16-Civ-B5 Water Supply and Wastewater Treatment · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Examination, May 2016 — 98-Civ-B5 Water Supply and Wastewater Engineering. Three hours; closed book, one two-sided aid sheet and an approved calculator permitted. Question 1 is compulsory and the candidate attempts any three of Questions 2–5; every question carries 25 marks, so a complete paper is 100 marks. All five questions are worked here, because this set is a study resource rather than a timed sitting.
Reference texts.
Check: representative design data. Questions 1 to 4 are discussion questions and the source paper prints no numbers at all in them. Every number that appears in those four answers is a representative Canadian municipal value chosen by the solver so that each definition or mechanism can be made concrete and checkable; each is labelled where it is used, and all of them. Only Question 5(b) uses data given by the examiner. The graded content of Questions 1–4 is the reasoning, not the arithmetic.
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.
Given. The five characteristics are explained against one representative Canadian municipal data set, so that each definition is followed by a number a designer would actually act on.
| Quantity | Value |
|---|---|
| Membrane-filter plate count, 10 mL portion | 68 colonies |
| Calcium and magnesium in the raw water | 80 mg/L Ca2+; 24 mg/L Mg2+ |
| Total alkalinity of the raw water | 220 mg/L as CaCO3 |
| Raw sewage five-day BOD and rate constant | 200 mg/L; $k = 0.23\ \text{d}^{-1}$ (base e, 20 °C) |
| Total ammonia nitrogen, pH and temperature of a treated effluent | 20 mg N/L; pH 8.0; 20 °C |
Find. For each characteristic: what physical or chemical property is actually measured, how it is measured, and why the resulting number governs a design, an operating decision or a regulatory limit.
Turbidity is an optical property, not a mass concentration. It quantifies the extent to which suspended and colloidal matter — clay, silt, precipitated iron and manganese, algal cells, organic detritus — scatters a beam of light rather than allowing it to pass straight through. The standard measurement (Standard Methods 2130 B) is nephelometric: a tungsten or LED source illuminates the sample and a detector set at 90° to the incident beam measures scattered light, which is reported in nephelometric turbidity units (NTU) against a formazin or styrene-divinylbenzene calibration standard. Because scattering depends on particle size, shape and refractive index as well as on particle mass, turbidity is a surrogate parameter: two waters of identical suspended-solids concentration can differ several-fold in NTU.
Its significance is out of all proportion to its simplicity, for three reasons. First, it is the only water-quality parameter that can be measured continuously, cheaply and on every individual filter, which makes it the practical control variable for a treatment plant. Second, it is the accepted surrogate for the physical removal of pathogens: Cryptosporidium oocysts and Giardia cysts cannot be monitored in real time, but a filter that reliably produces low turbidity is demonstrably removing particles of that size class, and Canadian practice awards log-removal credit on that basis. The Guidelines for Canadian Drinking Water Quality require chemically assisted filtration to produce individual-filter effluent turbidity of 0.3 NTU or less in at least 95 per cent of measurements each month, and never above 1.0 NTU. Third, particles shield microorganisms from disinfectants and exert an oxidant demand, so high turbidity simultaneously raises the chlorine dose required and lowers the inactivation achieved by it. In wastewater practice turbidity tracks effluent suspended solids and is used as the automatic control and alarm signal for UV disinfection systems, whose delivered dose collapses when the water becomes cloudy.
Coliforms are a group of Gram-negative, non-spore-forming, lactose-fermenting bacteria that are used as indicator organisms rather than as pathogens in their own right. Total coliforms include environmental genera such as Enterobacter and Klebsiella; the thermotolerant (faecal) subgroup, and in particular Escherichia coli, is essentially exclusive to the intestinal tract of warm-blooded animals and is therefore the specific evidence of faecal contamination. The enumeration is a culture method: a measured volume is drawn through a 0.45 µm membrane filter, the filter is incubated on a selective medium such as m-Endo agar, and the discrete colonies that develop are counted. Each colony is presumed to have grown from one viable, culturable cell or clump, so the result is reported as colony-forming units per 100 mL — the “forming” is a deliberate admission that what is counted is a colony, not a cell. Where turbidity or background growth defeats the membrane method, the multiple-tube fermentation technique gives a statistical most-probable number (MPN) instead.
Worked illustration. Counts between roughly 20 and 80 colonies per plate are statistically reliable, so the analyst filters a volume that lands in that band. Filtering 10 mL of a chlorinated effluent and counting 68 colonies gives
$$\text{CFU per }100\ \text{mL} \;=\; \frac{68\ \text{colonies}}{10\ \text{mL}}\times 100\ \text{mL} \;=\; \boxed{680\ \text{CFU}/100\ \text{mL}}$$The significance of the number is regulatory and epidemiological. For drinking water the Canadian guideline is absolute: no detectable E. coli and no detectable total coliforms in any 100 mL sample of water leaving a treatment plant or in the distribution system, with total coliforms in distribution treated as a signal of a breach of integrity such as a main break or backflow. For treated wastewater the limits are risk-based and protect the designated use of the receiving water: values of the order of 200 CFU/100 mL of faecal coliforms are typical where recreational contact or bivalve harvesting must be protected, which is what forces disinfection onto an otherwise adequate secondary plant. The 680 CFU/100 mL result above would therefore fail such a permit and demands more chlorine contact time, or a UV system, before discharge.
Hardness is the total concentration of multivalent metallic cations in solution, which in almost every natural water means calcium and magnesium; strontium, iron and manganese contribute negligibly. Because the cations have different molar masses but act equivalently, hardness is conventionally expressed on a common basis, as an equivalent concentration of calcium carbonate. Each ion is converted by the ratio of the equivalent weight of CaCO3 to its own:
$$\text{Hardness as CaCO}_3 \;=\; C_{\text{ion}}\times\frac{M_{\text{CaCO}_3}}{M_{\text{ion}}}$$Worked illustration. For the representative raw water above,
$$\text{Ca hardness} = 80\times\frac{100.09}{40.08} = 199.8\ \text{mg/L as CaCO}_3, \qquad \text{Mg hardness} = 24\times\frac{100.09}{24.31} = 98.8\ \text{mg/L as CaCO}_3$$so the total hardness is $\boxed{298.6\ \text{mg/L as CaCO}_3}$, which places the water firmly in the “very hard” class (above 180 mg/L). The split between carbonate and non-carbonate hardness follows from the alkalinity: since the alkalinity of 220 mg/L as CaCO3 is less than the total hardness, the carbonate hardness equals the alkalinity, 220 mg/L, and the non-carbonate hardness is the remainder, $298.6-220=78.6$ mg/L as CaCO3.
That split is the whole engineering point of the measurement. Carbonate hardness is associated with bicarbonate and can be precipitated with lime alone, because the bicarbonate supplies the carbonate ion needed. Non-carbonate hardness is associated with sulphate and chloride and requires soda ash, an added chemical cost and an added sludge volume, so a plant designer sizes the softening chemical feed from the 220/78.6 split rather than from the 298.6 total. Hardness has no health-based Canadian guideline — it is an aesthetic and operational parameter — but it drives soap and detergent consumption, scale formation in water heaters, boilers and distribution mains, and consumer complaints. Softening is usually taken only to a practical limit of about 30–40 mg/L as CaCO3 of calcium hardness and 10 mg/L of magnesium hardness, and a finished hardness near 80–100 mg/L as CaCO3 is often deliberately left in place, because water that is too soft is corrosive to the distribution system.
Biochemical oxygen demand is the mass of dissolved oxygen consumed by microorganisms while they oxidise the biodegradable organic matter in a sample under prescribed conditions. It is a bioassay, not a chemical analysis: a diluted, nutrient-buffered and seeded sample is incubated in a sealed bottle in the dark at 20 °C for five days, and the dissolved oxygen is measured before and after. The five-day value BOD5 is an arbitrary but universal convention, chosen historically because British rivers reach the sea in under five days. Suppressing the nitrifying organisms with allylthiourea gives the carbonaceous demand cBOD5, which is what Canadian effluent regulations actually specify; without suppression a well-nitrified effluent registers a spuriously high BOD.
Exertion of the demand is first order in the remaining biodegradable organic matter, so
$$\text{BOD}_t \;=\; L_0\left(1-e^{-kt}\right)$$where $L_0$ is the ultimate carbonaceous demand and $k$ the rate constant.
Worked illustration. With $k = 0.23\ \text{d}^{-1}$, the five-day test captures only
$$\frac{\text{BOD}_5}{L_0} = 1-e^{-0.23\times 5} = 0.683$$of the ultimate demand, so a raw sewage measuring 200 mg/L BOD5 actually carries $L_0 = 200/0.683 = \boxed{293\ \text{mg/L}}$ of ultimate carbonaceous demand. Two-thirds and not all of the oxygen debt appears in the standard test, and the remaining third is still exerted in the receiving water.
The significance is threefold. BOD sets the organic loading on a biological process and therefore the aeration power, the reactor volume and the sludge production of a treatment plant. It is the parameter in which the national secondary-treatment standard is written — the Wastewater Systems Effluent Regulations cap the average carbonaceous demand at 25 mg/L cBOD5. And it is the input to the receiving-water dissolved-oxygen analysis: it is the ultimate demand $L_0$, not the five-day value, that drives the Streeter–Phelps oxygen sag, which is precisely why the distinction above matters.
Total ammonia nitrogen (TAN) is the sum of the two forms in which ammonia exists in water, the un-ionised molecule NH3 and the ammonium ion NH4+, reported together on a nitrogen basis as mg N/L. That is what the laboratory measures, because the analytical method drives the equilibrium to one side before quantification. The two species interconvert instantaneously,
$$\mathrm{NH}_4^{+} \rightleftharpoons \mathrm{NH}_3 + \mathrm{H}^{+}$$and the split between them is fixed by pH and temperature. Free (un-ionised) ammonia is the fraction present as NH3, and it is the species that matters biologically, because the neutral molecule diffuses across gill membranes while the charged ammonium ion does not. The equilibrium constant is given by the Emerson correlation, with $T$ in kelvin:
$$\mathrm{p}K_a = 0.09018 + \frac{2729.92}{T}, \qquad f_{\mathrm{NH_3}} = \frac{1}{1+10^{\,(\mathrm{p}K_a-\mathrm{pH})}}$$Worked illustration. At 20 °C, $T = 293.15$ K and $\mathrm{p}K_a = 9.40$. At pH 8.0 the free fraction is
$$f_{\mathrm{NH_3}} = \frac{1}{1+10^{\,(9.40-8.00)}} = 0.0381 \quad (3.81\ \text{per cent})$$so an effluent carrying 20 mg N/L of TAN presents $0.0381\times 20 = \boxed{0.76\ \text{mg/L of free ammonia as N}}$ to the receiving water. The CCME water-quality guideline for the protection of aquatic life is 0.019 mg/L of un-ionised ammonia as N, so this effluent exceeds the guideline by a factor of 40 even though its total ammonia looks unremarkable.
That sensitivity is the significance of separating the two numbers. A single unit rise in pH multiplies the free-ammonia concentration roughly tenfold while TAN is unchanged, so a discharge that is compliant in the winter at pH 7 can be acutely lethal in the summer at pH 8.5 in a receiving water warmed and alkalinised by algal photosynthesis. Beyond toxicity, TAN carries a nitrogenous oxygen demand of 4.57 g O2 per gram of nitrogen nitrified, it consumes chlorine stoichiometrically and therefore controls the breakpoint dose, it is a nutrient contributing to eutrophication, and in a drinking-water distribution system it feeds nitrifying bacteria that destroy a chloramine residual.
| Characteristic | Illustrative result | What it decides |
|---|---|---|
| Turbidity | 0.3 NTU in 95 % of readings; never > 1.0 NTU (GCDWQ) | Filter performance and pathogen log-removal credit |
| Coliform forming units | 680 CFU/100 mL from 68 colonies on a 10 mL portion | Faecal contamination; disinfection adequacy |
| Hardness | 298.6 mg/L as CaCO3 (220 carbonate + 78.6 non-carbonate) | Lime versus soda-ash demand; scale and soap consumption |
| Biochemical oxygen demand | BOD5 200 mg/L $\Rightarrow$ $L_0$ = 293 mg/L at $k = 0.23\ \text{d}^{-1}$ | Aeration power, reactor size, permit compliance, DO sag |
| Total ammonia and free ammonia | TAN 20 mg N/L $\Rightarrow$ 0.76 mg/L NH3-N at pH 8.0, 20 °C | Aquatic toxicity, nitrogenous oxygen demand, chlorine demand |
