The Dissolved Methane Limit at 0.14mg/l and Methane Removal Explained

Methane comes from:

The landfill gas in the landfill

Range of methane concentrations found in raw leachate:

1mg/l to 20mg/l

Hazards to Sewer Workers/ Sewers Generally:

Explosion Hazard: oOccurs if the gas and air mixture is within the Explosive Limit,

Health Hazard: If methane concentrations are in exceedance of WHO Workplace concentrations in air, and

Ashyxiation Hazard: If methane is present at above the Explosive Limit

1. Public Sewer Discharge of Leachate

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.

But, unfortunately, most landfill leachates contain dissolved methane, unless it is removed.

2. Why Dissolved Methane Must be Removed

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 inevitably will, very soon after it flows into the sewer.

Even small quantities of methane in the confined space of a sewer can cause explosions, it would also be unhealthy for sewer workers to breathe for long periods, and if highly concentrated it can cause suffocation.

In the UK and elsewhere, it is illegal to discharge anything that can cause explosions or fires, like methane gas, into a sewer.

3. Accidents Have Happened in the Past

Disastrous accidents, involving explosions and multiple losses 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.

4. Consequences of Failure to Remove Methane

It has been illegal to discharge to public sewers, materials which may cause fire, or explosions in most national jurisdictions, for more than 50 years.

There may not be explicit limits written into discharge consents and permits, however, this does not relieve dischargers of their liability for such discharges.

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 be likely to be seen as negligence.

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Why Sewer Operators Usually Set a Maximum Permissible Dissolved Methane Limit in Leachate at 0.14mg/l1. A Lower Explosive Limit of 1.4mg/l was Calculated and Divided by 10.

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 the sewer.

Therefore, it is normal practice for the maximum permissible dissolved methane level of 0.14 mg/l to be specified within discharge consents/ permits/ 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.

* By applying basic scientific principles to the calculation of the vapour pressure equilibrium calculation for methane in still air.

2. The Reason for Applying a Safety Factor of 10

Sewer systems operators require a Factor of Safety of 10, on the allowable maximum dissolved methane concentration discharged to sewer due to unknown factors, which may contribute to the presence of methane in sewers.

For example, some sewers already contain methane gas, and any contribution from the discharge of leachate should not be allowed to appreciably worsen existing methane risks.

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 used in the process industries 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 for efficient operation. This is due to the wide variability found in leachate contaminants from site to site.