In the design of municipal landfill leachate collection systems, some state regulatory agencies require carbonate content of leachate collection system aggregate not to exceed 15 percent by weight. This requirement comes from a legitimate concern about the possibility of aggregate degradation, or loss of mass due to contact with leachate.
Most involved in landfill design and development will have experienced as a result, the fact that in some areas it is difficult to find carbonate free stone within an reasonably economic distance from the site. Many potential aggregate sources have been eliminated for supplying drainage material, due to this stipulation in the specification, but is it really warranted?
While leachate in MSW landfills is capable of dropping to pHs of 6.5, and sometimes 6, it rarely falls below this other than for short periods. This does not seem to be so low that problems would necessarily be serious, and if any of the carbonate dissolved from the stone, the amount would presumably be low as the reaction would be self limiting due to the dissolved carbonate caused by the reaction being bound to raise the pH. High pH will not erode the carbonate so the problem is corrected.
There is not a huge amount of research work on this that we have been able to find. We would be very interested to receive comments if our readers have sources to research on this matter which are more authoritative than the paper I am about to refer to.
The best paper we have found which sets out to by experimentation over a reasonably extended time period (in this case just under 6 months) to investigate whether carbonate drainage stone, when submerged in leachate, will suffer damage, is the following paper:
Suitability of Carbonate Aggregate in Land fill Leachate Collection Systems; Christopher G. Rubak, PE John,O. Starke, PE William D. Upman, PG M. Merrill Stevens, PhD: Presented to the Nineteenth International Madison Waste Conference, September 25-26 1996, Dept of Engineering Professional Development, University of Wisconsin – Madison.
This paper summarizes a research project which evaluated the suitability of a carbonate aggregate with a municipal solid waste leachate. The tests were conducted over a 20 week period using site specific landfill leachate and collection aggregate. Laboratory bench reactors were constructed to simulate landfill conditions with leachate flowing through carbonate aggregate.
The reactors consisted of 12-inch diameter plexiglass cylinders each charged with 80 pounds of carbonate aggregate. Leachate was then circulated through the reactors. An anaerobic environment was maintained in the reactors by applying 0.5 Atmosphere of CO2.
Fresh leachate was added to the reactors on a regular basis to maintain a constant concentration level during the test. Leachate samples were analyzed to determine the change in dissolved solids throughout the test period. Aggregate material was measured before and after the test to determine net mass change. Chemical equilibrium speciation modelling was also performed and compared to the bench test results.
On the face of it this experiment showed that there was no need for concern about carbonate deterioration even down to the exceptional pH 3.0 (exceptional for an MSW landfill under good regulatory control, built to good current standards).
However, the strange thing about the experiment to the writer is that the leachate used was changed on only, I think, 3 occasions; other than on these occasions the leachate was simply recirculated.
I would have preferred to see results which would ensure that the natural circumstances of a landfill were replicated more closely, and that would have meant allowing fresh leachate to pass through the system all the time.
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