g for Mixture 2), at which values the amounts of methane obtained during and at the end of the co-digestion were practically indistinguishable between the two mixtures. The results will be used to design, construct, and establish the optimal operating conditions of a continuous complete-mixture biodigester. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction With demographic and industrial growth, and the agglomeration of the population in large cities, the generation of agro-industrial wastes has been rising to such alarming levels that the public has become sensitized to the problems caused by these wastes if they are not properly managed. Until a few years ago, the waste generated by the small populations and the few industries that existed posed no great problem, since nature itself was able to recycle it into the environment. Today, however, the generation of waste biomass is so abundant and so localized that there is insufficient capacity for its natural degradation, and various treatment techniques have to be applied. Slaughterhouses represent one of the most important sectors of the meat industry. The average composition of their liquid effluent, once separated from the voluminous solids, is: total solids 4000 mg l 1, volatile solids 2000 mg l 1, chemical oxygen demand (COD) 2500 mg l 1, 5-day biochemical oxygen demand (BOD5) 1000 mg l 1, and nitrogen 250 mg l 1 [6]. Slaughterhouse effluent has the additional problem of its great variability in composition
* Corresponding author. Tel.: +34 924289600; fax: +34 924289601. E-mail addresses: [email protected] (A. Marcos), [email protected] (A.A. Mohamad). 0306-2619/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2009.09.037
and concentration, not only from one day to another, but even over the course of a single day according to the operations being carried out at any given time. The methods most commonly used to treat the effluent are: fine sieving, settlement, chemical precipitation, biological filters (or bacterial beds), and activated sludges [13,16]. Such treatments are insufficient, however, and there have been few studies of the consequences of the microbial flora that they inject into the environment. The problem seems particularly grave when one considers how little is known about the effects on health when these poorly treated effluents with their great amounts of biological contaminants are discharged into public water courses. Successful decontamination of these effluents will depend on collaboration between many experts in different fields, with the goal of constructing treatment plants that are best adapted to the particular circumstances of each case. Slaughterhouses use large volumes of steam and hot water to cleanse and sterilize the carcasses. Often the resulting wastewater is discharged untreated with the blood into the city’s sewers, or in many cases directly into open drains and, even worse, into surface water courses. The solid waste is sometimes incinerated or buried. In the latter case, the end result is generally similar to the liquid effluent case, since the organic components filter into groundwater systems.
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This situation has led to the establishment of ever more stringent environmental legislation impeding the discharge into the environment of contaminating wastes from the meat industry, and setting out the requirements for the appropriate treatment of the liquid effluent before it enters the sewage system. Anaerobic digestion and composting are the two biological processes that are most widely used on a commercial scale to decontaminate wet residual biomass and take advantage of its energy potential. In anaerobic digestion, the organic matter is degraded by a group of bacteria in the absence of oxygen into a series of gaseous products (biogas) and other products that contain most of the mineral content (N, P, K, Ca, etc.), leaving a series of other substances that are refractory to degradation [6]. Biogas contains a high percentage of methane, and can thus be used as an energy source, as indeed is the case in many installations. Many works have been published about the anaerobic digestion of the liquid fraction of slaughterhouse wastes (see for example [12,5]) including some studies on solid poultry waste fractions (see for example [15]), but it is difficult to see data about the codigestion of solid (fat, intestines, rumen, bowels, whiskers, etc.) and liquid (blood, washing water, manure, etc.) slaughterhouse wastes. Here, it is analyzed different mixtures of solid and liquid wastes from a typical slaughterhouse of a mean European city where are scarified different species of animals (bovine, porcine, ovine and caprine). As it is well known the anaerobic digestion of solid and liquid offals of different species gives different biodegradation and biogas productions. Other studies have been made on the effect of heating on thermodamageable materials [1]. Also, works have been done on the modeling of thermal reactive barrier [2]. The principal objectives of the present work were: (i) to analyze the decontamination resulting from co-digestion of the liquid and solid wastes generated over the course of a year in the municipal slaughterhouse of Badajoz; (ii) to quantify the methane production from the anaerobic digestion of the wastes; and (iii) to determine the optimal values of the COD loading rate to give the greatest biodegradation of the waste. The present study constitutes the first of a series of works devoted to the characterization and energy valuation of the meat industry’s liquid and solid wastes in Extremadura. The ultimate goal of this line of research is to model mathematically the codigestion process of this type of waste, so that the laboratory-scale results can be reliably extrapolated to situations different, technically and geographically, from those considered here. The Region of Extremadura is located in SW Spain, bordering Portugal. It has an important meat industry, with 41 slaughterhouses. Table 1 lists the numbers of animals butchered in 2000 and 2001 [14]. Table 2 lists the amounts of offal generated by each class of animal obtained from the municipal slaughterhouse of Badajoz. In preparing the table, the statistical data from the Badajoz slaughterhouse were used to calculate the mean quantities of by-products generated by butchering an animal of each of the four livestock classes. The resulting data are coherent with those given in the specialist literature [7].
Table 1 Number of animals butchered in 2000 and 2001 in the Autonomous Community of Extremadura, and the percentage change from one year to the next. Carcasses
2000 Number
2001 Number
% Variation 2001/2002
Bovine Porcine Ovine Caprine Total
16,741 609,239 180,908 47,606 854,494
20,678 695,444 191,933 43,675 951,730
23.52 14.15 6.09 8.26 11.36
On the other hand, researches have been done on the generation of alternative combustibles, such as those presented by [11], (Morin et al., 2006) and [3]. 2. Materials and methods 2.1. Characterization of effluent Moreover, the corresponding biochemical analysis to determine the BOD5 of the by-products was also performed. Table 3 summarizes the biochemical analysis of the waste. The contaminant load of the bovine solid wastes is notably less than that of the other three species. All four species, however, are given exactly the same liquid effluent (wastewater plus blood) BOD5, since the estimate was made for a mean annual quantity on the basis of data supplied by the slaughterhouse staff. The BOD5 results for the different classes of wastes differ slightly from those found by other literature sources [10]. On the other hand, the EU Council Directive of 21 May 1991 91/ 271/CEE concerning urban wastewater treatment establishes the mean contamination due to an inhabitant of an average European city—the population equivalent (p.e.)—as the organic biodegradable load having a BOD5 of 60 g of oxygen per day. It was estimated the contamination in p.e. deriving from the butchering of the four classes of animal in the Region of Extremadura, collected in Table 4, that rise to 4,047,440 p.e. Contamination important enough, for needing to be properly managed by all individuals and organisms responsible for preserving the environment. 2.2. Experimental setup Fig. 1 shows the apparatus used to carry out the processes of anaerobic biological degradation. It basically comprised a 2-l glass flask, in the mouth of which was a piece with a central tube that was submersed in the reaction medium to extract samples from the interior of the reactor, and to allow the completely methanized sample to be withdrawn. The flask had a side-arm in the neck to evacuate the gases that were produced. The reactor was immersed in a thermostatic water bath at 38 °C fitted with a magnetic stirrer. The temperature was regulated via a 500 W heating resistance and an adjustable contact thermometer. To determine the volume of methane produced, the gases were first passed through an airtight washing column containing a 20% by weight solution of sodium hydroxide in order to eliminate the carbon dioxide. The remaining methane then passed to a 1-l reservoir of a Boyle–Mariotte type, displacing the water which overflowed into a measuring cylinder. The measured water volume is the same of the volume of methane generated in each experiment. 2.3. Methods 2.3.1. Inoculum The inoculum used in the present experiments was a mixture of cow dung, which is rich in methanogenic bacteria, and anaerobic sludge from an urban wastewater treatment plant. 2.3.2. Acclimation The first step in all the experiments was the acclimation of the anaerobic population to the degradation of the liquid and solid wastes from the slaughterhouse. To this end, over one month, increasing volumes of liquid effluent were added to the inoculum. Then, the addition of liquid effluent was stopped, until the stabilization of the COD. This was then followed the beginning of the trials with the liquid and solid wastes.
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A. Marcos et al. / Applied Energy 87 (2010) 1729–1735 Table 2 Estimate of the quantity of offal generated per carcass, based on data provided by the staff of the Badajoz municipal slaughterhouse.
Bovine Porcine Ovine Caprine
Gross weight (kg)
Carcass weight (kg)
Total by-products (kg)
Blood (kg)
Fat (kg)
Intestines (kg)
Other by-products (kg)
540 160 20 12
300 115 11 6
240 45 9 6
39.4 11.2 1.4 0.8
32.6 1.7 0.5 0.3
26.6 1.3 1.9 1
141.4 30.8 5.2 3.9
Table 3 Estimate of the contamination in terms of BOD5 of the offal generated by the butchery of the different species of animals. Type of animal
Fat BOD5 (g l
Bovine Porcine Ovine Caprine
45.7 61.4 61.5 61.5
1
)
Intestines BOD5 (g l
1
)
Blood + wastewater BOD5 (g l
45.7 72.7 61.5 61.5
1
)
21 21 21 21
Table 4 Estimate of the contamination, in p.e., generated by the butchery of the different species of animals in the Community of Extremadura during 2001. Type of animal
Animal butchered in 2001
BOD5 (g l
Bovine Porcine Ovine Caprine
20,678 695,444 191,933 43,675
3533 199 148 80
1
)/carcass
Contamination in p.e. 58.8 3.3 2.5 1.3
Total contamination (p.e.) in the Community of Extremadura 1,215,866 2,294,965 479,832 56,777 Total: 4,047,440
Fig. 1. Schematic diagram of the discontinuous biodigester used in the present work.
2.3.3. Preparation of the samples In preparing the samples for the different experiments, the mean annual water consumption of the slaughterhouse and the
mean numbers of animals of each class butchered per year were taken into account. The field work carried out at the slaughterhouse has been used to determine for each animal class the mean
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Table 5 Physical–chemical parameters of the liquid effluent (wastewater plus blood, Mixture 1), and the mix of liquid effluent (97%) plus solid wastes (3%) (Mixture 2) from the Badajoz municipal slaughterhouse. Parameters
pH
CODsoluble (mg/l)
CODtotal (mg/l)
BOD5 (mg/l)
Alkalinity (g CaCO3/l)
Mixture 1 Mixture 2
6.36 6.91
111,700 122,400
125,600 131,400
65,800 77,080
5.12 7.80
quantities of blood, offal, and fat generated, that would subsequently be rendered in the slaughterhouse, obtained from the municipal slaughterhouse of Badajoz (see Table 2). 2.3.4. Parameters controlled The duration of experiments is showed in Tables 6 and 8. Each day were taken three samples at intervals of 6 h, except during night. The parameters, study object, were COD remaining and volume of methane produced. The values of BOD5, pH, acidity, alkalinity, and volatile suspended solids (VSS) were also measured using the standard methodology [4]. The microorganism concentration remained practically constant in each experiment, and very close to the initial value of 12 ± 0.1 g l 1. This result is explained by the slow cell growth rate in anaerobic processes, which varies between 0.02 and 0.05 g VSS/g COD degraded [8,9], reason why the variation in cell concentration would be so small as to be negligible in practice. 3. Results 3.1. Initial physico-chemical characterization of Mixture 1 consisting only of liquid wastes (wastewater plus blood) and of Mixture 2 (97% wastewater plus blood and 3% solid wastes, v/v) Mixture 1 contained a sample of the mean annual composition of the slaughterhouse liquid effluent, with 98% wastewater and 2% blood (v/v), but no solid waste. Its physico-chemical parameters at the beginning of the biochemical reaction are given in Table 5. Mixture 2 corresponded to a sample of the mean annual total waste from the slaughterhouse, with 97% liquid effluent (wastewater plus blood) and 3% solid offal. Its physico-chemical parameters at the beginning of the biochemical reaction are also given in Table 5.
3.2. Anaerobic digestion experiments carried out with Mixtures 1 and 2 Table 6 lists the results of the five experiments carried out with Mixture 1 (wastewater plus blood). The physico-chemical parameters of the mixture at the end of the process are given in Table 7. Tables 8 and 9 list the corresponding results for Mixture 2. Tables 6 and 8 give the values of the chemical oxygen demand at the beginning, CODO, and at the end, CODf, of the digestion process, the percentage degradation attained, XCOD, and the volume of methane, VF, produced by the end of each of the five experiments corresponding to Mixtures 1 and 2, respectively. For both mixtures, the COD load, CODl, was increased to the maximum that it was possible to degrade without inhibiting the reaction (see Tables 6 and 8). These loads were 0.5, 1.0, 2.0, 3.0, and 4.5 g CODl for the five experiments with Mixture 1 (liquid effluent: wastewater plus blood), and 0.5, 1.0, 2.0, 3.0, and 4.0 g CODl for the five experiments with Mixture 2 (liquid effluent plus solids), i.e., for Mixture1, the digestion process was inhibited for CODl values above 4.5 g, and for Mixture 2 the co-digestion was inhibited for CODl values above 4.0 g. The experiments were halted when the CODf had stabilized. The pH was also monitored throughout the operating period. It may be observed in Tables 7 and 9 that the pH in the digester at the end of both the Mixture 1 and the Mixture 2 experiments varied little, being around 7.5. Together with the pH, the concentration of volatile fatty acids and the alkalinity are also reactor control parameters. The alkalinity is directly related to pH. Indeed, in the operating range of anaerobic digesters, the carbon dioxide/bicarbonate system is the principal buffer. To have sufficient buffer capacity and achieve stable operation of the digester, it is necessary to work with values of the alkalinity greater than 1000 mg CO3Ca l 1. Given the data of Tables 7 and 9, one can establish mean final values of the volatile acidity and the alkalinity of 2.5 g CH3COOH l 1 and 7.9 g CO3Ca l 1, respectively for Mixture 1 (see Table 7), and of 5.5 g CH3COOH l 1 and 8.3 g CO3Ca l 1, respectively for Mixture 2 (see Table 9).
Table 6 Biodigestion experiments carried out with a liquid mixture (98% wastewater, 2% blood; Mixture 1), and column 6 the volume of methane, VF, produced. Experiment Mixture 1
Duration (h)
Organic load (g CODl)
Initial concentration COD0 (g l 1)
Final concentration CODf (g l 1)
% Removed XCOD (%)
Volume of methane VF (ml)
1 2 3 4 5
30 71 71 95 97
0.5 1.0 2.0 3.0 4.5
8.45 14.50 11.90 25.80 41.90
4.36 6.25 9.75 17.9 34.10
48.4 56.9 18.2 30.6 18.6
157 224 480 489 900
Table 7 Characteristic parameters of the liquid effluent of the Badajoz municipal slaughterhouse (98% wastewater, 2% blood; Mixture 1) at the end of each of the five biodigestion experiments. Experiment Mixture 1
Organic load (g CODl)
BOD5 (g l
1 2 3 4 5
0.5 1.0 2.0 3.0 4.5
3.88 3.88 7.76 17.46 27.16
1
)
pH
VSS (g l
7.73 7.76 7.29 7.13 7.34
27.1 25.0 26.1 20.0 23.0
1
)
Acidity (g CH3COOH l 1.2 1.4 1.8 3.9 4.0
1
)
Alkalinity (g CO3Ca l 6.2 6.9 6.9 10.4 9.1
1
)
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Table 8 Biodigestion experiments carried out with a mixture of 97% liquid effluent (wastewater plus blood) and 3% solid waste (v/v) from the Badajoz municipal slaughterhouse (Mixture 2). Experiment Mixture 2
Duration (h)
Organic load CODl (g)
Initial concentration COD0 (g l 1)
Final concentration CODf (g l 1)
% Removed XCOD (%)
Volume of methane VF (ml)
1 2 3 4 5
30 48 56 77 99
0.5 1.0 2.0 3.0 4.0
14.45 19.55 26.55 37.15 49.75
12.05 16.45 21.50 30.75 42.70
16.6 15.9 19.1 17.2 14.2
360 454 715 820 990
Table 9 Characteristic parameters of a mixture of 97% liquid effluent and 3% solid waste (Mixture 2), whose composition corresponds to the mean annual discharge from the Badajoz slaughterhouse, at the end of each of the five co-digestion experiments. Experiment Mixture 2
Organic load (g CODl)
BOD5 (g l
1 2 3 4 5
0.5 1.0 2.0 3.0 4.0
10.67 14.55 19.40 22.31 40.70
1
)
pH
VSS (g l
7.51 7.60 7.40 7.65 7.68
3.3 3.4 3.4 3.9 4.4
3.3. Substrate degradation and volume of methane produced during the biodigestion process 3.3.1. Substrate degradation In order to analyze in depth the biodigestion processes followed by Mixture 1 (wastewater plus blood) and Mixture 2 (liquid effluent plus solids), remaining COD and volume of methane produced were measured at different time intervals. Fig. 2 shows the evolution of the substrate concentration (in mg l 1 of COD) over the time of digestion of Mixture 1. It may be observed that the degradation of the organic load was greater at the initial reaction times, with the decline in the COD stabilizing over the course of the experiments until stationary values were reached. The final COD values of Fig. 2 of course correspond to those listed in column 4 of Table 6. Similarly, Fig. 3 shows the evolution of the co-digestion process of the liquid effluent (97%) and solids (3%). In this case, the final COD values of the figure correspond to those listed in column 4 of Table 8. Fig. 6 shows the temporal evolution of the substrate degradation, COD, for one experiment of Mixture 1 and one of Mixture 2.
50
1
)
Acidity (g CH3COOH l
1
2.7 3.5 5.6 7.3 8.6
)
Alkalinity (g CO3Ca l
1
)
5.2 6.5 7.9 9.5 12.5
The two experiments were those that gave the greatest degradation percentage, XCOD. In experiment 2, Mixture 1 (liquid effluent only), for example, there was a degradation of 56.9% (see Table 6 and Fig. 6). This suggests that if the goal is to obtain the greatest degree of degradation of the liquid effluent (wastewater plus blood) then the COD load fed to the 2-l biodigester should be that corresponding to 1 g. As may be observed in Table 6, the optimal biodegradation of the liquid effluent (Mixture 1) occurred at low values of the COD feed. For Mixture 2 (liquid effluent plus solids), the optimal value of the COD feed for the maximum biodegradation was 2 g (see Table 8 and Fig. 6). In this case, however, the percentage biodegradation was only 19.1%. In this case too, the percentage degradation declined with increasing COD feed.
3.3.2. Methane yield Figs. 4 and 5 show the values of the volume of methane produced during the biodigestion processes. Fig. 4 corresponds to the digestion of the liquid effluent (Mixture 1) and Fig. 5 to the co-digestion of liquid effluent (97%) and solids (3%) (Mixture 2). In all the experiments, it may be observed a high rate of gas production at the initial times of the reaction, with declining rates of formation as the reaction proceeds. The final values of the methane
45 40
70
35
60 50 -1
25
COD (g l )
-1
COD (g l )
30
20
Experiment 1 Experiment 2
15
Experiment 3 Experiment 4 Experiment 5
10 5 0
40 30
Experiment 1 Experiment 2
20
Experiment 3 Experiment 4
10
Experiment 5 0
0
20
40
60
80
100
120
t (h) Fig. 2. Temporal evolution of the substrate concentration (in mg l 1 COD) with error band of ±5, during the anaerobic biodigestion of the liquid effluent (wastewater plus blood), Mixture 1 for the series of five experiments.
0
20
40
60
80
100
120
t (h) Fig. 3. Temporal evolution of the substrate concentration (in mg l 1 COD) with error band of ±5%, during the anaerobic co-digestion of a mixture of liquid effluent 97% and solid waste (3%), Mixture 2 for the series of five experiments.
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1200
1200 Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5
1000
800
CH4 (ml)
CH4 (ml )
800
1000
600 400
600 400
200
Mixture 1, Experiment 5
200
0 0
20
40
60
80
1 00
Mixture 2, Experiment 5
12 0
0
t (h) Fig. 4. Temporal evolution of the volume of methane with error band of ±5%, produced during the anaerobic digestion of the liquid effluent (wastewater plus blood), Mixture 1.
0
50
100
150
t (h) Fig. 7. Temporal evolution of the methane production with error band of ±5%, during the experiments (no. 5) of greatest methane production for each of the two mixtures.
1000 900
2 (liquid effluent plus solids). It may be observed from Tables 6 and 8 that in both cases there is the greatest production of methane for the highest values of the COD feed (Expt. 5). The figure shows that the two cases present very similar kinetics, although there is a slight difference in favour of Mixture 2 throughout the time.
800
CH4 (ml)
700 600 500 Experiment 1 Experiment 2
400 300
4. Conclusions
Experiment 3 Experiment 4 Experiment 5
200 100 0 0
20
40
60 t (h)
80
100
120
Fig. 5. Temporal evolution of the volume of methane with error band of ±5%, produced during the anaerobic co-digestion of a mixture of liquid effluent (97%) and solid waste (3%), Mixture 2.
Mixture 1, Experiment 2
40
Mixture 2, Experiment 3
35
-1
COD (mg l )
30 25 20 15 10 5 0 0
10
20
30
40
50
60
70
80
t (h)
Fig. 6. Experiments with greatest biodegradation: experiment 2 for Mixture 1 (wastewater plus blood) with a COD load fed into the reactor of 1 g; and experiment 3 for Mixture 2 (liquid effluent plus solids) with a COD load of 2 g, with error band of ±5%.
volume in Figs. 4 and 5 correspond to the values given in the last column of Tables 6 and 8, respectively. Finally, Fig. 7 shows the evolution of the methane produced in the biodigestion of Mixture 1 (wastewater plus blood) and Mixture
1. The calculated total contamination of meat industry in Extremadura during 2001 is equivalent to 4 million p.e., if the disposal of the waste products had been directly into the environment without treatment. The small municipal slaughterhouse of Badajoz would, in such a case, have generated contamination equivalent to a city of 180,000 inhabitants. 2. The following physico-chemical parameters were measured at the beginning, during, and at the end of each biodigestion experiment: pH, alkalinity and volatile acidity, COD, and BOD5. The pH remained within the interval between the initial and final values of 7.73 and 7.34, respectively for Mixture 1 (liquid effluent), and of 7.51 and 7.68, respectively for Mixture 2 (liquid effluent plus solids). The alkalinity oscillates between 6200 and 10,400 mg CO3Ca l 1 for Mixture 1, and 5200 and 12,500 mg CO3Ca l 1 for Mixture 2. With respect to the volatile acidity, the mean values calculated at the end of each series of five experiments were 2.5 and 5.5 g CH3COOH l 1 for Mixtures 1 and 2, respectively. 3. For the biodigester that was used, the anaerobic digestion of the liquid effluent was inhibited for values of the chemical oxygen demand load fed into the reactor, CODl, above 4.5 g, and the anaerobic co-digestion of the liquid effluent plus solids in the proportion of 97% liquid and 3% solids corresponding to the discharge from the Badajoz municipal slaughterhouse was inhibited for CODl above 4 g. 4. Better COD degradation yields were obtained in the Mixture 1 (wastewater plus blood) experiments than in the case of Mixture 2 (liquid effluent plus solids), i.e., a greater level of decontamination of the organic load was obtained for Mixture 1. This is because of the high organic demand present in the solid waste used in Mixture 2, which limits the degrading action of the bacterial population present in the anaerobic sludge.
A. Marcos et al. / Applied Energy 87 (2010) 1729–1735
5. The experiments performed with one type of load, the greatest degradation for Mixture 1 was obtained for an organic load fed into the reactor of 1 g of COD, and for Mixture 2, 2 g of COD. Above these optimal values, the yields were poorer due to the build-up of the organic load, and hence of fatty acids, inhibiting the growth of the bacterial population. 6. With respect to the production of methane, higher productions resulted from greater loads fed into the reactor. Hence, for the two types of load dealt with in the present work, the optimal feed into the 2-l capacity discontinuous digester from the standpoint of methanization of the sample would be 4.5 g of COD for Mixture 1, and 4 g of COD for Mixture 2. Acknowledgements The authors express their gratitude to the Excma. Diputación de Badajoz and to the Junta de Extremadura (Consejería de Agricultura y Medio Ambiente) for the financial support granted for the performance of the present work. References [1] Bouvet A, Bennacer R. Effect of heating rate on Arrhenius law: thermodamageable material. J Energy Inst 2006;79(4):228–31. [2] Bouvet A, Demange D, Bennacer R, Herve Ph. Modelling and characteristic of thermal reactive barrier. J High Temp Mater Process 2006;10:317–25. [3] Hilkiah Igoni A, Ayotamuno MJ, Eze CL, Ogaji SOT, Probert SD. Designs of anaerobic digesters for producing biogas from municipal solid-waste. Appl Energy 2008;85(6):430–8.
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Combustible gas production (methane) and biodegradation of solid and liquid mixtures of meat industry wastes A. Marcos a,*, A. Al-Kassir a, A.A. Mohamad b, F. Cuadros a, F. López-Rodríguez a a b
School of Engineering, University of Extremadura, Avda. De Elvá, s/n, 06071, Badajoz, Spain Department of Mechanical and Manufacturing Engineering, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada T2N 1N4
a r t i c l e
i n f o
Article history: Received 20 April 2009 Received in revised form 10 September 2009 Accepted 24 September 2009 Available online 2 December 2009 Keywords: Decontamination Meat industry Anaerobic biodigestion
a b s t r a c t This work is devoted to determine the optimal operational conditions on the methane production as well as on the biodegradation obtained from the anaerobic codigestión of solid (fat, intestines, rumen, bowels, whiskers, etc.) and liquid (blood, washing water, manure, etc.) wastes of meat industry, particularly the ones rising from the municipal slaughterhouse of Badajoz (Spain). The experiments were performed using a 2 l capacity discontinuous digester at 38 °C. The loading rate were 0.5, 1, 2, 3, and 4.5 g COD for wastewater (washing water and blood; Mixture 1), and 0.5, 1, 2, 3, and 4 g COD for the co-digestion of a mixture of 97% liquid effluent and 3% solid wastes v/v (Mixture 2) which represents the annual mean composition of the waste generated by the slaughterhouse. The maximal biodegradation rates obtained were: Mixture 1, 56.9% for a COD load of 1 g; and Mixture 2, 19.1% for a COD load of 2 g. For both mixtures, the greatest methane production was for the maximum COD load (4.5 g for Mixture 1, and 4 g for Mixture 2), at which values the amounts of methane obtained during and at the end of the co-digestion were practically indistinguishable between the two mixtures. The results will be used to design, construct, and establish the optimal operating conditions of a continuous complete-mixture biodigester. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction With demographic and industrial growth, and the agglomeration of the population in large cities, the generation of agro-industrial wastes has been rising to such alarming levels that the public has become sensitized to the problems caused by these wastes if they are not properly managed. Until a few years ago, the waste generated by the small populations and the few industries that existed posed no great problem, since nature itself was able to recycle it into the environment. Today, however, the generation of waste biomass is so abundant and so localized that there is insufficient capacity for its natural degradation, and various treatment techniques have to be applied. Slaughterhouses represent one of the most important sectors of the meat industry. The average composition of their liquid effluent, once separated from the voluminous solids, is: total solids 4000 mg l 1, volatile solids 2000 mg l 1, chemical oxygen demand (COD) 2500 mg l 1, 5-day biochemical oxygen demand (BOD5) 1000 mg l 1, and nitrogen 250 mg l 1 [6]. Slaughterhouse effluent has the additional problem of its great variability in composition
* Corresponding author. Tel.: +34 924289600; fax: +34 924289601. E-mail addresses: [email protected] (A. Marcos), [email protected] (A.A. Mohamad). 0306-2619/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2009.09.037
and concentration, not only from one day to another, but even over the course of a single day according to the operations being carried out at any given time. The methods most commonly used to treat the effluent are: fine sieving, settlement, chemical precipitation, biological filters (or bacterial beds), and activated sludges [13,16]. Such treatments are insufficient, however, and there have been few studies of the consequences of the microbial flora that they inject into the environment. The problem seems particularly grave when one considers how little is known about the effects on health when these poorly treated effluents with their great amounts of biological contaminants are discharged into public water courses. Successful decontamination of these effluents will depend on collaboration between many experts in different fields, with the goal of constructing treatment plants that are best adapted to the particular circumstances of each case. Slaughterhouses use large volumes of steam and hot water to cleanse and sterilize the carcasses. Often the resulting wastewater is discharged untreated with the blood into the city’s sewers, or in many cases directly into open drains and, even worse, into surface water courses. The solid waste is sometimes incinerated or buried. In the latter case, the end result is generally similar to the liquid effluent case, since the organic components filter into groundwater systems.
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This situation has led to the establishment of ever more stringent environmental legislation impeding the discharge into the environment of contaminating wastes from the meat industry, and setting out the requirements for the appropriate treatment of the liquid effluent before it enters the sewage system. Anaerobic digestion and composting are the two biological processes that are most widely used on a commercial scale to decontaminate wet residual biomass and take advantage of its energy potential. In anaerobic digestion, the organic matter is degraded by a group of bacteria in the absence of oxygen into a series of gaseous products (biogas) and other products that contain most of the mineral content (N, P, K, Ca, etc.), leaving a series of other substances that are refractory to degradation [6]. Biogas contains a high percentage of methane, and can thus be used as an energy source, as indeed is the case in many installations. Many works have been published about the anaerobic digestion of the liquid fraction of slaughterhouse wastes (see for example [12,5]) including some studies on solid poultry waste fractions (see for example [15]), but it is difficult to see data about the codigestion of solid (fat, intestines, rumen, bowels, whiskers, etc.) and liquid (blood, washing water, manure, etc.) slaughterhouse wastes. Here, it is analyzed different mixtures of solid and liquid wastes from a typical slaughterhouse of a mean European city where are scarified different species of animals (bovine, porcine, ovine and caprine). As it is well known the anaerobic digestion of solid and liquid offals of different species gives different biodegradation and biogas productions. Other studies have been made on the effect of heating on thermodamageable materials [1]. Also, works have been done on the modeling of thermal reactive barrier [2]. The principal objectives of the present work were: (i) to analyze the decontamination resulting from co-digestion of the liquid and solid wastes generated over the course of a year in the municipal slaughterhouse of Badajoz; (ii) to quantify the methane production from the anaerobic digestion of the wastes; and (iii) to determine the optimal values of the COD loading rate to give the greatest biodegradation of the waste. The present study constitutes the first of a series of works devoted to the characterization and energy valuation of the meat industry’s liquid and solid wastes in Extremadura. The ultimate goal of this line of research is to model mathematically the codigestion process of this type of waste, so that the laboratory-scale results can be reliably extrapolated to situations different, technically and geographically, from those considered here. The Region of Extremadura is located in SW Spain, bordering Portugal. It has an important meat industry, with 41 slaughterhouses. Table 1 lists the numbers of animals butchered in 2000 and 2001 [14]. Table 2 lists the amounts of offal generated by each class of animal obtained from the municipal slaughterhouse of Badajoz. In preparing the table, the statistical data from the Badajoz slaughterhouse were used to calculate the mean quantities of by-products generated by butchering an animal of each of the four livestock classes. The resulting data are coherent with those given in the specialist literature [7].
Table 1 Number of animals butchered in 2000 and 2001 in the Autonomous Community of Extremadura, and the percentage change from one year to the next. Carcasses
2000 Number
2001 Number
% Variation 2001/2002
Bovine Porcine Ovine Caprine Total
16,741 609,239 180,908 47,606 854,494
20,678 695,444 191,933 43,675 951,730
23.52 14.15 6.09 8.26 11.36
On the other hand, researches have been done on the generation of alternative combustibles, such as those presented by [11], (Morin et al., 2006) and [3]. 2. Materials and methods 2.1. Characterization of effluent Moreover, the corresponding biochemical analysis to determine the BOD5 of the by-products was also performed. Table 3 summarizes the biochemical analysis of the waste. The contaminant load of the bovine solid wastes is notably less than that of the other three species. All four species, however, are given exactly the same liquid effluent (wastewater plus blood) BOD5, since the estimate was made for a mean annual quantity on the basis of data supplied by the slaughterhouse staff. The BOD5 results for the different classes of wastes differ slightly from those found by other literature sources [10]. On the other hand, the EU Council Directive of 21 May 1991 91/ 271/CEE concerning urban wastewater treatment establishes the mean contamination due to an inhabitant of an average European city—the population equivalent (p.e.)—as the organic biodegradable load having a BOD5 of 60 g of oxygen per day. It was estimated the contamination in p.e. deriving from the butchering of the four classes of animal in the Region of Extremadura, collected in Table 4, that rise to 4,047,440 p.e. Contamination important enough, for needing to be properly managed by all individuals and organisms responsible for preserving the environment. 2.2. Experimental setup Fig. 1 shows the apparatus used to carry out the processes of anaerobic biological degradation. It basically comprised a 2-l glass flask, in the mouth of which was a piece with a central tube that was submersed in the reaction medium to extract samples from the interior of the reactor, and to allow the completely methanized sample to be withdrawn. The flask had a side-arm in the neck to evacuate the gases that were produced. The reactor was immersed in a thermostatic water bath at 38 °C fitted with a magnetic stirrer. The temperature was regulated via a 500 W heating resistance and an adjustable contact thermometer. To determine the volume of methane produced, the gases were first passed through an airtight washing column containing a 20% by weight solution of sodium hydroxide in order to eliminate the carbon dioxide. The remaining methane then passed to a 1-l reservoir of a Boyle–Mariotte type, displacing the water which overflowed into a measuring cylinder. The measured water volume is the same of the volume of methane generated in each experiment. 2.3. Methods 2.3.1. Inoculum The inoculum used in the present experiments was a mixture of cow dung, which is rich in methanogenic bacteria, and anaerobic sludge from an urban wastewater treatment plant. 2.3.2. Acclimation The first step in all the experiments was the acclimation of the anaerobic population to the degradation of the liquid and solid wastes from the slaughterhouse. To this end, over one month, increasing volumes of liquid effluent were added to the inoculum. Then, the addition of liquid effluent was stopped, until the stabilization of the COD. This was then followed the beginning of the trials with the liquid and solid wastes.
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Bovine Porcine Ovine Caprine
Gross weight (kg)
Carcass weight (kg)
Total by-products (kg)
Blood (kg)
Fat (kg)
Intestines (kg)
Other by-products (kg)
540 160 20 12
300 115 11 6
240 45 9 6
39.4 11.2 1.4 0.8
32.6 1.7 0.5 0.3
26.6 1.3 1.9 1
141.4 30.8 5.2 3.9
Table 3 Estimate of the contamination in terms of BOD5 of the offal generated by the butchery of the different species of animals. Type of animal
Fat BOD5 (g l
Bovine Porcine Ovine Caprine
45.7 61.4 61.5 61.5
1
)
Intestines BOD5 (g l
1
)
Blood + wastewater BOD5 (g l
45.7 72.7 61.5 61.5
1
)
21 21 21 21
Table 4 Estimate of the contamination, in p.e., generated by the butchery of the different species of animals in the Community of Extremadura during 2001. Type of animal
Animal butchered in 2001
BOD5 (g l
Bovine Porcine Ovine Caprine
20,678 695,444 191,933 43,675
3533 199 148 80
1
)/carcass
Contamination in p.e. 58.8 3.3 2.5 1.3
Total contamination (p.e.) in the Community of Extremadura 1,215,866 2,294,965 479,832 56,777 Total: 4,047,440
Fig. 1. Schematic diagram of the discontinuous biodigester used in the present work.
2.3.3. Preparation of the samples In preparing the samples for the different experiments, the mean annual water consumption of the slaughterhouse and the
mean numbers of animals of each class butchered per year were taken into account. The field work carried out at the slaughterhouse has been used to determine for each animal class the mean
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Table 5 Physical–chemical parameters of the liquid effluent (wastewater plus blood, Mixture 1), and the mix of liquid effluent (97%) plus solid wastes (3%) (Mixture 2) from the Badajoz municipal slaughterhouse. Parameters
pH
CODsoluble (mg/l)
CODtotal (mg/l)
BOD5 (mg/l)
Alkalinity (g CaCO3/l)
Mixture 1 Mixture 2
6.36 6.91
111,700 122,400
125,600 131,400
65,800 77,080
5.12 7.80
quantities of blood, offal, and fat generated, that would subsequently be rendered in the slaughterhouse, obtained from the municipal slaughterhouse of Badajoz (see Table 2). 2.3.4. Parameters controlled The duration of experiments is showed in Tables 6 and 8. Each day were taken three samples at intervals of 6 h, except during night. The parameters, study object, were COD remaining and volume of methane produced. The values of BOD5, pH, acidity, alkalinity, and volatile suspended solids (VSS) were also measured using the standard methodology [4]. The microorganism concentration remained practically constant in each experiment, and very close to the initial value of 12 ± 0.1 g l 1. This result is explained by the slow cell growth rate in anaerobic processes, which varies between 0.02 and 0.05 g VSS/g COD degraded [8,9], reason why the variation in cell concentration would be so small as to be negligible in practice. 3. Results 3.1. Initial physico-chemical characterization of Mixture 1 consisting only of liquid wastes (wastewater plus blood) and of Mixture 2 (97% wastewater plus blood and 3% solid wastes, v/v) Mixture 1 contained a sample of the mean annual composition of the slaughterhouse liquid effluent, with 98% wastewater and 2% blood (v/v), but no solid waste. Its physico-chemical parameters at the beginning of the biochemical reaction are given in Table 5. Mixture 2 corresponded to a sample of the mean annual total waste from the slaughterhouse, with 97% liquid effluent (wastewater plus blood) and 3% solid offal. Its physico-chemical parameters at the beginning of the biochemical reaction are also given in Table 5.
3.2. Anaerobic digestion experiments carried out with Mixtures 1 and 2 Table 6 lists the results of the five experiments carried out with Mixture 1 (wastewater plus blood). The physico-chemical parameters of the mixture at the end of the process are given in Table 7. Tables 8 and 9 list the corresponding results for Mixture 2. Tables 6 and 8 give the values of the chemical oxygen demand at the beginning, CODO, and at the end, CODf, of the digestion process, the percentage degradation attained, XCOD, and the volume of methane, VF, produced by the end of each of the five experiments corresponding to Mixtures 1 and 2, respectively. For both mixtures, the COD load, CODl, was increased to the maximum that it was possible to degrade without inhibiting the reaction (see Tables 6 and 8). These loads were 0.5, 1.0, 2.0, 3.0, and 4.5 g CODl for the five experiments with Mixture 1 (liquid effluent: wastewater plus blood), and 0.5, 1.0, 2.0, 3.0, and 4.0 g CODl for the five experiments with Mixture 2 (liquid effluent plus solids), i.e., for Mixture1, the digestion process was inhibited for CODl values above 4.5 g, and for Mixture 2 the co-digestion was inhibited for CODl values above 4.0 g. The experiments were halted when the CODf had stabilized. The pH was also monitored throughout the operating period. It may be observed in Tables 7 and 9 that the pH in the digester at the end of both the Mixture 1 and the Mixture 2 experiments varied little, being around 7.5. Together with the pH, the concentration of volatile fatty acids and the alkalinity are also reactor control parameters. The alkalinity is directly related to pH. Indeed, in the operating range of anaerobic digesters, the carbon dioxide/bicarbonate system is the principal buffer. To have sufficient buffer capacity and achieve stable operation of the digester, it is necessary to work with values of the alkalinity greater than 1000 mg CO3Ca l 1. Given the data of Tables 7 and 9, one can establish mean final values of the volatile acidity and the alkalinity of 2.5 g CH3COOH l 1 and 7.9 g CO3Ca l 1, respectively for Mixture 1 (see Table 7), and of 5.5 g CH3COOH l 1 and 8.3 g CO3Ca l 1, respectively for Mixture 2 (see Table 9).
Table 6 Biodigestion experiments carried out with a liquid mixture (98% wastewater, 2% blood; Mixture 1), and column 6 the volume of methane, VF, produced. Experiment Mixture 1
Duration (h)
Organic load (g CODl)
Initial concentration COD0 (g l 1)
Final concentration CODf (g l 1)
% Removed XCOD (%)
Volume of methane VF (ml)
1 2 3 4 5
30 71 71 95 97
0.5 1.0 2.0 3.0 4.5
8.45 14.50 11.90 25.80 41.90
4.36 6.25 9.75 17.9 34.10
48.4 56.9 18.2 30.6 18.6
157 224 480 489 900
Table 7 Characteristic parameters of the liquid effluent of the Badajoz municipal slaughterhouse (98% wastewater, 2% blood; Mixture 1) at the end of each of the five biodigestion experiments. Experiment Mixture 1
Organic load (g CODl)
BOD5 (g l
1 2 3 4 5
0.5 1.0 2.0 3.0 4.5
3.88 3.88 7.76 17.46 27.16
1
)
pH
VSS (g l
7.73 7.76 7.29 7.13 7.34
27.1 25.0 26.1 20.0 23.0
1
)
Acidity (g CH3COOH l 1.2 1.4 1.8 3.9 4.0
1
)
Alkalinity (g CO3Ca l 6.2 6.9 6.9 10.4 9.1
1
)
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Table 8 Biodigestion experiments carried out with a mixture of 97% liquid effluent (wastewater plus blood) and 3% solid waste (v/v) from the Badajoz municipal slaughterhouse (Mixture 2). Experiment Mixture 2
Duration (h)
Organic load CODl (g)
Initial concentration COD0 (g l 1)
Final concentration CODf (g l 1)
% Removed XCOD (%)
Volume of methane VF (ml)
1 2 3 4 5
30 48 56 77 99
0.5 1.0 2.0 3.0 4.0
14.45 19.55 26.55 37.15 49.75
12.05 16.45 21.50 30.75 42.70
16.6 15.9 19.1 17.2 14.2
360 454 715 820 990
Table 9 Characteristic parameters of a mixture of 97% liquid effluent and 3% solid waste (Mixture 2), whose composition corresponds to the mean annual discharge from the Badajoz slaughterhouse, at the end of each of the five co-digestion experiments. Experiment Mixture 2
Organic load (g CODl)
BOD5 (g l
1 2 3 4 5
0.5 1.0 2.0 3.0 4.0
10.67 14.55 19.40 22.31 40.70
1
)
pH
VSS (g l
7.51 7.60 7.40 7.65 7.68
3.3 3.4 3.4 3.9 4.4
3.3. Substrate degradation and volume of methane produced during the biodigestion process 3.3.1. Substrate degradation In order to analyze in depth the biodigestion processes followed by Mixture 1 (wastewater plus blood) and Mixture 2 (liquid effluent plus solids), remaining COD and volume of methane produced were measured at different time intervals. Fig. 2 shows the evolution of the substrate concentration (in mg l 1 of COD) over the time of digestion of Mixture 1. It may be observed that the degradation of the organic load was greater at the initial reaction times, with the decline in the COD stabilizing over the course of the experiments until stationary values were reached. The final COD values of Fig. 2 of course correspond to those listed in column 4 of Table 6. Similarly, Fig. 3 shows the evolution of the co-digestion process of the liquid effluent (97%) and solids (3%). In this case, the final COD values of the figure correspond to those listed in column 4 of Table 8. Fig. 6 shows the temporal evolution of the substrate degradation, COD, for one experiment of Mixture 1 and one of Mixture 2.
50
1
)
Acidity (g CH3COOH l
1
2.7 3.5 5.6 7.3 8.6
)
Alkalinity (g CO3Ca l
1
)
5.2 6.5 7.9 9.5 12.5
The two experiments were those that gave the greatest degradation percentage, XCOD. In experiment 2, Mixture 1 (liquid effluent only), for example, there was a degradation of 56.9% (see Table 6 and Fig. 6). This suggests that if the goal is to obtain the greatest degree of degradation of the liquid effluent (wastewater plus blood) then the COD load fed to the 2-l biodigester should be that corresponding to 1 g. As may be observed in Table 6, the optimal biodegradation of the liquid effluent (Mixture 1) occurred at low values of the COD feed. For Mixture 2 (liquid effluent plus solids), the optimal value of the COD feed for the maximum biodegradation was 2 g (see Table 8 and Fig. 6). In this case, however, the percentage biodegradation was only 19.1%. In this case too, the percentage degradation declined with increasing COD feed.
3.3.2. Methane yield Figs. 4 and 5 show the values of the volume of methane produced during the biodigestion processes. Fig. 4 corresponds to the digestion of the liquid effluent (Mixture 1) and Fig. 5 to the co-digestion of liquid effluent (97%) and solids (3%) (Mixture 2). In all the experiments, it may be observed a high rate of gas production at the initial times of the reaction, with declining rates of formation as the reaction proceeds. The final values of the methane
45 40
70
35
60 50 -1
25
COD (g l )
-1
COD (g l )
30
20
Experiment 1 Experiment 2
15
Experiment 3 Experiment 4 Experiment 5
10 5 0
40 30
Experiment 1 Experiment 2
20
Experiment 3 Experiment 4
10
Experiment 5 0
0
20
40
60
80
100
120
t (h) Fig. 2. Temporal evolution of the substrate concentration (in mg l 1 COD) with error band of ±5, during the anaerobic biodigestion of the liquid effluent (wastewater plus blood), Mixture 1 for the series of five experiments.
0
20
40
60
80
100
120
t (h) Fig. 3. Temporal evolution of the substrate concentration (in mg l 1 COD) with error band of ±5%, during the anaerobic co-digestion of a mixture of liquid effluent 97% and solid waste (3%), Mixture 2 for the series of five experiments.
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1200
1200 Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5
1000
800
CH4 (ml)
CH4 (ml )
800
1000
600 400
600 400
200
Mixture 1, Experiment 5
200
0 0
20
40
60
80
1 00
Mixture 2, Experiment 5
12 0
0
t (h) Fig. 4. Temporal evolution of the volume of methane with error band of ±5%, produced during the anaerobic digestion of the liquid effluent (wastewater plus blood), Mixture 1.
0
50
100
150
t (h) Fig. 7. Temporal evolution of the methane production with error band of ±5%, during the experiments (no. 5) of greatest methane production for each of the two mixtures.
1000 900
2 (liquid effluent plus solids). It may be observed from Tables 6 and 8 that in both cases there is the greatest production of methane for the highest values of the COD feed (Expt. 5). The figure shows that the two cases present very similar kinetics, although there is a slight difference in favour of Mixture 2 throughout the time.
800
CH4 (ml)
700 600 500 Experiment 1 Experiment 2
400 300
4. Conclusions
Experiment 3 Experiment 4 Experiment 5
200 100 0 0
20
40
60 t (h)
80
100
120
Fig. 5. Temporal evolution of the volume of methane with error band of ±5%, produced during the anaerobic co-digestion of a mixture of liquid effluent (97%) and solid waste (3%), Mixture 2.
Mixture 1, Experiment 2
40
Mixture 2, Experiment 3
35
-1
COD (mg l )
30 25 20 15 10 5 0 0
10
20
30
40
50
60
70
80
t (h)
Fig. 6. Experiments with greatest biodegradation: experiment 2 for Mixture 1 (wastewater plus blood) with a COD load fed into the reactor of 1 g; and experiment 3 for Mixture 2 (liquid effluent plus solids) with a COD load of 2 g, with error band of ±5%.
volume in Figs. 4 and 5 correspond to the values given in the last column of Tables 6 and 8, respectively. Finally, Fig. 7 shows the evolution of the methane produced in the biodigestion of Mixture 1 (wastewater plus blood) and Mixture
1. The calculated total contamination of meat industry in Extremadura during 2001 is equivalent to 4 million p.e., if the disposal of the waste products had been directly into the environment without treatment. The small municipal slaughterhouse of Badajoz would, in such a case, have generated contamination equivalent to a city of 180,000 inhabitants. 2. The following physico-chemical parameters were measured at the beginning, during, and at the end of each biodigestion experiment: pH, alkalinity and volatile acidity, COD, and BOD5. The pH remained within the interval between the initial and final values of 7.73 and 7.34, respectively for Mixture 1 (liquid effluent), and of 7.51 and 7.68, respectively for Mixture 2 (liquid effluent plus solids). The alkalinity oscillates between 6200 and 10,400 mg CO3Ca l 1 for Mixture 1, and 5200 and 12,500 mg CO3Ca l 1 for Mixture 2. With respect to the volatile acidity, the mean values calculated at the end of each series of five experiments were 2.5 and 5.5 g CH3COOH l 1 for Mixtures 1 and 2, respectively. 3. For the biodigester that was used, the anaerobic digestion of the liquid effluent was inhibited for values of the chemical oxygen demand load fed into the reactor, CODl, above 4.5 g, and the anaerobic co-digestion of the liquid effluent plus solids in the proportion of 97% liquid and 3% solids corresponding to the discharge from the Badajoz municipal slaughterhouse was inhibited for CODl above 4 g. 4. Better COD degradation yields were obtained in the Mixture 1 (wastewater plus blood) experiments than in the case of Mixture 2 (liquid effluent plus solids), i.e., a greater level of decontamination of the organic load was obtained for Mixture 1. This is because of the high organic demand present in the solid waste used in Mixture 2, which limits the degrading action of the bacterial population present in the anaerobic sludge.
A. Marcos et al. / Applied Energy 87 (2010) 1729–1735
5. The experiments performed with one type of load, the greatest degradation for Mixture 1 was obtained for an organic load fed into the reactor of 1 g of COD, and for Mixture 2, 2 g of COD. Above these optimal values, the yields were poorer due to the build-up of the organic load, and hence of fatty acids, inhibiting the growth of the bacterial population. 6. With respect to the production of methane, higher productions resulted from greater loads fed into the reactor. Hence, for the two types of load dealt with in the present work, the optimal feed into the 2-l capacity discontinuous digester from the standpoint of methanization of the sample would be 4.5 g of COD for Mixture 1, and 4 g of COD for Mixture 2. Acknowledgements The authors express their gratitude to the Excma. Diputación de Badajoz and to the Junta de Extremadura (Consejería de Agricultura y Medio Ambiente) for the financial support granted for the performance of the present work. References [1] Bouvet A, Bennacer R. Effect of heating rate on Arrhenius law: thermodamageable material. J Energy Inst 2006;79(4):228–31. [2] Bouvet A, Demange D, Bennacer R, Herve Ph. Modelling and characteristic of thermal reactive barrier. J High Temp Mater Process 2006;10:317–25. [3] Hilkiah Igoni A, Ayotamuno MJ, Eze CL, Ogaji SOT, Probert SD. Designs of anaerobic digesters for producing biogas from municipal solid-waste. Appl Energy 2008;85(6):430–8.
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