Process Schematic for a Hard, Turbid, Iron-Bearing Raw Water: 16-Civ-B5 Water Supply and Wastewater Treatment, May 2014, Q5

Question 5: Process Schematic for a Hard, Turbid, Iron-Bearing Raw Water (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.

Given.

Raw water characteristics and the treatment challenge each one sets
Parameter	Raw water value	Interpretation	Process required
Turbidity	150–200 NTU	High; a silt-laden surface water, probably a river	Coagulation, flocculation, sedimentation, filtration
Total hardness	300 mg/L as CaCO₃	Very hard	Lime–soda ash softening with recarbonation
Iron	1.0 mg/L	Above the 0.3 mg/L GCDWQ aesthetic objective	Oxidation to Fe(III), then settling and filtration
Taste and odour	Seasonal	Algal metabolites (geosmin, MIB) during bloom season	Pre-oxidation plus powdered or granular activated carbon
Pathogens	Assumed present (surface source)	Bacteria, viruses, protozoan cysts and oocysts	Filtration credit plus chemical disinfection to a $CT$ target

Find. A process schematic showing every liquid and solid stream and every chemical injection point, together with the water quality expected after treatment for hardness, turbidity, iron and pathogens.

Approach. The raw water presents four independent problems, and the flowsheet is built by ordering the unit processes so that each one is placed where it does the most work and does not undo another. Oxidation must come first, so that iron is precipitated before the settling step rather than after it. Softening and coagulation are then combined in a single high-pH solids-contact stage, because the same lime that precipitates hardness also assists iron and turbidity removal and produces a heavy, fast-settling floc. Recarbonation follows to bring the pH back down and stabilise the water before filtration, because a supersaturated water would otherwise cement the filter media. Filtration then removes the residual floc, activated carbon polishes taste and odour, and disinfection is placed last, on the clearest water, where the chlorine demand is lowest and the $CT$ credit is cheapest to earn.

Figure 5.1 — Process schematic. Blue: raw-water and pre-treatment liquid train. Green: treated-water train and recycle. Brown: solids train. Red numbered circles are chemical injection points, keyed in Table 5.1 below.

Table 5.1 — Chemical injection points keyed to Figure 5.1
Point	Chemical	Purpose	Typical dose
1	Potassium permanganate or chlorine	Pre-oxidation of Fe(II) to insoluble Fe(III); partial destruction of taste-and-odour compounds; algae control	0.64 mg/L Cl₂ or 0.94 mg/L KMnO₄ per mg/L Fe
2	Hydrated lime, Ca(OH)₂	Raises pH to 10.3–11.0 and precipitates carbonate hardness as CaCO₃ and magnesium as Mg(OH)₂	150–250 mg/L, set by a jar test
3	Soda ash, Na₂CO₃	Supplies carbonate to precipitate the non-carbonate fraction of the hardness	as required by the carbonate/non-carbonate split
4	Ferric sulphate or alum coagulant	Destabilises the 150–200 NTU clay and silt turbidity so it can be flocculated	20–40 mg/L
5	Powdered activated carbon (seasonal) and anionic polymer	PAC adsorbs geosmin and MIB during bloom season; polymer strengthens the floc	PAC 5–20 mg/L seasonally; polymer 0.1–0.5 mg/L
6	Carbon dioxide	Recarbonation: lowers pH from about 10.5 to 8.5–9.0 and converts residual carbonate to bicarbonate so the water does not deposit scale on the filters and mains	to the stabilisation target
7	Chlorine (primary disinfection)	Inactivation credit earned in the baffled contact tank against the $CT$ target	1.5–3.0 mg/L to hold the design residual
8	Ammonia	Converts the free residual to monochloramine for a stable distribution residual and lower THM formation	Cl₂:N of 4:1 to 5:1
9	Fluoride (where the community fluoridates)	Dental-caries prevention to the Health Canada optimum	to 0.7 mg/L
10	Orthophosphate or polyphosphate corrosion inhibitor	Forms a passivating film in the distribution system; important after softening, which leaves a less-buffered water	0.5–1.0 mg/L as PO₄

Liquid and solid streams

The liquid train runs intake and bar screens, rapid mix with pre-oxidation and chemical addition, tapered three-stage flocculation, the solids-contact softener-clarifier, recarbonation, dual-media filtration, granular activated carbon adsorbers, the baffled chlorine contact tank, the clearwell and high-lift pumps, and finally the distribution system. The solids train collects two waste streams: the softening and coagulation sludge drawn continuously from the clarifier floor, which is by far the larger, and the spent backwash water from the filters, which is intermittent and dilute and is therefore equalised before it joins the thickener. Thickened sludge is dewatered mechanically and the cake goes to agricultural land application, where its calcium carbonate content has liming value, or to landfill. The recycle stream returns thickener supernatant, centrifuge centrate and equalised backwash water to the head of the plant; it must be returned at a controlled, steady rate rather than in slugs, because it carries concentrated pathogens and it is the one place in a plant where a Cryptosporidium breakthrough can be recirculated.

A note on sludge quantity, because it governs the site layout: softening produces roughly two kilograms of dry calcium carbonate for every kilogram of hardness precipitated. For a 20 000 m³/d plant removing 215 mg/L of hardness, that is of the order of $20\,000 \times 2 \times 0.215 = 8\,600$ kg/d of dry solids, an order of magnitude more than the coagulation sludge alone would generate. This is the principal reason a utility weighs central softening against point-of-entry alternatives.

Expected treated-water quality

Final Results — expected finished-water quality against the Canadian guidelines
Parameter	Raw water	After treatment	Removal	Governing unit processes	Canadian benchmark
Turbidity	150–200 NTU	under 0.1 NTU	99.9 per cent	Coagulation, flocculation, clarification, dual-media filtration	GCDWQ: 0.3 NTU in 95 per cent of measurements and never above 1.0 NTU for chemically assisted filtration
Total hardness	300 mg/L as CaCO₃	80–100 mg/L as CaCO₃ (design target 85)	about 72 per cent	Lime–soda softening, split treatment, recarbonation	No health-based guideline; 80–100 mg/L is the accepted service target
Iron	1.0 mg/L	under 0.05 mg/L	95 per cent or better	Pre-oxidation, co-precipitation at high pH, filtration	GCDWQ aesthetic objective 0.3 mg/L
Taste and odour	Seasonal geosmin and MIB	Below the odour threshold, about 5 ng/L	—	Pre-oxidation, seasonal PAC, GAC adsorbers	GCDWQ: inoffensive to consumers
Pathogens	Present (surface source)	3-log Giardia, 4-log virus, 3-log Cryptosporidium; total coliforms absent per 100 mL	99.9 to 99.99 per cent	2.5-log filtration credit plus chlorine $CT$ credit	GCDWQ minimum treatment goals: 3-log protozoa, 4-log virus; free residual of at least 0.2 mg/L maintained in distribution

Two design points deserve emphasis. Hardness is deliberately not driven to the practical chemical limit of about 50 mg/L as CaCO₃: a fraction of the raw water — about 14 per cent to land on 85 mg/L — is bypassed around the softener and blended back downstream. This split-treatment arrangement saves lime, cuts the sludge volume, and leaves the finished water with enough calcium carbonate saturation to form a protective film rather than attacking the mains. Second, the softening step is a substantial pathogen barrier in its own right: the excess-lime pH of 11 is lethal to bacteria and viruses within minutes, and the calcium carbonate floc enmeshes protozoan cysts, so a lime-softening plant typically earns log credits that a purely coagulation-based plant of the same size would not.

Topic. Integrated water treatment plant design — sequencing unit processes so that one multi-problem raw water is brought to drinking-water standard, with the residuals and recycle streams closed.

Key relations. Fe(II) oxidation, $4\mathrm{Fe}^{2+} + \mathrm{O_2} + 10\mathrm{H_2O} \rightarrow 4\mathrm{Fe(OH)_3}\!\downarrow + 8\mathrm{H^+}$, requiring 0.14 mg O₂ or 0.64 mg Cl₂ per mg Fe; lime softening, $\mathrm{Ca(HCO_3)_2} + \mathrm{Ca(OH)_2} \rightarrow 2\mathrm{CaCO_3}\!\downarrow + 2\mathrm{H_2O}$; split-treatment blending, $C_{\text{blend}} = fC_{\text{raw}} + (1-f)C_{\text{softened}}$; disinfection credit, $CT = C\,t_{10}$.

Why this works. The order of the processes is doing most of the work, and it is chosen so that each step leaves the next one an easier problem. Oxidising the iron first converts a dissolved species, which no clarifier can catch, into a particle, which the clarifier is already there to catch — so iron removal becomes free. Combining softening with coagulation exploits the fact that a lime floc is dense and abundant, so it sweeps the clay turbidity down with it and does the work of two clarifiers in one. Recarbonation is placed between clarification and filtration for a reason that is easy to overlook: a high-pH, carbonate-supersaturated water will continue to precipitate calcium carbonate wherever it goes next, and if that is inside a sand bed the media cements together within months. Putting disinfection last means the chlorine meets a water with almost no demand and almost no organic precursor left, which both minimises the dose and minimises trihalomethane formation. Finally, the recycle line closes the mass balance — without it a softening plant would discharge several per cent of its intake as waste, which is neither permittable nor economic.

Common pitfall. The classic exam error is drawing a conventional coagulation train and simply adding a box labelled softening, without recarbonation and without addressing what the high pH does to the chlorine that follows: at pH 10.5 only about one per cent of the free residual is the germicidal HOCl species, so a plant that disinfects before recarbonating will fail its $CT$ requirement. The second is drawing only the liquid train, since the question explicitly asks for solid streams — sludge, backwash and recycle carry marks. The third is claiming a finished hardness at the theoretical minimum; the practical limit is about 40 mg/L as CaCO₃ of calcium plus 10 mg/L of magnesium, and a well-run plant aims above it on purpose.

Source. Crittenden et al., MWH's Water Treatment: Principles and Design, 3rd ed., Ch. 9 (coagulation and flocculation), Ch. 11 (granular filtration), Ch. 13 (disinfection and $CT$), Ch. 14 (lime–soda softening and recarbonation), Ch. 22 (residuals management); Davis & Cornwell, Introduction to Environmental Engineering, 5th ed., Ch. 6; Health Canada, Guidelines for Canadian Drinking Water Quality — technical documents on turbidity, iron, chlorine and enteric protozoa.

Question 5 of 5: Process Schematic for a Hard, Turbid, Iron-Bearing Raw Water

Question 5: Process Schematic for a Hard, Turbid, Iron-Bearing Raw Water (25 marks)

Liquid and solid streams

Expected treated-water quality