18-Env-A6 Solid Waste Engineering and Management · December 2016
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.); Freeze & Cherry, Groundwater; CCME, Guidance Document on Landfill Gas Management; Canadian Environmental Protection Act, 1999.
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.
Key factors: (1) leachate strength and composition relative to the WWTP's design influent (BOD5, COD, ammonia-nitrogen, heavy metals, chlorinated organics) — young/fresh leachate is typically high-strength and biodegradable, while mature leachate is lower-BOD but relatively higher in recalcitrant COD and ammonia, which can be inhibitory or non-biodegradable in a conventional biological process; (2) hydraulic and organic loading impact on the plant's available capacity — leachate must be a small enough fraction of total influent flow/load that it does not upset the biological process (shock-load a nitrifying population) or exceed hydraulic capacity; (3) presence of inhibitory or toxic constituents (heavy metals, high ammonia, chlorinated solvents) that could pass through untreated or poison the biomass; (4) sewer-use bylaw / discharge permit limits set by the receiving municipality, which the leachate must meet at the point of discharge (often requiring on-site pre-treatment, e.g. equalization, ammonia stripping, or metals precipitation, before trucking or piping to the WWTP); (5) transport method and distance (piped connection vs. tanker-truck haul) and its cost; and (6) seasonal variability in both leachate flow (precipitation-driven) and WWTP available capacity.
These factors interact rather than acting independently — a plant with abundant hydraulic capacity but a nitrifying process already operating near its ammonia-loading limit may still be unable to accept leachate without pre-treatment, while a plant with spare biological capacity but limited peak hydraulic capacity may only be able to accept leachate during low-flow (dry-weather) periods. A responsible engineer negotiates an acceptance agreement that specifies a maximum leachate fraction of total flow, a maximum organic/ammonia loading rate, and a pre-treatment requirement triggered whenever the leachate's own quality exceeds those limits, rather than treating WWTP acceptance as an unconditional, permanent disposal pathway.
