18-Env-A4 Water and Wastewater Engineering · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved calculator permitted. The paper instructs that Question 1 is compulsory and any three of the remaining four questions are required; all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — activated-sludge kinetics, solids/hydraulic retention time, nitrogen and phosphorus forms; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — discrete settling theory, indicator organisms, coagulation chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — process selection, softening, rapid and slow sand filtration; Guidelines for Canadian Drinking Water Quality (Health Canada).
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.
Chlorine demand. Demand is the share of an applied dose consumed by reactions with reducing substances in the water and therefore not left over as measurable residual: $\text{Demand}=\text{Dose}-\text{Residual}$. Graphically it is the vertical gap between the dashed dose = residual reference (zero demand) and the actual measured curve. In Zone I nearly the whole dose is consumed almost instantly by fast-reacting reducing agents (dissolved $\text{Fe}^{2+}$, $\text{Mn}^{2+}$, $\text{H}_2\text{S}$, readily oxidizable organics), so residual stays near zero and demand equals essentially the whole dose applied.
Formation of chloramines and organochlorines. Once the immediate reducing demand is satisfied, further chlorine reacts with ammonia (Zone II) in stepwise substitution — $\text{NH}_3+\text{HOCl}\rightarrow\text{NH}_2\text{Cl}+\text{H}_2\text{O}$ (monochloramine), then $\text{NH}_2\text{Cl}+\text{HOCl}\rightarrow\text{NHCl}_2+\text{H}_2\text{O}$ (dichloramine) — producing "combined available chlorine" that shows up as rising residual through Zone II. In parallel, chlorine also reacts with natural organic matter and any surviving T&O precursors to form organochlorines, including regulated disinfection by-products such as trihalomethanes and haloacetic acids; this pathway runs alongside chloramine formation at every dose level where chlorine and organic precursors coexist.
Breakpoint chlorination. Continuing to add chlorine past the chloramine peak (Zone III) oxidizes the chloramines further toward nitrogen gas and other end products ($2\text{NH}_2\text{Cl}+\text{HOCl}\rightarrow\text{N}_2\!\uparrow+3\text{HCl}+\text{H}_2\text{O}$, approximately), so measured residual actually falls even as more chlorine is applied — the added chlorine is being spent destroying chloramines already formed. The minimum reached, once essentially all ammonia-derived combined chlorine is oxidized away, is the breakpoint. Any dose beyond it (Zone IV) has no ammonia left to react with and appears directly as free available chlorine ($\text{HOCl}/\text{OCl}^-$), so the curve rises again on a slope close to parallel with the reference line. Dosing just past the breakpoint is standard potable-water practice because free chlorine is a far faster, more effective disinfectant per unit dose than the combined chlorine it replaces.
Natural colloidal turbidity (clays, silica, organic detritus) stays suspended largely because particles carry a net negative surface charge and repel one another through their surrounding diffuse ionic double layer rather than colliding and agglomerating under Brownian motion. Charge neutralization occurs when the coagulant supplies positively charged species (hydrolyzed aluminum/iron from alum or ferric salts, or a cationic polymer) that adsorb onto the negative particle surface, reducing or reversing its net charge and so reducing inter-particle repulsion — the dominant mechanism for typical alum coagulation of natural waters. Ionic layer (double layer) compression instead raises the water's ionic strength (adding an indifferent electrolyte), compressing the diffuse counter-ion cloud without necessarily changing the particle's surface charge; a thinner double layer lets particles approach closer before repulsion dominates, letting short-range van der Waals attraction take over. Hydrolyzing metal coagulants act through a mix of both mechanisms (plus sweep-floc enmeshment at higher dose), while simple electrolyte addition illustrates pure double-layer compression on its own.
The schmutzdecke ("dirty skin") is the thin biologically active mat of algae, bacteria, protozoa and trapped particulate matter that develops on the surface of a slow sand bed, where it performs most of the actual treatment — straining plus genuine biological degradation of organics and pathogens — at the very low loading rates (∼0.1–0.4 m/h) and long filter runs (weeks to months) characteristic of slow sand filtration.
Rapid sand filters, by contrast, operate at loading rates one to two orders of magnitude higher (∼120–200 m/h) and are hydraulically backwashed every 24–72 hours, which strips the bed and prevents a mature, stable schmutzdecke from ever establishing. Rapid filtration therefore relies primarily on physical/chemical removal mechanisms distributed through the depth of the bed — straining, sedimentation, interception and adsorption of coagulated floc onto the media grains — not on a biologically active surface layer. What rapid filters do exhibit is a short filter-ripening period immediately after each backwash, during which effluent turbidity is briefly elevated until a thin coating of previously-removed floc re-establishes on the media surface and improves capture efficiency; this ripening coating is sometimes loosely described as analogous to a schmutzdecke, but it is a transient physical floc layer, not the stable, weeks-old biological mat that gives slow sand filtration its name. A candidate answer should state plainly that a true schmutzdecke does not persist in rapid sand filtration and explain why (loading rate and backwash frequency), rather than assuming the term transfers unchanged.
