18-Env-A6 Solid Waste Engineering and Management · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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; ISO 14040/14044, Environmental Management — Life Cycle Assessment.
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) Biological treatment — leachate's high BOD/COD and ammonia load is amenable to conventional aerobic biological treatment (aerated lagoons, sequencing batch reactors, or membrane bioreactors), which oxidizes dissolved organics and, with a sufficiently long solids retention time, nitrifies ammonia; anaerobic pre-treatment (UASB or anaerobic lagoon) is often used first on young, high-strength leachate to reduce the organic load cheaply before aerobic polishing. (2) Physical-chemical treatment — coagulation-flocculation (alum or ferric salts) removes colour, suspended solids and some heavy metals; air stripping removes ammonia directly (raising pH to convert NH₄⁺ to volatile NH₃, then stripping in a packed tower); and activated-carbon adsorption removes the refractory (biologically non-degradable) organic fraction typical of older, more stabilized leachate that biological treatment alone cannot reduce further. (3) Membrane treatment (reverse osmosis/nanofiltration) — produces a high-quality permeate suitable for direct discharge or reuse by rejecting dissolved salts, heavy metals and residual organics, at the cost of generating a concentrated reject stream that itself needs further management (evaporation, deep-well injection, or co-disposal in the landfill).
Selecting among these three in practice follows the leachate's own biodegradability, measured by its BOD:COD ratio: young leachate (high BOD:COD, typically above 0.5) responds well and cheaply to biological treatment alone, while older, more stabilized leachate (BOD:COD often below 0.1) has largely exhausted its readily-biodegradable fraction and needs physical-chemical polishing or membrane treatment to meet a discharge permit. A facility's treatment train therefore often shifts over the landfill's operating life — starting biological-only on young leachate and adding a physical-chemical or membrane stage as the site matures and its leachate strength and biodegradability decline.
