1. Leachate Management: Incineration & Gasification
Dr. Prabha Joshi, Asst. Professor
Department: B.E. Civil Engineering
Subject: Environmental Engineering - I
Semester: VI
Teaching Aids Service by KRRC Information Section

2. Leachate
Leachate consists of water and water soluble compounds in the refuse that accumulate as water moves through the landfill. This water may be from rainfall or from the waste itself. Leachate may migrate from the landfill and contaminate soil and ground water, thus presenting a risk to human and environmental health. The greatest threat to ground water posed by modern landfills is leachate.
This happens when the dumped refuse contains non-biodegradable and carcinogenic substances such as plastics, unused medicines, paints, insecticides, sanitary napkins which may start troubling on coming in contact with rain water seeping through the dumped garbage producing a coloured liquid called leachate.

3. Leachate
This highly poisonous and polluted leachate containing organic compounds like chlorinated hydrocarbons, such as benzine, toluene, xylene etc and toxic elements such as arsenic, copper, uranium etc. is likely to seep to the ground water table to pollute ground water, leading to diseases like cholera, typhoid, polio etc.
Landfill liners are designed and constructed to create a barrier between the waste and the environment and to drain the leachate to collection and treatment facilities. This is done to prevent the uncontrolled release of leachate into the environment.

4. Society produces many different solid wastes that pose different threats to the environment and to community health. Different disposal sites are available for these different types of waste. The potential threat posed by the waste determines the type of liner system required for each type of landfill. Liners may be described as single (also referred to as simple), composite, or double liners.

5. quantity of leachate produced depend on factors:
Soil type
Waste composition
Degree of compaction
Rainfall
Evaporation
Landfill type and age

6. Leachate collection conduits
Photo: project_jak.htm

7. Ref: O’Leary, Philip & Patrick Walsh. 2002
Ref: O’Leary, Philip & Patrick Walsh Land Disposal of MSW: Protecting Health & Environment. Waste Age- February 2002, pp

9. Leachate collection systems
Clay: to protect the ground water from landfill contaminants, clay liners are constructed as a simple liners that is two-to five feet thick. In composite and double liners the compacted clay layers are 2-5 feet thick depending on the characteristics of the underlying geology and type of liner to be installed. Regulations specify that the clay used can only allow water to penetrate at a rate of less than 12 inch per year. The effectiveness of clay liners can be reduced by fractures induced due to freeze-thaw cycle , drying out and the presence of some chemicals. In theory, one foot of clay is enough to contain the leachate. The reason for the additional clay is to safeguard the environment in the case of some loss of effectiveness in part of clay layer. The effectiveness of clay liners can be maximized by laying the clay down in 4-6 inches and then compacting each layer with heavy roller. In addition clays compacted at high moisture content act as effective barriers than the clays compacted at low moisture content.

10. Geomembranes : Geomembranes are also called flexible membrane liners
Geomembranes : Geomembranes are also called flexible membrane liners. These liners are constructed from various plastic materials including polyvinyl Chloride (PVC) and high density polyethylene (HDPE).
The preferred material for MSW and secure landfills is HDPE. This material is strongly resistant to most chemicals and considered to be impermeable to water. Therefore HDPE minimizes the transfer of leachate from the landfill to the environment.
Geotextiles: In landfill liners geotextiles are used to prevent the movement of small soil and refuse particles into the leachate collections layers and to protect geomembranes from punctures These materials allow movement of water but trap particles to reduce clogging in the leachate collection system.

11. Geosynthetic Clay Liner: Geosynthetic clay liners becoming more common in landfill liner designs. These liners consists of a thin clay layer (4-6mm) between two layers of geotextile. These liners can be installed more quickly than traditional compacted clay liners and efficiency of these liners is impacted less by freeze-thaw cycles.
Geonet: A geonet is a plastic net like drainage blanket which may be used in landfill liners in place of sand or gravel for the leachate collection layer, sand and gravel are usually used due to cost considerations and because geonets are more susceptible to clogging by small particles. This clogging would impair the performance of the leachate collection system. Geonets do however convey liquid more rapidly than sand and gravel.

12. Control of Lechate Movement
Single Liner System: Single liners consists of a clay liner a geosynthetic clay liner or a geomembrane (specialized plastic sheeting). Single liners are sometimes used in landfills designed to hold construction and demolition debris.
These landfills are not constructed to contain paint, liquid tar, municipal garbage or treated lumber; consequently, single liner system are usually adequate to protect the environment. It is cheaper to dispose of construction material in a C&DD landfill than in a MSW landfill because C&DD landfills use only a single liner and are therefore cheaper to build and maintain than other landfills.

14. Composite Liner Systems
A composite liner consists of a geomembrane in combination with a clay liner. Composite liner systems are more effective at limiting leachate migration into the subsoil than either a clay liner or a single geomembrane layer. Composite liners are required in a Municipal solid waste landfill.
MSW landfills contain waste collected from residential commercial and industrial sources. These landfills may also accept C&DD debris but not hazardous waste.
The minimum requirement for MSW landfills is a composite liner . Frequently landfill designers and operators will install a double layer system used in some MSW landfills and in all hazardous waste landfills.

16. Double liner systems
A double liner consists of either two single liners, two composite liners or a single and a composite liner. The upper (primary liner) usually functions to collect the leachate while the lower (secondary liner) acts as a leak-detection system and backup to the primary liner.
Double layer system used in some MSW landfills and in all hazardous waste landfills.
Hazardous waste landfills (also referred to as secured landfills) are constructed for the disposal of waste that once were ignitable, corrosive, reactive, toxic or are designated as hazardous.
These wastes can have adverse effect on human health and the environment if improperly managed.
Hazardous wastes are produced by industrial commercial, and agricultural activities.

17. Hazardous waste must be disposed of in hazardous waste landfills and these landfills must have a double liner system with a leachate collection system above the primary composite liner and a leak detection above the secondary composite liner

18. Combustion Types
Incineration (energy recovery through complete oxidation)
Mass Burn
Refuse Derived Fuel
Pyrolysis
Gasification
Plasma arc (advanced thermal conversion)

19. Incineration
It is a chemical reaction in which carbon, hydrogen and other elements in the waste mix with oxygen in the combustion zone and generates heat.
Aim
Volume reduction
Stabilization of waste
Recovery of energy from waste
Sterilization of waste

20. Incineration Air is supplied to the waste
Ash, tar and a gas product are formed
Gas is utilized by direct combustion to produce heat or steam for electricity
Gas can be unclean and hence furnace must be designed to tolerate this
Gas can have varying compositions, hence furnace must be able to handle this.

23. Thermal Pyrolysis
Thermal degradation of carbonaceous matter in the absence of O2, requires lower temperature than gasification.
Can be used for MSW, tyres, plastics and biomass
Char, oil and combustible gases are the products – Used for energy production
End product is ash which must be land filled
Many types of Pyrolysis processes having different heating rates and maximum temperatures exist
Choice depends on the type of mix of solid, liquid and gas required.

24. Gasification
Partial oxidation process using air, pure oxygen, oxygen enriched air, or steam
Carbon converted into syngas
More flexible than incineration
More public acceptance

25. Waste Incineration - Advantages
Volume and weight reduced (approx. 90% vol. and 75% wt reduction)
Waste reduction is immediate, no long term residency required
Destruction in seconds where LF requires 100s of years
Incineration can be done at generation site
Air discharges can be controlled
Ash residue is usually non-putrescible, sterile, inert
Small disposal area required
Cost can be offset by heat recovery/ sale of energy

26. Environmental Considerations
One Ton of waste creates 3.5 MW of energy (eq. to 300 kg of fuel oil) powers 70 homes
Biogenic portion of waste is considered CO2 neutral (tree uses more CO2 during its lifecycle than released during combustion)
Should not displace recycling

27. Waste Incineration - Disadvantages
High capital cost
Skilled operators are required (particularly for boiler operations)
Some materials are noncombustible
Some material require supplemental fuel
Public disapproval
Risk imposed rather than voluntary
Incineration will decrease property value (perceived not necessarily true)
Distrust of government/industry ability to regulate