What is leachate and how is it treated in landfills?

Leachate is liquid that percolates through landfill waste, dissolving contaminants including organics, heavy metals, and ammonia. Treatment options depend on characteristics: biological treatment (activated sludge, SBR) for biodegradable fraction; physical-chemical treatment (coagulation, precipitation) for metals; membrane processes (RO, NF) for dissolved solids; and activated carbon for refractory organics. Young leachate has high BOD/COD ratio (>0.5), old leachate is less biodegradable (<0.1). Often discharged to municipal treatment plants after pretreatment.