Factors influencing leachate quantity
a) Amount of precipitation received.
b) Ground water interaction when the landfill base is below groundwater table.
c) Moisture content of waste increases biodegradation and increases leachate production. Such a scenario is mostly applicable in the case of municipal solid waste and due to sludge that are disposed.
d) Final cover reduces leachate quantity due to low percolation through compacted covers. Also vegetation in the top soil of final cover reduces infiltration by increased evapotranspiration.
Estimation of leachate quantity
The quantity of leachate is directly dependent on precipitation received. Pre-closure and post-closure leachate generation from a landfill vary significantly. Pre-closure leachate generation rate is required for designing leachate collection pipes in the landfill, fixing the size of leachate collection tank and treatment plant. Post-closure leachate generation rate is required to plan the management of leachate and cost incurred for it. Leachate quantity considerably reduces after closure and construction of covers.
Leachate volume (Lv) is given by Eq. 3.1.
L v = P + S - E - AW |
3.1 |
Where P is the precipitation volume, S is the volume of pore liquid squeezed from the waste, E is the volume lost by evaporation and AW is the volume of liquid lost through absorption in waste.
Pore squeeze leachate volume (S)
When sludge in disposed, liquid within the pores gets squeezed due to self- weight of sludge and weight of waste dump and cover soil. Such an action is similar to the consolidation process occurring in a saturated soil. Primary consolidation of waste accounts for the majority of pore squeeze leachate. The primary consolidation properties of sludge are used to predict leachate generation rate.
Loss due to evaporation depends on ambient temperature, wind velocity difference in vapour pressure etc. Leachate absorbed in waste (AW) is depended on field capacity (FC) of waste. FC is the maximum moisture content that waste can retain against gravitational force without producing down ward flow. When the moisture content is within FC, the waste has the capacity to retain water without causing downward flow.
Post closure leachate generation rate
Only water that can infiltrate through the final cover of the landfill percolates through the waste and generates post closure leachate. Water balance method expressed by Eq. 3.2 is a popular method for estimating post closure leachate generation.
L'V = P - ET - R - S |
3.2 |
Where L'V is the volume of post closure leachate, P is the volume of precipitation, ET is the volume lost though evapotranspiration, R is the volume of run off and S is the volume of moisture stored in soil and waste. Potential ET is obtained based on appropriate empirical equation.
R = Cr I A |
3.3 |
Where Cr is the run off coefficient, I is the rainfall intensity and A is the area of landfill surface.
Soil moisture storage (S): A portion of infiltrating water is stored by soil and only a part of this is used for vegetation. Soil moisture storage capacity is the difference between field capacity and wilting point. Wilting point is the moisture content at which plants cannot draw moisture and starts wilting. Normally, moisture content corresponding to 1500 kPa matric suction is taken as wilting point.
Water balance method if not done properly results in large errors especially when used for long term leachate generations rate. The disadvantages of water balance method are: (i) it does not account permeability of cover layer (ii) evapotranspiration is sometimes wrongly calculated due to over prediction of root length in vegetation layer. In reality root would not have penetrated entire thickness of vegetation layer. Some of the freely downloadable software such as hydrologic evaluation of landfill performance (HELP) model by US Environmental Protection Agency (USEPA) is a handy tool for performing water balance studies.
Gas generation rate
Gas generation rate is mostly valid for municipal solid waste (MSW) landfill where organic matter decomposition results in the production of gases. Gas production in MSW landfill occurs due to anaerobic degradation resulting from hydrolysis and fermentation (attributed to bacterial activities), acetogenesis and dehydrogenation, and methanogenesis. Hydrogen gas is produced due to the oxidation of soluble products to organic acids. Some of the other gases produced from MSW are methane, carbondioxide, hydrogen sulphide and nitrogen. Gas production reaches a stable rate and then decreases as biological activity in MSW landfill start decreasing. The assessment of time dependent percentage production of methane from a MSW landfill is important for recovering methane as an energy source, and there by reducing greenhouse gas effect.