16-Civ-B5 Water Supply and Wastewater Treatment · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Examination, May 2018 — 16-Civ-B5 Water Supply and Wastewater Engineering. Three hours, closed book, one two-sided aid sheet and an approved Casio or Sharp calculator permitted. Question 1 is compulsory and candidates attempt any three of Questions 2–5; every question carries 25 marks. Marks are shown at the end of each question and the paper explicitly invites candidates to state any assumptions they make. All five questions are worked below, because the set is a study resource rather than a three-hour sitting.
Reference texts.
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. A single raw-water source whose quality is characterised by the four parameters below; the plant is to be a conventional Canadian municipal surface water works.
| Parameter | Raw value | Governing objective |
|---|---|---|
| Turbidity | 150–200 NTU | < 0.3 NTU in 95 % of filtered samples (GCDWQ) |
| Total hardness | 300 mg/L as CaCO3 | 80–100 mg/L, aesthetic objective |
| Iron | 1.0 mg/L | < 0.3 mg/L aesthetic objective; target < 0.1 |
| Taste and odour | seasonal (algal) | no detectable T&O at the tap |
| Pathogens | assumed present (surface source) | 3-log Giardia/Cryptosporidium, 4-log virus |
Find. A process train that meets all five objectives at once, drawn as a flowsheet showing every liquid and solid stream and every chemical injection point, together with the water quality expected after treatment.
Approach. Take the contaminants in the order in which they are most economically removed — oxidise the iron and the taste-and-odour precursors first, remove turbidity and hardness together in a single coagulation/softening train because both are precipitation-and-settling duties, polish on a filter that also carries adsorptive capacity, then disinfect the now low-turbidity water where CT credit is cheapest.
Intake and pre-oxidation. Raw water is screened at the intake and then dosed with potassium permanganate (or ozone, where the taste-and-odour load justifies the capital cost) in a contact basin. Permanganate at roughly 1 mg per mg of iron oxidises soluble ferrous iron to insoluble ferric hydroxide,
$$\mathrm{3\,Fe^{2+}+MnO_4^{-}+7\,H_2O\rightarrow 3\,Fe(OH)_3\downarrow+MnO_2\downarrow+5\,H^{+}}$$so that the iron leaves the plant as a settleable and filterable solid rather than oxidising in the distribution system and producing red water. The same oxidant destroys much of the geosmin and 2-methylisoborneol responsible for the seasonal earthy-musty odour. Pre-oxidation is placed ahead of coagulation, not after, so the iron floc is available to be swept up in the subsequent clarification.
Coagulation and flocculation. Alum (or ferric sulphate) plus a small anionic polymer is injected into a rapid-mix basin at a velocity gradient of 600–1000 s−1 for 10–30 seconds, which destabilises the colloids responsible for the 150–200 NTU turbidity. Because the raw hardness is 300 mg/L as CaCO3, lime and soda ash are added in the same train and the plant is operated as an excess-lime softening works; carbonate hardness is precipitated as calcium carbonate and non-carbonate magnesium hardness as magnesium hydroxide,
$$\mathrm{Ca(HCO_3)_2+Ca(OH)_2\rightarrow 2\,CaCO_3\downarrow+2\,H_2O}$$ $$\mathrm{MgSO_4+Ca(OH)_2\rightarrow Mg(OH)_2\downarrow+CaSO_4},\qquad \mathrm{CaSO_4+Na_2CO_3\rightarrow CaCO_3\downarrow+Na_2SO_4}$$Combining softening with coagulation is the key economy in this design: the same tanks and the same clarifier serve both duties, the dense calcium carbonate crystals act as a weighting agent that improves floc settling, and the high pH of the softening reaction (10.5–11) itself provides substantial disinfection credit and drives the iron removal to completion. Flocculation follows in three tapered stages at G falling from about 60 to 20 s−1 over 20–30 minutes, so that the pin floc grows into settleable aggregates without being sheared apart.
Sedimentation. The flocculated, softened water passes to rectangular or circular clarifiers at a surface overflow rate of 20–40 m3 m−2 d−1, which removes upwards of 95 per cent of the incoming solids and drops the turbidity from 150–200 NTU to roughly 2–5 NTU. The underflow is a mixed chemical sludge of calcium carbonate, magnesium hydroxide, aluminium hydroxide, ferric hydroxide and manganese dioxide with the original raw-water silt — a large solids stream, which is why the residuals handling on a softening plant is a major part of the capital cost.
Recarbonation. Carbon dioxide is injected downstream of sedimentation to bring the pH from about 10.8 back to 7.8–8.3, converting residual caustic alkalinity to bicarbonate and preventing after-precipitation of calcium carbonate in the filters and the distribution mains. This step is also what makes the subsequent chlorination effective, for the speciation reason set out in Question 2(a).
Filtration. Dual-media (anthracite over sand) rapid filters at 5–12 m/h polish the water to below 0.3 NTU, capture the last of the ferric floc, and provide the physical barrier against Cryptosporidium oocysts that no chemical disinfectant can be relied upon to inactivate. A granular activated carbon cap, or powdered activated carbon dosed ahead of the filters during the algal season, removes the residual taste-and-odour compounds that survived pre-oxidation. Filters are backwashed on head loss or turbidity breakthrough; the spent wash water is equalised and returned to the head of the works.
Disinfection and clearwell. Chlorine is applied to the filtered water in a baffled clearwell sized to deliver the required CT, and ammonia is added at the outlet to convert the free residual to monochloramine for the distribution system, which resists decay and produces fewer trihalomethanes over long residence times. Where the source is Cryptosporidium-prone, ultraviolet reactors ahead of the clearwell supply the protozoan credit at a fraction of the chlorine dose.
Residuals. Softening sludge, clarifier sludge and thickened backwash solids are combined, thickened, dewatered on a centrifuge or belt press, and landfilled or land-applied; some plants recalcine the calcium carbonate to recover lime. The clarified backwash supernatant returns to the plant inlet, and its recycled solids and pathogen load are the reason that return stream must be equalised rather than returned in slugs.
| Parameter | Raw | After clarification | After filtration | Finished water |
|---|---|---|---|---|
| Turbidity | 150–200 NTU | 2–5 NTU | < 0.3 NTU | < 0.3 NTU (typ. 0.05–0.1) |
| Total hardness | 300 mg/L as CaCO3 | 80–120 | 80–100 | 80–100 mg/L as CaCO3 |
| Iron | 1.0 mg/L | < 0.2 mg/L | < 0.1 mg/L | < 0.05–0.1 mg/L |
| Taste and odour | seasonal, strong | much reduced | none detectable | none detectable |
| Pathogens | present | 1–2 log removed (incl. high-pH credit) | 2–2.5 log Giardia/Crypto. | ≥ 3-log protozoa, ≥ 4-log virus; 0.2–0.5 mg/L residual |
| pH | ~7.8 | 10.5–11 | 7.8–8.3 | 7.8–8.3, positive LSI |
Check: the paper gives no flow, no alkalinity and no split between calcium and magnesium hardness, so the train above is specified qualitatively and the performance figures are the ranges a conventional excess-lime softening plant achieves. Two design choices would change with that missing data: if most of the 300 mg/L is carbonate hardness with little magnesium, single-stage lime treatment suffices and the soda ash is unnecessary; and if the source is a small, consistently low-alkalinity stream, ion exchange or membrane softening may beat lime on life-cycle cost despite the larger chemical footprint of the brine. The finished hardness target of 80–100 mg/L is deliberately not zero — fully softened water is aggressive to the distribution system.
