18-Env-B5 Industrial & Hazardous Waste Management · December 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); Montgomery & Runger, Applied Statistics and Probability for Engineers (for Q1–Q5's basic-statistics content).
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.
Three well-established substance classes toxic to biological oxidation (activated-sludge) systems are: (1) heavy metals (Cu, Zn, Cr6+, Ni, Cd, Pb), which denature microbial enzymes and accumulate in the biomass over repeated small discharges rather than washing straight through; (2) cyanide, which binds the iron in cytochrome oxidase and blocks cellular respiration directly, so even a few mg/L can sharply depress the oxygen-uptake rate; and (3) phenolic and chlorinated organic compounds (phenol, chlorophenols, chlorinated solvents), which denature proteins and disrupt cell membranes at elevated concentration, and, for the chlorinated species specifically, can also be resistant to biodegradation and persist through the system. All three share a common design implication: because activated sludge cannot be economically "protected" once these substances reach the aeration basin at an inhibitory concentration, the appropriate response is source control or pretreatment upstream (precipitation for metals, chemical oxidation for cyanide, dilution/equalization or adsorption for phenolics) rather than relying on the biological process itself to tolerate them. An industrial waste survey (Question 9) that specifically screens for these three classes, rather than only the standard BOD/TSS/pH panel, is what allows a designer to catch a toxicity risk before it causes a biological upset rather than diagnosing it after a treatment failure has already occurred. This screening cost is small relative to the cost of recovering a biological system that has genuinely crashed. Recovery from a toxic upset can take weeks (re-establishing a healthy microbial population), during which the plant may be forced to operate on relaxed permit terms or bypass treatment altogether, so prevention through pretreatment is consistently the more economical path.
