Question 13 of 22: Design Factors for a Landfill Leachate Collection System

Question 13: Design Factors for a Landfill Leachate Collection System (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(1) Design leachate generation rate, derived from a site water balance (precipitation, runoff, evapotranspiration and infiltration through the cover — see Q16), which sizes every downstream component. (2) Drainage-layer permeability and slope above the liner — must convey leachate to the collection pipes quickly enough to keep the head of leachate on the liner below the regulatory maximum (commonly 300 mm). (3) Collection pipe network design — pipe spacing, diameter and slope, sized (via Manning's equation) to convey the peak leachate flow without surcharging. (4) Clog resistance — granular drainage-media gradation and geotextile filter compatibility with the specific leachate/refuse characteristics, since biological growth and chemical precipitation (e.g., calcium carbonate scaling) can clog pipes and drainage media over a multi-decade operating and post-closure life. (5) Access for inspection and maintenance — cleanouts and riser pipes to allow jetting/inspection of the collection pipes, and use of chemically-durable materials (HDPE) compatible with leachate over the design life.