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18-Env-A6 Solid Waste Engineering and Management · May 2013

Question 14 of 17: Leachate Quantity and Time to Saturation

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Examination, May 2013 — 04-Env-A6 / 18-Env-A6, Solid Waste Engineering and Management. 3 hours duration, closed book, one letter-sized aid sheet permitted. All 17 questions constitute a complete paper (100 marks total).

Reference texts: Tchobanoglous, Theisen & Vigil, Integrated Solid Waste Management: Engineering Principles and Management Issues; Vesilind, Worrell & Reinhart, Solid Waste Engineering; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); CCME, Guidance Document on Landfill Gas Management.

Check: several questions on this paper (Q1, Q13, Q14, Q17) supply an incomplete data set and explicitly invite the candidate to "make and state" assumptions. Every assumed value below is called out where it is introduced and is chosen from standard solid-waste-engineering practice; the governing METHOD, not the specific assumed number, is what the exam is testing.

Question 14: Leachate Quantity and Time to Saturation (6 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.

Given. Compacted refuse density 600 kg/m³, depth 9 m, initial moisture content 20% by volume, 1 m clay cover at 2% slope.

Find. 14.1 Annual leachate quantity (once generated); 14.2 Time before a given year's lift saturates and starts producing leachate.

Clay cover + refuse (water balance)(CV boundary — dashed)Runoff150 mm/yrEvapotranspiration500 mm/yrPrecipitation P1000 mm/yrPercolation → leachate350 mm/yr
Field-capacity water-balance method: infiltration less runoff and evapotranspiration percolates into the refuse; leachate begins once the refuse's field capacity is reached.

Approach. Apply the field-capacity water-balance method: (a) compute net percolation into the refuse from a climate water balance on the cover, which becomes the steady-state leachate rate once the refuse is saturated; (b) compute the extra water storage the refuse can hold before it reaches field capacity, and divide by the percolation rate to get the time to saturation.

Check: rainfall (P = 1000 mm/yr), evapotranspiration (ET = 500 mm/yr) and a runoff coefficient (C = 0.15 for a gently-sloped 2% clay cover) are assumed Canadian-temperate-climate values, as invited by the question. Field capacity of the compacted refuse is assumed at 25% by volume (typical range 20–35% for compacted MSW at this density) — 5 points above the stated 20% initial moisture content.
  1. Net percolation (climate water balance on the cover). $$\text{Percolation} = P - RO - ET = 1000 - (0.15\times1000) - 500 = \boxed{350\ \text{mm/yr}}$$ This is also the steady-state leachate generation rate once the refuse below reaches field capacity — expressed per unit area, $3{,}500\ \text{m}^3/\text{ha/yr}$.
  2. Additional water storage capacity of the refuse before saturation. $$\Delta S = (FC - MC)\times\text{depth} = (0.25 - 0.20)\times 9\ \text{m} = 0.45\ \text{m} = 450\ \text{mm}$$
  3. Time to saturation. $$t = \dfrac{\Delta S}{\text{Percolation rate}} = \dfrac{450\ \text{mm}}{350\ \text{mm/yr}} = \boxed{1.29\ \text{yr}\ (\approx 15\ \text{months})}$$
QuantityValue
14.1 Annual leachate quantity (once saturated)350 mm/yr ≈ 3,500 m³/ha/yr
14.2 Time to saturation of a given year's lift≈1.29 yr (≈15 months)