Mechanisms of clogging in granular drainage systems permeated with low organic strength leachate

Leachate drains in municipal solid waste (MSW) landfills are susceptible to biological and (or) chemical clogging. This paper describes clogging of drainage aggregates permeated with leachates representative of those from landfills containing wastes with a low organic content — such as low-level radioactive waste repositories — which may occur as a result of microbiological activity causing formation of bacterial biofilms (microbiological clogging) and precipitation of low-solubility inorganic salts (chemical clogging). The balance between these depends on the leachate composition. Biological deposits appeared to reach a pseudo steady state, proportional to the nutrient loading, for the range of conditions investigated and as such can be considered to be self-limiting and the clog material reasonably permeable. Harder inorganic deposits of calcium carbonate occurred if the Ca2+ concentration in the leachate exceeded the local solubility limit under the prevailing conditions, i.e., partial pressures of CO2 between 3.7 and 6 kPa and pH of 6.7–6.8. CaCO3 clog was observed to bind the granular aggregates together and be effectively impermeable, and was, unlike a pure microbial clog, observed not to be self-limiting. Hard CaCO3 clog could be reduced by not co-disposing wastes that are high in calcium with wastes having a high organic content, and generally keeping the Ca2+ concentration in the leachate low