Question 18 of 18: Developing Information to Treat a Landfill Leachate

Question 18: Developing Information to Treat a Landfill Leachate (7 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.

Because the site is abandoned and (being a secure hazardous landfill) its exact original waste inventory may be incompletely documented, the investigation has to reconstruct what is actually in the leachate and how the site behaves hydraulically before any treatment can be selected, in a staged approach. (1) Historical records review. Compile whatever waste-acceptance records, site design/closure reports and prior regulatory correspondence exist, to identify the categories of waste disposed and any known contaminants of concern — this narrows the analytical scope of everything that follows. (2) Site hydrogeological investigation. Install monitoring wells (both within and around the site, at multiple depths) to characterize the local groundwater flow direction, gradient and the aquifer(s) potentially affected, and to distinguish leachate that is being actively captured/collected from any that may already be migrating off-site. (3) Leachate characterization (sampling and analysis). Sample the leachate itself (and any affected groundwater) over a representative period — since leachate strength and composition change markedly with a landfill's age — for the full parameter set relevant to a hazardous-waste leachate: BOD/COD, TOC, TDS, pH, heavy metals, specific organic priority pollutants (VOCs, SVOCs) consistent with the waste history from step 1, and any parameters exceeding groundwater or discharge standards. (4) Flow-rate and seasonal-variability assessment. Measure or estimate the leachate generation/collection rate and its seasonal pattern (typically correlated with precipitation infiltration through the cap), since a treatment system must be sized on both peak and low-flow conditions, not just the average. (5) Risk assessment against applicable standards. Compare the characterized leachate/groundwater data against the applicable regulatory limits (drinking-water and/or groundwater standards for any receptor, discharge-permit or sewer-use limits if the leachate is to be conveyed off-site for treatment) to define exactly which parameters must be reduced and by how much — this is what actually drives treatment-process selection rather than treating "clean it up" as a generic goal. (6) Bench/pilot treatability testing on representative leachate samples for the candidate treatment technologies identified from steps 3–5 (e.g. biological treatment for the organic fraction, chemical precipitation for metals, activated carbon or advanced oxidation for recalcitrant organics), to confirm the selected process train actually achieves the required removal before committing to full-scale design. Only once these six information sets are in hand — history, hydrogeology, characterization, flow, risk/standards, and treatability — can a technically defensible and appropriately-sized leachate treatment solution be selected.