Landfill Site Selection
The siting of a landfill should be based on both operational and risk factors. Key considerations when identifying suitable sites include:
Proximity to source of waste to satisfy the proximity principle and to keep transport costs to a minimum
Accessibility to suitable road and possibly rail
Presence to sensitive human and protected ecological receptors
Location of and distance to sensitive groundwater resources, especially where taking biodegradable waste.
Landfill Chemistry and Biology
The processes and reactions that take place in a landfill depends on the type of material or waste deposited in it. To better explain these processes and reactions, a Municipal Solid Waste (MSW) landfill would be the best scenario to look at. Municipal waste is made up mainly of organic and inorganic waste. The organic contents include paper, kitchen, garden waste and textile while the in-organics include glass, plastics, metals and minerals. The organic contents are the main culprits in terms of reactions in and emissions from landfills.
The organic or better put, biodegradable material in waste are mainly made up of: a. Carbohydrates (simple and complex) such sugars, cellulose, etc; b. Lipids or fats which are made up of long hydrocarbon chains with carboxylic acid group at one end; c. Protein which are generally in the form of complex polymers made up of amino acid groups; d. Lignin which are organic polymers with complex molecular structures. There are five main phases in the decomposition of land filled waste:
Aerobic decomposition Decomposition is the process by which organic substances are broken down into simpler forms of matter. Aerobic decomposition therefore simply means decomposition in air. This phase is important for the formation of gas and leachate. As in the title, this stage is aerobic. These pockets of air are present in the gaps between deposited waste. Due to the type of waste deposited (MSW), water is also present. These conditions are suitable for aerobic micro-organism which begin to break down the waste in a series of hydrolysis reaction. As this continues, carbohydrates are broken down to into simple sugars (glucose), carbon dioxide and water. Gas and leachate formation increases at this stage. The leachate is characterized with high concentration of ammonium while the gas is made up of approximately 70-80% carbon dioxide and 15-20% hydrogen with nitrogen making up the rest. As the process of decomposition continues and in addition to subsequent deposition of waste as cover material, the oxygen level is depleted causing the aerobic microbes to become inactive. At this stage, anaerobic conditions begin to set in.
Hydrolysis and Fermentation At this stage, oxygen level would have dropped significantly and facultative micro-organism become more active. In a series of hydrolysis reactions, the micro-organisms break down carbohydrates and lipids into sugars and the sugars are further broke down to carbon dioxide and water. The break down of lipids result in gradual release of acetic acid. Proteins which are made up of long chains of amino acids are firstly broken down into simpler amino acids and then further broken down to ammonia and fatty acids such as acetic acids. Some of the amino acid is used up by the micro-organisms for growth. Acetogenesis As the name implies, this stage is dominated by the formation of organic acids principally acetic acid. Anaerobic conditions are fully established and Acetogenic micro-organisms dominate to convert amino acids to simpler organic acids (acetic acid), carbon dioxide and hydrogen . Any remaining carbohydrates are further broken down to produce more acetic acids as demonstrated below: C6H12O6 + 2H2O --> 2CH3COOH + 2CO2 + 4H2 Other reactions takes place involving carbohydrate, carbon dioxide and hydrogen to produce more acetic acid while reducing carbon dioxide and hydrogen levels in the gas. This reduction in levels is followed by the development of methane forming micro-organisms which is discussed below. The leachate produced during this stage is highly acid with a pH as low as 4. Leachate produced during this stage is highly toxic with high levels of heavy metals dissolved in it. Methanogenesis This is characterised by high level of methane and carbon dioxide (approximately 60% and 40% respectively) and an increase in the leachate pH mainly due to further break down of organic acids (acetic acid). This stage normally begins at least six months from the date of waste tipping and continues for a number of years to become fully established. Methane and carbon dioxide are formed in two main reaction processes. The Methanogenic micro-organisms are predominant during this stage and generate methane and carbon dioxide from the acetic acid (equation 1). At the same time, carbon dioxide and hydrogen formed during the hydrolysis of lipids are converted by other methanogenic bacteria to form additional methane (equation 2): Equation 1 2CH3COOH --> 2CH4 + 2CO2 Equation 2 4H2 +CO2 --> CH4 + 2H2O Signifcant levels of methane is produced from 3-12 months after the start of this phase and can continue for up to 30 years after site closure. It should be noted that gas production in a landfill is a long term process and may continue for a far longer period beyond the specified 30 years. This is why landfill’s design life extends many years beyond the time when it is closed. Oxidation This is the final stage of the landfill reaction and decomposition process. As the rate of organic acid production and break down reduces, the levels of methane and carbon dioxide also begin to decline. Air begins to diffuse into the waste mass and aerobic conditions becomes established. Residual methane is further oxidized by the action of aerobic bacteria.
Landfill Site Design
The landfill directive imposes a number of operational and engineering requirements on all newly constructed landfills and existing landfills (pre-landfill directive) that wish to continue accepting waste. The landfill directive also requires that all landfills must be classified as either inert waste, non hazardous or hazardous waste sites. Regardless of the classification, sites must:
be provided with a geological barrier to prevent water from entering and leachate from escaping the site;
be designed to prevent ingress of groundwater, surface water and precipitation;
be designed to enable collection of contaminated surface waters for treatment;
be provided with an artificial sealing liner and drainage layer to collect leachate;
have gas collection system and the gas must be used for energy production or flared.
Each site will have it's own characteristics and sensitivities therefore the engineering and operational requirements should be designed on a site-by-site basis. An assessment and understanding of the geological and hydrogeological setting, the waste types to be accepted and their behavior in a landfill environment and the risk of emission and potential impact on receptors are all factors to consider in this case.
The landfill directive requires that all landfills must be provided with an artificial sealing liner and drainage layer to collect leachates. A landfill liner is intended to be a low permeable barrier, which is laid down under engineered landfill sites. Until it deteriorates, the liner retards migration of leachate, and its toxic constituents, into underlying aquifers or nearby rivers, causing pollution of the local water resources. With this in mind, we can agree that site liners need to be designed on a site-by-site basis and part of the design process will involve understand a risk assessment of the fate of any discharged leachate through a given liner.
In all cases, all landfill site liner system should consist of the following:
Leachate collection layer
This consists of 0.5m thick layer of porous material such as gravel. It is located immediately below the deposited waste and meant to provide a free drainage region. A network of perforated pipes leading to a collection sump is also provided to support drainage. The collected leachate is pumped out and collected for treatment at regular intervals.
Liner protection layer
This is a flexible geotextile layer which lies immediately beneath the leachate collection layer. It acts as a filtration system to retain fines and smaller particles and also protects the main barrier from being punctured by the gravel that forms the drainage collection layer.
As stated previously, the landfill directive requires that all landfill must be provided with a geological barrier to prevent water from entering and leachate from escaping the site . The geological barrier is a natural geological low permeability formation between the underside of the artificial sealing liner and any groundwater that may be present. It is intended to provide a longer lasting protection by preventing any harm to the soil or groundwater after the artificial sealing liner has degraded.
Artificial sealing liner
This is made up of a layer of compacted natural clay with a minimum required thickness and a maximum allowable hydraulic conductivity, overlaid by a high-density polyethylene / polymer membrane. This together with the existing site geological barrier prevents the ingress of surface water into the landfill and release of leachate from it. In some instances there may also be a groundwater collection system located below the liner to reduce the risk of groundwater ingress.
In the absence of the above barrier systems, the effects of landfills will be disastrous as is still the case with a number of historic landfills. While these systems provide high level of protection to the environment, there will always be some seepage through them. This can be calculated using Darcy's law which is used to predict the flow of fluids through beds of granular material. It is expressed in the following way:
Q = kAi
Q = ki
Q = flow rate in cubic meters per day
k = permeability in meters per day
i = hydraulic gradient
A = cross sectional area in square meters perpendicular to the flow
Permeability is a measure of how rapidly a fluid will flow through the granular bed
Hydraulic gradient is defined as the sum of the head or depth of fluid and the thickness of the geological barrier, divided by the thickness of the geological barrier.
Site capping is required on all landfill types. The landfill directive requires that on completion of tipping, each cell must be capped for the following reasons:
to prevent pest and vermin into the site;
to prevent rainfall and surface water entering the site;
to control the release of landfill gas into the environment;
to prevent ingress of air into the landfill thereby sustaining anaerobic conditions.
A typical cap will consist of five layers:
Surface gas drainage layer
The comprises of a layer of porous materials that enables gas to be pumped and moved to the edge of the site for collection.
Surface artificial sealing liner
This is only mandatory for hazardous waste landfills and is made up of a membrane or geo-textile material to prevent ingress of water.
Impermeable mineral surface layer A layer of compacted clay used to protect the artificial sealing liner and to reduce gas release from and water ingress into the landfill.
Surface drainage layer
This is constructed in such a way as to allows free flow of surface water to the site perimeter for collection and/or treatment.
The topsoil layer serves two main purpose; to protect the capping layer and to allow grasses and other plants to be established without the danger of root penetration and damage to the underlying layer. It is recommended to use well aerated soils with good organic content. This has the added advantage of promoting microbial communities which are able to oxidize and methane seeping through the capping layers. It should be noted that the above requirements can be relaxed depending on the type of waste and local conditions.
Surface water protection
Where present within or beside the landfill site, surface water including running water (river), static water (lakes) must be protected. Failure to do this may result in costly pollution and damage to them.
Where running water cannot be diverted, an impervious seal should be carefully constructed between it and the landfill. In the case of static water like lakes, this should be pumped out where possible before filling commences. Where this is not possible then it should be infilled with inert material for several meters above the water table to provide an unsaturated base. Running water from the site may be contain contaminants from the site. This should be prevented from leaving the site and collected for treatment or off-site disposal.
Leachate Collection and Treatment
Leachate is any liquid that, in passing through matter, extracts solutes, suspended solids or any other component of the material through which it has passed. Leachate removal from a landfill base is necessary for the following reasons:
a. to reduce leakage; the lower the leachate head, the lower the leakage rate;
b. to reduce damage to the liner from corrosive substances in the leachate;
c. to allow the leachate to be collected for treatment before release into the environment.
Sumps are constructed in landfills to enable leachate to drain into. The collected leachate is then collected through a network of vertical wells which lead from the button of each sump to the surface. Each well is fitted with pipes and pumps to remove the leachate for treatment.
There are different processes available for treating leachate all of which are discussed in the pollution control section.
Landfill Gas Collection and Treatment
Landfill gases are mainly made up of methane and carbon dioxide both of which are greenhouse gases. Methane is also odorless and explosive which makes it even more dangerous. For these and other reasons, landfill gasses must be removed.
Landfill gas removal systems normally consists of a network of vertical boreholes connected to a network of pipes at the surface and a system of pumps. The collected gas is passed through a condensate to remove the moisture and water droplets before passing through a gas engine to generate power or flared through a stack. Flaring converts the methane to carbon dioxide which a less potent greenhouse gas and also destroys odor-forming compounds.
It should be noted that gas engines and flares are not exactly end of pipe treatment systems. Burning or combustion of landfill gas generate pollutants in their own right. This will be discussed further in the pollution control section.