18-Env-A4 Water and Wastewater Engineering · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2014 — 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.
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.
Chlorides occur naturally from mineral dissolution but elevated concentrations usually flag road-salt runoff, saline groundwater intrusion, or sewage/industrial contamination; their main significance is aesthetic (salty taste above roughly 250 mg/L, the Health Canada aesthetic objective) and infrastructural, since chloride-rich water accelerates corrosion of metallic pipe and appurtenances. Sulfates similarly derive from mineral dissolution (gypsum, sulfide oxidation) and industrial discharge; at elevated concentrations (above the 500 mg/L aesthetic objective) they impart a bitter taste and can cause an osmotic laxative effect, particularly as magnesium or sodium sulfate, and sulfate-reducing bacteria in sulfate-rich waters generate hydrogen sulfide and promote concrete/pipe corrosion. Nitrates are the most health-critical of the three: they arise from agricultural fertilizer runoff, septic-system leachate and animal waste, and are regulated at a Maximum Acceptable Concentration of 45 mg/L as NO₃⁻ (10 mg/L as N) in Canada because infants under six months can reduce ingested nitrate to nitrite in their gut, causing methemoglobinemia ("blue-baby syndrome") by oxidizing haemoglobin's iron and impairing oxygen transport; nitrate is also the key indicator of agricultural/septic contamination of groundwater and a driver of surface-water eutrophication.
Measuring chlorides is classically done by argentometric (Mohr) titration: a measured sample is titrated with standardized silver nitrate (AgNO₃) in the presence of potassium chromate (K₂CrO₄) indicator. Silver chloride (the less soluble precipitate) forms preferentially and white AgCl precipitates first; once essentially all chloride is consumed, further Ag⁺ reacts with the chromate indicator to form a reddish-brown silver chromate (Ag₂CrO₄) precipitate, marking the end point (chloride-selective electrode potentiometric titration and ion chromatography are the modern instrumental alternatives). Sulfate is most commonly measured by the turbidimetric method: barium chloride is added under controlled acidic, conditioning-reagent conditions to precipitate a uniform suspension of fine barium sulfate crystals, and the resulting turbidity is read on a spectrophotometer or turbidimeter against a calibration curve of known sulfate standards (a gravimetric method — precipitating, drying, and weighing the BaSO₄ — is used where higher precision at higher concentrations is required, and ion chromatography is now the standard laboratory method for simultaneous anion analysis).
Ozone (O₃) cannot be stored or transported because it is unstable, so it is generated on site by passing dry oxygen (or air) through a high-voltage corona discharge, then dispersed into the water in a multi-chamber contact basin (fine-bubble diffusers or venturi injectors) sized for the required contact time. As a very strong oxidant (standard potential 2.07 V) ozone disinfects both directly — attacking cell walls, membranes and nucleic acids on contact — and indirectly, by decomposing (especially at higher pH) into hydroxyl radicals (•OH), an even more powerful, less selective oxidant; off-gas leaving the contactor is destroyed (thermally or catalytically) before venting, since ozone is toxic and cannot be released to atmosphere.
Two advantages over chlorination: (1) Ozone achieves far lower CT values against chlorine-resistant protozoan (oo)cysts — Giardia and especially Cryptosporidium, which are highly resistant to free chlorine at achievable doses/contact times — making it the disinfectant of choice where these organisms are a concern. (2) Because no chlorine is involved, ozonation does not form chlorinated DBPs (THMs, HAAs); it also oxidizes taste-, odour- and colour-causing compounds, generally improving aesthetic water quality as a side benefit. Two disadvantages: (1) Ozone decomposes rapidly in water (half-life of minutes) and leaves essentially no persistent residual, so it cannot protect the distribution system the way a chlorine or chloramine residual does — a secondary disinfectant must still be added downstream. (2) Ozone systems have substantially higher capital cost (on-site generators, high-voltage equipment, off-gas destruction) and electrical energy demand than chlorination, and where bromide is present in the source water, ozonation can form bromate, a regulated, suspected human carcinogen, requiring careful dose control.
