Assessment of biogeochemical deposits in landfill leachate drainage systems phase II

Land disposal of solid waste is a vital component of any solid waste management system. Design, operation and closure of municipal solid waste (MSW) landfills are required by regulations to control leachate and gases generated during the life, closure,and post-closure of the facility. Clogging of leachate drainage and removal systems in landfills is a common phenomenon and has been acknowledged in several landfills throughout the United States and abroad. This project was conducted in two phases. Phase I was completed in February of 2005 and Phase II was completed in August of 2006. Leachate characteristics data obtained in Phase I was processed and analyzed, along with supplementary data obtained in Phase II on liquid and solid phase testing. Leachate samples from the landfill and lysimeters indicated the presence of iron and sulfate-reducing bacteria. These bacteria are known to facilitate biologically induced precipitate formation.The mechanism by which biologically ind uced precipitate may form begins with oxidizing acetate by iron and sulfate-reducing bacteria, reducing sulfate to sulfide and ferric iron to ferrous, and then forming calcium carbonate, iron sulfate, and possibly dolomite and other minerals.The results show that the clogging mechanism is driven by two major processes: transformation of volatile acids to substrates by iron and sulfate-reducing bacteria causing local pH and total carbonate to increase, which accelerate calcium carbonate precipitation, and thermodynamically favored reactions in supersaturated conditions based on saturation indices of calcium, sulfide, iron, and other species with respect to minerals. For each 1 mg of consumed volatile acids there were 1.7 mg of calcium, 0.28 mg of sulfate, and 0.03 mg of iron removed. Field and lysimeter precipitate samples were analyzed (using X-Ray Diffraction, Scanning Electron microscopy, and Electron Dispersive Spectroscopy) and correlated with geochemical modeling of leachate const ituents. Precipitate analyses showed the presence of calcium carbonate, brushite (calcium phosphate),and dolomite, where as geochemical modeling showed that calcium carbonate, hydroxyapatite (complex of calcium phosphate), dolomite, pyrite, and siderite may be formed from field and lysimeter leachate constituents. The results also showed that submerged and stagnant conditions in the leachate collction systems accelerate the precipitation process