1. Landfilling

2. Leachate
Leachate is defines as the liquid that has percolated through solid waste and has extracted dissolved or suspended materials. The liquid usually enters the landfill from external sources such as drainage, precipitation and ground water.

3. Composition of Leachate
Function of the time i.e. landfill phase: BOD/COD ratio .5 initially, in mature landfills. The BOD is consumed and the mature landfills contain humic and fuvic acids which are not readily biodegradable.
Mature landfill: BOD mg/l, pH , Total Iron mg/l

4. Water Balance Water entering as precipitation
Water entering in solid waste - typically MSW is 20% moisture
Water entering in cover material. Field Capacity(FC) is the maximum amount of moisture that can be contained in the cover material: 6-12% for sands, for clay loams. FC is the quantity of water than can be held against the pull of gravity, eq

5. Movement of Leachate
Darcy's Law eq predicts the rate of seepage of leachate from the bottom of the landfill.
K is the coefficient of permeability also and is 1333ft/day for uniform coarse sand and ft/day for colloidal clay.

6. Fate of Constituents of Leachate in Subsurface Migration
The constituents, of course, are the main reason for concern.
heavy metals - Removal is a function of the cation exchange capacity (CEC) of the soil and defined as meq of cations/100 grams of soil e.g. 100 meq/100g for organic colloids and 10meq/100g for clays. Should not be a problem.
trace organics - adsorption (the process of collecting soluble substances that are in solution on a suitable surface) is the most common removal mechanism.

7. Control of Leachate
As the leachate travels through the soil many of its harmful characteristics will be lessened by filtration and adsorption, however, best practice calls for elimination or containment.
The geotextile is a thin synthetic material which minimizes the intermixing of different soil layers. The geonet is a thin synthetic material which serves as a drainage layer, instead or sand. The geomembrane is a thin synthetic material which serves as an impervious barrier.

8. Control of Leachate
Liner Systems :geomembrane, clay, geosynthetic clay, combinations

9. Recycle
Evaporate
Treatment

10. . Settlement of Landfills
Nominal angle of repose for waste material is 1.5 to 1. 3:1 most commonly used for side slope.
If height is over 50', use benches which not only help slope stability but serve as roadways, places to put gas and water pipes.
Settlement occurs as the material decomposes and weight is lost as gas and leachate (as much as 40% of original mass).
As the overburden increases, compaction and settlement occurs. Maximum specific weight from 1750 to 2150 lb/yd3.. 90% of the ultimate occurs in the first 5 years.

11. (water is required for this reaction to go forward)
Landfill Gases
organic matter + H2O (bacteria) biodegraded organic matter + CH4 + CO2 + other gases
(water is required for this reaction to go forward)

12. Composition and Generation of Gases
Primarily CH4 (methane) and CO2 (carbon dioxide); methane (.7167 g/L) has a density less than air ( g/L) and will rise perhaps to building basements where it may collect and be explode; CO2 is heavier than air ( g/L) and will sink to the GWT and perhaps cause chemical problems.
Also N, O, Sulfides (odors) etc. and trace gases.
Trace gases such as acetone, toluene etc. are associated with the deposition of hazardous wastes, more in older landfills.

13. Generation takes place in five phases:
- Phase I - Initial Adjustment - aerobic phase, mainly N2 and O2 being generated.
- Phase II - Transition- going anaerobic, pH of leachate (if any) will drop.
- Phase III - Acid -
- hydrolysis - a chemical reaction, higher molecular compounds (proteins, lipids etc.) are transformed into simpler compounds that the MO's can metabolize.
- acidogenesis - microbial conversion of above compounds into lower mass compounds such as acetic acid.
- Phase IV - Methane Fermentation -A second group of microbes (strict anaerobes) converts the phase III material into CH4 (methane) and CO2 (carbon dioxide).
- Phase V - Maturation - die off
Volume ft3/lb of biodegradable organic solids destroyed (see eq 11-2). Function of lignin content. Lignin is the organic glue that binds trees and plants together.

14. Movement of Landfill Gas
Mechanisms include diffusion, migration of a gas from a higher to a lower concentration and convection, the internal pressure of the landfill is greater than atmospheric pressure.
Convection velocity 1-15 cm/d.
Most methane (lighter than air) is vented to the atmosphere, however, both CH4 (methane) and CO2 (carbon dioxide) have been found in concentrations of up to 40%

15. Movement of Landfill Gas
at lateral distances of up to 400' from the edges of unlined landfills. CO2 is heavier than air and sinks perhaps to the GWT where it is readily soluble in water and forms carbonic acid. The carbonic acid lowers the pH which increases the hardness and mineral content via solubilization.

16. Control of Landfill Gases
1.) Passive Control
Vents
Trenches (interceptor to collect)
Trenches/walls (barrier to redirect)
2.) Active
Extraction wells
Extraction trenches
Air dams

17. Management of Landfill Gas
Flaring - thermal destruction, 1500F and a residence time of .3-.5s.
Energy Recovery Systems - direct use of gas in turbines to produce electricity, heat water/steam for traditional steam electrical generating plant.