Filling it Up
A modern sanitary landfill is divided into cells at the design stage. During the landfill’s life, only a single cell is active at a time. An individual cell may range from several hundred square feet to several acres in size, but it’s intended to hold a single day’s waste. The active life of a landfill, then, will depend on how many cells it will have, how closely the waste stream matches predictions, and other factors relating to design and management.
Breaking it Down
There are four stages to the breakdown of waste in a sanitary landfill. Since waste is added continuously over the active years of the landfill, waste will be found in various stages throughout the site, typically organized by individual cells.
- Stage 1 (aerobic processes)
As waste is dumped in an active landfill cell, there is plenty of oxygen available throughout the mass, although it is reduced by repeated compaction as new waste is added. In this environment, aerobic bacteria thrive. Oxygen is consumed while complex carbohydrates, proteins and lipids are broken down, releasing carbon dioxide. Meanwhile, oxygen content decreases. This initial stage lasts until all available oxygen is depleted, which can take anywhere from days to months.
- Stage 2 (anaerobic conversion to organic acids)
As oxygen is depleted by aerobic bacteria and other microbes, decreasing concentrations of oxygen lead to widespread anaerobic conditions within the layers. As this continues, anaerobic bacteria take over and effluent gas quickly transitions from O2 to CO2 and the environment becomes highly acidic. During this stage, when insoluble organic polymers such as carbohydrates are exposed to water, they quickly break down into derivatives that can be processed by other bacteria. Acidogenic bacteria gain ascendance at this stage, converting sugars and amino acids into CO2, hydrogen, ammonia, and organic acids.
- Stage 3 (acetate formation)
In acetogenesis, anaerobic bacteria convert organic acids to acetic acid, bringing pH to a more neutral state and permitting methane-producing bacteria to get established. Methane- and acid-producing bacteria function in syntropy, where the acid-producing bacteria generate compounds that methane-producing bacteria consume, while methanogenic bacteria in turn consume CO2 and acetate, keeping levels within a range that’s tolerable to the acid-producing bacteria.
In the early aerobic phases of decomposition, biodegradable organic matter is rapidly broken down and the volume of waste decreases quickly. This permits more compaction and the addition of more layers on top, which restricts access to oxygen while the leachate’s chemical oxygen demand increases, promoting conversion to the anaerobic state.
Ultimately, it’s the establishment of syntropy between bacterial species that determines the success of the landfill’s mission.
The usable life of a sanitary landfill is highly dependent on consistently achieving a maximum compaction rate in order to save space and promote the anaerobic conditions that long-term decomposition processes require. In today’s landfills, a useful life expectancy of 30-50 years is typical.
Some newer styles of landfill, known as bioreactors, reuse leachate to support bacterial breakdown. This practice can significantly increase the rate of waste decomposition and ultimately, waste consolidation. This means that the waste occupies a smaller volume and could potentially reduce the span of post-closure management.
Closing it Down
Once a sanitary landfill has reached maximum capacity, the final cell(s) are capped with an impermeable cover, covered with at least two feet of soil, and planted with grass and bushes that do not have deep, invasive roots, so that the liner below is not punctured in future years. This cover will also have a gas venting layer and a stone or synthetic biotic layer to discourage burrowing animals.
Since anaerobic breakdown is such a slow process, landfills continue to produce methane and CO2 for decades after they have stopped accepting waste. Unmonitored accumulation of these gases can be dangerous, so decommissioned landfills typically require active monitoring for 30 years or more after closure. Gas collection systems are required by the EPA to direct gas to collection points, where it can be burned off, filtered and transported for use as a drop-in replacement for natural gas, or used on-site as an alternative fuel or a source of electricity.
Leachate continues to present a danger to the local environment and must be checked regularly to record the volume of leachate produced and ensure that barriers won’t be overwhelmed and release leachate during a major storm event, for example. Leaks in liners are always possible to some extent, no matter the type of liner employed. Soil and groundwater monitoring should be conducted frequently to ensure there’s no leachate contamination.
Land settlement is inevitable, but harder to manage. As anaerobic digestion and waste compaction continues, municipal landfills can slump as much as 20% over several decades. This isn’t an issue if the landfill has been converted to a natural area, but any roads, utilities, buildings, and sports fields are vulnerable to settlement.
