18-Env-B9 Environmental Chemistry and Microbiology · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 04-Env-B9, Environmental Chemistry/Microbiology. 3 hours duration; closed-book exam (approved Casio or Sharp calculator only). The paper has two sections — Section 1: Chemistry (11 questions, 50 marks) and Section 2: Microbiology (14 questions, 50 marks) — twenty-five questions constitute the complete exam and all are answered below. Total examination mark 100.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) (water chemistry, disinfection, water/wastewater microbiology, indicator organisms); Metcalf & Eddy (Tchobanoglous, Stensel, Tsuchihashi & Burton), Wastewater Engineering: Treatment and Resource Recovery (5th ed.) (chemical unit processes, chemical phosphorus precipitation, biomass stoichiometry, activated-sludge microbiology); Guidelines for Canadian Drinking Water Quality (Health Canada); MWH's Water Treatment: Principles and Design (3rd ed.) (chlorine disinfection, contact-tank sizing).
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.
Trihalomethanes (THMs) are a family of disinfection by-products — single-carbon compounds in which three of the four hydrogens of methane are replaced by halogen atoms (chlorine and/or bromine), the most common being chloroform ($\text{CHCl}_3$), bromodichloromethane, dibromochloromethane, and bromoform. They form when free chlorine reacts with naturally occurring organic matter (humic and fulvic acids from decaying vegetation) present in the source water, with bromide ion (common in coastal/saline-influenced sources) shifting the mix toward brominated species. THM formation increases with chlorine dose, contact time, temperature, pH, and NOM (organic precursor) concentration, which is why utilities minimize THMs by removing NOM upstream of chlorination (enhanced coagulation, GAC) or by switching to alternative disinfectants (Q4) that produce fewer or different by-products.
