Landfill leachate is contaminated liquid formed when rain or another source of water enters a landfill and comes in contact with waste, which may then flow into and contaminate local groundwater if not properly managed. Landfill leachate varies widely in composition depending on the age of the landfill and the type of waste that it contains. It usually contains both dissolved and suspended material. The generation of leachate is caused principally by precipitation percolating through waste deposited in a landfill. Once in contact with decomposing solid waste, the percolating water becomes contaminated, and if it then flows out of the waste material it is termed leachate. Additional leachate volume is produced during this decomposition of carbonaceous material producing a wide range of other materials including methane, carbon dioxide and a complex mixture of organic acids, aldehydes, alcohols and simple sugars.
The risks of leachate generation can be mitigated by properly designed and engineered landfill sites, such as those that are constructed on geologically impermeable materials or sites that use impermeable liners made of geomembranes or engineered clay. The use of linings is now mandatory within the United States, Australia and the European Union except where the waste is deemed inert. In addition, most toxic and difficult materials are now specifically excluded from landfilling. However, despite much stricter statutory controls, leachates from modern sites are often found to contain a range of contaminants stemming from illegal activity or legally discarded household and domestic products.
In a 2012 survey performed in New York State, all surveyed double-lined landfill cells had leakage rates of less than 500 liters per hectare per day. Average leakage rates were much lower than for landfills built according to older standards before 1992.
Composition When water percolates through waste, it promotes and assists the process of decomposition by bacteria and fungi. These processes in turn release by-products of decomposition and rapidly use up any available oxygen, creating an anoxic environment. In actively decomposing waste, the temperature rises and the pH falls rapidly with the result that many metal ions that are relatively insoluble at neutral pH become dissolved in the developing leachate. The decomposition processes themselves release more water, which adds to the volume of leachate. Leachate also reacts with materials that are not prone to decomposition themselves, such as fire ash, cement-based building materials and gypsum-based materials changing the chemical composition. In sites with large volumes of building waste, especially those containing gypsum plaster, the reaction of leachate with the gypsum can generate large volumes of hydrogen sulfide, which may be released in the leachate and may also form a large component of the landfill gas. The physical appearance of leachate when it emerges from a typical landfill site is a strongly odoured black-, yellow- or orange-coloured cloudy liquid. The smell is acidic and offensive and may be very pervasive because of hydrogen-, nitrogen- and sulfur-rich organic species such as mercaptans.
In a landfill that receives a mixture of municipal, commercial, and mixed industrial waste but excludes significant amounts of concentrated chemical waste, landfill leachate may be characterized as a water-based solution of four groups of contaminants: dissolved organic matter (alcohols, acids, aldehydes, short chain sugars, etc.), inorganic macro components (common cations and anions including sulfate, chloride, iron, aluminium, zinc and ammonia), heavy metals (lead, nickel, copper, mercury), and xenobiotic organic compounds such as halogenated organics, (PCBs, dioxins, etc.). A number of complex organic contaminants have also been detected in landfill leachates. Samples from raw and treated landfill leachate yielded 58 complex organic contaminants including 2-OH-benzothiazole in 84% of the samples and perfluorooctanoic acid in 68%. Bisphenol A, valsartan and 2-OH-benzothiazole had the highest average concentrations in raw leachates, after biological treatment and after reverse osmosis, respectively.
Management
In older landfills and those with no membrane between the waste and the underlying geology, leachate is free to leave the waste and flow directly into the groundwater. In such cases, high concentrations of leachate are often found in nearby springs and flushes. As leachate first emerges it can be black in colour, anoxic, and possibly effervescent, with dissolved and entrained gases. As it becomes oxygenated it tends to turn brown or yellow because of the presence of iron salts in solution and in suspension. It also quickly develops a bacterial flora often comprising substantial growths of Sphaerotilus natans.
History of collection In the UK, in the late 1960s, central government policy was to ensure new landfill sites were being chosen with permeable underlying geological strata to avoid the build-up of leachate. This policy was dubbed "dilute and disperse". However, following a number of cases where this policy was seen to be failing, and an exposee in The Sunday Times of serious environmental damage being caused by inappropriate disposal of industrial wastes, both policy and the law were changed. The Deposit of Poisonous Wastes Act 1972, together with The 1974 Local Government Act, made local government responsible for waste disposal and for the enforcement of environmental standards regarding waste disposal.
Proposed landfill locations also had to be justified not only by geography but also scientifically. Many European countries decided to select landfill sites in groundwater-free clay geological conditions or to require that the site have an engineered lining. In the wake of European advancements, the United States increased its development of leachate retaining and collection systems. This quickly led from lining in principle to the use of multiple lining layers in all landfills (excepting those truly inert).
Collection systems The primary criterion for design of the leachate system is that all leachate be collected and removed from the landfill at a rate sufficient to prevent an unacceptable hydraulic head occurring at any point over the lining system.
There are many components to a collection system including pumps, manholes, discharge lines and liquid level monitors. However, there are four main components which govern the overall efficiency of the system. These four elements are liners, filters, pumps and sumps.
Liners
Natural and synthetic liners may be utilized as both a collection device and as a means for isolating leachate within the fill to protect the soil and groundwater below. The chief concern is the ability of a liner to maintain integrity and impermeability over the life of the landfill. Subsurface water monitoring, leachate collection, and clay liners are commonly included in the design and construction of a waste landfill. To effectively serve the purpose of containing leachate in a landfill, a liner system must possess a number of physical properties. The liner must have high tensile strength, flexibility, and elongation without failure. It is also important that the liner resist abrasion, puncture, and chemical degradation by leachate. Lastly, the liner must withstand temperature variation, must resist UV light (which leads most liners to be black), must be easily installed, and must be economical.
There are several types of liners used in leachate control and collection. These types include geomembranes, geosynthetic clay liners, geotextiles, geogrids, geonets, and geocomposites. Each style of liner has specific uses and abilities. Geomembranes are used to provide a barrier between mobile polluting substances released from wastes and the groundwater. In the closing of landfills, geomembranes are used to provide a low-permeability cover barrier to prevent the intrusion of rain water. Geosynthetic clay liners (GCLs) are fabricated by distributing sodium bentonite in a uniform thickness between woven and non-woven geotextiles. Sodium bentonite has a low permeability, which makes GCLs a suitable alternative to clay liners in a composite liner system. Geotextiles are used as separation between two different types of soils to prevent contamination of the lower layer by the upper layer. Geotextiles also act as a cushion to protect synthetic layers against puncture from underlying and overlaying rocks. Geogrids are structural synthetic materials used in slope veneer stability to create stability for cover soils over synthetic liners or as soil reinforcement in steep slopes. Geonets are synthetic drainage materials that are often used in lieu of sand and gravel. Radz can take of drainage sand, thus increasing the landfill space for waste. Geocomposites are a combination of synthetic materials that are ordinarily used singly. A common type of geocomposite is a geonet that is heat-bonded to two layers of geotextile, one on each side. The geocomposite serves as a filter and drainage medium.
Geosynthetic clay liners are a type of combination liner. One advantage to using a geosynthetic clay liner (GCL) is the ability to order exact amounts of the liner. Ordering precise amounts from the manufacturer prevents surplus and over-spending. Another advantage to GCLs is that the liner can be used in areas without an adequate clay source. On the other hand, GCLs are heavy and cumbersome, and their installation is very labor-intensive. In addition to being arduous and difficult under normal conditions, installation can be cancelled during damp conditions because the bentonite would absorb the moisture, making the job even more burdensome and tedious.
Drainage system The leachate drainage system is responsible for the collection and transport of the leachate collected inside the liner. The pipe dimensions, type, and layout must all be planned with the weight and pressure of waste, and transport vehicles in mind. The pipes are located on the floor of the cell. Above the network lies an enormous amount of weight and pressure. To support this, the pipes can either be flexible or rigid, but the joints to connect the pipes yield better results if the connections are flexible. An alternative to placing the collection system underneath the waste is to position the conduits in trenches or above grade.
The collection pipe network of a leachate collection system drains, collects, and transports leachate through the drainage layer to a collection sump where it is removed for treatment or disposal. The pipes also serve as drains within the drainage layer to minimize the mounding of leachate in the layer. These pipes are designed with cuts that are inclined to 120 degrees, preventing entry of solid particles.
Filters The filter layer is used above the drainage layer in leachate collection. Two types of filters are typically used in engineering practices: granular and geotextile. Granular filters consist of one or more soil layers or multiple layers having a coarser gradation in the direction of the seepage than the soil to be protected.
Sumps or leachate well