Intro
The sanitary landfill consists of solid waste and an inert cover material (generally soil).
The leachate (usually black liquid) is generated from the decomposition of the solid waste and through the infiltration of the rain. The landfill gases are also produced which are released through a series of vertical pipes.
Calculation of leachate
The leachate needs engineered treatment for it to be safe to be released into the environment. For this, the estimation of the leachate quantity is crucial. The steps applied in estimating leachate include doing the mass balance of liquid in a landfill cell.
The steps include:
Step 1:
Know the basic things like
- Waste quantities (e.g. waste deposited per day)
- Waste Characteristics (e.g. Moisture content of the solid waste, density)
- Landfill characteristics (Lift height, waste-to-cover ratio, number of lifts)
- Cover material and gas production
- Field capacity (i.e the maximum amount of water that a landfill can hold with zero leachate)
- Rainfall quantities (infiltration)
Step 2:
Do the mass balance for each year from year 1 to year 5 (as there are five lifts chosen in this) (*Shown in Excel sheet). These include:
(a) Determine the amount and weight of the gas produced (This gas is produced from the anaerobic decomposition of the organic waste)
(b) Determine the weight of the water consumed in the production of landfill gas (Water is consumed in the anaerobic decomposition process).
(c) Determine the water vapor weight as some of the water is in the form of water vapor.
(d) Determine the weight of the water in the solid waste ( i.e moisture content in solid waste plus water infiltrated from rainfall)
(e) Determine the dry weight of the solid waste
(f) Determine the average weight of the solid waste (This is needed for the calculation of field capacity, which is determined with an empirical formula).
(g) Calculate the field capacity
(h) Determine the maximum amount of water that can be held in the solid waste
(i) Finally, determine the amount of the leachate formed (i.e water in the solid waste- field capacity of the solid waste)
(j) Determine the water remaining in the landfill cell
(k) Determine the total weight of the lift at the end of the year (it is assumed that one landfill cell fills up in one year)
Step 3:
The calculation is the same up to 5 lifts (only the average weight increases and leachate needs to be balanced).
From 6th year and onwards, the calculation is a little different as no further leachate is added. The decomposition of the waste takes new leachate and infiltration of rainfall also occurs creating more leachate. (*Shown in excel)
Step 4:
The total leachate is estimated by determining the total area occupied by the landfill. This is done as the load of solid waste per unit area is known and the total quantity of the waste is known.
The weight of the leachate in kg or pounds(lb) is then listed for each year.
Step 5:
Ideally, the leachate plant is designed for the maximum leachate production. Leachate production started to increase after a few years. Then, it decreases and leachate production continues for many years even after the closure of the landfill.
The calculation is an ideal estimation of the conditions. However, the landfill cover is not so perfect practically. Again, many factors come into play in practical application.
The engineered design is useful for estimating the leachate from landfills.
Here’s the Excel sheet of the complete estimation of landfill leachate.
*Note: The design is based on the Integrated Solid Waste Management Book by George Tchobanoglous.