Biogeochemical evaluation of mechanisms controlling CaCO_3(s) precipitation in landfill leachate-collection systems

A common failure mode for landfills is clogging of the leachate-collection system. The reduction in hydraulic conductivity associated with clogging causes a buildup of leachate head on the underlying liner, potentially increasing advective contaminant transport from the landfill and contaminating adjacent groundwater. In this paper, the biogeochemical model CCBATCH is used to link a primary cause of leachate collection system failure-CaCO_3(s) precipitation-to anaerobic degradation of volatile fatty acids (VFAs) in column reactors used to study the clogging phenomena. One key to applying CCBATCH correctly was dividing the VFA conversion into two steps: conversion of propionate to acetate, carbonic acid, and methane; and acetate conversion to methane and carbonic acid. The primary driver for CaCO_3(s) precipitation in the columns was acetate fermentation to CH₄ and H₂CO₃, which increased the total carbonate concentration in the leachate and shifted the acid/base control to a weaker acid system, which caused an increase in solution pH. A second key to proper modeling was adding CO_2(g) gas transfer to CCBATCH. The modeling results indicate that the kinetics of CO_2(g) gas transfer was a key control over leachate chemistry once acetate fermentation was nearly complete. These results suggest that the best approach for the long-term control of CaCO_3(s) clogging may be to enhance CO_2(g) gas transfer from the leachate while buffering the leachate pH to near neutral. Taken together, these actions should decrease the yield of CaCO_3(s) precipitated per mass of acetate removed.