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.
Beyond conventional free-chlorine disinfection, three alternative technologies are in widespread water/wastewater use: (1) ultraviolet (UV) irradiation, which inactivates organisms by damaging nucleic acids rather than by chemical oxidation and leaves no disinfection by-products; (2) ozonation ($\text{O}_3$), a very strong oxidant generated on-site by corona discharge that inactivates chlorine-resistant protozoa (Giardia, Cryptosporidium) effectively but leaves no lasting residual, usually followed by a small chlorine/chloramine dose for distribution-system protection; and (3) chlorine dioxide ($\text{ClO}_2$), an oxidant/disinfectant generated on-site that produces fewer trihalomethanes than free chlorine and is effective against biofilms, though it can form the regulated by-products chlorite and chlorate. Chloramination (combined chlorine, using ammonia to convert free chlorine to monochloramine) is a fourth common alternative valued for its persistent, low-THM residual in long distribution systems.
