31 Structure and operation of solid waste landfill
Dr. Yogalakshmi K. N
Objectives:
1. To define landfill
2. To understand the components of landfill
3. To know the different types of lining material and liner system used in landfill
4. To discuss the types of waste emplacement within the landfill
5. To understand the structure of a landfill capping or cover
6. To discuss the process of waste degradation within the landfill
7. To know the various factors influencing the waste degradation in landfill
Disposal of solid waste
Solid wastes are any discarded or abandoned materials that is discarded by the human society as it is of no further use. According to the source of generation and type they are classified as urban wastes, agricultural wastes, biomedical wastes, bulky waste, open area waste, radioactive wastes, e wastes and many more. The term refuse is also used for denoting solid wastes. The solid waste must be properly managed to avoid environmental and health hazards. The waste management involves various steps namely waste collection, segregation, transport, processing, treatment and disposal. Disposal is the final step in the waste management. Ultimately all solid waste ends in disposal site either directly or after some processing and treatment it ends in disposal sites. Most of the developing and under developed countries, dispose or dump the solid waste in an open land without any protection or environmental control. Such dumping is not safe and is called as open dumps or open dumping. Solid waste is also disposed off in water bodies eg. Sea, ocean, lake, river etc. Sometimes they are also burned in open areas, the phenomenon termed as open burning. Developed countries are practicing a system of safe or controlled dumping called the landfill. Landfill is an engineered structure where the waste is safely disposed and contained.
Controlled dumping or landfills
Landfills are the largest route of disposal. Landfill is an engineered structure for disposal or containment of solid waste. It is considered to be the safe method of waste disposal. Landfills are designed and operated according to acceptable standards. It is designed in a way to protect the environment and health of people. Landfills are not homogeneous. They are usually made up of cells in which a discrete volume of waste is kept isolated from adjacent waste cells by a suitable barrier. This landfill option of waste disposal is suitable only when the land is available at an affordable price, availability of adequate workforce, availability of technical resources to operate and manage the site.
Types of landfill
Landfills are of three types. They are
• Waste landfills for hazardous waste or secured landfill – This type of landfill is used for containment of hazardous waste that possess great potential in causing health and environmental effects.
• Waste landfills for non-hazardous waste or sanitary landfill – This type of landfill is generally used for municipal solid waste that does not cause dangerous effects on environment and living beings.
• Waste landfills for inert waste – Used mainly for wastes that does not react and remains inert. Eg. Construction and demolition waste.
Structure of landfill
As mentioned earlier, landfills are engineered structures to contain the waste (Figure 1). This helps in protecting the environment from various effects. The main purpose of designing a landfill is to contain the waste and protect the environment through a geological barrier and liner system at the base, sides and on top named the cap of the system landfill. The requirement and type of the barrier system depends on the nature of waste being loaded in the landfill. The barrier system is selected based on few criteria. They are
• prevent the pollution of soil, groundwater or surface water
• ensure efficient collection of the leachate
The barrier used are both natural and synthetic. Natural or geological barrier used for a particular landfill is selected based on the geological and hydrogeological conditions below the landfill and availability of material in the vicinity of the landfill site.
The landfills are composed of three main components. They are
• A liner system – used at the bottom to protect the soil and ground water.
• Waste emplacement cells – pattern by which waste is loaded into the landfill
• Cover or capping – It is the top most protection layer that prevents the entry of water or other animals into the waste pile
Figure 1 Landfill structure
Liner system:
As discussed earlier, landfill liner is a barrier that is laid at the bottom and sides of the landfill to protect the contaminants from entering into the soil and ground water. In addition to protection they are also designed to collect the leachate and control the accumulation and migration of landfill gas.
Thus liner system is a combination of many layers. The liner system is made of both natural and artificial or synthetic materials. A liner system consists of the following layers
– natural liner,
– a synthetic liner,
– a leachate collection system and
– geofabric for preventing the entry of solid material into the leachate collection tube
The permeability is one important factor based on which the liner materials are chosen. The materials used should provide sufficient permeability and thickness. Permeability is the ease with which the leachate flows through the liner material. The permeability or hydraulic conductivity of the material is calculated using Darcy’s law that describes the flow of a liquid through a porous material. Table 1 summarizes the hydraulic conductivity of few liner materials.
Table 1 Hydraulic conductivity of few materials used as liners
– the type of waste
– geological and hydrogeological conditions in the surrounding environment
– the properties of the derived leachate and
– the resistance of the liner to the leachate.
The properties that need to be considered for a good liner system include density, tensile strength, puncture resistance, tear resistance, resistance to ultraviolet (UV) light, ozone and chemical resistance. Resistance of membrane to chemicals is very important since the leachate may contain a range of organic and inorganic acids and alkali and organic hydrocarbons.
Natural clay
Clays is an unconsolidated rock material that is composed of clay minerals such as montmorillanite, illite and kaolinite ormed during the weathering of rocks. The clay minerals are extremely fine which determines the porosity and permeability of the material. The permeability and porosity should be very low for good liner material. The clay liner possess low permeability. The clay is collected from the nearby areas, homogenized, sieved to remove large rocks and spread on the landfill by bulldozers or scrapers. The large roller vehicles compacts the materials and reduces the void space between the pieces. It also increase the density of the clay.
Bentonite-enhanced soils
Bentonite is a mixture of clay minerals, principally of the montmorillante type. The bentonite-type clay minerals have a high swelling characteristic on absorbing moisture. When the naturally occurring clay soil does not have a high enough level of clay minerals to produce a suitably low permeability, bentonite clay is added to form a bentonite-enhanced soil. The permeability of the bentonite-enhanced soil gets reduced further by swelling under pressure due to mass of waste overlying in the landfill.
Bentonite exists in two forms namely sodium bentonites and calcium bentonites. Sodium form possess lower permeabilities. Around 5-15% sodium bentonite is added to the original soil.
Geosynthetic clay liner
Geosynethetic clay liners is a mixture of bentonite clay mechanically or chemically adhered to a geotextile fabric. The clay is sandwiched between two layers of the geotextile fabric where the layers are joined by adhesives, needlepunching or stitching. The thickness of these liners were around 1 cm thick. Geosynthetic clay liner are often used as alternatives to natural compacted clay liners.
Flexible membranes
Synthetic, polymeric plastic materials with extremely low permeabilities are used as Flexible membranes. High-density polyethylene and PVC are commonly used as flexible membranes. The thickness of these membrane range from 0.75mm to 3 mm
Geotextiles
Geotextiles are fabric materials used as protection for polymeric plastic membranes and filtration material to filter out fine-grained particles from the leachate so that drainage layers do not get blocked. They possess high permeability and are composed of polypropylene or polyester fibres which are manufactured to form a fabric-type material.
Geonets
They are porous sheets of plastic netting used as drainage layers to carry leachate or landfill gas. They prevent the net from clogging due to particles in the leachate hence always associated with geotextiles. They act as an alternative to gravel in liner system. Geonets are made of polyethylene with a thickness of 5 mm.
Liner system
Four types are liner system is used in construction of landfills. They are
1. Single liner system
2. Composite liner system
3. Double liner system
4. Multiple liner system
The liner system is selected based on the following criteria.
– Nature and composition of waste
– Cost
– Geology and hydrogeology of the surrounding area
– Should not allow the ground water to seep into the landfill
– Prevent the escape of leachate and gas from the landfill
– Should be resistant to chemical reactions
– The liner system should have proper stability and good lifetime
Single liner system
Single liner system is used for low risk waste where the generation of leachate is very negligible. The system comprises of a primary barrier made of clay or/and bentonite enhanced soil. Above and below the primary barrier is a layer of geotextile. It filters out suspended solids and provides protection. Between the waste and the protection layer would be the leachate collection system which consists of perforated drainage pipes that facilitates collection of leachate. The leachate is then pumped out and treated in municipal wastewater treatment plants. Beneath the liner system there is groundwater collection system which consists of gravel covered by a geonet. This layer is an alternative and not compulsory. Figure 2 depicts the single liner system.
Composite liner system
In a composite liner system (Figure 3), two different types of liner material is used namely: clay based mineral layer and polymeric membrane layer. The primary barrier here is synthetic polymeric flexible membrane with a separation layer of thin geotextile or polymeric material. The polymeric material can be HDPE, LDPE, PVC and many more. In addition to primary barrier, a secondary barrier made of clay, bentonite enhanced soil or geosynthetic clay is used. Similar to single liner, between the waste and the protection layer leachate collection system which consists of drainage pipes is layered. The composite liner system is used for more secure containment of waste.
Double liner system
Figure 4 depicts the double liner system. Similar to composite liner system the primary barrier is a synthetic polymeric flexible membrane with a separation layer of thin geotextile or polymeric material (HDPE, LDPE, PVC etc). Secondary barrier is composed of clay, bentonite enhanced soil or geosynthetic clay. A leachate collection tubes are layered between the waste and the protection layer.
Figure 2 Single liner system
Figure 3 Composite liner system
An intermediate high permeability drainage layer is incorporated between primary and secondary barrier. This layer is for the collection of leachate and gas. Every barrier is separated by geotextile fabric.
Figure 4 Double liner system
Multiple liner system
Multiple liner system combines few layers of double and composite liner system. Three barriers are made in this type of system. They are primary, secondary and tertiary barriers. Between these barriers intermediate layer is also attached for leachate and gas collection and maintenance. The primary barrier (synthetic polymer material) is in intimate contact with the secondary barrier (clay, bentonite enhanced soil etc.). This is done to prevent lateral movement of leachate and gas. All other layers are similar to other liner systems.
Waste emplacement
Waste is loaded in the landfill is a systematic way. Four methods are used for loading the waste in the landfill. They are
- Trench method: In this method, a trench is excavated into which the waste is deposited. The material that is excavated will used as liner material or later for the purpose of cover.
- Area method: In area method, the wastes is deposited in layers. After deposition, the waste pile looks like terraces over the deposited area. The drawback of this method is generation of excessive leachate which is very difficult to be controlled.
- Cell method: This method involves the deposition of wastes in cells that either imaginary or preconstructed. The waste is dumped in a sequential and tidy manner. This method is the most preferred method as it encourages the concept of progressive filling and restoration.
- Canyon/depression: This method refers to the placing of suitable wastes against lined canyon or ravine slide slopes. In this method the slope stability and leachate gas emission control are critical issues.
Everyday after the waste is loaded onto the landfill, the waste is covered with one layer of soil. This would prevent the wind-blown litter and act as inoculum for degradation. Green waste or other refuse materials can also be used as an alternative to soil.
Landfill cap
When the landfill capacity is full with waste then it has to be closed with a cover layer called the landfill cap. Landfill cap is the final cover or capping material of the landfill. The purpose of the landfill cover is to protect the waste, prevent the entry of rain and surface water into the waste, control the release of landfill gas, to prevent ingress of air which would disturb the anaerobic degradation process and enable the growth of short rooted plant when covered with top soil. Once again, the design of cover depends on the waste material. The cover layer is as follows:
- Overlaying the waste is a gas collection layer. It consists of a porous layer i.e geonet, geotextile or coarse sand that facilitates gas to permeate and move into the gas collection system
- Above this is a barrier layer made of plastic geomembrane, geosynthetic clay liner of bentonite and geotextile or natural compacted clay. It is used to protect ingress of air and water into the waste layer.
- Above this layer is the drainage layer which is again used to collect water from precipitation
- Next to drainage layer is the protection layer to prevent the burrowing animals, entry of roots and man-made intrusions
- The last layer of cover is the top soil used for the growth of short rooted plants. This is mainly done for the restoration and beautification of closed landfill.
Figure 5 Structure of landfill cap
Waste degradation in landfills
The waste inside the landfill undergoes physical, chemical and biological changes. The Waste degradation inside the landfill is time consuming process and hence takes long time to degrade or undergo changes. Due to these changes the waste gets compacted and its volume gets reduced. The various physiochemical process that takes place inside the landfill is chemical precipitation, adsorption, and many more. The biological process of waste degradation inside the landfill occurs in phases which is elaborated below. Three broad biological groups namely carbohydrates, proteins and lipids are present in organic portion of the waste. Degradation of waste occurs in five stages. Figure 6 and 7 depicts the waste degradation by biological route within the landfill.
Stage I – Hydrolysis/ aerobic degradation
This is the first stage of waste degradation and it occurs in the presence of oxygen conditions. The oxygen in the void spaces and those locked in the waste are used in this stage for degradation. It occurs immediately after waste is placed in the landfill and lasts for few days or weeks depending on the availability of oxygen for the process. The aerobic micro-organisms metabolize the available oxygen and degrade the organic fraction of the waste into hydrocarbons, carbon dioxide, water and heat. The heat increases the temperature of the waste to up to 70-90 °C. The water and carbon dioxide react to form carbonic acid thereby increasing the acidity of the leachate.
Stage II – Hydrolysis and Fermentation
This stage begins as soon as the oxygen in the waste pile gets depleted. During this stage the facultative anaerobes establishes themselves in the waste pile due to reduced oxygen conditions. Carbohydrates, proteins and lipids are hydrolysed to sugars which are then further decomposed to carbon-dioxide, hydrogen, ammonia and organic acids. Likewise, proteins are decomposed via deaminisation to form ammonia, carboxylic acids and carbon dioxide. Organic acids namely acetic acid, but also propionic, butyric, lactic and formic acids are formed. The leachate produced at this phase contains ammonical nitrogen in high concentration. The temperature at this phase will be around 30 and 50 °C. The gas concentrations in the waste in this stage was 80% carbon dioxide and 20% hydrogen.
Stage III – Acetogenesis
The organic acids produced in stage II are converted by acetogen micro-organisms to acetic acid, carbon dioxide and hydrogen. After few days the hydrogen and carbon dioxide levels continue to decrease in the landfill. It is at this point the methane-generating micro-organisms, the methanogens grow and results in the production of methane and carbon dioxide. The pH at this phase is acidic (pH
4) which leads to increase in the solubility of metal ions which can be analyzed in the leachate. Ions such as chloride, ammonium and phosphate are also found in high concentration in the leachate.
Figure 6 Stages of waste degradation within the landfill
Stage IV – Methanogenesis
Stage IV is the main stage for landfill gas generation (i.e) 60% methane and 40% carbon dioxide is produced. This phase is very slow and takes many years for completion. The waste is devoid of oxygen and anaerobic condition exits in this phase. There are two classes of micro-organisms which are active in the methanogenic stage, the mesophilic bacteria which are active in the temperature range 30-35 °C and thermophilic bacteria active in the range 45-65 °C. When temperatures drops below 15 °C in cold weather the rate of biological degradation drops. In addition to temperature, pH also plays an important
Figure 7 Waste degradation inside the landfill
role in this phase. The ideal conditions for the methanogenic micro-organisms are a pH range from 6.8 to 7.5. Moisture also determines the level of waste degradation in this phase.
Stage V – Oxidation
The last and final stage of waste degradation is oxidation. During this phase new aerobic micro-organisms slowly replace the anaerobic forms and re-establish aerobic conditions. The residual methane is converted to carbon dioxide and water.
All the different stages may be progressing simultaneously until all the waste reaches stage five and stabilization occurs.
Figure 8 depicts the gas generation during the five stages of waste degradation in the landfill.
Figure 8 Gas generation during the five stages of waste degradation in the landfill
Factors influencing waste generation
The factors that influence waste degradation in landfill include
• Site characteristics: depths exceeding 5 m develops anaerobic conditions that results in large quantity of gas generation. Shallower sites allow air interchange and lower anaerobic activity,
• Waste characteristics: The ratio of biodegradable matter determines the degree of biodegradation of the waste. Shredded waste enhances the rates of biological degradation via enhanced surface area. However, compaction inhibits the waste degradation as it will limit the percolation of water and nutrients to the microorganisms.
• Moisture of the waste: Moisture of around 15 to 40% with an average of 30% is essential for good degradation. Moisture determines the rate of landfill gas and leachate production. The moisture content within the site will depend on moisture content of the waste, precipitation in the location and percolation of surface and groundwaters into the site
• Temperature: It indicates the type of micro-organisms population within the waste pile and landfill. Compacted waste achieves lower temperatures due to the limited availability of oxygen and substrate in the waste.
• Acidity: The acidity of the landfill determines the rate of biodegradation. The pH of a typical landfill site would initially be neutral, followed by acidic phases (pH 4) during acidogenesis or acetogenesis where organic acids are produced. The organic acids provide the nutrients for the methanogenic bacteria and as the acids are consumed, the pH rises. The methanogenic bacteria are most active in the pH range 6.8-7.5. pH outside this optimum range decreases gas production
Post closure monitoring
After the closure of the landfill, they should be monitored continuously through gas and leachate monitoring stations and wells respectively. The leachate monitoring wells are located at 5 km radius in concentric circles. The ground water is collected and periodically analyzed to assess the damage in the liner system.
Summary
To summarize, in this module we have studied about
- Landfill structure and its components
- Liner material and lining systems
- Types of waste emplacement
- Landfill cover
- Process of waste degradation inside the landfill
- Factors influencing waste degradation in the landfill
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References
- Williams, Paul T. (2013) Waste treatment and disposal, John Wiley Publishers.