16-Civ-B5 Water Supply and Wastewater Treatment · December 2015

Question 1 of 7: Treatment Train for a Lake Supply with TSS, Hardness and Taste-and-Odour Problems

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. 98-Civ-B5 Water Supply and Wastewater Engineering — National Examination, December 2015. Three hours; closed book, one aid sheet written on both sides; an approved calculator is permitted. Question 1 is compulsory and the candidate attempts any five of the remaining six, so 100 marks are written out of the 115 printed (Q1 = 25 marks, Q2 to Q7 = 15 marks each; Q2 splits 12 + 3). Every one of the seven questions is solved below, because this set is a study resource rather than an exam script.

Reference texts.

Metcalf & Eddy / AECOM, Wastewater Engineering: Treatment and Resource Recovery, 5th ed. — solids fractionation, nitrogen chemistry, sedimentation theory, anaerobic digestion.
Crittenden, Trussell, Hand, Howe & Tchobanoglous, MWH's Water Treatment: Principles and Design, 3rd ed. — coagulation and flocculation theory, softening, filtration, disinfection.
Davis & Cornwell, Introduction to Environmental Engineering, 5th ed. — treatment-train selection, hardness chemistry, residuals management.
Mihelcic & Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design, 3rd ed. — mass balances, reaction kinetics, water-quality indicators.
CCME, Canadian Environmental Quality Guidelines (ammonia and receiving-water criteria) and Health Canada, Guidelines for Canadian Drinking Water Quality (treated-water objectives).
APHA/AWWA/WEF, Standard Methods for the Examination of Water and Wastewater — Method 2540 D/E for the solids series.

Check: representative design data. Questions 1, 3, 4 and 7 are discussion questions and print no numbers. Where a number appears in those answers it is a representative Canadian municipal value chosen by the solver to make the argument concrete; it is labelled as such at the point of use, and every one of them. The graded content of those questions is the reasoning, not the arithmetic.

Question 1: Treatment Train for a Lake Supply with TSS, Hardness and Taste-and-Odour Problems (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. A lake source whose raw water carries three simultaneous problems — high suspended solids and turbidity, high hardness, and seasonal taste and odour — is to be treated to potable standard. The paper states the water-quality problems qualitatively, so the design basis below is the representative Canadian prairie-lake water assumed by the solver.

Assumed design basis for the proposed plant
Parameter	Value	Consequence for the train
Design flow	20 000 m³/d	Sets unit sizes; not needed for the process selection itself
Turbidity	25 to 120 NTU (seasonal)	Coagulation, flocculation, clarification, filtration all required
TSS	30 to 150 mg/L	Sizeable solids/residuals train
Total hardness	320 mg/L as CaCO₃ (Ca 220, Mg 100)	Chemical (lime–soda) softening
Total alkalinity	260 mg/L as CaCO₃	Ample buffer for alum; no supplemental alkali needed
Geosmin and 2-MIB	20 to 60 ng/L, late summer	Pre-oxidation plus adsorption
Ammonia	0.4 mg/L as N	Chloramine residual is the natural choice for distribution

Find. A defensible liquid treatment train and its matching solids/residuals train, drawn as a process flow diagram, with every chemical named at the point where it is injected, and the governing chemical reactions written out for coagulation and for chlorine disinfection.

Figure 1.1 — Proposed process flow diagram. Upper line: the liquid train, with green arrows marking chemical injection points. Lower line: the solids/residuals train. Broken red lines are solids withdrawals; the broken green line is the thickener decant returned upstream of coagulation.

Approach. Each raw-water problem is assigned to the unit process that is the accepted best answer for it — particles to coagulation/flocculation/clarification/filtration, hardness to lime–soda softening placed so that its own precipitate is removed by the same clarifier, taste and odour to pre-oxidation plus adsorption — and the units are then ordered so that each one protects the one downstream of it.

Key processes in the liquid train

The lake intake is a multi-level screened structure. Drawing from more than one depth is not a detail: a lake stratifies, and in late summer the epilimnion carries the algae that generate geosmin and 2-methylisoborneol while the hypolimnion carries dissolved iron and manganese released from anoxic sediments. Being able to select the draw depth is the cheapest taste-and-odour control the plant will ever own. Coarse bar screens and a travelling band screen protect the raw-water pumps.

Aeration and pre-oxidation follow. Cascade or diffused aeration strips dissolved gases and part of the volatile odour load and begins the oxidation of iron and manganese. Potassium permanganate is the preferred chemical pre-oxidant here because it destroys taste-and-odour compounds and oxidises manganese without forming the halogenated by-products that pre-chlorination would create in a water this rich in natural organic matter. Where an ozone contactor can be afforded, ozone does the same job faster and also improves downstream coagulation.

Rapid mix disperses the coagulant into the flow within one to two seconds. The coagulant chemistry is what actually destabilises the colloids, and it is described in the reactions below. A cationic polymer added as a coagulant aid at the same point strengthens the resulting floc. Flocculation then follows in three tapered compartments (for example velocity gradients of roughly 60, 35 and 20 s⁻¹) over about 25 minutes, so that the floc grows without being torn apart as it becomes more fragile.

Lime softening with sedimentation is where the hardness is removed. Placing the softening basin here, rather than in a separate downstream train, is the key economy of this design: the calcium carbonate and magnesium hydroxide precipitates are heavy, they sweep the coagulated turbidity down with them, and one set of clarifiers therefore does two jobs. The basin is operated as a solids-contact unit so that recirculated precipitate provides the crystal surface that the reactions need, and the practical floor of about 40 mg/L as CaCO₃ of residual calcium hardness is respected rather than chased.

Recarbonation and filtration come next. Softened water leaves the clarifier at pH 10.5 or above and is supersaturated with calcium carbonate; carbon dioxide is injected to bring it back to about pH 8.5 to 9.0, otherwise the filter media and every downstream pipe wall will be cemented with scale. Powdered activated carbon may be dosed just ahead of the filters during an odour episode. Dual-media anthracite-over-sand filters then polish the water to below 0.1 NTU, which is the practical prerequisite for reliable disinfection.

Granular activated carbon adsorption is the dedicated taste-and-odour barrier. Geosmin and 2-MIB are detectable by a human palate at a few nanograms per litre, far below any level that coagulation or filtration can address, and adsorption is the only unit process that reliably removes them. A GAC contactor also removes the natural-organic-matter precursors that would otherwise become disinfection by-products.

Finally the chlorine contact tank and clearwell deliver primary disinfection. The contact time is sized on the CT concept — the product of residual concentration and contact time at the plant's lowest temperature and highest pH — and ammonia is added afterwards to convert the free residual to monochloramine, which is far more persistent in the distribution system and forms fewer trihalomethanes. Fluoride and a corrosion inhibitor such as orthophosphate are dosed at the clearwell outlet.

Key processes in the solids and residuals train

Two residual streams leave the liquid train, and they are quite different. The clarifier underflow is a chemical sludge of calcium carbonate, magnesium hydroxide, aluminium hydroxide and the raw-water solids, at perhaps 2 to 5 % solids. The spent filter backwash is a large, dilute, intermittent stream at well under 1 % solids. It is equalised first so that the rest of the train sees a steady flow, and it is a mistake to combine the two before equalisation.

A gravity thickener raises the combined sludge to roughly 6 to 10 % solids. Its decant is the largest single recycle in the plant, and it is returned upstream of the rapid mix rather than to the intake, so that anything it carries is re-coagulated rather than simply re-circulated. Conditioning with a cationic polymer then flocculates the sludge so that it will release water mechanically, and a centrifuge or belt filter press dewaters it to a 20 to 30 % solids cake. Lime sludge cake is a genuinely useful material: it can be land-applied as an agricultural liming agent, sent to a cement kiln, or recalcined on a large plant to recover quicklime and close the lime loop. Alum-rich sludge has fewer outlets and normally goes to landfill. Filtrate and centrate return to the head of the solids train.

Chemicals, injection points and purpose

Chemical dosing schedule for the proposed plant
Chemical	Injection point	Purpose	Typical dose
Potassium permanganate (KMnO₄)	Raw-water main, ahead of aeration	Taste and odour, Fe and Mn oxidation	0.5 to 2 mg/L
Alum, Al₂(SO₄)₃·14H₂O	Rapid mix	Coagulation	20 to 45 mg/L
Cationic polymer	Rapid mix (coagulant aid)	Floc strength	0.2 to 0.5 mg/L
Hydrated lime, Ca(OH)₂	Softening-basin inlet	Carbonate hardness and Mg removal	120 to 180 mg/L
Soda ash, Na₂CO₃	Softening-basin inlet	Non-carbonate hardness	0 to 60 mg/L
Carbon dioxide (CO₂)	Recarbonation basin	pH stabilisation, scale control	15 to 30 mg/L
Powdered activated carbon	Ahead of filtration (episodic)	Taste and odour peaks	5 to 20 mg/L
Chlorine (Cl₂ or NaOCl)	Contact-tank inlet	Primary disinfection	1.5 to 3 mg/L
Ammonia (NH₃)	Contact-tank outlet	Chloramine residual for distribution	0.3 to 0.6 mg/L as N
Fluoride and orthophosphate	Clearwell outlet	Dental health, corrosion control	0.7 mg/L F; 1 mg/L as P
Cationic polymer	Sludge conditioning tank	Dewaterability	3 to 8 kg/t dry solids

Reactions in coagulation

Alum dissolves and hydrolyses. In a water with adequate alkalinity the protons released are neutralised by bicarbonate, and the overall stoichiometry is

$$\mathrm{Al_2(SO_4)_3\cdot 14H_2O} + 6\,\mathrm{HCO_3^-} \longrightarrow 2\,\mathrm{Al(OH)_3(s)} + 3\,\mathrm{SO_4^{2-}} + 6\,\mathrm{CO_2} + 14\,\mathrm{H_2O}$$

If the alkalinity is exhausted the same hydrolysis proceeds against the water itself, and the pH falls sharply:

$$\mathrm{Al^{3+}} + 3\,\mathrm{H_2O} \rightleftharpoons \mathrm{Al(OH)_3(s)} + 3\,\mathrm{H^+}$$

That second form is why alkalinity has to be checked before a dose is set. Each mole of alum destroys six equivalents of alkalinity, and it is worth putting a number on it.

Given. An alum dose of 30 mg/L on a water of alkalinity 260 mg/L as CaCO₃; the molar mass of Al₂(SO₄)₃·14H₂O is 594.4 g/mol and the equivalent weight of CaCO₃ is 50 g/eq.

Find. The alkalinity destroyed and the dry chemical sludge generated, to confirm that no supplemental alkali is required.

$$\text{Alkalinity destroyed} = 30\ \frac{\mathrm{mg}}{\mathrm{L}}\times\frac{6\times 50}{594.4} = \boxed{15.1\ \mathrm{mg/L\ as\ CaCO_3}}$$

That is under six per cent of the available alkalinity, so the buffer is ample and no caustic or soda ash is needed for the coagulation step. The same stoichiometry gives the solids load: two moles of Al(OH)₃ at 78.0 g/mol per mole of alum is $30\times 2\times 78.0/594.4 = 7.87$ mg/L of dry aluminium hydroxide, roughly 157 kg/d at the design flow, which the solids train must handle on top of the raw-water solids and the softening precipitate.

The softening reactions run in the same basin and are given here for completeness, since they generate the bulk of the residuals:

$$\mathrm{Ca(HCO_3)_2} + \mathrm{Ca(OH)_2} \longrightarrow 2\,\mathrm{CaCO_3(s)} + 2\,\mathrm{H_2O}$$ $$\mathrm{Mg(HCO_3)_2} + 2\,\mathrm{Ca(OH)_2} \longrightarrow 2\,\mathrm{CaCO_3(s)} + \mathrm{Mg(OH)_2(s)} + 2\,\mathrm{H_2O}$$ $$\mathrm{CaSO_4} + \mathrm{Na_2CO_3} \longrightarrow \mathrm{CaCO_3(s)} + \mathrm{Na_2SO_4}$$

and recarbonation reverses the excess:

$$\mathrm{Ca(OH)_2} + \mathrm{CO_2} \longrightarrow \mathrm{CaCO_3(s)} + \mathrm{H_2O}, \qquad \mathrm{CaCO_3} + \mathrm{CO_2} + \mathrm{H_2O} \longrightarrow \mathrm{Ca(HCO_3)_2}$$

For the assumed carbonate hardness of 180 mg/L as CaCO₃, one equivalent of lime per equivalent of carbonate hardness gives $\left(180/50\right)\times 74.1/2 = 133$ mg/L of Ca(OH)₂ before any excess-lime allowance for magnesium.

Reactions in chlorine disinfection

Chlorine gas hydrolyses essentially completely in dilute solution:

$$\mathrm{Cl_2} + \mathrm{H_2O} \rightleftharpoons \mathrm{HOCl} + \mathrm{H^+} + \mathrm{Cl^-}$$

Hypochlorous acid is a weak acid, and this single equilibrium governs the whole disinfection performance of the plant:

$$\mathrm{HOCl} \rightleftharpoons \mathrm{H^+} + \mathrm{OCl^-}, \qquad \mathrm{p}K_a = 7.54 \text{ at } 25\,{}^{\circ}\mathrm{C}$$

HOCl is the far stronger biocide — between forty and eighty times more effective than the hypochlorite ion, because the neutral molecule crosses the cell membrane while the anion is repelled by it. The practical consequence is best shown numerically.

Given. Free chlorine in water at 25 °C, $\mathrm{p}K_a = 7.54$, at pH 7.0 and at pH 8.0.

Find. The fraction of the free residual present as HOCl at each pH.

$$f_{\mathrm{HOCl}} = \frac{1}{1+10^{\,\mathrm{pH}-\mathrm{p}K_a}} \;\Rightarrow\; f(7.0)=\boxed{0.776}, \qquad f(8.0)=0.257$$

Holding the contact tank near pH 7 therefore delivers three times the active disinfectant of the same dose at pH 8, and this is precisely why the recarbonation step is placed before the contact tank rather than after it. Two further reaction sets complete the picture. Chloramine formation, deliberate here for the distribution residual, is

$$\mathrm{NH_3} + \mathrm{HOCl} \longrightarrow \mathrm{NH_2Cl} + \mathrm{H_2O}, \qquad \mathrm{NH_2Cl} + \mathrm{HOCl} \longrightarrow \mathrm{NHCl_2} + \mathrm{H_2O}$$

and if instead the ammonia is to be destroyed, chlorine is pushed past the breakpoint:

$$2\,\mathrm{NH_3} + 3\,\mathrm{Cl_2} \longrightarrow \mathrm{N_2} + 6\,\mathrm{HCl}$$

a stoichiometry of 7.6 mg of Cl₂ per mg of ammonia nitrogen, which is why breakpoint chlorination is expensive on a water carrying appreciable ammonia.

Question 1 — summary of the proposed design
Item	Answer
Liquid train	Intake and screens → aeration/pre-oxidation → rapid mix → flocculation → lime softening and sedimentation → recarbonation and filtration → GAC adsorption → chlorine contact and clearwell
Solids train	Backwash equalisation → gravity thickener → polymer conditioning → mechanical dewatering → cake to reuse or landfill; decant and filtrate returned ahead of the rapid mix
Turbidity and TSS barrier	Coagulation, flocculation, clarification, dual-media filtration
Hardness barrier	Lime–soda softening with recarbonation
Taste-and-odour barrier	Multi-level intake, KMnO₄ pre-oxidation, PAC, GAC contactor
Alkalinity destroyed by 30 mg/L alum	15.1 mg/L as CaCO₃ (of 260 available)
Chemical sludge from alum alone	7.87 mg/L as Al(OH)₃, about 157 kg/d
Lime for 180 mg/L carbonate hardness	133 mg/L as Ca(OH)₂
Free chlorine as HOCl	77.6 % at pH 7.0; 25.7 % at pH 8.0
Breakpoint chlorine demand	7.6 mg Cl₂ per mg NH₃-N

Topic. Integrated design of a conventional surface-water treatment plant: matching unit processes to raw-water problems, and closing the residuals loop.

Key relations. Alum stoichiometry, one mole consuming six equivalents of alkalinity; the lime–soda softening reactions; chlorine hydrolysis and the HOCl/OCl⁻ equilibrium with $\mathrm{p}K_a = 7.54$; the CT concept for primary disinfection.

Why this works. A treatment train is not a menu of processes but an ordered sequence in which each unit conditions the water for the next. Coagulation destabilises colloids so that flocculation can grow them, flocculation grows them so that gravity can remove them, clarification removes the bulk so that the filters are not overloaded, and the filters produce the low-turbidity water without which disinfection cannot be relied upon, since particles shield micro-organisms from the disinfectant. Placing the softening precipitation in the same basin as the coagulated turbidity exploits the fact that calcium carbonate crystals are dense and act as ballast, and recarbonation immediately afterwards protects everything downstream from scale. Returning the thickener decant ahead of the rapid mix rather than to the intake means the recycled load is coagulated once more instead of accumulating in a loop.

Common pitfall. Two errors recur. The first is pre-chlorinating a high-organic lake water in order to control taste and odour: chlorine reacts with natural organic matter to form trihalomethanes and haloacetic acids, so the odour problem is traded for a regulated by-product problem, and permanganate or ozone should be used instead. The second is treating the residuals as an afterthought — a softening plant generates a very large mass of sludge, and returning an unequalised, unthickened backwash stream to the head of the works simply recycles the solids the plant has just spent chemicals removing.

Source. Crittenden et al., MWH's Water Treatment: Principles and Design, 3rd ed., Ch. 9 (coagulation and flocculation), Ch. 13 (lime softening) and Ch. 13–14 (disinfection); Davis & Cornwell, Introduction to Environmental Engineering, 5th ed., Ch. 6.

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