Question 1 of 5: Water and Wastewater Characteristics

Question 1: Water and Wastewater Characteristics (25 marks)

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.

Each of the five parameters below is a surrogate: none of them measures the hazard directly, and each earns its place in a regulation or a design because it is cheap, reproducible and correlates reliably with something that matters. The significance of a parameter is therefore always two statements — what it physically measures, and what it is being used to stand in for.

Part (i) — Turbidity (5 marks). Turbidity is the optical property by which suspended and colloidal matter scatters rather than transmits light. It is measured nephelometrically, at 90° to an incident beam of defined wavelength, and reported in nephelometric turbidity units (NTU) against a formazin standard; it is a light-scattering measurement and not a mass measurement, so two waters of identical suspended-solids concentration can differ several-fold in turbidity if their particles differ in size, shape or refractive index. Its significance in water supply is threefold. First, it is the only continuous, real-time indicator of filter performance available to an operator: particles in the 1 to 10 µm range that shelter Cryptosporidium oocysts and Giardia cysts also scatter light, so a rise in filtered-water turbidity is an early warning that the physical barrier has been breached, long before any microbiological result returns from the laboratory. Second, particles exert a chlorine demand and physically shield micro-organisms from both chlorine and ultraviolet light, so turbidity control is a precondition for reliable disinfection rather than an aesthetic nicety. Third, it is the enforceable performance standard: the Guidelines for Canadian Drinking Water Quality require chemically assisted filtration to produce water at or below 0.3 NTU in at least 95 percent of measurements each month, never exceeding 1.0 NTU, with tighter values for slow sand and diatomaceous-earth filtration. In wastewater, effluent turbidity is used as the surrogate for the tertiary-filtration performance that precedes UV disinfection, because UV transmittance collapses when solids pass.

Part (ii) — Coliform forming units (5 marks). The intent of the question is the enumeration of coliform bacteria as colony-forming units (CFU). Total coliforms are a group of Gram-negative, non-spore-forming, lactose-fermenting rods; thermotolerant (faecal) coliforms and, more specifically, Escherichia coli are the fraction associated with the gut of warm-blooded animals. Enumeration is by membrane filtration — a measured volume is drawn through a 0.45 µm membrane, the membrane is incubated on a selective medium, and the colonies that develop are counted and reported as CFU per 100 mL — or by a defined-substrate most-probable-number method reported as MPN per 100 mL. The unit is deliberately named "colony forming" because one colony may arise from a clump rather than a single cell, and because only culturable, viable organisms are counted. Their significance is that they are indicator organisms, not pathogens. Enteric pathogens are episodic, present in small numbers, and expensive to assay individually, whereas coliforms are shed continuously in enormous numbers, survive in water on roughly the same timescale as bacterial pathogens, and are detected by a simple, standard, inexpensive test. In Canada the drinking-water guideline is E. coli and total coliforms none detectable per 100 mL; a total-coliform detection without E. coli signals a loss of distribution-system integrity or regrowth in a biofilm, while an E. coli detection signals faecal contamination and triggers a boil-water advisory. The known limitation — that chlorine-resistant protozoan cysts and enteric viruses can survive when coliforms do not — is precisely why the multi-barrier approach pairs the indicator with turbidity and CT criteria rather than relying on it alone.

Part (iii) — Alkalinity (5 marks). Alkalinity is the acid-neutralising capacity of a water: the quantity of strong acid required to titrate a sample to a defined endpoint, conventionally pH 4.5 (the methyl-orange or total alkalinity endpoint), with the pH 8.3 phenolphthalein endpoint distinguishing carbonate and hydroxide fractions. In natural waters at ordinary pH it is carried almost entirely by bicarbonate, with contributions from carbonate, hydroxide and, in wastewaters, from ammonia, phosphate and organic bases:

$$\text{Alk} = [\text{HCO}_3^-] + 2[\text{CO}_3^{2-}] + [\text{OH}^-] - [\text{H}^+]$$

It is reported as an equivalent mass of calcium carbonate, because that convention lets alkalinity, hardness and chemical doses be added and subtracted directly. One milliequivalent per litre equals $100.09/2 = 50.04$ mg/L as CaCO₃.

Given. A raw water carrying 145 mg/L bicarbonate ($M = 61.02$ g/mol, $z = 1$), to be coagulated at an alum dose of 40 mg/L. Find. The alkalinity as CaCO₃ and the residual after coagulation. Converting on an equivalent basis,

$$\text{Alk} = 145 \times \frac{50.04}{61.02} = 118.9\ \text{mg/L as CaCO}_3$$

Each milligram per litre of alum, $\mathrm{Al_2(SO_4)_3 \cdot 14H_2O}$ ($M = 594.4$), consumes three equivalents of alkalinity, that is $3 \times 50.04/594.4 = 0.2525$ mg/L as CaCO₃, so 40 mg/L of alum destroys 10.1 mg/L and leaves

$$\boxed{\text{Alk}_{\text{residual}} = 118.9 - 10.1 = 108.8\ \text{mg/L as CaCO}_3}$$

The significance is buffering. A water with adequate alkalinity absorbs the acid released by hydrolysing coagulants without a pH excursion that would drive alum out of its optimum window (pH 5.5 to 7.5), and it resists the pH swings that would otherwise dissolve lead and copper from service lines and household plumbing. Alkalinity is also one of the two inputs to the Langelier index that governs whether a distribution system will lay down or dissolve a protective calcium-carbonate film. Soft, poorly buffered surface waters — common on the Canadian Shield and on much of coastal British Columbia — often need supplemental alkalinity (lime, soda ash or sodium bicarbonate) purely for corrosion control. In wastewater treatment alkalinity is consumed stoichiometrically by nitrification, at 7.14 mg as CaCO₃ per mg of ammonia nitrogen oxidised, and a nitrifying plant that runs out of alkalinity loses pH, loses its nitrifiers, and fails.

Part (iv) — Chemical oxygen demand (5 marks). COD is the oxygen equivalent of the organic matter in a sample that is susceptible to oxidation by a strong chemical oxidant. In the standard closed-reflux method the sample is digested for two hours at 150 °C in excess potassium dichromate with concentrated sulphuric acid, silver sulphate as catalyst and mercuric sulphate to mask chloride; the dichromate consumed is measured colorimetrically or by titration and expressed as mg/L of O₂. Its significance is operational speed and completeness. The five-day biochemical oxygen demand measures only what a mixed microbial seed will oxidise in five days at 20 °C, takes five days to report, and is sensitive to seed condition, toxicity and nitrification; COD returns in a few hours, is far more reproducible, and captures biodegradable and refractory organics alike. That difference is what makes the ratio diagnostic rather than the value alone. For a typical raw municipal sewage at COD 430 mg/L and BOD₅ 205 mg/L,

$$\frac{\text{COD}}{\text{BOD}_5} = \frac{430}{205} = 2.10$$

which sits squarely in the 1.9 to 2.5 band expected of readily biodegradable domestic wastewater. A ratio near 4, as in the pulp-mill effluent at COD 1600 and BOD₅ 400 mg/L, says that three-quarters of the organic carbon will not be removed biologically at all and warns the designer that a conventional activated-sludge plant will meet a BOD limit while failing a COD limit. Once a plant-specific ratio has been established by parallel testing, COD becomes the day-to-day control parameter, with BOD₅ retained for compliance reporting. In modern design practice COD is also the fundamental currency: influent COD is fractionated into readily biodegradable, slowly biodegradable, soluble inert and particulate inert components, and every activated-sludge simulation model is written on a COD balance because, unlike BOD, COD is conserved through synthesis, oxidation and endogenous decay.

Part (v) — Total nitrogen and TKN (5 marks). Total Kjeldahl nitrogen is the sum of organic nitrogen and ammonia nitrogen, measured by acid digestion that converts organic nitrogen to ammonium, followed by distillation and titration or by a colorimetric finish. Total nitrogen adds the oxidised forms:

$$\text{TKN} = \text{Org-N} + \text{NH}_3\text{-N}, \qquad \text{TN} = \text{TKN} + \text{NO}_3^-\text{-N} + \text{NO}_2^-\text{-N}$$

Given. A raw sewage at Org-N 14, NH₃-N 26, NO₃-N 0.4 and NO₂-N 0.1 mg/L as N. Find. TKN and TN. Direct substitution gives TKN $= 14 + 26 = 40$ mg/L and TN $= 40 + 0.4 + 0.1 = 40.5$ mg/L as N; the oxidised forms are negligible in raw sewage because the collection system is anaerobic, which is why TKN alone is an adequate influent descriptor and TN is the parameter that matters in the effluent. All forms are reported as nitrogen so they can be added without conversion.

The significance is that the two numbers answer different questions. TKN measures the nitrogen the plant must still oxidise, so it sizes the aerobic sludge age, the oxygen supply (4.57 kg O₂ per kg N nitrified) and the alkalinity demand. TN measures what the receiving water actually receives, so it drives the anoxic volume and the internal recycle needed for denitrification and it is the parameter written into a discharge permit. Their difference also diagnoses a process: an effluent with low TKN and high TN is nitrifying fully but not denitrifying. In the receiving environment nitrogen matters on three counts — un-ionised ammonia is acutely toxic to fish at fractions of a milligram per litre, nitrification of discharged ammonia exerts an oxygen demand downstream, and nitrogen is the limiting nutrient in most Canadian coastal and estuarine waters, so it governs eutrophication where phosphorus governs it in fresh water. Finally, TKN is checked against the carbonaceous load for nutrient sufficiency: at the conventional BOD₅:N:P ratio of 100:5:1, a BOD₅ of 205 mg/L needs 10.3 mg/L of nitrogen for cell synthesis, so the 40 mg/L available here is in large excess and the surplus is what must be removed.