18-Env-B5 Industrial & Hazardous Waste Management · May 2016
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.
What happens. Nitrification is the sequential, two-step biological oxidation of ammonia to nitrate carried out by two distinct groups of autotrophic, chemolithotrophic bacteria. In the first step, ammonia-oxidizing bacteria (classically Nitrosomonas) oxidize ammonium (NH4+) to nitrite (NO2−). In the second step, nitrite-oxidizing bacteria (classically Nitrobacter) oxidize the nitrite further to nitrate (NO3−). Both groups derive their energy from these oxidation reactions (rather than from organic carbon) and consume alkalinity and dissolved oxygen in the process (roughly 4.57 g O2 and 7.14 g CaCO3 alkalinity per g N oxidized).
What controls the rate. Dissolved oxygen must stay above roughly 2 mg/L, since nitrifiers are strict aerobes with a low oxygen affinity relative to heterotrophs. Temperature strongly affects nitrifier growth rate, which slows sharply below about 10–15°C. Solids retention time (SRT) must exceed the nitrifiers' slow minimum doubling time (they grow far more slowly than heterotrophic BOD-removing bacteria) or they are washed out of the system. pH is optimal near 7.5–8.5; nitrification slows markedly below pH 6.5. Available alkalinity must be sufficient to buffer the acid produced, or the pH itself will fall and further suppress the rate. Un-ionized ammonia and certain industrial toxicants (heavy metals, some organics) can inhibit or kill the nitrifying population outright, since it is far more sensitive to shock toxicity than ordinary heterotrophic biomass.
