The ENVIROS METHANE STRIPPING WEB SITE
In the UK and many other countries, the most preferred option for landfill leachate disposal is via the sewer, where the contaminants in leachate can be treated in combination with domestic sewage.
Disastrous accidents, such as at Abbeystead in North West England, have sensitised local water companies to the dangers of dissolved methane within pumping schemes, but only now is its control in leachate discharges becoming widely adopted. 
Explosion Hazard
Failure to consider the possibility of an explosive hazard, arising within the confined atmosphere of a sewer, as a result of the introduction of leachate containing high levels of dissolved methane, would therefore represent negligence on the part of a landfill operator.
Where leachate has been pumped directly from within landfilled wastes, it can contain relatively high concentrations of dissolved methane gas. In theory, concentrations might exceed 25 mg/l of dissolved methane, but since landfill atmospheres typically contain only up to 60% methane gas by volume, dissolved concentrations recorded are usually up to 10 - 15 mg/l of dissolved methane.
Such levels are typical at a wide range of sites, but even at landfills where relatively diluted leachates are collected from groundwater, perimeter ditches etc., concentrations of methane in the order of 2 - 5 mg/l are often measured, and values can vary considerably on a daily basis.
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Figure 1. The pilot-scale methane stripping unit
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(All EU installations must also be compliant with the ATEX Directive since 2005, and (in the UK) with the DSEAR Regulations.) Enviros Consulting provides DSEAR risk assessments, and gas hazard zoning plans etc.)
Safe Concentrations
A concentration of dissolved methane as low as 1.4 mg/l in leachate is known to be capable of giving rise to explosive methane levels in atmospheres in contact with it. In accordance with mine safety procedures, a safety factor of 10-fold is routinely applied to discharges of leachate being made to sewer; maximum dissolved methane levels of 0.14 mg/l are often specified within discharge consents.
If an on-site treatment plant is to achieve this standard (which often represents a 99% removal of dissolved methane), reliably and consistently, then a scientific approach must be used in its design.
In a number of instances, methane stripping systems have been marketed as ‘packages’ for use with leachates, but have not performed adequately or reliably.
Experimental Trials
Over the last 7 years, Enviros Consulting) has carried out studies at a number of landfill sites in the UK and Ireland, to investigate the controlled stripping of methane from a variety of landfill leachates, using a purpose-built, pilot-scale system. These studies have provided fundamental and practical data, which now have been used to design several full-scale stripping plants on landfill sites.
Trials were undertaken using a portable treatment system, which could be set up rapidly, and is capable of treating in excess of 20 litres of leachate per minute (>1 m3/hr), at accurately controlled and measured leachate and air flow rates (see Figures 1 and 2).
The unit comprises of four identical reactors, each constructed from a standard wheeled bin. Each reactor contains a coarse aeration device in the base, that allows air to bubble through the contents at a rate which can be controlled and measured. Leachate is pumped into the first chamber, by means of twin electric pumps, directly from a leachate sump at the landfill, and flow rate can be determined accurately using a large measuring cylinder and stopwatch.
Effluent from chamber 1 overflows into chamber 2, and passes through the system in a similar manner, finally flowing by gravity into an effluent tank, from where it is pumped to discharge by a low level-controlled pump. When all of the chamber lids are closed, the concentration of methane in ‘off-gas’ from each individual chamber can be determined, by testing the flow of gas within a horizontal vent in the lid of each reactor.
The system was operated for 2 – 3 hours for each leachate tested, to allow more than 4 volumes (approximately 3 m3) of leachate to pass through. Regular measurements were conducted to ensure that the system had achieved ‘steady state’ conditions before final samples are taken, and tested for dissolved methane. If needed, small controlled amounts of a silicone-based anti-foam solution were added to the first reaction chamber, to calculate likely requirements of a full-scale system.
Concentrations of methane were measured regularly in leachate and liquid leaving each reactor using a rapid indirect method throughout the trial. The measurement of methane in reactor off-gases also provided a good indication of the stability of the overall treatment system during the trials.
Trial Results
Five landfills were visited in England and Ireland, and 5 leachates (labelled A to E) were tested using the pilot equipment. Concentrations of dissolved methane varied from 1.28 - 13 mg/l, and both acetogenic and methanogenic leachates were tested.
The stripping trial results for each leachate (mean values of several analyses taken after steady-state conditions) are summarised and presented in Figure 3. Results are expressed as reductions in actual concentration of dissolved methane against cumulative volumes of air used to strip each volume of leachate. Results in all tests were stable and consistent. In all cases, effluents from the fourth treatment reactor were well below the sewer discharge limit of 0.14 mg/l dissolved methane. For those leachates containing lower initial levels of dissolved methane (< 3.2 mg/l) this limit was reached after passage through only 3 reactors, and use of five or less volumes of air per volume of leachate in total.
ENVIROS Consulting is an independent UK environmental consultancy.
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