18-Env-A4 Water and Wastewater Engineering · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2013 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved calculator permitted. The paper instructs candidates to attempt any two questions from Part A and any two from Part B (100 marks); all six are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — activated-sludge kinetics, nitrification, aeration, anaerobic digestion; 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 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.
(a) Chlorine demand. Chlorine demand is the portion of an applied chlorine dose that is consumed by reactions with reducing substances in the water and therefore does not remain as measurable residual: $\text{Demand}=\text{Dose}-\text{Residual}$. On the curve this is the vertical distance between the dashed "dose = residual" reference line (which represents zero demand, i.e. every bit of applied chlorine surviving as residual) and the actual measured curve at any given dose. In Zone I, essentially the entire dose is consumed almost immediately by fast-reacting reducing agents (dissolved Fe²⁺, Mn²⁺, H₂S, and readily oxidizable organics), so residual stays near zero and demand equals the whole dose applied.
(b) Formation of chloramines and organochlorines. Once the immediate reducing demand is satisfied, further chlorine reacts with ammonia present in the water (Zone II) to form chloramines in a 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), and potentially $\text{NHCl}_2+\text{HOCl}\rightarrow\text{NCl}_3+\text{H}_2\text{O}$ (trichloramine/nitrogen trichloride) as the Cl:N ratio rises — this "combined available chlorine" is measured as rising residual through Zone II and peaks near the top of Zone II/start of Zone III. Simultaneously, chlorine also reacts with natural organic matter (humic/fulvic substances) and any residual T&O precursors to form organochlorines, including regulated disinfection by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs); this reaction pathway runs in parallel with chloramine formation and continues at every dose level where free or combined chlorine and organic precursors coexist.
(c) Breakpoint chlorination. Continuing to add chlorine past the chloramine peak (Zone III) oxidizes the di- and tri-chloramines further, driving them to nitrogen gas, nitrate 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) — residual actually falls through this zone even as more chlorine is applied, because the added chlorine is being consumed destroying the chloramines already formed. The minimum point reached, where essentially all ammonia-derived combined chlorine has been oxidized away, is the breakpoint. Any dose applied beyond the breakpoint (Zone IV) is no longer consumed by ammonia (there is none left to react with) and appears directly as free available chlorine (HOCl/OCl⁻), so the curve rises again, this time on a slope close to parallel with the "dose = residual" reference. Operating just past the breakpoint is the standard practice for potable water disinfection because free chlorine is a far more effective (and faster-acting) disinfectant than the combined chlorine it replaces, for a given applied dose.
Natural colloidal turbidity particles (clays, silica, organic detritus) are stabilized in suspension primarily because they carry a net negative surface charge, which causes them to repel one another electrostatically (via their surrounding diffuse electric double layer) rather than colliding and agglomerating under Brownian motion. Coagulation must overcome this repulsion, and does so through two distinct mechanisms. Charge neutralization occurs when a coagulant contributes positively charged species (e.g. hydrolyzed aluminum or iron species from alum/ferric salts, or a cationic polymer) that adsorb directly onto the negative particle surface, reducing or reversing its net surface charge and thereby reducing the electrostatic repulsion between particles — this is the dominant mechanism for typical alum/polymer coagulation of natural waters. Ionic layer (double layer) compression instead works by increasing the ionic strength of the water (adding indifferent electrolyte, e.g. a simple salt), which compresses the diffuse counter-ion cloud surrounding each particle without necessarily changing its surface charge; a thinner double layer lets particles approach much closer before repulsion becomes significant, allowing van der Waals attraction to dominate at shorter range and permitting aggregation. In practice, hydrolyzing metal coagulants (alum, ferric chloride) act through a combination of both mechanisms plus a third (sweep-floc enmeshment at higher dose), whereas simple electrolyte addition alone illustrates pure double-layer compression.
Discrete (Type I) settling describes particles that settle as individual, non-interacting entities whose size, shape and density remain constant throughout the fall — each particle has one fixed terminal settling velocity (governed by Stokes' law for small, low-Reynolds-number particles), and the ideal-basin overflow-rate theory derived in Question A1(i) applies directly. It is representative of grit/sand removal and the settling of dilute, non-flocculating discrete solids. Flocculent (Type II) settling applies to particles that DO interact as they fall — typically the coagulated floc produced downstream of chemical coagulation — where collisions during settling cause smaller particles to agglomerate into larger ones, so settling velocity increases with depth and time of fall rather than staying constant. Because the settling velocity is not a single fixed value, flocculent settling cannot be sized from a single Stokes velocity; it is instead characterized empirically with a settling column test (percent removal measured at multiple depths and times) and design curves of overall removal vs. overflow rate and detention time, which is exactly the behaviour expected in the primary/secondary clarifiers downstream of the coagulation-flocculation train described in Question A2.
