18-Env-B5 Industrial & Hazardous Waste Management · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Nemerow & Dasgupta, Industrial and Hazardous Waste Treatment, 2nd ed.; Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery, 5th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 6th ed.; LaGrega, Buckingham & Evans, Hazardous Waste Management, 2nd ed.; CCME, Guidelines for the Management of Biomedical Waste in Canada (1992); Canadian Environmental Protection Act (CEPA), 1999; provincial Environmental Protection / Hazardous Waste Regulations (e.g. BC's Hazardous Waste Regulation, O.Reg. 347 in Ontario).
All eighteen questions are compulsory on this paper and are answered in full below.
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.
Plug-flow and complete-mix are the two idealized flow regimes for an activated-sludge (or similar suspended-growth) reactor, and each carries a distinct, predictable set of trade-offs directly traceable to how substrate concentration varies through the tank:
| System | Advantages | Disadvantages |
|---|---|---|
| Plug-flow | High substrate concentration at the inlet drives a high initial reaction rate, giving good overall BOD removal efficiency and effluent quality for a given tank volume; tapered aeration is straightforward to apply (matching air supply to the declining oxygen demand along the tank length), improving energy efficiency; generally better settling sludge (lower filamentous bulking tendency) because the high substrate gradient favours floc-forming over filamentous organisms. | Poor resistance to shock/toxic loading — a slug entering the head of the tank sees the full undiluted concentration, which can upset or kill the biomass there before it is diluted; more sensitive to hydraulic short-circuiting if the tank geometry/baffling is imperfect; oxygen demand is highly non-uniform along the tank length, requiring more complex (tapered) aeration control to avoid under- or over-aerating sections. |
| Complete-mix | Instantaneous, uniform dilution of any incoming shock or toxic load across the whole tank volume, giving excellent resistance to hydraulic and organic shock loading; uniform substrate concentration and oxygen demand throughout the tank simplifies aeration system design and control; more forgiving of a variable industrial influent. | Lower reaction driving force (the whole tank operates at the LOW effluent-level substrate concentration rather than a high inlet concentration), which for the same kinetics generally requires a larger tank volume for equivalent removal efficiency; more prone to filamentous bulking (poorer settling sludge) because the uniformly low substrate concentration favours filamentous organisms with a higher affinity at low substrate levels; effluent quality is more sensitive to any upset in mixing energy/pattern. |
In practice, many industrial and municipal activated-sludge plants adopt an intermediate step-feed or plug-flow-with-recycle configuration precisely to combine plug-flow's efficiency and settleability with complete-mix's shock-load tolerance — a direct consequence of the trade-offs shown in the table above.
