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The optimization of methane gas recovery from waste material and possibilities for its utilization
Conservation
& Recycling.
Vol. 4. No. 3, pp. I29 - 136, 198
I
0361-3658/81/030129-08$02.~/0
Printed in Great Britain.
Pergamon
Press Ltd.
THE OPTIMIZATION OF METHANE GAS RECOVERY FROM WASTE MATERIAL AND POSSIBILITIES FOR ITS UTILIZATION KOO CHEUL SHIN University of Stuttgart, Institut fiir Siedlungswasserbau, Wassergiite- und Abfallwirtschaft
THE NECESSITY FOR SEWER GAS UTILIZATION Sewer gas (biogas) can be generated from anaerobic decomposition of different waste substances, e.g. from sludge obtained in sewage works, from household refuse and from agricultural waste. In order to run a sewage works economically the managers of the plants are now obliged to show more interest in the maximum usage of this gas. Even though in most of the municipal waste water treatment plants in the Federal Republic of Germany the digesters are available, one quarter of the annual gas production remains unused. In view of the so-called ‘energy crisis’, it seems foolish to burn off sewer gas, a valuable source of energy and one, moreover, produced at high cost. Even in former times sewer gas was used in various ways, for instance as fuel gas for motor vehicles or in agriculture. The cheap oil of the 1960s precluded the utilization of methane gas or prevented its development. Increased oil prices now make sewer gas utilization more and more economical. Further, sewer gas is a constant source of energy, whereas coal and oil, which are used almost exclusively for the production of energy, are not inexhaustible.
GAS QUANTITIES
OBTAINABLE
FROM SEWAGE SLUDGE
Development of sewer gas production and utilization Figure 1 shows the development of sewer gas production and utilization in the Federal Republic of Germany between 195 1 and 1978[1, 21. The enormous increase in sewer gas production can be explained by the fact that, in many waste water treatment plants constructed during the last few years, digesters have also been constructed in spite of the high costs. According to the statement of the ‘Statistisches Bundesamt’ in the Federal Republic of Germany in 1978 some 308 million m3 of sewer gas was produced. As per this statement one can expect a gas production of 340 million m3 in 1980. In the usage of the gas, the heating of digesters and other equipment predominates, consuming approximately 46070, followed by generation of power, at approximately 28%. However, in 1978, a quarter of the entire quantity of gas produced in municipal waste water treatment plants was not utilized. Nowadays, the use of methane as fuel for motor vehicles, is no longer significant. In 1953, the production of methane for fuel was top of the list but, since 1967, it has not been used for this purpose. Even the delivery of sewer gas to gasworks and other users has decreased continuously and is at present insignificant.
Paper presented at the Third Recycling World Congress, Basle, 29 September - 1 October, 1980. 129
130
KOO CHEUL
SHIN
Situation of sewer gas production and utilization At the end of 1979, a general survey of sewage sludge treatment was carried out. Questionnaires, sent to 230 waste water treatment plants, included specified questions on the type of plant and the type of sludge treatment, as well as the quantity and utilization of sewer gas. Of the questionnaires, about 130 forms, (a response rate of 57%) were filled in and returned, giving data on waste water treatment plants which treat 24.4 million population equivalents, equal to 27% of the 91 million population equivalents treated in the municipal sewage works in the Federal Republic of Germany. Table 1 shows the distribution of the treatment plants, examined in this survey, based on the size and population equivalents treated. Table
1. Size, number and population
Size G F E D C B A, A, A,
Size of plants x lo3 p.e.
Number
< 5 510 LO20 2050 50100 loo200 200500 5Ot- 1000 > 1000
Total
equivalents
of sewage works
Total p.e. per class
%
3 7 16 25 24 26 16 8 5
8600 48 000 242 090 791 000 1 853 000 4 069 950 4 830 100 5760000 6 825 000
0.03 0.2 1.0 3.2 7.6 16.7 19.8 23.6 21.9
130
24 427 740
of plants
100.0
According to Table 2, the percentage of sewage works which possess digesters is very high, about 90%. Not only do these plants possess digesters but they also treat 90% of the population equivalents included in this survey. Further, the number of small plants which do not, as a rule, possess digesters, is very small. The high percentage of sewage works with digesters can be explained by the fact that the questionnaires were directed particularly to treatment plants which use digester gas for generation of power. Table 2. Size G F E D C B A, A, At A,-G
Plants
Sewage works with digestion
with dig. (Q)
Pop.
equiv. (%I
100.0 100.0 87.5 92.0 91.7 92.3 81.3 75.0 100.0
100.0 100.0 90.2 94.2 90.6 92.9 83.7 78.3 100.0
90.0
89.5
Dig. volume (m’) -2270 5893 16 358 55 449 91 095 159 435 113 460 139 700 159 850 743 510
Dig. gas (IO@m3/yr) 0.2 0.4 1.3 5.7 9.2 21.9 23.3 26.4 43.1 131.5
In each of the sewage works examined by the survey, all of the raw sludge is digested. The solids content of this sludge was 1 - 6% and the organic space loading of the digesters was 0.15 -4.82 kg org.d.m./mJ,d/. The total digestion volume was 743 510 m3. The annual gas production in this digestion volume amounted to 131.5 million m 3, From these values one can calculate the entire gas production of the Federal Republic of Germany for 1979. Out of 24.43 million population
THE OPTIMIZATION
OF METHANE GAS RECOVERY
FROM
WASTE
MATERIAL
131
equivalents (p.e.) included in the survey under reference (Table l), 89.5% were treated in plants with digesters. This means 24.43 million p.e. x 0.895 = 21.86 million p.e. are treated in plants which possess digesters. Thus, one can calculate the specific gas quantity produced for per population equivalent per year as (131.5 million m3 gas)/(21.86 million p.e.) = 6.02 m3 gas/p.e., yr. The minimum specific gas yield was 1.42 and the maximum 13.29 m3/p.e., yr. Of the examined treatment plants 62% have a specific gas yield of 3.65 - 7.3 m3/p.e., yr. For the total Federal Republic of Germany the amount of digester gas is computed as follows[2]: the waste water of 83% of the population of 61 million is treated in sewage works. On average, 1.8 population equivalents are treated per inhabitant. That means: 61 x 10’ x 1.8 p.e. x 0.83 = 91.1 x lOs p.e. is treated in sewage plants. In 1975, a total of 42.30 x lo6 m3 of raw sludge were obtained, of which 26.37 x 10’ m3 were then digested. That is: (26.37 x lo6 m3)/(42.30 x lo6 m3) x 100 = 62% of the sludge obtained had been digested, a quantity of raw sludge which equals 56.5 x lo6 p.e. 6= 91.1 x 10s p.e. x 0.62). The specific gas production yielded in 1979, as already shown, is 6.02 m3 per p.e.; the entire gas production in 1979 could thus amount to 6.02 m3 gas/p.e., yr x 56.5 x lo6 p.e. = 340 x 10’m3 gas/yr. This value tallies with the expected quantity of gas to be produced in 1979 as shown in Fig. 1: 330 x lo6 mJ,
Fig. 1. Digester
gas production
and utilization
in the Federal
Republic
of Germany.
132
KOO CHEUL SHIN
The generation of power in municipal waste water plants is shown in Table 3. Overall engine power is calculated as 59 164 horsepower. Table 3. Size G F E D c B A, A, A,
Generation of power in sewage works
Engine output (hp) 1793 4574 11 015 12 161 18 583 11 038
~_________ A,-G
Sewage works Population equiv. (%) (Q) -_ . 34.7 35.4 45.5 46.6 47.8 49.9 50.0 53.8 100.0 100.0 40.0 45. I
59 164
37.6
58.0
It is worth noting that power generation takes place only in the bigger treatment plants, i.e. those treating more than 20 000 population equivalents. Even though only 38% of the plants surveyed possess gas motors for the production of power, 58% of the population equivalents are treated in these plants.
GAS QUANTITIES
DERIVED FROM REFUSE
Laboratory digestion tests were carried out to examine the processes of gas evolution from household refuse. A maximum specific gas yield of 320 ml gas/g organic dried matter was achieved at the optimum space loading with organic substances, amounting to 3.9 g organic dried matter per litre per day. If one kg of household refuse contains 34% dried organic matter, one could achieve 320 l/kg 0rg.d.m. x 0.34 = 109 l/kg refuse. The possible gas yield, theoretically, amounts to 1855 l/kg carbon[3]. If in 1 kg of household refuse 12% of effective carbon is present and 50% of it is decomposed, the gas quantity obtained per kg domestic refuse amounts to 1855 l/kg x 0.12 x 0.5 = 111 l/kg refuse. This shows that the quantity of gas yielded in laboratory tests approached the theoretical value. Assuming that, at present, the refuse of 17 x lo6 inhabitants is incinerated, that of 3 x lo6 composted and that of 41 x lo6 is landfilled, we can calculate the possible gas production from household refuse in the Federal Republic of Germany. If the average refuse yield per inhabitant per year is 220 kg for 41 x IO6 inhabitants the gas production amounts to 0.109 m3/kg x 220 kg/person,yr x 41 x lo6 = 983 x lo6 m3 gas. If the gas from only 50% of the tipped refuse could be collected, some 492 x 10s m3 of gas per year would be available. Present research activities show new trends in the use of gas obtained from landfills. These research programs are testing the use of this gas for heating greenhouses, as fuel for motor vehicles, and for anaerobic digestion of pulverized refuse.
QUANTITY
OF GAS OBTAINABLE
FROM AGRICULTURAL
REFUSE
In the field of agriculture, one can generate gas not only from animal waste or excrements but also from vegetable refuse. Table 4 gives a survey of the different kinds of refuse, their quantities and the specific quantities of gas obtainable in the FRG[4]. The obtainable gas
THE OPTIMIZATION
133
OF METHANE GAS RECOVERY FROM WASTE MATERIAL
quantities, as shown in Table 4, are based on a digestion temperature of 30°C and a digestion period of about 15 days, which is also found to be the optimum period for anaerobic decomposition (see section 5). If 5OVoof the agricultural refuse of the FRG were to be used for gas production, one could obtain 3978 x lo6 m3 of gas per year, 60% from vegetable refuse and 40% from animal excrement from stock-farming. Thus, from agricultural refuse one could obtain 8 times as much gas as from household refuse. While considering the values in Table 4, one must also consider that gas yield varies according to the type and composition of the residual substances. One must also consider that the quantity and quality of refuse varies according to place and season. Table 4.
Gas quantities from agricultural substances in the Federal Republic of Germany.
Kind 1978
Stock (IO?
cows Pigs Hens
15.37 23.20 87.63
Excrement (lo6 tonnes/yr 175.0 34.9 6.6
Organic matter (070) 9 7 17
Spec. gas quantity (m3/t.org.m.) 150 240 300
Wheat Rye Barley Mixed corn Oats Maize Potatoes Sugarbeet Turnips
2363 586 337
-~
3286
Total
1975
Total gas quantity ( 108m’/yr)
50%
1643
Products (10’ tonnes/yr)
Waste (10’ tonnes/yr)
Organic matter (Q)
Spec. gas quantity (m3/t org. m.)
Total gas quantity (10’ m3/yr)
7.01 2.12 6.97 1.06 3.44 0.53 10.85 18.20 23.05
10.0 3.5 7.8 1.3 4.4 0.7 4.3 4.7 2.2
70 71 70 70 69 68 51 8 8
200 200 200 200 200 200 300 250 250
1400 497 1092 182 607 95 658 94 44 4669
Total 50%
2335 3978
Total
The expenses involved in the utilization of the energy contained in this refuse cannot be ignored. The agriculturists will introduce biogas plants and thus use their energy potential only if these bring them economic advantages. Further factors include what other facilities for refuse disposal are available to each agricultural plant and what they aim to achieve: (a) (b) (c)
elimination of decomposable substances, deodorizing, further use in agriculture or utilization of agricultural waste in energy production.
OPTIMIZATION
OF THE DIGESTION
PROCESS
Laboratory tests were carried out with sludge and household refuse to analyse the anaerobic digestion process. The digestion vessels were stored in a heated room equipped with heat
134
KOO CHEUL SHIN
insulated to a constant temperature of 33°C. Stirrers sufficed to keep the solids in suspension. The digested sludge was removed every 24 h and an equal quantity of raw sludge was fed in. To establish the limits of loading capacity the addition of raw sludge was increased step by step. Figure 2 shows the gas quantities obtainable for three different types of sludge as a function .--=--
550
-h
‘\ ‘\ 0
h,
i
,
450
;
\
t
\\
u
\
350_y-:.-.-.-.-*.-
j
id ;250= P
.
~,, , , , ,, \ 1
0
??
‘\ \
04
\\X
6 0 Munfclpal 0 lndustriil Huwtes
150 -
sludge sludge
??
I 30
I 20
I 40 Space
I 60
I 50 looding,
I 70
I 80
g org/l,d
Fig. 2. Specific gas yield as a function of the space loading.
of the space loading. For sewage sludge with a high industrial component (‘industrial sludge’) the highest specific gas production 373 ml/g org., day was decidedly less than that produced from municipal sludge, which was 555 ml gas/g org., day. A maximum gas yield of 395 ml/g org., day was achieved from the digestion of humates contained in residues obtained from the food production industry. Table 5 shows the respective digestion periods and load conditions for the different sewage sludges and the corresponding values of the experiments carried out with household refuse. Table 5. Maximum specific gas yield and corresponding loadings Sample
Max gas ’ ml ( ___ g::;d ., )
.
Sewage sludge Small industrial component Large industrial component Humates
(d)
Sy (I,b)
loading (Y)
Subst. loading
12.5
0.080
3.9
0.193
373 395
11.1 10.4
0.090 0.096
3.9 4.1
0.182 0.169
11.3
0.089 _____~
14.3
0.070
320
CH,
($$k)‘“’
555
Average Household refuse
Digestion period
78.5 67.3
4.0 0.181 ~.__. ~~~ ~_~... -. ~~ 3.9
_
64.1
The maximum specific gas production from domestic refuse was 320 ml gas per gram organic dried matter/d, which was significantly less than that derived from industrial sludge. Moreover,
THE OPTIMIZATION
OF METHANE GAS RECOVERY
FROM WASTE
MATERIAL
135
Table 5 draws our attention to the fact that the optimum load of the digestion chamber for sewage sludge and domestic refuse tally with each other. This value amounts to 3.9--4.0 g.org.d.m./l,d. Even in the digestion periods, there is not much difference since they are 11.3 and 14.3 days respectively. To obtain the results that were achieved in the laboratory tests, one must maintain optimum working conditions: viz. sufficient agitation of the sludge in the digestion chamber and removal of scum, continuous charging of the digester and the exact observance of the appropriate digestion temperature.
BIOGAS PLANTS IN KOREA In Korea today about 30 000 biogas plants are already in operation; the gas is used only for cooking. After China and India, Korea has the largest number of biogas plants but most are very simple in construction and, therefore, have to contend with the following shortcomings:
(4 (b)
irregular gas production, a surplus of scum.
Another difficulty is that, to a large extent, gas production is dependent For satisfactory operation a biogas plant requires thorough mixing and Better technical equipment ensures better digestion of waste substances, investment is required which the rural population will hardly be able to Figure 3 shows the energy requirement of a rural household in Korea
on the temperature. continuous charging. but a higher capital afford. and the fuels used to
Fig. 3. Energy demand of a rural household in Korea.
fulfil this need. About 48% is used for heating and 52% for cooking. A household has to spend US $500 per year for energy, comprising US $240 for heating and US $260 for cooking. By using biogas, the cost of energy for cooking could be reduced by 65% as shown in Table 6. If 50070,or 1.25 million of the rural households in Korea used biogas for cooking, US $211 x IO6 per year would be saved. Apart from this, the wood conserved could be used for other purposes.
136
KOO CHEUL SHIN Table 6.
Energy savings through use of biogas in Korea
Energy requirements for cooking (US $/household,yr) Straw Wood Coal ___Oil .~~
Energy saving by biogas (Per household,yr) 1.25 households = 50% (US $/yr) (US $) (070)
104 127
67 82
26 31
83.8 102.5
29
20
H
25.0
260
169
65
211.3
CONCLUSION Following rapid increases in the price of energy, and increased awareness of potential shortages, increasing attention is being devoted to sewer or biogas as a source of energy. The minimum price of fuel oil which makes the utilization of gas economical in the Federal Republic of Germany has already been exceeded. Therefore, the use of sewer gas will inevitably increase, in spite of all the above-mentioned limitations.
REFERENCES 1. Abwassertechnische
2. 3. 4. 5.
Vereinigung: Lehr- und Handbuch der Abwassertechnik Band Ill, 2. Auflage, Verlag W Ernst (1978). Statistisches Bundesamt: Angaben der Abteihmg IV B 4. K. C. Shin, Biologische Reinigung hochkonzentrierter Abwiisser durch anaerobe alkalische Giirung, Dissertation, TH Stuttgart (1961). W. Baader, E. Dohne and M. BrenndBrfer, Biogas in Theorie und Praxis: KTBL-Schrift, 229 (1978). S. Y. Ahn and M. H. Kim, Verwertung von Methangas, Jeilmunha-Verlag, Korea (1973).
& Recycling.
Vol. 4. No. 3, pp. I29 - 136, 198
I
0361-3658/81/030129-08$02.~/0
Printed in Great Britain.
Pergamon
Press Ltd.
THE OPTIMIZATION OF METHANE GAS RECOVERY FROM WASTE MATERIAL AND POSSIBILITIES FOR ITS UTILIZATION KOO CHEUL SHIN University of Stuttgart, Institut fiir Siedlungswasserbau, Wassergiite- und Abfallwirtschaft
THE NECESSITY FOR SEWER GAS UTILIZATION Sewer gas (biogas) can be generated from anaerobic decomposition of different waste substances, e.g. from sludge obtained in sewage works, from household refuse and from agricultural waste. In order to run a sewage works economically the managers of the plants are now obliged to show more interest in the maximum usage of this gas. Even though in most of the municipal waste water treatment plants in the Federal Republic of Germany the digesters are available, one quarter of the annual gas production remains unused. In view of the so-called ‘energy crisis’, it seems foolish to burn off sewer gas, a valuable source of energy and one, moreover, produced at high cost. Even in former times sewer gas was used in various ways, for instance as fuel gas for motor vehicles or in agriculture. The cheap oil of the 1960s precluded the utilization of methane gas or prevented its development. Increased oil prices now make sewer gas utilization more and more economical. Further, sewer gas is a constant source of energy, whereas coal and oil, which are used almost exclusively for the production of energy, are not inexhaustible.
GAS QUANTITIES
OBTAINABLE
FROM SEWAGE SLUDGE
Development of sewer gas production and utilization Figure 1 shows the development of sewer gas production and utilization in the Federal Republic of Germany between 195 1 and 1978[1, 21. The enormous increase in sewer gas production can be explained by the fact that, in many waste water treatment plants constructed during the last few years, digesters have also been constructed in spite of the high costs. According to the statement of the ‘Statistisches Bundesamt’ in the Federal Republic of Germany in 1978 some 308 million m3 of sewer gas was produced. As per this statement one can expect a gas production of 340 million m3 in 1980. In the usage of the gas, the heating of digesters and other equipment predominates, consuming approximately 46070, followed by generation of power, at approximately 28%. However, in 1978, a quarter of the entire quantity of gas produced in municipal waste water treatment plants was not utilized. Nowadays, the use of methane as fuel for motor vehicles, is no longer significant. In 1953, the production of methane for fuel was top of the list but, since 1967, it has not been used for this purpose. Even the delivery of sewer gas to gasworks and other users has decreased continuously and is at present insignificant.
Paper presented at the Third Recycling World Congress, Basle, 29 September - 1 October, 1980. 129
130
KOO CHEUL
SHIN
Situation of sewer gas production and utilization At the end of 1979, a general survey of sewage sludge treatment was carried out. Questionnaires, sent to 230 waste water treatment plants, included specified questions on the type of plant and the type of sludge treatment, as well as the quantity and utilization of sewer gas. Of the questionnaires, about 130 forms, (a response rate of 57%) were filled in and returned, giving data on waste water treatment plants which treat 24.4 million population equivalents, equal to 27% of the 91 million population equivalents treated in the municipal sewage works in the Federal Republic of Germany. Table 1 shows the distribution of the treatment plants, examined in this survey, based on the size and population equivalents treated. Table
1. Size, number and population
Size G F E D C B A, A, A,
Size of plants x lo3 p.e.
Number
< 5 510 LO20 2050 50100 loo200 200500 5Ot- 1000 > 1000
Total
equivalents
of sewage works
Total p.e. per class
%
3 7 16 25 24 26 16 8 5
8600 48 000 242 090 791 000 1 853 000 4 069 950 4 830 100 5760000 6 825 000
0.03 0.2 1.0 3.2 7.6 16.7 19.8 23.6 21.9
130
24 427 740
of plants
100.0
According to Table 2, the percentage of sewage works which possess digesters is very high, about 90%. Not only do these plants possess digesters but they also treat 90% of the population equivalents included in this survey. Further, the number of small plants which do not, as a rule, possess digesters, is very small. The high percentage of sewage works with digesters can be explained by the fact that the questionnaires were directed particularly to treatment plants which use digester gas for generation of power. Table 2. Size G F E D C B A, A, At A,-G
Plants
Sewage works with digestion
with dig. (Q)
Pop.
equiv. (%I
100.0 100.0 87.5 92.0 91.7 92.3 81.3 75.0 100.0
100.0 100.0 90.2 94.2 90.6 92.9 83.7 78.3 100.0
90.0
89.5
Dig. volume (m’) -2270 5893 16 358 55 449 91 095 159 435 113 460 139 700 159 850 743 510
Dig. gas (IO@m3/yr) 0.2 0.4 1.3 5.7 9.2 21.9 23.3 26.4 43.1 131.5
In each of the sewage works examined by the survey, all of the raw sludge is digested. The solids content of this sludge was 1 - 6% and the organic space loading of the digesters was 0.15 -4.82 kg org.d.m./mJ,d/. The total digestion volume was 743 510 m3. The annual gas production in this digestion volume amounted to 131.5 million m 3, From these values one can calculate the entire gas production of the Federal Republic of Germany for 1979. Out of 24.43 million population
THE OPTIMIZATION
OF METHANE GAS RECOVERY
FROM
WASTE
MATERIAL
131
equivalents (p.e.) included in the survey under reference (Table l), 89.5% were treated in plants with digesters. This means 24.43 million p.e. x 0.895 = 21.86 million p.e. are treated in plants which possess digesters. Thus, one can calculate the specific gas quantity produced for per population equivalent per year as (131.5 million m3 gas)/(21.86 million p.e.) = 6.02 m3 gas/p.e., yr. The minimum specific gas yield was 1.42 and the maximum 13.29 m3/p.e., yr. Of the examined treatment plants 62% have a specific gas yield of 3.65 - 7.3 m3/p.e., yr. For the total Federal Republic of Germany the amount of digester gas is computed as follows[2]: the waste water of 83% of the population of 61 million is treated in sewage works. On average, 1.8 population equivalents are treated per inhabitant. That means: 61 x 10’ x 1.8 p.e. x 0.83 = 91.1 x lOs p.e. is treated in sewage plants. In 1975, a total of 42.30 x lo6 m3 of raw sludge were obtained, of which 26.37 x 10’ m3 were then digested. That is: (26.37 x lo6 m3)/(42.30 x lo6 m3) x 100 = 62% of the sludge obtained had been digested, a quantity of raw sludge which equals 56.5 x lo6 p.e. 6= 91.1 x 10s p.e. x 0.62). The specific gas production yielded in 1979, as already shown, is 6.02 m3 per p.e.; the entire gas production in 1979 could thus amount to 6.02 m3 gas/p.e., yr x 56.5 x lo6 p.e. = 340 x 10’m3 gas/yr. This value tallies with the expected quantity of gas to be produced in 1979 as shown in Fig. 1: 330 x lo6 mJ,
Fig. 1. Digester
gas production
and utilization
in the Federal
Republic
of Germany.
132
KOO CHEUL SHIN
The generation of power in municipal waste water plants is shown in Table 3. Overall engine power is calculated as 59 164 horsepower. Table 3. Size G F E D c B A, A, A,
Generation of power in sewage works
Engine output (hp) 1793 4574 11 015 12 161 18 583 11 038
~_________ A,-G
Sewage works Population equiv. (%) (Q) -_ . 34.7 35.4 45.5 46.6 47.8 49.9 50.0 53.8 100.0 100.0 40.0 45. I
59 164
37.6
58.0
It is worth noting that power generation takes place only in the bigger treatment plants, i.e. those treating more than 20 000 population equivalents. Even though only 38% of the plants surveyed possess gas motors for the production of power, 58% of the population equivalents are treated in these plants.
GAS QUANTITIES
DERIVED FROM REFUSE
Laboratory digestion tests were carried out to examine the processes of gas evolution from household refuse. A maximum specific gas yield of 320 ml gas/g organic dried matter was achieved at the optimum space loading with organic substances, amounting to 3.9 g organic dried matter per litre per day. If one kg of household refuse contains 34% dried organic matter, one could achieve 320 l/kg 0rg.d.m. x 0.34 = 109 l/kg refuse. The possible gas yield, theoretically, amounts to 1855 l/kg carbon[3]. If in 1 kg of household refuse 12% of effective carbon is present and 50% of it is decomposed, the gas quantity obtained per kg domestic refuse amounts to 1855 l/kg x 0.12 x 0.5 = 111 l/kg refuse. This shows that the quantity of gas yielded in laboratory tests approached the theoretical value. Assuming that, at present, the refuse of 17 x lo6 inhabitants is incinerated, that of 3 x lo6 composted and that of 41 x lo6 is landfilled, we can calculate the possible gas production from household refuse in the Federal Republic of Germany. If the average refuse yield per inhabitant per year is 220 kg for 41 x IO6 inhabitants the gas production amounts to 0.109 m3/kg x 220 kg/person,yr x 41 x lo6 = 983 x lo6 m3 gas. If the gas from only 50% of the tipped refuse could be collected, some 492 x 10s m3 of gas per year would be available. Present research activities show new trends in the use of gas obtained from landfills. These research programs are testing the use of this gas for heating greenhouses, as fuel for motor vehicles, and for anaerobic digestion of pulverized refuse.
QUANTITY
OF GAS OBTAINABLE
FROM AGRICULTURAL
REFUSE
In the field of agriculture, one can generate gas not only from animal waste or excrements but also from vegetable refuse. Table 4 gives a survey of the different kinds of refuse, their quantities and the specific quantities of gas obtainable in the FRG[4]. The obtainable gas
THE OPTIMIZATION
133
OF METHANE GAS RECOVERY FROM WASTE MATERIAL
quantities, as shown in Table 4, are based on a digestion temperature of 30°C and a digestion period of about 15 days, which is also found to be the optimum period for anaerobic decomposition (see section 5). If 5OVoof the agricultural refuse of the FRG were to be used for gas production, one could obtain 3978 x lo6 m3 of gas per year, 60% from vegetable refuse and 40% from animal excrement from stock-farming. Thus, from agricultural refuse one could obtain 8 times as much gas as from household refuse. While considering the values in Table 4, one must also consider that gas yield varies according to the type and composition of the residual substances. One must also consider that the quantity and quality of refuse varies according to place and season. Table 4.
Gas quantities from agricultural substances in the Federal Republic of Germany.
Kind 1978
Stock (IO?
cows Pigs Hens
15.37 23.20 87.63
Excrement (lo6 tonnes/yr 175.0 34.9 6.6
Organic matter (070) 9 7 17
Spec. gas quantity (m3/t.org.m.) 150 240 300
Wheat Rye Barley Mixed corn Oats Maize Potatoes Sugarbeet Turnips
2363 586 337
-~
3286
Total
1975
Total gas quantity ( 108m’/yr)
50%
1643
Products (10’ tonnes/yr)
Waste (10’ tonnes/yr)
Organic matter (Q)
Spec. gas quantity (m3/t org. m.)
Total gas quantity (10’ m3/yr)
7.01 2.12 6.97 1.06 3.44 0.53 10.85 18.20 23.05
10.0 3.5 7.8 1.3 4.4 0.7 4.3 4.7 2.2
70 71 70 70 69 68 51 8 8
200 200 200 200 200 200 300 250 250
1400 497 1092 182 607 95 658 94 44 4669
Total 50%
2335 3978
Total
The expenses involved in the utilization of the energy contained in this refuse cannot be ignored. The agriculturists will introduce biogas plants and thus use their energy potential only if these bring them economic advantages. Further factors include what other facilities for refuse disposal are available to each agricultural plant and what they aim to achieve: (a) (b) (c)
elimination of decomposable substances, deodorizing, further use in agriculture or utilization of agricultural waste in energy production.
OPTIMIZATION
OF THE DIGESTION
PROCESS
Laboratory tests were carried out with sludge and household refuse to analyse the anaerobic digestion process. The digestion vessels were stored in a heated room equipped with heat
134
KOO CHEUL SHIN
insulated to a constant temperature of 33°C. Stirrers sufficed to keep the solids in suspension. The digested sludge was removed every 24 h and an equal quantity of raw sludge was fed in. To establish the limits of loading capacity the addition of raw sludge was increased step by step. Figure 2 shows the gas quantities obtainable for three different types of sludge as a function .--=--
550
-h
‘\ ‘\ 0
h,
i
,
450
;
\
t
\\
u
\
350_y-:.-.-.-.-*.-
j
id ;250= P
.
~,, , , , ,, \ 1
0
??
‘\ \
04
\\X
6 0 Munfclpal 0 lndustriil Huwtes
150 -
sludge sludge
??
I 30
I 20
I 40 Space
I 60
I 50 looding,
I 70
I 80
g org/l,d
Fig. 2. Specific gas yield as a function of the space loading.
of the space loading. For sewage sludge with a high industrial component (‘industrial sludge’) the highest specific gas production 373 ml/g org., day was decidedly less than that produced from municipal sludge, which was 555 ml gas/g org., day. A maximum gas yield of 395 ml/g org., day was achieved from the digestion of humates contained in residues obtained from the food production industry. Table 5 shows the respective digestion periods and load conditions for the different sewage sludges and the corresponding values of the experiments carried out with household refuse. Table 5. Maximum specific gas yield and corresponding loadings Sample
Max gas ’ ml ( ___ g::;d ., )
.
Sewage sludge Small industrial component Large industrial component Humates
(d)
Sy (I,b)
loading (Y)
Subst. loading
12.5
0.080
3.9
0.193
373 395
11.1 10.4
0.090 0.096
3.9 4.1
0.182 0.169
11.3
0.089 _____~
14.3
0.070
320
CH,
($$k)‘“’
555
Average Household refuse
Digestion period
78.5 67.3
4.0 0.181 ~.__. ~~~ ~_~... -. ~~ 3.9
_
64.1
The maximum specific gas production from domestic refuse was 320 ml gas per gram organic dried matter/d, which was significantly less than that derived from industrial sludge. Moreover,
THE OPTIMIZATION
OF METHANE GAS RECOVERY
FROM WASTE
MATERIAL
135
Table 5 draws our attention to the fact that the optimum load of the digestion chamber for sewage sludge and domestic refuse tally with each other. This value amounts to 3.9--4.0 g.org.d.m./l,d. Even in the digestion periods, there is not much difference since they are 11.3 and 14.3 days respectively. To obtain the results that were achieved in the laboratory tests, one must maintain optimum working conditions: viz. sufficient agitation of the sludge in the digestion chamber and removal of scum, continuous charging of the digester and the exact observance of the appropriate digestion temperature.
BIOGAS PLANTS IN KOREA In Korea today about 30 000 biogas plants are already in operation; the gas is used only for cooking. After China and India, Korea has the largest number of biogas plants but most are very simple in construction and, therefore, have to contend with the following shortcomings:
(4 (b)
irregular gas production, a surplus of scum.
Another difficulty is that, to a large extent, gas production is dependent For satisfactory operation a biogas plant requires thorough mixing and Better technical equipment ensures better digestion of waste substances, investment is required which the rural population will hardly be able to Figure 3 shows the energy requirement of a rural household in Korea
on the temperature. continuous charging. but a higher capital afford. and the fuels used to
Fig. 3. Energy demand of a rural household in Korea.
fulfil this need. About 48% is used for heating and 52% for cooking. A household has to spend US $500 per year for energy, comprising US $240 for heating and US $260 for cooking. By using biogas, the cost of energy for cooking could be reduced by 65% as shown in Table 6. If 50070,or 1.25 million of the rural households in Korea used biogas for cooking, US $211 x IO6 per year would be saved. Apart from this, the wood conserved could be used for other purposes.
136
KOO CHEUL SHIN Table 6.
Energy savings through use of biogas in Korea
Energy requirements for cooking (US $/household,yr) Straw Wood Coal ___Oil .~~
Energy saving by biogas (Per household,yr) 1.25 households = 50% (US $/yr) (US $) (070)
104 127
67 82
26 31
83.8 102.5
29
20
H
25.0
260
169
65
211.3
CONCLUSION Following rapid increases in the price of energy, and increased awareness of potential shortages, increasing attention is being devoted to sewer or biogas as a source of energy. The minimum price of fuel oil which makes the utilization of gas economical in the Federal Republic of Germany has already been exceeded. Therefore, the use of sewer gas will inevitably increase, in spite of all the above-mentioned limitations.
REFERENCES 1. Abwassertechnische
2. 3. 4. 5.
Vereinigung: Lehr- und Handbuch der Abwassertechnik Band Ill, 2. Auflage, Verlag W Ernst (1978). Statistisches Bundesamt: Angaben der Abteihmg IV B 4. K. C. Shin, Biologische Reinigung hochkonzentrierter Abwiisser durch anaerobe alkalische Giirung, Dissertation, TH Stuttgart (1961). W. Baader, E. Dohne and M. BrenndBrfer, Biogas in Theorie und Praxis: KTBL-Schrift, 229 (1978). S. Y. Ahn and M. H. Kim, Verwertung von Methangas, Jeilmunha-Verlag, Korea (1973).