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Leachate Treatment

Methane Stripping

Process Type: Physical Treatment

Definition: Leachate Methane Stripping is the process of removal of dissolved methane from a liquid by physical transfer of methane into air by bubbling air through leachate containing dissolved methane.

Description:

Methane is more soluble in water than oxygen. At 20°C, about 25mg of methane will dissolve in a litre of water, from a pure methane atmosphere. Leachates from within a biologically active landfill will generally be extracted from a gaseous environment comprising typically 60 percent methane, and 40percent carbon dioxide (by volume).

In these circumstances, at temperatures of between 40 and 20 degrees centigrade, methane can dissolve to concentrations of between 10 and 15 mg/l.

Such dissolved methane concentrations are routinely measured in landfill leachates (e.g. Robinson et. al., 1999). Even at landfills where relatively diluted leachates are collected from surface seepages, perimeter ditches etc, concentrations of methane in the order of 2 - 5 mg/l are often determined, and values can vary widely on a day-to-day basis.

Significant methane levels can even be measured in pools of surface water on capped landfill areas, where landfill gas is escaping by bubbling through them.

A concentration of dissolved methane as low as 1.4 mg/l is known to be capable of giving rise to an explosive level of methane gas, in confined atmospheres in contact with such liquid (Buswell and Larson 1937; Larson, 1938).

Actual (as opposed to potential) risks of sewer explosion from the presence of dissolved methane from landfill leachates in the UK have not been established by any recent technical papers, and none are described as such in the Health & Safety Executive lists of reported accidents (2006). However, there is now a presumption that measures should be applied to control levels of dissolved methane in discharges of leachate into the public sewerage system.

Therefore, in accordance with mine safety procedures, a factor of safety of ten times is normally being applied in the UK and elsewhere in many nations, by Regulators, for discharges of leachate into the public sewerage system. This is provided by compliance with an upper consented limit of 0.14 mg/l of dissolved methane, as is widely applied by receiving sewerage authorities.

In order to meet this consent limit, therefore, from initial dissolved methane levels of 15 mg/l in leachates, more than 99 percent removal must be achieved, reliably and consistently.

Advantages:

Methane stripping is a simple method of initial pre-treatment which will enable the discharge to sewer, in countries where the sewerage authority requires removal of all explosive substances. It will not remove other contaminants.

Disadvantages:

Concerns at leachate methane stripping plants relate closely to the composition of specific leachates. Foaming may sometimes be an issue, particularly in treatment of leachates from more recent wastes, and can require routine addition of small quantities of antifoam agents.

Calcification may present a problem and unless the stripping plant is designed to avoid the build-up of calcium deposits running costs have on many plants been far higher than anticipated, due to cleaning costs.

Continuous monitoring of levels of dissolved methane in final effluents using membrane probes has proved unreliable at present.

Alternative, indirect measurements, (such as PPM methane values in off-gases from the final stripping reactor) could be incorporated as could continual monitoring of dissolved oxygen, but simpler systems such as fail-safe shutdown and alarm systems when blowers fail to operate/ draw current are likely to be more reliable at present.

Where best used:

Landfill sites where adequate capacity is available for treatment in the local sewage treatment works. Small sites and sites where there would not be a suitable watercourse for a discharge of a high quality treated leachate. Also useful for old sites where leachate can be low in contaminants.

Costs comments:

Cost is only a few pence per cubic metre treated - as long as high calcification (scale removal jetting) costs can be avoided by expert design.

Sustainability comments:

The vented methane gas is lost to atmosphere in all the designs adopted to date in the UK, and the methane in the offgas may be considered to be significant in terms of carbon emissions. (See FAQs at www.methanestripping.uk.)

Energy usage comments:

Energy is used in the process, but energy use is low compared with aerobic SBR treatment.

Chemical usage/by-product production:

No chemicals are consumed in the methane stripping process, but may be used to prevent calcification and to control foaming.

Environmental Issues and Health & Safety (DSEAR) Concerns:

Provided that adequate volumes of air are used during the stripping process, concentrations of methane present in exhaust gases will be well below explosive levels. Of greater concern, especially in leachates from relatively- recently emplaced wastes, may be potential for release of odorous gases during the stripping process. The significance of such releases can rapidly be assessed by use of pilot-scale stripping trials, involving collection of gas samples for formal testing using odour panels (where members of the public determine at what dilution such odours are detectable, in controlled trials). Although at the great majority of full-scale methane stripping installations in the UK, such odour effects have been minimal and have not required specific treatment, at some sites gas biofilters (e.g. brushwood or heather filters) have been successfully installed.


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