18-Env-A4 Water and Wastewater Engineering · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams / EGBC — May 2016 — 04-ENV-A4 Water and Wastewater Engineering. Three-hour exam; Question 1 is compulsory (25 marks) and any three of the remaining four questions are required (25 marks each); all five are solved below for completeness. Closed book, one double-sided aid sheet permitted, approved calculator permitted.
Reference texts: Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — nitrification, BOD test theory, alkalinity/anaerobic digestion, phosphorus removal, disinfection chemistry; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — the Streeter–Phelps oxygen sag, pH and coagulation–flocculation chemistry, water hardness; MWH's Water Treatment: Principles and Design (3rd ed.) — granular filtration (headloss, backwash) and ion exchange.
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.
Chemical phosphorus removal doses a metal salt (alum or $FeCl_3$) or lime to precipitate soluble orthophosphate as an insoluble metal-phosphate (or calcium-phosphate) floc, which is captured with the sludge; it is fast and reliable at any time but adds chemical cost and inert chemical sludge, and can be dosed at several points (pre-, co-, or post-precipitation relative to biological treatment). Biological phosphorus removal (EBPR) cycles mixed liquor through an anaerobic zone, where phosphorus-accumulating organisms (PAOs) release stored polyphosphate while storing incoming VFAs internally, followed by an aerobic zone, where they oxidize those reserves and take up phosphorus in excess of ordinary growth needs; the P leaves the system in the wasted biomass, avoiding chemical cost but requiring adequate influent VFA/COD and no nitrate carry-over into the anaerobic zone.
Ion exchange removes dissolved ionic contaminants by passing water through a bed of resin beads carrying a fixed, exchangeable counter-ion; as the target ion in solution contacts the resin it displaces the resin's own counter-ion in a stoichiometric, reversible reaction — for water softening, a cation-exchange resin in the sodium form swaps hardness cations for sodium, $\text{Resin-Na}_2+Ca^{2+}\rightleftharpoons\text{Resin-Ca}+2Na^+$ (and similarly for $Mg^{2+}$). The resin has a finite exchange capacity (meq of ion exchanged per litre or kg of resin); once exhausted (breakthrough, detected by rising hardness in the effluent) the bed is taken offline and regenerated by passing a concentrated solution of the original counter-ion (e.g. brine, NaCl, for a softener) through the bed in reverse, which drives the exchange equilibrium back and restores capacity, producing a waste brine that must be disposed of. Ion exchange is also used for nitrate removal, demineralization (with both cation- and anion-exchange resins in series) and removal of specific trace contaminants.
Discrete (Type I) settling occurs when particles are present at low enough concentration and are non-agglomerating, so each particle settles independently at a constant velocity governed by a force balance between gravity, buoyancy and drag — for small spherical particles in laminar (Stokes) flow, $v_s=\dfrac{g(\rho_s-\rho_w)d^2}{18\mu}$; grit chambers are designed on this basis, since grit particles are dense and non-flocculating. Flocculent (Type II) settling occurs when particles DO agglomerate as they settle — collisions during settling increase particle mass and hence settling velocity, so the removal efficiency for a given detention time cannot be predicted from a single particle-size analysis and instead requires a laboratory settling-column test, which measures percent removal at a series of depths and times and is used directly to size the full-scale basin (e.g. primary clarifiers treating raw, coagulated or biological suspensions).
Surface overflow rate (SOR, also called surface loading rate) is the clarified flow divided by the tank's surface area, $\text{SOR}=Q/A$ (typically $\text{m}^3/\text{m}^2\cdot\text{d}$); it is the hydraulic design parameter and, for an ideal (Type I) settling tank, represents the settling velocity a particle must exceed to be captured — any particle settling faster than the SOR is removed regardless of depth. Solids loading rate (SLR) is the total mass of solids applied per unit surface area per unit time, $\text{SLR}=Q(MLSS)/A$ (typically $\text{kg/m}^2\cdot\text{h}$), and governs the CLARIFIER'S ability to thicken and compact the settled sludge (and to avoid rising sludge or blanket washout) rather than just clarify the overflow. Secondary (activated-sludge) clarifiers must satisfy BOTH criteria simultaneously — SOR controls effluent clarity while SLR controls whether the high-MLSS underflow can actually be compacted and returned (RAS) without the sludge blanket rising into the effluent — which is why SLR, not SOR alone, is usually the governing design parameter for secondary clarifiers (unlike primary clarifiers, where SOR alone typically governs).
