Emission of methane into the atmosphere from landfills in the former USSR

Chemosphere,Vol.26,Nos.l-4, pp 401-417, 1993 Printed in Great Britain

0045-6535/93 $6.00 + 0.00 PergamonPress Ltd.

EMISSION OF METHANE INTO THE ATMOSPHERE FROM LANDFILLS IN THE FORMER USSR

Alia N. Nozhevnikova .1, A.B. Lifshitz 2 V.S. Lebedev 3, and G.A. Zavarzin 1

1Institute of Microbiology, Acad. Sci. Russia, Pr. 60 let Octiabrya, 7, k. 2, Moscow, 117811, Russia 2Firm Geopolis, Podolsky Kursantov str., 22-A, 113545, Moscow, Russia 3All-Union Institute Geoinformsystem, Ramensky branch. Pos. Neftegasosjomka, Ramenskoe, Moscow reg., 140100, Russia (Received in USA 13 November 1991; accepted 15 June 1992)

ABSTRACT The annual production of solid domestic wastes by population of big cities of the USSR is about 37.5 million tons. The main method of disposal is burial in designated landfills. In the USSR large landfills occupy an area of more than 140,000 hectares. It has been calculated that the mass of the landfill deposits generating methane is today about 600 million tons. The studies carried out using geophysical, isotopic, and microbiological methods at different landfills of the Moscow region have shown that the emission of methane and other gases from the surface of landfills into the atmosphere is extremely irregular and considerably less than their generation in the anaerobic zone. The most important factors determining methane emission are the thickness of the layer of buried refuse, the heterogeneity of the deposit body, and the microbiological oxidation of gases in the upper aerated ground layer. It has been shown that the temperature in the anaerobic zone of big landfills is relatively constant and in most cases is 25-35°C. Methanogenesis often is most intensive in the upper part of anaerobic zone where the content of organic matter is rather high. The stable carbon isotope composition of the biogas generated from landfills is characteristic of the methanogenesis from organic wastes and depends on the concentration of organic matter and the age of the landfill. At first a lighter gas is generated and then a heavier one as the substrate is depleted. In the upper aerated ground layer of the landfill, about 1 meter in depth, methane, hydrogen, and carbon monoxide (CH4, HE, CO) are oxidized intensively. The number of bacteria oxidizing these gases reaches 1011 cells per gram of wet refuse. In this case the stable isotope composition of methane becomes heavier and, of carbon dioxide, lighter. It has been shown that at small landfills methane can be oxidized completely in aerobic zones. The gas-oxidizing ability of the microflora of the aerated ground layer of a landfill decrease considerably in the cold season of the year. The methane emission from landfills located in the USSR is estimated at 1.2 - 2.4 billion cubic meters per year. About two-thirds comes from the European part of the country. Effective methods of decreasing methane emission into the atmosphere are the extraction of biogas from big landfills and the maintenance of good aeration of the upper ground layer at the small ones.

401

402

1. INTRODUCTION The yearly emission of methane from the Earth's surface into the atmosphere is estimated to be 550 Tg (Khalil and Rasmussen, 1983, 1990; Cicerone and Oremland, 1988).

Significant impact is made by

landfills. The ratio of the landfill source in the CH 4 budget from the overland sources varies, according to the different authors, from 2% to 7% (Gorbatiuk et al., 1989). The anaerobic decomposition of municipal solid waste may account for as much as 7 to 20% of global anthropogenic sources of CH 4 emission (Khalil and Rasmussen, 1990; Richards, 1989; Bingemer and Crutzen, 1987). It has been calculated that landfills globally cause the annual emission into the atmosphere of about 9-20 Tg of CH 4 and 20-43 Tg of CO 2 (Minko et al., 1989; Richards, 1988). The amount of waste corresponds to the density distribution of the city population. The yearly production of municipal solid wastes (MSW) by the world population is about 450-500 million tons.

About 320 to 350 million tons are buried every year in the specially equipped

designated landfills and spontaneous open dumps followed by anaerobic degradation and methane production (Gorbatiuk et al., 1989). In addition to methane and carbon dioxide, the landfill sites are a powerful source of different gaseous products and volatile organic compounds, often extremely toxic ones. Among them are carbon monoxide, hydrogen sulfide, nitrogen oxides, and various hydrocarbons, including the halogenated ones, etc. At present more than 130 components of the dump biogas have been identified (Willumsen et al., 1988; Minko et al., 1990). The gas is a complex mixture of anaerobic process products and substances of anthropogenic origin and has no natural analogs. An already formed mature landfill is an analog of an industrial solid-phase bioreactor of geological scale that functions actively for 20-40 years (Gorbatiuk et al., 1989). The refuse depth can be conventionally divided vertically into several zones differing by the character of microbiological processes: aerobic (0 - 1.5 m), transition (0.5 - 2 m), and anaerobic (1.5 - 20 m and greater). Due to interzonal gas exchange, the nutritive structure of the landfill microbial symbiosis is formed. Anaerobic microorganisms have the role of gas producer, and aerobic ones have the role of consumer by carrying out the oxidation process. The upper aerated landfill cover layer is a biogeochemical barrier for penetration of the atmospheric air into the lower landfill layers on one side and of gaseous products of their vital activity on the other.

In this way the

"bacterial filter" is formed. Gaseous oxidation products and the unoxidized products of the anaerobic zone are emitted into the atmosphere.

2. UTILIZATION O F MUNICIPAL SOLID WASTES IN T H E USSR The yearly production of municipal solid wastes (MSW), according to the norms existing in the former USSR, was estimated to be 1 m 3 or about 300 kg per capita of urban population (Gorbatiuk et al., 1988). The most significant organic wastes are paper (60%) and food (25%); the amount of plastic waste is estimated to be 5% (Gorbatiuk et al., 1988, 1989). The annual production of solid domestic wastes by the 124.6 million population of big cities of the USSR is about 37.5 million tons. The main method of disposing

403

of the waste is by burying it in designated landfills. Part of the waste is accumulated at open dumps. It is estimated that more than 80% of MSW is degraded in anaerobic conditions. In the USSR big landfills occupy an area of more than 140,000 hectares. Assuming that landfills are active for 20-25 years after closing (Gorbatiuk et al., 1989), it has been calculated that the mass of the landfill deposits generating methane is today about 600 million tons. The areas of single landfills vary from a few hectares to several hundred hectares. The depth of the refuse layer usually does not exceed 30-50 m, and the organic content of the layer reaches 60%. Until the present time, industrial solid wastes were often buried together with domestic wastes. Only recently bottom soil compaction and isolation of the areas intended for landfills has begun in the former USSR. Besides the especially equipped landfills, a part of municipal and industrial solid wastes were buried in open dumps close to the cities, usually in ravines. As communities increase in size, such dumps often are found within the town. In Moscow, with a population of 12 million, more than 80 small spontaneous dumps have been registered. More than 50 big landfills are situated around Moscow.

3. OBJECTS AND METHODS Several big landfills and small dumps situated in the Moscow region were investigated in order to study the production and content of biogas in an anaerobic zone and its migration and oxidation in an aerated upper layer, and to determine gas flows from the surface to the atmosphere. Studies of gas emission rates have been carried out at 10 large and small landfills around Moscow (Gorbatiuk et al., 1988); some of them were investigated in more detail. This report is based on data obtained at three landfills of different types. The exploitation of big old landfill "Kuchino" was over 15 years ago. Its refuse thickness was 8 to 20 m.; the occupied area was 60 hectares; volume was 24 million cubic meters; the content of organic carbon (Corg) was 5-12% (mean value 8.85%).

Landfill "Mitino" was closed a year and a half before the

measurement of gas emission. It occupies an area of more than 100 hectares; the average refuse thickness is 15 m; the content of Corg is 11-12%. The spontaneous dump "Ramenki" was formed on the site of a former ravine in southwest part of Moscow. The ravine was filled with a sand-clay mixture combined with municipal and construction solid wastes in 1977-1980. The dump area is 6 hectares; the thickness of the refuse is 2-5 meters, up to 10 meters in the central part; the content of Corg is less than 5%. More detailed characteristics of investigated landfills and dumps are described elsewhere (Nozhevnikova et al., 1989, 1992). At each landfill evenly-spaced wells were drilled, and gas and refuse samples were taken from different depths for laboratory studies. During the investigation of the upper ground layers (to the depth of 1 m) samples were taken with a tube sampler. Under natural conditions, surface-to-atmosphere gas emissions were determined with accumulative l-liter volume vessels with tubes for gas sampling. Accumulative vessels were placed on the surface of the landfill at specific points. Samples of gas were taken periodically (Nozhevnikova et al., 1992). The camera-kinetic method was used for establishing gas content also (Orlow and Minko, 1987). The gas samples were analyzed by gas chromatograph (GC) using a Chrom-5

404

with a flame-ionization detector; the gas carrier was argon; the stationary phase was silica gel (Nozhevnikova et al., 1989). In order to determine the microcomponent content of the landfill, gas samples were taken into adsorber tubes and analyzed by GC-mass-spectrometry (Minko et al., 1990). The stable carbon isotopic content of biogas was measured by the relative compensation method; results were expressed in per mil (%0) 613C to standard PDB (Lebedev et al., 1988). Solid refuse samples were collected by drilling. Soon after the drills were withdrawn, pH and temperature were measured and the samples placed in containers filled to the top and hermetically sealed. Total suspended solids (TSS) in ground samples were determined by drying the samples to a stable weight at 105°C. The content of organic carbon was determined on a Rock Avel pyrolysis apparatus. Microbial biomass was determined by the modified Irdzhens method according to the amount of DNA (Vedenina and Slobodkin, 1988). Volatile fatty acids were analyzed with a Chrom-5 chromatograph with a flame-ionization detector; helium was the gas carrier; Chromosorb-101 was the stationary phase. The intensity of methane formation by specimens of natural landfill soil samples was determined at 25°C in 3-liter laboratory digesters completely filled with fresh ground samples. The volume of gas being formed was measured by collecting it over water saturated with NaC1. The methane concentration was determined by gas chromatography.

Activity of methanogenic microbial association, degrading organic

substances, was characterized according to the methane formation from organic material of landfill refuse and when specific substrates (methanol, acetate, formate, methylamine, hydrogen-carbon dioxide mixture) were added. The original refuse samples (5-gram) were diluted to a moisture content of 97% with mineral medium and cultivated in 120-ml bottles. The gas space of the bottles was filled with nitrogen to establish anaerobic conditions. Enrichment cultures of anaerobic bacteria were obtained with specific substrates (Nozhevnikova and Yagodina, 1982). The intensity of the methanogenesis was also characterized by the radioisotope method with 14C-acetate and 14C-methylamine as methane precursors (Nozhevnikova et al., 1989). All experiments and measurements were performed in triplicate. The activity of gas-oxidizing microflora of the upper landfill ground layer was investigated in experiments with fresh soil samples from the depth 10 to 90 cm. Five gram soil samples, moistened with 10 ml of sterile water or phosphate buffer, were put into 120-ml bottles in triplicate; the gas space was filled with the necessary gas mixture of methane, hydrogen, carbon monoxide, nitrogen, and oxygen. Samples were cultivated at 25°C and 6°C. Consumption of gases was measured by gas chromatography. To determine the number of methanotrophic bacteria, to obtain enrichment cultures, and to cultivate isolates, mineral medium of Wittenbury in gas mixture of methane and air (1:1) was used (Galchenko et a1.,1986). Mineral medium of Schlegel in gas atmospheres of hydrogen, carbon dioxide, oxygen (70:10:20) or carbon monoxide and air (1:1) respectively was used for determinating the number, cultivation, and isolation of hydrogen-oxidizing bacteria and carboxydobacteria (Nozhevnikova and Yurganov, 1978). The concentration of different gasoxidizing bacteria in soil samples was measured by the inoculation of a measured amount of soil and dilutions on an elective medium. Identification of hydrogen-oxidizing bacteria and carboxydobacteria was made after isolation of colonies on agar media. Identification of methanotrophic bacteria was made with immunoserums

405

(Galchenko, 1990).

4. M E T H A N E PRODUC"I'ION IN THE ANAEROBIC ZONE The landfill is an extremely heterogeneous formation, resulting from the practice of deposition of wastes. The intensity of methane production depends on different factors. The most important of these are anaerobic conditions, concentration and quality of organic material, and the state of methanogenic microbial community, which must be well balanced. It sometimes takes several years before methanogenic microflora are accumulated and good equilibrium between all microbial groups involved in methanogenesis from complex organic compounds is achieved. This why sometimes in "fresh" landfills acidic fermentations and butyrate, propionate, acetate, butanol, acetone, hydrogen, etc., are produced as end products.

Their

concentration in landfill gas sometimes reaches hundreds, even thousands mg per m 3. In biogas from some parts of Kuchino landfill concentrations of these compounds were (in mg/m 3 of gas): acetic acid, 10.0; butyric acid, 15.0; acetone, 2.5; iso-pentanol, 4.5; n- and iso-butanol, 0.3; ethylacetate and n-butylacetate, 0.5 (Minko et al., 1990). Investigation of the gas probes taken from a pit from the depth of 2-20 m (usually every 2-3 m) has shown that in the anaerobic zone of the "mature" landfills the gas phase is represented by the mixture of methane (50-70%) and carbon dioxide (30-50%).

Microimpurities, the main components of which are

nitrogen, hydrogen, carbon monoxide, hydrogen sulfide, ammonia, may constitute only a few per cent or less. The hydrogen concentration usually is not higher than 0.05%, sometimes reaching 1%; the carbon monoxide concentration varies from 0 to 0.01%. Maximum concentrations of hydrogen 8% and carbon monoxide 2% were observed at a landfill where municipal and industrial wastes were buried together. The total amount of the fermentation volatile products in the landfill gas may reach 30-40 mg/m 3 (Minko at al., 1990). Because of the high concentration of organic material, microbiological processes inside landfills are very active. Temperatures at different depths in big landfills usually are about 30°C and up to 55°C at some points of the near-surface layer. The character of the vertical distribution of the CH 4 generation rate in different parts of the landfill may be different. The active methanogenic zones may be located in different points of the landfill body. The laboratory research of ground samples has shown that the character of the vertical distribution of the rate of methane generation may be different in distinct sectors of the same landfill. There are sectors in the thickest part of the mature Kuchino landfill where methane production increases with the depth. But more characteristic for this landfill was that maximum methanogenic activity is in the upper layer of the anaerobic zone, relatively rich in fresh organic substance. Laboratory investigation of the Kuchino landfill ground samples from different sampling pits at depths from 0.5 to 18 m has shown that the layer from the depth of 2 - 6 m is the most active (Figure 1). In samples from lower horizons the rate of methanogenesis decreased. The content of total solids (TS) in soil samples from different depths was more or less constant. The content of organic carbon, microbial biomass,

406

and volatile fatty acids also decreases with depth, indicating deep degradation of organic substance (Table 1). The

methanogenesis rate from 14C-acetate (calculated for total acetate on the basis of the known

addition of labeled acetate) was 15 times higher for the probe from 5.5 to 8 m than for the probe from 13 to 16 m.

Changes in the activity of cellulolytic and methanogenic bacteria utilizing acetate, methanol,

methylamines, formate, and H 2 + CO 2 mixture coincided with the type of methanogenesis rate distribution. At the same time the numbers of these bacteria in samples from deeper horizons of refuse was high, and activity of anaerobic microflora rapidly recovered after the addition of corresponding substrates (Nozhevnikova et al., 1989).

depth, m

6 8 10

qZ 16

I

0

200

/+00

'

I

600

CHz,cm3. kg -I. month -I Fig. 1. Rate of methane production by ground samples from different depths of Kuchino landfill (cm 3 CH 4 /kg of wet ground per month).

Table 1. The biochemical characteristics and methanogenic activity of refuse samples from different depths of the Kuchino landfill. Depth

TSS

Corg

Microbial biomass

VFA

Rate of CH 4 production

m

kg.m 3

% of TSS

% of VSS

mol.kgq

mol.kg 1 TSS.d "1

0.5 - 1.0

0.35

8.8

n.d.

n.d.

0.28

2.5 - 5.0

0.33

11.4

35

3.3-5.0

2.17

8.0- 11.0

0.37

7.4

19

0.33-0.5

1.22

16.0- 18.0

0.34

4.9

21

0.015-0.016

0.8

407

The temperature in most pits was 25-31°C, almost unchanging with depth.

Investigation of the

temperature and humidity effect on the intensity of methanogenesis by the refuse microflora showed maximum activity at 35-40°C and the total solid (TS) content in the fermented mass of 4-10% (Nozhevnikova et al., 1989). The rate of methanogenesis by microflora of the isolated refuse samples from the Ramenki dump, from 2 - 7 m depth, was almost 10 times lower than at the same depths of the Kuchino landfill. The temperature of the refuse in the Ramenki dump did not exceed 15°C. Usually the concentration of methane and carbon dioxide in the biogas of lower horizons were at ratio 1.5-2, mean value 1.75. The composition of gas generated in Ramenki was calculated as 63% CH4, 36% CO2, and 1% N2, with the ratio 1:0.57:0.016. Isotopic composition of carbon of biogas of all landfills was typical for methanogenesis from organic materials. B13C varied for CH 4 from -50 to -60%0 and for CO 2 from -10 to +2.7%0. In the Ramenki dump 6 13C for methane was -50 to -55%0 and for carbon dioxide 0 to -10%o. The carbon of CH 4 from the gas generating horizons of the Kuchino landfill was much lighter, 813C = -58 to -60%0 and that of CO 2 was 813C = +1.4 to +2.7%0. This may be due to a higher organic substance concentration in the Kuchino landfill. It was shown earlier that methanogenic bacteria form a lighter methane in the presence of a substrate excess, while a heavier methane is formed when the substrate is exhausted (Lebedev et al., 1989).

mmot CH .g-I.TSS

200-

100

0

I

0

j

20

t

I

~

[

/,0 60 temperature, °C

Fig. 2. Production of CH 4 by microflora of Kuchino landfill ground at different temperatures (mmol CH4/g

TS). The study of the methanogenic microflora of the landfill ground has shown that it consists of mesophilic methanobacteria with the development optimum at 35-40°C (Figure 2).

In the enrichment

408

cultures obtained on different substrates (precursors of methane: acetate, methanol, methylamine) the abundant development of Methanosarcina

has been observed.

The rod-like forms belong to

Methanobacterium and Methanobrevibacter genera have developed on such substrates as formate and H 2 + CO 2 mixture. Isolates of Methanobacterium fofmicicum, Mb. briantii, Mb. spp., Methanosarcina barked, Ms. spp., and Methanococcus spp. from landfill refuse have been obtained by other researchers also (Campbell et al., 1985; Fielding et al., 1988).

5. OXIDATION OF GASES IN THE AEROBIC ZONE At the boundary of anaerobic and aerobic conditions a transition zone is formed. Its thickness and depth at which it is situated determines the intensity of gas flow from the anaerobic zone. Because of the heterogeneity of the landfill structure, the intensity of gas flows inside landfills is very unevenly distributed. Maps of the concentrations of methane and carbon dioxide at a depth of about 70 cm in the Kuchino landfill area are represented in Figures 3 and 4. Gas concentration varies widely, but the CH4:CO 2 ratio and isotopic composition on the whole are close to the values of gases in the generating horizons.

distance, rn

600

400

200

.

0

200

~00

600 disfance, m

Fig. 3. CH 4 concentrations (% vol.) measured in the soil gas at a depth of 70 cm at different sites of the Kuchino landfill area.

409

distance, m

600

400

200"

.

0

200

400

600 distance, m

Fig. 4. C O 2 concentrations (% vol.) measured in the soil gas at a depth of 70 cm at different sites of the Kuchino landfill area.

distance, m

4(-1'6 0 "X'I

~*

4<1 /'~

~ .X1.1 Z.2

~

1.4ak

~I-3 .~.).--~. "; (~?~-~P..~)'

~

25 * ~1.c,

..~.~

200

"1("1

T .1(2.7

l-X-

1246

200-

0

3,1 *

1.~ ~

*

1.3 ~1

.X.1 2

46

.xll

• .,

4-

400

600 distance, m

Fig. 5. Aeration coefficients (N2:O2:3.74) measured in the soil gas at a depth of 70 cm at different sites of the Kuchino landfill area.

410

In order to characterize the extent of aeration in the horizon, the aeration coefficient (K) was used: K = N2:O2:3.74 where N2:O 2 is the nitrogen-oxygen ratio in the soil sample gas. The value of the ratio N2:O 2 is equal to 3.74 for atmospheric air. We believe that the "K" parameter may characterize the aerobic process intensity in the near surface layer of the landfill soil. When K is about 1, the aerobic processes are absent or go on at a rate that is not more than that of oxygen. The ratio increase indicates the oxygen consumption by the aerobic microflora; a very high value of K means anaerobic conditions. At a depth of 0.6 - 0.7 m in the Kuchino landfill the aeration coefficient varies from 26.07 to 1 (Figure 5). This is due not only to the structure of the upper ground layer and activity of the aerobic microflora but also to the intensity of the biogas stream from lower horizons. At the large landfills the air displacement with the biogas stream may be observed and emission of a gas corresponding to the generated one in its composition. Closer to the surface a sharp decrease of methane concentration due to the oxidation processes has been observed (Table 2, Figure 7). It is interesting that the active gas oxidizing and methanogenic microflora at times are present simultaneously in the ground of the transition zone (Table 2). Methane, hydrogen, and carbon monoxide are oxidized in the presence of oxygen, while methanogenesis takes place with samples from a depth of 50-90 cm when the oxygen is exhausted. In the same samples the presence of a denitrifying bacterium, Micricoccus denitrificans, has been found. Due to methane oxidation by methanotrophic bacteria, the isotopic carbon composition of CH 4 and CO 2 is changed and the carbon of methane becomes heavier, while that of CO z becomes lighter. The relationship between CH 4 concentration and values of 8 13C of methane and carbon dioxide for the Kuchino landfill gas is shown in Figure 6. Curves of methane and carbon dioxide concentration changes and methane oxidation rates by microflora at two points of the upper refuse layer in the Ramenki dump are shown in Figure 7. Concentrations of gases were measured in soil gas in situ; microbiological activity was determined in the laboratory. Methane here undergoes complete oxidation; no methane escaping from the surface of the Ramenki dump was observed. The highest microflora activity, determined in the laboratory in the refuse samples, was at the depth of 40-60 cm, which coincides with the methane concentration decrease and sharp increase of isotopicly lighter carbon dioxide. At the first sampling point (Fig. 7.1,) the 8 13C value of carbon dioxide has changed from -2%0 at the depth of 80 cm to -10%o at 50 cm; at the second point (Fig 7.2) it was equal to 0%0 at the depth of 80 cm; and at the depth of 40 cm it was equal to -8%0. The data show the existence of good correlation in changes of concentration and microbiological and isotopic parameters. The activity of the methane-oxidizing microflora of the landfill upper refuse layers varied in rather broad limits and depended on the place and depth of sampling. The methane-oxidation rate, measured in the laboratory at 25°C in the refuse samples at depths from 10 to 90 cm, was from 0.12 to 0.60 mmol of CH4/day for 1 g of total solids (TS). On the whole, refuse samples of the Kuchino landfill were more active, oxidizing on an average 0.40 mmol CHJday.g TS, while the average methane oxidation rate in the samples taken from the Ramenki dump was equal to 0.20 mmol CHa/day.g TS. The number of methane-oxidizing

9.0

19

19

10-20

50-60

70-90

1.8

2.1

1.8

CO

0 days

20

20

20

H2

16

16

20

02

16

18

1.3

CH 4

1.1

1.1

0

CO

1 days

11.1

13.5

2.5

H2

15

10

0

CH 4

0

0

0

CO

6 days

0

0

0

H2

0

0

0

02

35.4

14.1

0

CH 4

0

0

0

CO

20 days

0

tr.

0

H2

55

28

0

CH 4

50 d

0

'

/

20

I

.

'

•

o

40

I

o

\ i

o

CH4, %

ox"x,,

/

/.

0

-10

--20

613Cc0 %o

Fig. 6. The ratio of CH 4 concentration and ~ 13C values of CH 4 and CO 2 in the near-surface gases of Kuchino landfill.

-40

-50

-60

%0

X 0

_80CH,

613C

~__

0 i

I

20 L

I

I

40 o~

0.4

20 I

I

I

I

%

mmot CH4.g-I~TSS

60 06

I

F-HL,

40% I

0.8 mmo[ [H4.dI.g-I.TSS 0.6

I

Fig. 7. Changes of CH 4 and CO 2 concentrations and methane oxidizing microflora activity in the upper layer of Ramenki landfill ground.

02

i

I

~.....,~

~

gO de ~th (cm)

40

20-

80

60

40

20

0

Data were obtained with 5 g soil samples moistened with 10 ml water in 120 ml bottles filled with gas mixture; experiments were performed in triplicate.

CH 4

depth (cm)

Time (days):

Table 2. Change of concentration of gases (%) by microflora of upper layer of "Kuchino" landfill.

412

bacteria in the landfill refuse, determined by inoculation of serial 10-fold dilutions of samples into an elective medium, was 107-109 cells/g of wet refuse. In fact, this value was 1-2 orders higher when the numbers of hydrogenotrophs were determined by the immunoserum method (Galchenko, 1990).

Morphological and

cultivation peculiarities of isolated cultures show that they belong to different genera of methanotrophic bacteria.

Identified by the immunological method were Methylococcus capsulatus, Methylomonas albus,

Methylobacter bovis, Mb. hroococcum, Mb. capsulatus, Methylosinus sporium, Ms. trichosporium, Methylocistis parvus, Mcs. minimus, Mcs. pyriformis, Mcs. echinoides. In deposits at the depth of 10-80 cm an active hydrogen- and carbon monoxide-oxidizing microflora has been found. The hydrogen oxidizing rate at 25°C varied from 0.2 to 1.75 mmol H2/day.g TS when the number of hydrogen-oxidizing bacteria was equal to 107-1011 cells/g of wet refuse. The CO-oxidation rate was 0.02 - 0.18 mmol CO/day.g TS and the number of bacteria was 106-108 cells/g of wet refuse. The isolated cultures of hydrohenobacteria and carboxydobacteria belong to the Pseudomonas, Alcaligenes, Comamonas,

Mocrococcus genera. In our experiments, if the gas mixture contained CH4, H2, and CO, H 2 was utilized first, then CO (at moderate concentration), and only after that was CH 4 consumed (Table 2). When temperatures decreased to 5-7°C, the methane and hydrogen oxidation rate of the same refuse samples decreased by 3-5 times and the CO oxidation by 10 times. The enrichment cultures of methaneoxidizing bacteria were obtained. Their methane consumption rate at 6°C was 2.5 times lower than that of methane-oxidizing cultures developing at 25°C. Methylomonas albus, Methylobacter boviL Methylobacter

chroococcum, Methylosinus sporium, Methylocystis parvus, and Methylocistis minimus were identified in psychrotrophic enrichment cultures. The temperature of landfill refuse at the depth of the anaerobic zone does not depend on air temperature, and is more or less constant and rather high, suggesting the likelihood that the rate of methane production is constant. Some increase of methane emission into the atmosphere during the cold season of the year may be due to a decreased activity of the gas-oxidizing microflora.

6. GAS EMISSION F R O M T H E LANDFILL SURFACES INTO T H E A T M O S P H E R E Numerous large landfills of the Moscow region were examined in order to study the concentration, field structure, and emission rate of their gaseous products into the atmosphere. Instant emission surveys and regime observations of the concentration of mineralization products were carried out. In all studied areas, CH4, CO2, and often H 2 and CO emissions were found.

The gases' flux strength and ambient

concentration in the air above the landfill surface are distributed unevenly. In our investigations the flux rates ranged from 0 to a maximum for each gas (in 10.4 m 3 .h-1/m 2 ): 19.8 for methane, 46.3 for carbon dioxide, 1.2 for hydrogen, and 0.75 for carbon monoxide. These data show that emission rates for gases of anaerobic origin from landfills into the atmosphere exceed those for natural areas by 10-1000 times. Intensive hydrogen fluxes have been observed in "fresh" landfills where the anaerobic process is not completely established and for which emission of such fermentation products as volatile fatty acids, alcohols,

413

x 10-6

Z

~

0

H~

70

a.

b.

7O

C.

Fig. 8. The gas emission from Mitino landfill surface. x,y axes: the length and width of the area, meters. z axis: the flux of the gas components, m3/min/m2.

414

acetone, etc., into the atmosphere is also typical. The flux of carbon monoxide in such landfills is significant (Figure 8 a,b,c,). The escape of CO from low parts of landfills has been registered at all stages of waste mineralization, thus showing the existence of a continuous biogenic source this substance. As one can see from Figures 8 and 9, the flux of methane from the surface of the "mature" Kuchino landfill is 10 times more intensive than from "fresh" Mitino landfill. There was practically no release of hydrogen and CO registered from the surface of Kuchino landfill; traces were registered in a few points only. The fluxes of hydrogen and carbon monoxide from the Mitino landfill to the atmosphere are intense. Maximal heterogeneity is typical for "fresh" landfills. The analysis of structure of the emission fields of gaseous products, formed inside the landfills, has shown that different parts of the refuse deposits may be at different stages of the microbiological process. For example, in one part of the landfill only CO 2 emission occurs, indicating the existence of an intense oxidative process, while in another part methane emission is found, and in others there is practically no gas production. During the cold part of the year the relative methane emission increased compared to carbon dioxide.

x 10-~

x 10-~ 5J

24

3~

<

1.

0.2 5OO

5

0~

a. CH 4 Fig. 9. The gas emission from Kuchino landfill surface,

b. CO 2

x,y,z, - as in Fig. 8.

Investigations at the small open dump Ramenki inside the Moscow city limits have shown that in the case of small capacity of refuse and low organic content, CO 2 emissions dominate. CO 2 fluxes to 6 x 10 m3/h/m2 were observed, but the CO 2 emission rate and concentration in the ambient air over the landfill surface were extremely uneven. CO 2 concentration reached 0.25%, and methane concentration did not exceed 3 x 103%. A slight increase of methane emission in this dump has been observed only in winter and only if the upper refuse layer was disturbed. These results are in good agreement with data from the

415

microbiological investigation of the Ramenki dump refuse. The isotopic composition of CO 2 carbon, sampled from air above the surface of the Ramenki dump, appeared to be equal to 6 13C = -19.3%o, which is vastly different from the values typical for CO 2 in the air, where 6

13C ranges from -7 to -10%o.

Landfills are an important source of methane, carbon dioxide, and gaseous microcomponents. When estimating the contribution of landfills to gas emissions from the Earth's surface, it is necessary to account for the activity of aerobic microflora in the upper refuse layer. The scale of gas oxidation in the landfill aerobic zone may be quite significant. For the Kuchino landfill it has been calculated that every year about 9.5 106 m 3 of methane and 6.2 106 m 3 of CO 2 pass from the anaerobic refuse zone to the borders of the aerobic zone. In summer over 70% of the methane is oxidized and about 50% in an average year; therefore, 4.8 106 m 3 of methane and about 11 106 m 3 of CO 2 are emitted into the atmosphere from the landfill surface. We estimate that the volume of MSW buried in landfills that actively generate methane now is about 600 million tons, and the yearly value of methane emissions from the landfills situated on the territory of the former USSR is 1.2-2.4 billion m 3 or 0.9-1.7 teragrams. We estimate the amount of municipal solid wastes produced by in cities is 300 kg per capita per year, according to the norms existing in the former USSR (Rules of San. Mgt., 1980; Gorbatiuk, 1988); the urban population in the former USSR is 124.6 million; the yearly production of MSW in big cities in the former USSR is 37.4 million tons; 80% of it is degraded anaerobically. According to the literature and experimental data obtained in the Moscow region, we assumed also that the average gas specific activity of landfill refuse in the anaerobic zone is 200 m3/tons; the rate of biogas production is 5-10 m3/tons per year; the time needed for intensive degradation of organic material and methane formation is 20 years; the content of methane in landfill biogas is 50-70%; 30% of methane is oxidized in the aerobic zone of the landfills. Taking into account the density of urban population of the former USSR, the impact of some regions was estimated (Table 3).

Table 3. The emission of methane from landfills in different regions of the former USSR. Region

CH 4 Emission billion m3/year

Tg/year

1.10 0.27 0.52 0.32 0.12

0.79 0.19 0.37 0.23 0.09

European part of Russia Siberia Ukraine and Moldavia Middle Asia Caucasus

7. CONCLUSIONS Gas formation within landfill deposits has environmental significance, on both local and global scales. Natural soil systems may cover substantial areas while the fluxes of gases of anaerobic origin per unit of the

416

total surface are relatively small. In the case of landfills the situation is inverse: their relative surface area is small and the gas emission is several orders higher than that of the natural systems. Thus, 1 hectare of the Kuchino landfill emits the same amount of C H

4 as 2000 -

4000 hectares of sod-podzol soil, which makes

the basic background soil type of the Moscow region. In consequence, in an urban region the landfill deposits may emit to the atmosphere as many gas impurities as the entire natural ecosystems under anaerobic conditions conserved within city limits. Two ways of mitigating the emissions of existing landfills follow from the results we obtained. For small open dumps, enhancing the aerobic oxidative process in the upper layer of landfill refuse seems reasonable. This may be attained by rather simple approaches: mounding th e friable ground, planting grassy and bushy flora with powerful root systems, digging drains for enlargement of the aerobic zone surface. In this case not only methane but other gaseous and volatile products of the anaerobic zone are oxidized. To detoxify large landfills the biogas should by pumped out and utilized. The productive power of large landfills is comparable with the small gas fields (Richards, 1989). In order to minimize the fuel gas losses and to lower emission of other volatile components into the atmosphere, the upper layer of the landfill should be thicker.

ACKNOWLEDGMENTS We thank V.K. Nekrasova, D.V. Ivanov, Dr. O.I. Minko, Dr. V.F. Galchenko, for assistance with the experiments and Dr. O.V. Gorbatiuk for discussions.

We also thank Prof. G. Lettinga and Ing. J. v. Lier

from the Department of Environmental Technology of Agriculture University of Wageningen for discussion and for help with getting up the text and illustrations. We gratefully acknowledge Prof. M.A.K. Khalil, Dr. J. Bogner, Ms. M.J. Shearer and Ms. Edie Taylor who edited our paper for English usage.

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