Landfill gas venting for agricultural restoration

Waste Management & Research (1987) 5, 1-12

LANDFILL GAS VENTING FOR AGRICULTURAL RESTORATION W . John Spreull * and Susan Cullum t (Received 17 March 1986) A sand and gravel quarry at Great Sunnings near Upminster in Essex was backfilled with industrial and commercial waste (non-household municipal waste) in the process of restoration to agriculture . These waste types contain significantly less putrescible materials than domestic waste, nevertheless previous experience of farming such refilled land has resulted in crop damage and lowering of yields in early years following restoration due to the adverse effects of landfill gases . These effects may be two-fold : (a) removal of oxygen from the soil and (b) toxic effects of the gases, e .g . carbon dioxide and ethylene . A landfill gas venting system in conjunction with a capping layer was installed at the site before soil replacement and cultivations, following which winter wheat was sown in the same year as soil replacement . Data was recorded on the gas venting scheme and the agricultural activities with a view to establishing whether such a concept is a practical proposition for future restoration under similar circumstances and with comparable conditions . It is considered that the trials show the scheme to be workable . Key Words Landfill gas, venting, agricultural restoration, crop yields, industrial commercial waste, methane, ethylene, soil anaerobism, U .K . 1 . Introduction The disposal of waste to landfill sites has been taking place for many years, and because this method of disposal is still by far the cheapest, it is likely to continue for a very long time . Sand and gravel workings offer potential sites for waste disposal, but with increasing frequency consents for sand and gravel extraction are being granted on better quality agricultural land because this is where the remaining reserves are located, at least in England (Verney 1976) . Current mineral consents often require that the land be restored within the 5 year "after care" period to its former agricultural standard or performance, and future mineral consents on higher grades of land will place even greater demands on the mineral operator and the restorer . The availability of inert fill is limited, but the pressure from local authorities, residents and environmentalists for quick re-filling and restoration is mounting . It is therefore in the interests of the extractive industry, as well as those concerned with waste disposal, to be able to utilise degradable fills in the process of restoration, and to establish engineering practices which will reduce the problems resultant from using these fills- principally landfill gases and leachate . If this can be achieved, filling can be completed incorporating degradable fill so that the community's waste disposal requirements can be satisfied at the lowest possible cost to the rate payers . Scientists have recognized for many years that in the process of degradation organic waste will produce gases but the problem, if one was identified, was usually associated * St . Albans Sand & Gravel Company Ltd, Delamare Road, Cheshunt, Hertfordshire EN8 9SJ, U .K . t R.M .C.(U .K .) Ltd ., RMC House, High Street, Feltham, Middlesex TWI3 4HA, U .K . 0734-242X/87/010001 + 13 $03 .00/0

© 1987 ISWA

W. J . Spreull and S . Cullum with what is loosely called household waste or domestic refuse, however many industrial and commercial wastes also have a high capacity for gas production . Motivation for the trials outlined in this paper was to resolve the last hurdle in the achievement of good and speedy restoration to high grade agricultural land following sand and gravel extraction and filling with degradable waste. Good restoration practices have been developed, but the temporary adverse effects of landfill gases although limited in extent have not yet been eliminated (Gilman et al. 1984) . The adverse effects on crops due to landfill gases can be extensive and severe, but this tends to be the exception . More usually there are small patches of bare ground or stunted growth . In these latter cases the reduction in crop yield due to landfill gas effects can be minimal, as can be seen from the yields of both the Bush Farm Experiment (DOE 1982) and McRae Field at Hatfield (McRae 1982) . Although it is clear that landfill gas production can be maintained for many years, experience indicates that certainly in the shallower sand and gravel workings adverse effects on agriculture and indeed any obvious effects at all, can be limited to 3 or 4 years, as illustrated by the quick recovery of Coopers Field at Hatfield (see Figs I and 2) . No data is available to evaluate quantitatively the effects on crop yields attributable to landfill gases . In 1984, the interim agricultural grading of the Bush Farm Experiment [carried out by the Ministry of Agriculture, Fisheries and Food (MAFF)] was downgraded due to soil anaerobism related to landfill gases (see Fig . 3) . However crop yields from the site were similar to those obtained from unexcavated fields of high agricultural grading . In the past, attempts were made at Bush Farm and Hatfield to combat the effects of landfill gases on crop growth by capping the waste with clays prior to soil replacement, and leaving the margins of the site unsealed to encourage lateral migration of the gases . These attempts reduced, but did not eliminate, crop die-back . Since the restored fields are returned to normal cultivations and early cereal production, the more conventional vertical venting pipe system over the surface of the field was impractical since they interfere with agricultural operations . The trials reported in this paper suggest a practical solution to the problem in agricultural restoration where the requirement is

Fig . 1 . A severe example on Coopers Field Hatfield 1979 of landfill gas die hack . Filling of the site with commercial waste was completed in 1976 . In 1979 the then farmer described the field as being unfit for agricultural use .



Landfill gas venting and agriculture

3

Fig. 2 . Coopers Field Hatfield 1984, (Norman winter wheat) at the exact position of extreme die back shown in Fig . 1, 5 years earlier . No special venting to remove gases was installed . The fill surface gave no response to gas detecting equipment at an inspection pit . Yield 7 .96 tonnes ha -1 , parish average 7 .50, national average, England, 7 .71 .

Fig . 3 . Bush Farm Experiment 1984, Landfill gas induced die back in oil seed rape .



4

W . J . Spreull and S. Culluni

to achieve high productivity on better quality land following backfilling with degradable materials, but the concept could also have an application to the prevention of gas migration from landfill sites into properties and service ducts where their presence could have more serious consequences . 2 . Technical background It has long been recognized that the biodegradation of wastes deposited in landfills, for example food wastes, vegetable matter, wood, paper, cardboard, etc . . will lead to the production of two major by-products, leachate and landfill gas . The biological processes involved in the breakdown of waste are a complex series of inter-related reactions which are strongly dependent on a number of factors including composition of the fill, moisture content, temperature, compaction and acidity, as described by Farquhar & Rovers (1973) . Rees (1980) recognized a relationship between efficient gas production in wastes and improvement in leachate quality, which is now leading to further research into biological methods of controlling landfill reactions . The major components of landfill gas are methane (CH 4 ) and carbon dioxide (CO,) generally at concentrations of approximately 55 and 40%, respectively . Other components may be nitrogen and hydrogen, particularly in the early stages of gas generation, and a large number of trace gas components including ethylene (C 2 H 4 ) ( Smith & Russell 1969), hydrogen sulphide and many organic and organosulphur compounds . Although methane itself is odourless, some of the trace gas constituents have extremely obnoxious smells at very low concentrations and are responsible for the characteristic odour of landfill gases (Young & Parker 1983) . Landfill gases pose several problems for the site operator . Since methane is both explosive and inflammable, safety considerations are of prime importance, both on, and near to, landfill sites . Following restoration of the landfill site, the gases are frequently found to have an adverse effect on the quality of the crop(s) grown . The rate of gas production varies significantly with time and although the actual rates differ substantially from site to stie . the same pattern of gas evolution is found at most sites . There is an initial phase following fill placement of perhaps 6 months to 2 years during which gas production becomes established . Gas production rates reach a peak shortly afterwards and may maintain a high level of production for a further 2-3 years . Subsequently rates of production gradually decline, although depending on site factors gas may continue to be produced at relatively low levels for many years (Stearns et al . 1984) . It is commonly thought that wastes containing high percentages of putrescible waste, e .g . domestic waste, produce significantly higher quantities of landfill gas than industrial and commercial waste . However research carried out recently by the Harwell Laboratory Waste Management Team (Emberton et al. 1985), has shown that certain industrial wastes are capable of producing comparable quantities of gas to many domestic landfill sites, and must therefore be treated similarly with regard to gas control measures . The effects of landfill gases on crops grown on restored soils are not fully understood, and research is being undertaken at Wye College, London University (Hewitt 1985, Hewitt et al. 1985) into the mechanisms involved . One of the more obvious effects is soil anaerobism caused by the physical exclusion of oxygen from the soil by the sheer volume of landfill gases moving upwards through the soil profile . Methane can also cause anaerobism if the soil develops bacteria that can metabolize CH 4 using O z from the soil pores (Mueller 1969) . Although methane itself is not thought to be toxic to plants, carbon dioxide and several of the trace gases present in landfill gas, e .g . ethylene, are



5

Landfill gas venting and agriculture

known to be toxic, and this may be an important factor in the adverse effect of gases . The trials subsequently described in this report are a logical progression of the work already carried out on the Bush Farm Experiment Site, and are a further attempt to prevent the migration of landfill gases into the restored soil profile . 3. Trials layout and results 3 .1 .

Preamble

In 1984 it was decided to take two sites, one at Hatfield in Hertfordshire and one at Great Sunnings in Essex, and lay out on each of them an identical gas venting system designed to prevent the upward migration of landfill gases from the fill into the soil zones . This consisted of a system of horizontal perforated plastic pipes overlain by a clay capping layer . This report concerns the details of the Great Sunnings Experiment, although a replicated trial is being conducted at Beech Farm, Hatfield . The trials were carried out by the St . Allans Sand & Gravel Company, a Company which has been actively involved in agricultural restoration for many years . 3 .2 .

Site details and filling operations

The parcel of land the subject of the trial is 3 .55 ha (8 .77 acres), and is located within 250 m of the Bush Farm Experiment Site, 8 ha (20 acres) . The soils, hydrology and geology of the two sites are comparable, the site geology consists of an average depth of 1 .2 m of overburden overlaying 3 .7 m of terrace river gravels . The basement geology is London Clay . Following excavation, and prior to filling, the bottom of the site was levelled and the sides of the excavations were clay lined . Filling, which started in February 1983, was completed in mid-1984 . Fill consisted of industrial and commercial wastes (non-household municipal waste) and inert excavated materials, placed in layers not exceeding 2 .5 m in depth by a steelwheeled compactor . The depth of fill was between 4 and 5 m . Composition and emplacement of fill was very similar to the 2nd, 3rd and 4th quarters of the Bush Farm Experiment Site . Table 1 gives an analysis of imported fills at Great Sunnings for the six months TABLE 1 Great Sunnings : fill imports for the first six months imports 25, 145 m')

of 1984

(average monthly

m' Transfer Station Clean Fill General Packaging Miscellaneous Builders Rubble Excavated Muck Timber Glass Tins Total

including paper

75,053 20,031 17,105 15,545 10,136 8090 4309 538 68 150,875

49 .7 13 .3 11 .3 10 .3 6 .7 5 .4 2 .9 0 .3 99 .9



6

W. J. Spreull and S. Cullum

Fig . 4. Great Sunnings landfill area showing gas venting pipes .

ending June, 1984 . Although the putrescible fraction of household or domestic refuse was hardly present, there was some in the transfer station loads . Volumes of fill are shown as on entering the site, and are therefore in varying degrees of compaction . Transfer station loads were well compacted on arrival, and on emplacement 1 .2 m 3 filled I m of void . On emplacement it is estimated that densities of I tonne m -3 were achieved . Excavated muck was mainly clay, which was sometimes contaminated with other materials, whereas the better soils were designated "clean fill" . 3 .3 . Installation o! vents Following discussions with the contractors, in order to reduce costs and for ease of installation, the decision was made to use conventional land drainage plastic slotted pipes . The method of installation was to employ a trenchless drainage machine which is capable of working in rough conditions, in preference to digging a ditch and laying the pipes in separately . This machine opens up a slot and lays the pipes and stone in a single operation . Continuous slotted plastic pipes (60 mm diameter) were laid in the top of the fill at 20 m intervals horizontally to a depth of approximately 0 .6 metres in 40 mm gravel surround, the extremities venting through vertical pipes at the end of the haul road on the West of the site and into a ditch to the East of the Site (Figs 4 and 5) . Some difficulty was experienced with the trenching due to the fact that some large obstructions and sheets of plastic were close to the surface . In one case the tine on the machine was broken, on other occasions obstructions caused the machine to stop, and several times resulted in dragging large quantities of fill materials to the surface . Clearly for ease of implementation clean unobstructed fill only should be used in the top 0 .75 m of the site . Following vent installment and the tidying up of the landfill surface, the sandy clay sealing cap was applied to a depth of 0 .3 m and compacted with two passes of a vibrating roller . It was known at the time that the density of this cap was poor, especially on the northern section, but no better material was available in the quantity required . The sealing cap was similar in placement and compaction to that which was applied to the 3rd and 4th quarters of the experiment site .



Landfill gas venting and agriculture

1 .2 m

7

Soil horizon

r' r*

Perforated plastic pipe with 40 mm stone surround Landfill

GENERAL CROSS SECTION

A

B Perimeter ditch serving

tsrf Central haul road

land drainage and gas venting

Soil profile

Land draina ge pipe

I

Clay seal -Gas venting pipe Landfill

CROSS SECTION A-B Fig . 5 . Great Sunnings gas venting scheme .

Soil replacement took place in July 1984, which consisted of an 800 mm thick layer of subsoil and a maximum of 300 mm of topsoil . In mid October 1984 a land drainage system was installed . The cost of laying the gas venting pipe in a gravel surround was £0 .95 per metre run, which is equivalent to £316 per hectare (£128 per acre), but 1985/ 1986 costs are likely to be nearer £618 642 per hectare (£250-260 per acre) . 3 .4 . Gas vent monitoring Monitoring of the gas vents was undertaken at approximately six-weekly intervals from February 1985, although an initial survey was carried out in November 1984 . On-



8

W. J . Spreull and S. Cullum West side

East side

Extensior area

Soils replaced 1985 Extension area

Soils not replaced

Soils replaced 1984 Land drains installed vi

First harvest, 1985

V6

Crcp of wheat

V5

V4

vs Site p office

V2

vi

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Fig . 6 . Schematic diagram of location of gas vents at Great Sunnings (as at 8 .7 .85) .

site measurements of methane concentration were taken from each vent with either a Gas-Tec (capable of reading from 0-10,000 ppm CH,) or a Gascoseeker (capable of 0100% CH 4 ) . Where the above readings were greater than 1% methane, duplicate gas samples were withdrawn from each vent into greased glass syringes which were then taken back to Wye College for gas chromatographic analysis . The chromatograph readings provide information of methane, carbon dioxide, oxygen, nitrogen and ethylene concentrations in each sample . On site measurement of the rate of gas flow out of the vents were measured with a Casella airflow indicator . This meter gives only approximate results of gas flows and is not capable of detecting very low flow rates . On each monitoring occasion the land drains installed in the soil profile and which open into the ditch on the east side of the site were checked for methane . 3 .5 . Trial results The layout of the gas vents at the trial site is shown schematically in Fig . 6 . The gas monitoring results for methane, carbon dioxide and ethylene are presented in Table 2, and the gas flow results in Table 3 . Figure 7 shows changes in gas composition with time from vent 10 . There is a significant difference in both the concentration and quantity of gases being emitted from the two sides of the site . The western vents adjacent to the haul road in the centre of the site have shown consistently high levels of methane and carbon dioxide with concentrations generally between 37 and 60% for methane, and



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10

TABLE 3 Great Sunnings venting : Flow (m min - ') in 60 mm diameter pipe (BP = Barometric pressure . London Weather Centre, MSL . 1200 GMT in kP)

Vent No .* V1W V2W V3W V4W V5W V6W V7W V8W V9W VIOW VIIW V12W V13W

Total flow (m 3 min - ')

13 2 .85 100 .87

BP

26 .3 .85 99 .86

BP

28 0 28 40 11 10 11 0 24 20 15 3 32 0 .49

* No measurable flow was recorded t Meter not functioning .

19 0 25 28 11 22 12 0 25 2'_ 14 2 29 0 .46

22 .5 .85 100 .68

BP

8 .7 .85 t 102 .31

BP

20 0 37 23 10 9 12 0 11 14 9 10 24 0 .39

16 .8 .85 101 .68

BP

2 .10 .85 101 .50

BP

0 0 14 20 9

0 0 21 19 20

Buried vents 16 12 17 16 16 14 31 0 .36

0 16 20 16 12 0 34 0 .35

from the eastern vents on any occasion .

20-40% for carbon dioxide . In addition there have been measurable rates of flow of gas from all but one of these vents on every monitoring occasion . In contrast to this, the vents on the eastern side of the site have shown more variable results . In November 1984 methane levels in the eastern vents were high, 20 50% in nine of the 13 vents . However on this occasion the western ends of the vents were not open to the atmosphere . Since the western end of the vents have been opened up the concentration of methane being emitted from the eastern vents has varied from 0 to 52% . There is no consistency in the data from any one vent on the different monitoring occasions, with the same vent showing zero methane on one occasion, but 37% methane the next . This is consistant with the report of Gilman et al . 1982 . There have never been measurable rates of gas flow emitted from any of the eastern vents . It is clear from the results that the majority of the landfill gases are venting through the western ends of each vent . It is not immediately apparent why this should be the case . The western vents, are at a slightly higher elevation than the eastern vents . The mixture of landfill gases has a calculated density which is slightly less than air and it may be warmer than air due to anaerobic activity (Rees 1980), therefore it may migrate upwards in response to this density difference . It is clear from the concentrations of methane and carbon dioxide being emitted from the western vents that the fill material is well into the stable gas production phase caused by the anaerobic degradation of the wastes . It is to be expected that these concentrations of gases will stay fairly constant for up to 5 years or more (Stearns et al. 1984) . The concentrations of ethylene detected in the vents has ranged from zero to as high as 104 .2 ppm, an ethylene concentration of only 10 ppm is known to severely inhibit plant growth (Smith et al. 1969), therefore the concentration of ethylene detected in



11

Landfill gas venting and agriculture 65

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55

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16 .11,84

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Date Fig . 7 . Carbon dioxide and methane levels found in vent 10, Great Sunnings, during the period 16 .11 .842 .10.85 .

the vents would be capable of severely affecting crop growth were the gases to reach the root zone of the crops . The quantities of gas being emitted from the western vents has been calculated by summing the rates of flow in metres per minute from the 13 vents and multiplying this by the cross-sectional area of the vents (see Table 3) . The total flow has varied between 0 .35 and 0 .49 m 3 min - ' (12 .4 and 17 .3 ft 3 min - ') . This figure does not necessarily represent the total quantity of gases being generated in the site as the gases may also be migrating horizontally to the north and west into adjacent fill areas . However continued monitoring of the rates of flow from the vents should indicate whether gas generation rates are changing with time . There have been no visible signs in the wheat crop of any landfill gases in the soil . Random checks over the soil surface with the Gas-Tec have also failed to detect any measurable concentrations of methane . The monitoring of the land drains on the eastern side of the site have shown consistently low readings . In general there is no detectable methane being emitted from these drains, although on one occasion a reading of 1 % methane was detected when the adjacent gas vent was blocked . These consistently low readings are another indication that the gas venting scheme is working to control gas migration satisfactorily, since monitoring of land drains on the adjacent Bush Farm experiment site has frequently shown high concentrations of landfill gases . 5 . Conclusions

The industrial and commercial wastes used to backfill the site have the potential for producing large quantities of landfill gas, and the site is well into the anaerobic phase



12

W. J. Spreull and S . Cullum

of methane and carbon dioxide production . Regular monitoring of the gas venting system has shown that to date the vents are channelling substantial quantities of landfill gases to the atmosphere which might otherwise have permeated into the soil and adversely affected plant growth . As well as the high percentages of methane, which is not considered to be toxic to plants, and carbon dioxide, which is, significant quantities of ethylene have been measured in the vents well above its toxicity level of 10 ppm . To date there have been no visible indications of landfill gas on the surface of the site, and none has been detected . Gas readings from land drains installed in the soil profile also give low or zero readings . Despite the unusual practice of sowing wheat as an initial crop following soil replacement the yields were in line with the Parish average for the harvest year . The success of the venting experiment to date has been most encouraging . Similar work is being carried out on one other site at Hatfield . It is considered that on sites where high quality agricultural restoration is required, such venting systems will be a vital part of the restoration procedures . It might be desirable to carry out control experiments where randomized blocks are set up with and without gas vents . In this work adverse effects at the unvented Bush Farm (Fig . 3) are in marked contrast to the excellent first year growth of winter wheat on the vented Great Sunnings field . References DOE (1982), Joint Agricultural Land Restoration Experiments, Progress Report No . 2 : 1977-1981 for Bush Farm, Upminster, Essex . Department of the Environment, MAFF, SAGA . Emberton, J . R . Parker, A ., McGahan, D . J . & Worrallo, S . N . (1985), Biological and Chemical Characterisation of Landfills (X) . R11900 . Hatfield Quarry, Hertfordshire, U .K . Farquhar, G . J . & Rovers, F . A . (1973), Gas production during refuse decomposition, Water Air and Soil Pollution, 2, 483-495 . Flower .. F . B ., Gilman, E . F . & Leone, I . A . (1981), Landfill gas, what it does to trees and how its injurious effects may be prevented, Journal of Arboriculture, 7, 43-52 . Gilman, E . F ., Leone, L A . & Flower, F . B . (1982), Influence of soil gas contamination on tree root growth, Plant and Soil. 65, 3--10 . Gilman, E . F ., Flower, F . B . & Leone, I . D . (1984) . Standardized procedures for planting vegetation on completed sanitary landfills, Waste Management & Research, 3, 65---80 . Hewitt, A . K . J . (1985), The Effect of Landfill Gas on Soils and Vegetation . A Report for the Ready Mixed Concrete Group Ltd ., U .K . Hewitt, A . K . J . & McRae, S . G . (1985), The effect of gases emitted from landfills on soils and crops . In Proceedings of the 1st International TNO Conference on Contaminated Soil, pp . 251-253 . Leone, 1 . A ., Flower, F . B ., Gilman, E . F. & Arthur, J . J . (1977), Damage to woody species by anaerobic landfill gases, Journal of Arboriculture, 3, 221-226, McRae, S . G . (1982), The Agricultural Restoration ofSand and Gravel Quarries--A Case Stud r at Hatfield Quarry, Hertfordshire . Occasional Paper No . 9 . Wye College, University of London . Mueller. J . C . (1969), 'Preferential utilization of the methane component of natural gas by a mixed culture of bacteria, Canadian Journal of Microbiology . 15. 1114--1116 . Rees, J . F . (1980), Optimization of methane production, and refuse decomposition in landfills by temperature control, Journal Chemical Technology and Biotechnology, 30, 458-465 . Smith, K . A . & Russell, R . S . (1969), The occurrence of ethylene and its significance in anaerobic soil, Nature, London, 222, 769-771 . Stearns, R . P ., Wright, T . D . & Stirrat, B . A . (1984), Landfill gas recovery and utilization at Industry Hills, California, Waste Management & Research . 3, 65---80 . Verney, R . B . (1976), Report of the Advisory Committee on Aggregates . `Aggregates : the war ahead'. H .M .S .O ., London . Young, P . J . & Parker, A . (1983), The identification and possible environmental impact of trace gases and vapours in landfill gas, Waste Management & Research, 1, 213-226 .