Can we help you solve your Dissolved Methane problem?
We provide process designs and other consultancy services for the planning, design, tendering, construction supervision commissioning, and operation of this type of plant.
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The IPPTS Associates METHANE STRIPPING WEB SITE
In the video below we explain what a methane stripping plant is, and why they are needed. It is under 2 minutes long, but if you are in a hurry we have repeated the text below.
In the United Kingdom and many other countries, the preferred option for landfill leachate disposal for the landfill operator, is to the public sewer without any treatment first.
This may be acceptable to the operator of the sewage works, and if so, the contaminants in the leachate are treated in combination with sewage.
Unfortunately, landfill leachate almost always contains dissolved methane from landfill gas in the landfill. This is an explosive substance when it comes out of the leachate as it flows in the sewer. Even small quantities of methane in the confined space of a sewer can cause explosions.
In the UK and elsewhere it is illegal to discharge anything like methane which is a gas and may cause an explosion, into a sewer.
Disastrous accidents, involving explosions and multiple loss of life have occurred due to methane in confined spaces,
For this reason, the authorities responsible for licensing leachate discharges to public sewers will only permit leachate to be discharged from which the methane has been removed.
The removal, or "stripping of methane" out of the leachate is carried out in a methane stripping plant.
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 landfill 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 frequently measured, and values can vary considerably on a daily basis.
(All EU installations must also be compliant with the ATEX Directive since 2005, and (in the UK with the DSEAR Regulations.) IPPTS Associates provides DSEAR risk assessments, and gas hazard zoning plans etc.)
Safe Methane Concentrations to Sewers
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.
Therefore, it is normal practice for the maximum permissible dissolved methane level of 0.14 mg/l to be specified within discharge consents/agreements.
If an on-site treatment plant is to achieve this standard (which often represents a 99% removal of dissolved methane), reliably and consistently, a scientific approach must be used in its design.
Only the use of a tried and tested scientifically based process design will ensure the discharge consent limit will be achieved, or better removal achieved, under all operating conditions.
In a number of instances, methane stripping systems have been marketed as standard ‘packages’ for use with leachates, but many have not performed adequately or reliably.
IPPTS Associates recommend that a bespoke design, tailored to each site, is essential for each installation, for reliability and economy, and efficient operation. This is due to the wide variability found in leachate contaminants from site to site.
Experimental Trials
Over the last 10 years, our designer 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 have been used by this designer to provide successful process designs for 11 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.
The unit comprised of four identical reactors, each constructed from a standard wheeled bin. Each reactor contained 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 was pumped into the first chamber, by means of twin electric pumps, directly from a leachate sump at the landfill, and flow rate was 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
The stripping trial results for each leachate (mean values of several analyses taken after steady-state conditions) were summarised. Results are expressed as reductions in actual concentration of dissolved methane against cumulative volumes of air used to strip each volume of leachate.
The 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.
The trial results therefore provided a scientifically based method of producing a process design for a wide range of leachates and methane concentrations, and it is this data which IPPTS Associates uses to provide our current designs for our methane stripping plant clients..
IPPTS Associates is a UK based environmental engineering consultancy.
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