How LAndfills work
Landfills are more than just a pile of trash covered with dirt. The Fresno Municipal Sanitary Landfill, opened in California in 1937, was considered to be the first modern, sanitary landfill in the United States. It was the first to employ trenching, compacting, and the daily covering of waste with soil. But it wasn’t until 1979, over 40 years later, that the EPA developed criteria for sanitary landfills. These criteria were designed to protect the environment surrounding the landfill areas, specifically:
- Siting restrictions in floodplains, wetlands, etc.;
- Endangered species protection;
- Surface water protection;
- Groundwater protection;
- Disease and vector (rodents, birds, insects) control;
- Open burning prohibitions;
- Explosive gas (methane) control;
- Fire prevention through the use of cover materials; and
- Prevention of bird hazards to aircraft.
One thing is certain, just looking at this list of requirements, is that a landfill requires management since it remains a biologically active mound of both organic and inorganic waste. The two major biological activities of a “retired” landfill are the production of leachate (liquids), which have the potential to contaminate the groundwater; and the production of gases, primarily a 50/50 mixture of Carbon Dioxide and Methane.
LEACHATE MONITORING AND CONTROL
During operation and up to 30 years after the landfill is closed, there is an elaborate and detailed requirement for sampling of any liquid leachate. It must be captured both upstream and downstream of the landfill to determine what changes, if any, are happening as water works its way through the landfill. The number of sample stations, frequency of sampling, and target substances of the sampling program are specifically called out. This is known as the detection monitoring phase. If any substances of concern are found, then the sampling moves to an Assessment Monitoring Phase. This means more frequent monitoring on an expanded number of substances and a groundwater protection standard must be applied. Successive clean samples allows the landfill operator to move back down to the monitoring level. If, however, leachate contamination levels remain high then they must move to corrective measures that protect human health and the environment. Remedial actions must remain in place until three years of consecutive compliance with groundwater standards have been achieved. By law, landfills must be monitored for 30 years after the last bit of waste has been deposited and covered by soil.
The other active component produced by a landfill as it ages is a mixture of Carbon Dioxide and Methane; both greenhouse gases. Methane has 21 times the greenhouse gas effect of Carbon Dioxide. Recent figures in the US indicate that about 500 landfill installations collect the methane gas and burn it to convert to electricity. The EPA estimates that as many as 500 additional landfills could cost effectively convert their methane to energy as well. Although the burning of methane does produce carbon dioxide and water as byproducts, since methane has a much stronger greenhouse effect, converting the methane to carbon dioxide is a net positive for greenhouse gas emissions. One other alternative is for the landfill operator to scrub the landfill gas to convert it to pipeline grade methane and then sell off the methane directly.
MINING MODERN LANDFILLS
There is a growing trend of viewing landfills as a source of valuable mixed, recoverable material. Landfills usually have some construction waste such as wood and metal. Often the aluminum content in a landfill exceeds the concentrations that one would find in a surface mine of aluminum ore. This makes the economic recovery of aluminum very feasible. Wood can be used as fuel to generate power and some precious metals are available in used electronics. Recovery of precious metals from electronics has gained popularity under the name of
“Urban Mining”. In addition, a significant percentage of most landfills is waste food products that readily degrade into compost. They are not necessarily of high value, but removing them does produce a useful product and it also reduces the size of the landfill considerably. Most landfill mining operations consist of a front end loader that moves waste on to a conveyor which has a variety of sorting stations. The stations would vary depending on the contents in the landfill but various sizes of screens might be used to separate organic material from inorganics for example. Magnetic sorters would pull out any steel components and so forth. There are also researchers which are looking at novel ways to sort and recover plastics. The variety of plastics makes this a difficult task, but success in this area would go a long way toward reducing landfills and give a boost to the circular economy.
A new life for landfills
When a landfill passes its 30 year mark and is stable with no groundwater risk, it pretty much looks like an overgrown field or maybe a hilly area, and is hardly distinguishable from its surroundings. This is the post-closure and reclamation stage which is broken down into three categories or types:
Category 1 – Very little structures, looks like an open field; parks, cemeteries
Category 2 – Utilities, light structures, paving; golf course, baseball field
Category 3 – Major structures; stadiums, shopping centers, buildings