Monitoring of landfills

Merseyside Recycling and Waste Authority (MRWA) has a statutory obligation to conduct environmental monitoring at its closed landfill sites.

Environmental monitoring falls into three categories;

– Landfill Gas Monitoring

– Leachate Monitoring

– Groundwater and Surface Water Monitoring

Landfill Gas Monitoring

As waste within a landfill decomposes, microscopic bacteria break the waste down and produce a number of gases. Using a portable hi-tech infra-red gas analyser MRWA regularly monitor each of the seven closed landfill sites for landfill gas at fixed monitoring points around each site. Landfill gas is made up of several components, each at varying concentrations, these are:

– Methane

– Carbon Dioxide

– Oxygen

– Hydrogen Sulphide

– Carbon Monoxide

– Other trace gases

Landfill gas is extracted from two of MRWA’s closed landfill sites with the methane element of the gas at Billinge burned to produce electricity (enough to power approximately 1000 homes) which is fed into the national grid.

At sites where electricity production is not a viable option, but landfill gas is produced in moderate quantities, gas flaring units are used. The gas flare breaks the methane (a greenhouse gas) into water and carbon dioxide, therefore reducing its impact on the environment.

The remaining sites produce an insufficient quantity of landfill gas to sustain an active gas extraction system. These sites are regularly monitored for landfill gas, as well as engineered containment and dispersion measures, such as vent trenches and cowlings.

Leachate Monitoring

Alongside landfill gas monitoring, MRWA has a statutory obligation to monitor the landfill leachate produced within the sites.

Leachate is the liquid produced as rainwater and groundwater percolate through the waste. As the water passes through the decomposing waste it picks up chemical and biological components which, if not properly controlled, may be harmful to the environment.

Leachate is collected at the base of sites, where it is then pumped to the surface for treatment, before being discharged to a public foul sewer, where it is then treated in waste water treatment works. MRWA regularly monitors the leachate both within the site and before discharge, in order to ensure compliance with environmental legislation.

Leachate is monitored from several points within the landfill sites:

– boreholes located within the main body of the landfill

– specially designed sampling points before the discharge point

Leachate samples are collected from these points and monitored for a variety of chemical and biological determinants, including ammoniacal nitrogen, dissolved methane, and pH.

Groundwater and Surface Water Monitoring

To ensure that the control measures (engineered containment systems that prevent leachate migrating off site) are working correctly, groundwater and surface water samples are taken. The samples are sent for analysis in the laboratory, where they are tested for the same determinants as leachate samples. Samples are taken from upstream and downstream locations, this allows a comparison to be made, and therefore identify any contamination (if any) from the site.

The groundwater level at the gas monitoring boreholes is measured on a regular basis. This enables MRWA to compare groundwater levels over the year.