Moisture Consumption during Gas Production in Landfills: Implications to Permitting ET Caps
Michigan State University
Project Background & Objectives
In order to permit an earthen or evapotranspirative (ET) cap, often regulators rely upon percolation from instrumented lysimeters. Typically the percolation needs to be relatively small for permitting an ET cap. While the purpose behind low percolation is to minimize leachate generation in the landfill during the post-closure period and prevent it from “filling up” the landfill (“bath tub” effect), removal of moisture from the waste from various sources including during the active phase due to the chemical reactions that occur to produce CO2 and CH4 (landfill gases) are ignored. Preliminary calculations suggest that up to 3% of the moisture initially present in refuse is lost to the landfill gas due to the stoichiometric reactions. This is in addition to water lost during decomposition as explained in the next paragraph. This 3% moisture removal may seem low. However, it is equivalent to about 200 cm of water storage in a landfill that has about 65 m (200 ft) thick waste. 200 cm of water is equivalent to 100 to 200 years of infiltration from an earth cap assuming the percolation from the cap is 20 or 10 mm/year (Khire and Mijares 2008).
Systematically quantifying the moisture consumption in landfills will have profound impact on how water balance of landfills is evaluated and how ET caps could be permitted. Hence, the key objectives of this project are to: (1) quantify the moisture consumption in the waste during landfill gas production for municipal solid waste landfills; and (2) evaluate the net change in water storage of the landfill (primarily the waste) with an ET cap.