18-Env-B5 Industrial & Hazardous Waste Management · December 2017
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; Basel Convention on the Control of Transboundary Movements of Hazardous Wastes (1989); Canadian Nuclear Safety Commission (CNSC) regulations on radioactive waste; provincial hazardous waste regulations (e.g. BC's Environmental Management Act and Hazardous Waste Regulation).
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.
(i) Plug-flow is a good choice for a relatively consistent, non-toxic, moderate-strength wastewater with a steady flow — for example, treating a municipal or a well-equalized food-processing wastewater in a long, narrow aeration basin. Plug-flow provides a declining substrate gradient along the tank length, which favours good floc formation (better settling, less filamentous bulking) and typically achieves higher-quality, more consistent effluent than complete-mix for the same detention time, because the organisms near the outlet are working against a low, well-defined substrate concentration.
(ii) Complete-mix is a good choice for a wastewater subject to shock loading — a highly variable, intermittently toxic or high-strength industrial discharge (e.g. a batch dye-house or chemical-plant effluent). Because a complete-mix basin instantaneously dilutes any incoming shock load throughout the entire tank volume, the biomass at any point never sees the full incoming concentration, giving the process much greater resistance to upset or toxic shock than plug-flow, where the first section of the tank would bear the full undiluted shock.
