As recycling becomes more common, use of landfill is reducing, but it is still an important means of waste disposal. For example, 2012-13 figures from Defra (department of the environment, food and rural affairs) for England show that 8.51 million tonnes, or 33.9%, of waste collected by local authorities went to landfill.
Paper, vegetable matter, wood, textiles and plastic make up the majority of waste going into modern landfills, about 65% of which is biodegradable. When a landfill site is first set up, there is a high proportion of oxygen present in the mass of waste. As this waste becomes damp from the seepage of rainfall, aerobic degradation takes place, producing carbon dioxide and sometimes other gases, such as hydrogen. As the oxygen is used up, anaerobic degradation may take place, producing methane, hydrogen sulphide and more carbon dioxide. It takes approximately two years for a landfill to start steadily generating gases, and at this stage the main constituents are methane (70%) and carbon dioxide (30%) with small amounts of hydrogen, hydrogen sulphide and other trace gases, depending on conditions.
What are the hazards associated with landfill?
There are two primary associated hazards. One is the production of leachate, formed when water passes through the waste in the landfill picks up organic and inorganic compounds. This toxic liquid collects at the base of the landfill cell. If not properly controlled, it can contaminate the surrounding soil, groundwater and nearby watercourses.
The other main hazard is the methane, carbon dioxide, hydrogen sulphide and other gases released by the breakdown of organic materials; known as landfill gas. The majority of the gas is produced during the working life of a landfill and for about 20 years after a site has been capped. However, low levels of residual generation will occur for much longer, possibly in excess of 100 years. Methane poses a severe explosion risk, is damaging to plant life and is also a greenhouse gas. Carbon dioxide, also a greenhouse gas, is toxic and an asphyxiant, as it depletes local oxygen levels. Hydrogen sulphide is highly toxic, even at very low concentrations.
Different ways to monitor
Landfill gas samples can be extracted and analysed either using portable equipment or by installing a permanent monitoring system. The main options are:
The relative concentration of methane/carbon dioxide/oxygen in the headspace of a borehole indicates the evolution of the decomposition process. Sampling equipment is used for monitoring by regularly drawing samples from boreholes and measuring the concentrations of each gas. Fluctuations in atmospheric pressure affect gas evolution and concentration, so monitoring systems often also measures the pressure within each borehole. Gas concentrations and pressure readings can be taken manually using portable instruments, while fixed sampling systems provide an automated solution where by readings are regularly taken and logged for analysis purposes.
This method is primarily used to locate methane emissions through breaches in the cap of a closed landfill and to demonstrate compliance with the Landfill Directive, in particular, identifying faults in the gas management system at a site and prioritising the remediation required, and quantifying the total emissions of this important greenhouse gas from the site as a whole.
The air around the perimeter of an operating or closed landfill site is monitored to quantify the level of methane and other gases escaping into the surrounding environment. This is usually performed at night, when air conditions tend to be stiller and there is a thermal rise, which takes gas from the ground upwards. In the UK, the permitted limit for ambient methane escaping from a site is 10ppm. Samples may be taken at the site perimeter or up to 500 metres away. Any demonstrated to exceed limits will be subject to repeated tests.
Landfill sites are, and will be for a long time, an integral part of any waste management strategy. Their ongoing use requires good operational practice, of which gas management is an integral part. With a Gas Management Plan in place, operators can be sure that any landfill under their jurisdiction will be safe for generations to come.