16-Civ-B5 Water Supply and Wastewater Treatment · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 98-Civ-B5 Water Supply and Wastewater Engineering, National Examination, May 2014. Three hours; closed book with one aid sheet written on both sides; approved calculator permitted. Question 1 is compulsory and the candidate then attempts any three of Questions 2–5. Every question carries 25 marks, so the paper is marked out of 100. All five questions are solved below — the set is a study resource, not 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.
Deep-bed granular filtration is not a sieve. A rapid sand or dual-media bed has pore openings of the order of 50 to 150 micrometres, while the particles it must capture — floc fragments, clay, Cryptosporidium oocysts, bacteria — are one to two orders of magnitude smaller. Removal therefore happens inside the bed, on the surfaces of the grains, and is conventionally analysed in two stages: a transport stage that brings a particle from the bulk flow to within molecular reach of a grain, and an attachment stage that holds it there. The four key phenomena are three transport mechanisms and one attachment mechanism.
1. Straining and interception. Straining proper is the mechanical trapping of particles too large to pass a pore constriction; it acts mostly in the top few centimetres and is responsible for the surface mat that drives the early head-loss rise. Interception is its in-depth counterpart: a particle travelling along a streamline that passes within one particle radius of a grain surface touches the grain and is caught, without ever leaving its streamline. Interception dominates for the larger particles, roughly above one micrometre, and improves as the grain size falls.
2. Sedimentation. Particles denser than water settle across streamlines under gravity while they traverse a pore. Each pore behaves as a very small, very shallow settling basin with an enormous surface-area-to-volume ratio — a cubic metre of 0.5 mm sand offers thousands of square metres of grain surface — so residence times of a fraction of a second are enough. This mechanism, like interception, favours larger and heavier particles, and it is why filtration rate matters: doubling the approach velocity halves the time available for a particle to fall onto a grain.
3. Diffusion (Brownian motion). Particles below about one micrometre, and especially below 0.1 micrometre, are jostled off their streamlines by collisions with water molecules and wander onto grain surfaces. Diffusive transport increases as particle size falls, exactly opposite to interception and sedimentation. The two opposing trends produce the well-known minimum in filter efficiency at a particle size around one to two micrometres — unhelpfully, close to the size of the pathogens of greatest concern, which is why coagulation upstream matters so much.
4. Adsorption and attachment. Transport only delivers a particle to a grain; whether it stays is a surface-chemistry question. Natural particles and clean media both carry a negative surface charge, so an untreated suspension is electrostatically repelled and passes straight through no matter how good the transport. Once coagulation has neutralised that charge, short-range van der Waals attraction and, where a polymer is used, physical bridging hold the particle on the grain. This is the controlling step in practice: a properly coagulated water filters to below 0.1 NTU through the same bed that passes a raw water almost untouched.
Two further phenomena are worth naming as secondary contributors. Flocculation within the pores continues to build particle size as velocity gradients bring captured and travelling particles into contact, and biological activity — the schmutzdecke of a slow sand filter, or the biofilm on a biologically active carbon cap — adds straining and metabolic removal of dissolved organics.
Free residual chlorine is the fraction of the residual present as hypochlorous acid, hypochlorite ion and dissolved molecular chlorine. When chlorine gas or hypochlorite is dosed into water it hydrolyses essentially completely:
$$\begin{aligned}\mathrm{Cl_2} + \mathrm{H_2O} &\;\longrightarrow\; \mathrm{HOCl} + \mathrm{H^+} + \mathrm{Cl^-}\\ \mathrm{HOCl} &\rightleftharpoons \mathrm{H^+} + \mathrm{OCl^-}, \qquad \mathrm{p}K_a = 7.54\end{aligned}$$Combined residual chlorine is the fraction bound to nitrogen as chloramines, formed when hypochlorous acid meets ammonia. The three species form in sequence as the chlorine-to-nitrogen ratio and the pH rise:
$$\begin{aligned}\mathrm{NH_3} + \mathrm{HOCl} &\rightarrow \mathrm{NH_2Cl} + \mathrm{H_2O} &&\text{(monochloramine)}\\ \mathrm{NH_2Cl} + \mathrm{HOCl} &\rightarrow \mathrm{NHCl_2} + \mathrm{H_2O} &&\text{(dichloramine)}\\ \mathrm{NHCl_2} + \mathrm{HOCl} &\rightarrow \mathrm{NCl_3} + \mathrm{H_2O} &&\text{(nitrogen trichloride)}\end{aligned}$$Monochloramine dominates above pH 7 and is the species deliberately produced in chloramination; dichloramine appears in acidic waters and nitrogen trichloride only at high dose and low pH, both of them objectionable in taste and odour. The two residuals differ on five counts that matter operationally.
| Attribute | Free residual | Combined residual |
|---|---|---|
| Species | $\mathrm{HOCl}$, $\mathrm{OCl^-}$, $\mathrm{Cl_2(aq)}$ | $\mathrm{NH_2Cl}$, $\mathrm{NHCl_2}$, $\mathrm{NCl_3}$ |
| Germicidal strength | Strong; $\mathrm{HOCl}$ is roughly 80 times more potent than $\mathrm{OCl^-}$ | Weak; monochloramine needs of the order of 25 times the $CT$ of free chlorine for equal inactivation |
| Persistence in the mains | Decays quickly, hours to a day | Very stable, days to weeks — the reason it is used as a secondary disinfectant |
| By-products | Trihalomethanes and haloacetic acids with natural organic matter | Far fewer THMs and HAAs, but nitrosamines including NDMA, plus a nitrification risk in the distribution system |
| Measurement (DPD method) | Read immediately — only free chlorine has reacted | Total minus free, after potassium iodide addition releases the bound chlorine |
The practical division of labour follows from the table: free chlorine is used inside the plant, in a baffled contact tank, to earn the $CT$ credit for primary disinfection, and the residual is then converted to monochloramine by adding ammonia before the water enters distribution, where a long-lived residual matters more than a strong one. Note that one milligram of monochloramine carries $70.9/51.5 = 1.38$ mg of oxidising chlorine, which is why combined residual is still reported as mg/L as $\mathrm{Cl_2}$.
Given. A water carrying ammonia nitrogen and a background of reducing substances, to which chlorine is dosed in progressively larger amounts. The illustrative water below has a total ammonia nitrogen of 2.0 mg/L as N and an immediate chlorine demand of 2.0 mg/L from iron, manganese, sulphide and organic matter.
Find. The shape of the residual-versus-dose curve, the stoichiometry that fixes its two turning points, and why the process is carried out.
Zone 1 — immediate demand. The first chlorine added is consumed by fast reductants: ferrous iron, manganese(II), sulphide, nitrite and readily oxidisable organic matter. No measurable residual of any kind appears until this demand is satisfied, here at a dose of 2.0 mg/L.
Zone 2 — chloramine formation. Further chlorine reacts with ammonia one mole for one mole to form monochloramine, so the total (all combined) residual rises almost linearly with dose. The peak is reached when all the ammonia has been converted, at a mass ratio of
$$\frac{\mathrm{Cl_2}}{\mathrm{N}} \;=\; \frac{70.9}{14.0} \;=\; 5.06 \;\approx\; 5{:}1$$For 2.0 mg/L as N that is $5.06 \times 2.0 = 10.1$ mg/L of chlorine beyond the immediate demand, giving the peak at an applied dose of about 12.1 mg/L.
Zone 3 — chloramine destruction. Past the peak the added chlorine oxidises the chloramines already formed rather than making more, and the nitrogen is stripped out as nitrogen gas. The residual therefore falls as the dose rises, which is the counter-intuitive behaviour that makes this curve worth knowing. The governing overall reaction is
$$2\,\mathrm{NH_3} + 3\,\mathrm{HOCl} \;\longrightarrow\; \mathrm{N_2}\!\uparrow\; +\; 3\,\mathrm{H_2O} \;+\; 3\,\mathrm{HCl}$$which sets the theoretical break-point ratio at
$$\frac{\mathrm{Cl_2}}{\mathrm{N}} \;=\; \frac{3 \times 70.9}{2 \times 14.0} \;=\; \boxed{7.59 \approx 7.6{:}1 \ \text{by mass}}$$Zone 4 — free residual. At the break-point the combined residual reaches its minimum and only trace chloramines remain. Every milligram of chlorine added beyond this point survives as free residual, so the curve turns and rises at unit slope. In practice competing side reactions with organic nitrogen push the working break-point to a ratio of 8:1 to 10:1; for the illustrative water, 7.6:1 predicts $7.59 \times 2.0 = 15.2$ mg/L above the immediate demand, while a design allowance at 10:1 would call for 20 mg/L. This is why a break-point installation is sized on a bench-scale demand curve run on the actual water, never on stoichiometry alone.
Why it is done. Break-point chlorination is used to guarantee a free chlorine residual in a water that contains ammonia, to remove ammonia itself (a polishing step in some water and wastewater plants), and to destroy taste-and-odour compounds and chloramine odours that persist below the break-point. Its costs are a large chlorine dose, a substantial alkalinity consumption and acid production, elevated chloride, and increased formation of trihalomethanes once free chlorine appears — so it is a deliberate choice, not a default.
