18-Env-A4 Water and Wastewater Engineering · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2015 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved calculator permitted. Question 1 is compulsory; the paper instructs candidates to attempt any three of the remaining four (100 marks total); all five are solved below for completeness. (The source page prints an internal header inconsistency — "NATIONAL EXAMINATION, MAY 2015" beside a page footer reading "December 2015" — the period is taken as December 2015 per the footer; this does not affect any question content.)
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery; Davis & Cornwell, Introduction to Environmental Engineering; MWH's Water Treatment: Principles and Design; Guidelines for Canadian Drinking Water Quality (Health Canada).
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.
UV disinfection passes water through a reactor chamber where lamps — low-pressure mercury-vapour (near-monochromatic, peak germicidal output around 254 nm) or medium-pressure (polychromatic) — emit ultraviolet light that is absorbed directly by microbial nucleic acids (DNA/RNA). The absorbed photons form pyrimidine dimers, most commonly thymine dimers, which physically block replication and transcription; the organism is thereby inactivated — genetically disabled from reproducing — without any chemical reaction, additive or residual. Reactors are validated by bioassay and sized to deliver a required UV dose (intensity × exposure time, mJ/cm²) that meets a target log-inactivation credit (e.g., 3-log Giardia, 4-log virus under the multi-barrier treatment goals in the Guidelines for Canadian Drinking Water Quality).
Advantages over chlorination: no chlorinated disinfection by-products (THMs/HAAs) are formed, since there is no chemical reaction with natural organic matter; UV is highly effective against chlorine-resistant protozoan (oo)cysts — Cryptosporidium and Giardia — at achievable, practical doses, where chlorine is essentially ineffective; there is no chemical storage, handling or transport hazard; contact time is measured in seconds rather than minutes; and UV does not alter taste or odour. Disadvantages: UV leaves no residual disinfectant, so it provides no protection against regrowth or recontamination in the distribution system and a secondary chemical disinfectant (chlorine/chloramine) is normally still required downstream; performance depends strongly on water quality, since turbidity and particulates shield organisms from exposure, and some organisms can partially reverse UV damage through photoreactivation or dark repair; lamps require periodic cleaning (fouling reduces UV transmittance) and replacement; capital cost and continuous electrical dependency are higher, with no disinfection at all during a power outage unless backed up; and performance cannot be confirmed with a simple field residual test, relying instead on continuously monitored UV-intensity sensors and validated flow.
Coagulation adds a chemical coagulant (alum, ferric chloride, or a cationic polymer) that neutralizes the negative surface charge on colloidal particles (clay, natural organic matter, some pathogens) and/or forms metal-hydroxide precipitates that physically sweep particles from suspension, destabilizing them so they can aggregate. It is necessary whenever a source carries fine colloidal turbidity or colour that will not settle on its own — a surface-water intake during a high-turbidity runoff event, or an algae-laden reservoir.
Flocculation is gentle, prolonged mixing (a tapered, decreasing velocity gradient $G$) applied immediately after coagulation, bringing destabilized particles into physical contact so they collide and build into larger, readily-settleable flocs. It is necessary any time a coagulant has just been dosed, because charge neutralization alone does not make particles aggregate — they must actually collide, which is what flocculation's controlled mixing provides.
Filtration passes water through a granular media bed (single-medium sand, or dual-media sand-over-anthracite) that removes remaining particulate matter by straining, sedimentation within the pore spaces, and interception/adsorption onto the grain surfaces. It is necessary as a polishing step downstream of clarification (or as direct filtration where raw turbidity is already low) to meet turbidity/particle-removal targets and to provide a physical barrier against Cryptosporidium and Giardia that survive coagulation-sedimentation.
Adsorption removes dissolved organic contaminants — taste-and-odour compounds such as geosmin and 2-methylisoborneol from algal blooms, natural organic matter that would otherwise drive high disinfection-by-product formation, and specific trace organics or pesticides — onto the high-surface-area micropores of activated carbon, either as a granular contactor or as powdered carbon dosed ahead of filtration. It is necessary whenever conventional coagulation/filtration alone cannot remove a dissolved organic problem, such as a seasonal taste-and-odour episode or elevated DOC.
Ion exchange replaces a dissolved ionic species with a less-objectionable ion held on a resin bed — classically Ca²⁺/Mg²⁺ exchanged for Na⁺ in water softening, or nitrate, arsenate and similar anions exchanged on an anion resin. It is necessary for hardness removal from a groundwater supply, or for targeted removal of a specific dissolved ion (nitrate in agricultural groundwater, arsenic, uranium) that physical clarification and filtration do not touch at all, since these are true dissolved-ion problems rather than particulate ones.
