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.
Autotrophic bacteria obtain their cell carbon from inorganic carbon dioxide ($\text{CO}_2$/$\text{HCO}_3^-$) and their energy either from light (photoautotrophs, e.g. cyanobacteria) or from the oxidation of inorganic chemicals (chemoautotrophs) — the ammonia- and nitrite-oxidizing nitrifying bacteria (Nitrosomonas, Nitrobacter) central to biological nitrification are the classic wastewater-engineering example, deriving energy from oxidizing $\text{NH}_4^+$ or $\text{NO}_2^-$. Heterotrophic bacteria require organic carbon compounds both as their carbon source for new cell synthesis and as their energy source (via oxidation of that same organic substrate) — the dominant group responsible for BOD/carbonaceous-organic-matter removal in conventional activated sludge.
Because autotrophs derive far less energy per unit substrate oxidized than heterotrophs do from organic substrate, autotrophic growth (yield and rate) is markedly slower — nitrifiers have a much longer minimum solids retention time than heterotrophic BOD-removing organisms, which is the practical reason nitrification requires a longer SRT/larger aeration basin than carbonaceous BOD removal alone.
