Question 1 of 5: Water and wastewater quality parameters

Question 1: Water and wastewater quality parameters (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 given first as a definition — what is actually measured, and by what method — and then as a statement of why a design or operating engineer cares about it. The five are deliberately chosen to span the two halves of the course: three drinking-water parameters, two wastewater parameters.

i. Turbidity in water (5 marks)

Definition. Turbidity is an optical property of a water sample: the extent to which suspended and colloidal matter scatters a beam of light rather than transmitting it in a straight line. It is measured nephelometrically, by comparing the light scattered at 90° to the incident beam against that scattered by a formazin standard, and is reported in nephelometric turbidity units (NTU). It is emphatically not a measure of mass — it is a surrogate. A few milligrams per litre of fine clay or algal cells produce far more turbidity than the same mass of coarse sand, because scattering is most efficient when the particle size is comparable to the wavelength of the light.

Significance. Turbidity is the single most useful continuous indicator of particle-removal performance in a drinking-water plant, and this is why the Guidelines for Canadian Drinking Water Quality express the treatment goal in terms of it: filtered water from chemically assisted filtration must be at or below 0.3 NTU in at least 95 % of measurements and must never exceed 1.0 NTU. That limit exists because turbidity is a proxy for the physical barrier against Cryptosporidium and Giardia, organisms that chlorine at practical CT values does not reliably inactivate; a filter that breaks through on turbidity has lost the only barrier those pathogens face. Turbidity also matters operationally in three further ways: particles adsorb and shield micro-organisms from disinfectant, so a turbid water disinfects poorly; particulate matter exerts a chlorine demand and consumes dose that was intended for pathogens; and consumers judge water by its appearance, so turbidity carries an aesthetic weight out of proportion to its direct health risk. In raw water it also drives coagulant dose selection, since a low-turbidity, highly coloured water and a high-turbidity, uncoloured water need quite different coagulation strategies.

ii. pH and Alkalinity in water (5 marks)

Definition. These two parameters are frequently confused and are best defined against each other as intensity versus capacity. pH is an intensity factor — the negative base-ten logarithm of the hydrogen-ion activity, $\mathrm{pH} = -\log_{10}\{\mathrm{H}^{+}\}$ — and it says how acidic the water is at this instant, but says nothing about how hard it would be to change. Alkalinity is the capacity factor: the acid-neutralising capacity of the water, that is, the quantity of strong acid required to titrate a sample to a defined carbonic-acid end point (pH about 4.5). In natural waters it is dominated by the carbonate system:

and by universal convention it is reported as an equivalent concentration of calcium carbonate, in mg/L as CaCO₃. A water at pH 9 with 10 mg/L as CaCO₃ of alkalinity is alkaline but chemically fragile; a water at pH 7.5 with 200 mg/L as CaCO₃ is near neutral and extremely well buffered.

Significance. The pair governs almost every chemical decision in both water and wastewater treatment. In coagulation, hydrolysing metal coagulants are strongly pH sensitive — alum works best near pH 5.5 to 6.5 when the objective is natural organic matter removal and nearer pH 6.5 to 7.5 for turbidity removal — and each milligram of alum destroys roughly half a milligram of alkalinity as CaCO₃, so a soft, poorly buffered Canadian surface water will suffer a pH crash at design dose unless lime or soda ash is added. In corrosion control, the pH–alkalinity–calcium combination sets whether the distribution system deposits or dissolves calcium carbonate, and therefore whether lead and copper are released from service lines and premise plumbing. In disinfection, the split between hypochlorous acid and the far weaker hypochlorite ion turns on a pK_a of about 7.5, so raising pH by one unit can cost most of the disinfecting power of a given free-chlorine residual. In wastewater treatment, nitrification consumes 7.14 mg of alkalinity as CaCO₃ for every milligram of ammonia nitrogen oxidised, so a plant with a weakly buffered influent will lose its nitrifiers to a falling pH long before it runs out of aeration.

iii. Taste and odour in water (5 marks)

Definition. Taste and odour are sensory (organoleptic) parameters, quantified not by an instrument but by a trained panel. The classical measure is the threshold odour number — the dilution factor at which the odour of a sample just becomes undetectable — and modern practice supplements it with flavour profile analysis, in which panellists name and rank descriptors such as earthy, musty, swampy, chlorinous or medicinal. The parameter is unusual in that human detection thresholds are far below any routine analytical detection limit for the responsible compounds.

Significance. The two dominant causes in Canadian surface supplies are geosmin and 2-methylisoborneol (MIB), metabolites released by cyanobacteria and actinomycetes, both detectable by a sensitive nose at concentrations of only a few nanograms per litre. Chlorophenolic (medicinal) tastes arise when free chlorine reacts with phenolic precursors; swampy and rotten-egg odours arise from sulphides in anoxic reservoir hypolimnia; and chloraminous tastes arise from operating in the dichloramine region of the breakpoint curve. Taste and odour are aesthetic objectives in the GCDWQ rather than health-based maximum acceptable concentrations, which tempts engineers to treat them as trivial; that is a mistake for two reasons. First, they generate the overwhelming majority of customer complaints and are the parameter by which the public actually judges a utility. Second, and more seriously, a consumer who finds the tap water unpalatable may switch to an untreated well, a surface source or an uncontrolled bottled supply, and so exchange an aesthetic problem for a microbiological one. Design significance follows from the causative compounds: conventional coagulation, sedimentation and filtration remove essentially none of the dissolved geosmin or MIB, so control requires powdered activated carbon during bloom events, ozonation or an advanced oxidation process, granular activated carbon contactors, or upstream source control such as reservoir destratification.

iv. Biochemical oxygen demand in wastewater (5 marks)

Definition. The biochemical oxygen demand is the mass of dissolved oxygen consumed per unit volume by micro-organisms while they oxidise the biodegradable organic matter in a wastewater sample under standardised conditions — incubation in the dark at 20 °C for five days, giving the parameter universally written BOD₅ and reported in mg/L. It is an operationally defined, empirical parameter, not a chemical species. Oxygen uptake follows an approximately first-order approach to an ultimate value:

where L₀ is the ultimate carbonaceous demand and k is the rate constant, typically about 0.23 d⁻¹ for settled municipal wastewater, which is why five days captures roughly two-thirds of the ultimate value rather than all of it. Because nitrifying organisms exert their own oxygen demand after a lag of several days, the test is normally run with a nitrification inhibitor and reported as carbonaceous BOD (CBOD₅).

Significance. BOD is the classical measure of the strength of a wastewater and the quantity that regulation is written around: Canadaʹs Wastewater Systems Effluent Regulations set a national baseline of 25 mg/L CBOD₅ and 25 mg/L suspended solids in the effluent of a wastewater system. Its engineering significance is threefold. It is the load that determines the size of secondary treatment, entering directly into organic loading rates, food-to-micro-organism ratios and aeration demand. It is the quantity that determines the impact of a discharge on a receiving water, because oxidation of the discharged organics is what depresses downstream dissolved oxygen — the classical Streeter–Phelps oxygen sag. And it is a measure of biodegradability rather than of total organic content, so the ratio of BOD to chemical oxygen demand tells the designer how much of the load a biological process can actually reach. Its limitations are equally important: it takes five days, which makes it useless for real-time control; it is imprecise and seeding-dependent; and it can be badly misleading on industrial wastewaters containing inhibitory or toxic compounds. This is why plants run COD or total organic carbon for daily control and reserve BOD for compliance.

v. Total Kjeldahl Nitrogen in wastewater (5 marks)

Definition. Total Kjeldahl nitrogen is the sum of organic nitrogen and ammonia nitrogen in a sample, named after the digestion procedure that measures it: the sample is digested in hot concentrated sulphuric acid with a catalyst, which converts organic nitrogen to ammonium, and the total ammonium is then determined by distillation and titration or colorimetrically. The crucial point of the definition is what it excludes — the oxidised forms, nitrite and nitrate, are not captured — so total nitrogen must be built up as

Raw municipal wastewater typically carries 20 to 70 mg N/L of TKN, of which perhaps 60 % is already ammonia, the remainder being urea and proteinaceous organic nitrogen that hydrolyses quickly in the collection system.

Significance. TKN is the design parameter for every nitrogen-related decision in a treatment plant. It sets the nitrogenous oxygen demand, because complete nitrification consumes 4.57 g of oxygen per gram of nitrogen oxidised — an aeration burden that can rival the carbonaceous demand and that is routinely underestimated by designers who size blowers on BOD alone. It sets the alkalinity demand at 7.14 g as CaCO₃ per gram of nitrogen, which for a soft water frequently decides whether chemical addition is required. It determines whether an anoxic zone and internal mixed-liquor recycle are needed to meet a total nitrogen limit, and the ratio of influent BOD to TKN tells the designer whether there is enough readily biodegradable carbon to drive that denitrification or whether an external carbon source must be purchased. Finally it is an effluent-toxicity parameter in its own right: un-ionised ammonia is acutely toxic to fish at fractions of a milligram per litre, and the WSER accordingly limit un-ionised ammonia in the effluent to less than 1.25 mg/L as N at 15 °C, a limit that is met by nitrifying rather than by dilution.