18-Env-A4 Water and Wastewater Engineering · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams / EGBC — May 2015 — 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.) — nitrogen and solids characterization, BOD test theory, nitrification/alkalinity, disinfection chemistry, activated-sludge and sludge-processing design; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — the Streeter–Phelps oxygen sag, pH and coagulation–flocculation chemistry, jar testing; MWH's Water Treatment: Principles and Design (3rd ed.) — granular filtration (headloss, backwash) and chemical phosphorus removal.
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.
The jar test is a bench-scale simulation of the plant's coagulation–flocculation–sedimentation train, run in parallel beakers ("jars") dosed with a range of coagulant concentrations (and sometimes pH values) using a multi-paddle stirrer. Each jar undergoes a rapid mix (high shear, ~1–2 min) to disperse the coagulant and destabilize colloids, then a slow mix (gentle, ~15–30 min) to promote floc growth, followed by a quiescent settling period. The jar giving the fastest, largest, cleanest-settling floc and the lowest residual turbidity/colour identifies the optimum coagulant dose (and pH) to carry forward to full-scale dosing, avoiding both under-dosing (poor removal) and over-dosing (wasted chemical, re-stabilized colloids, elevated sludge production).
Headloss is the pressure (energy) drop across the filter media as water is forced through it; it starts near the clean-bed value and rises steadily through a filter run as captured particles accumulate in the pore spaces and constrict the flow paths, until a terminal headloss (set by the available driving head or a turbidity breakthrough) triggers the end of the run. Schmutzdecke ("dirty skin") is the thin biologically active mat that develops on top of a slow sand filter, made of algae, bacteria, and trapped particulate matter; it is the primary treatment mechanism in slow sand filtration, removing organics and pathogens by both biological action and fine physical straining (rapid sand filters, by contrast, rely on depth filtration/interception and have no equivalent layer). Backwash is the periodic reversal of flow through a (rapid) filter at a high enough rate to fluidize/expand the media bed (often combined with an air scour), which lifts and shears off the accumulated floc and solids so they can be flushed out in the wash-water, restoring the bed's headloss and capacity before the next filter run.
Discrete (Type I) settling describes particles that settle independently, without changing size, shape, or density as they fall — each particle reaches its own terminal settling velocity governed by Stokes' law, and the removal efficiency of a settling basin depends only on the surface overflow rate versus that velocity. It applies to dilute suspensions of granular, non-flocculating solids, such as grit chambers and pre-sedimentation basins. Flocculent (Type II) settling describes particles that agglomerate/coalesce with each other as they settle, growing in size (and settling velocity) with depth and time — typical of the dilute chemically coagulated suspension in a primary clarifier receiving coagulant, or a dilute activated-sludge mixed liquor. Because the settling velocity is not constant, flocculent settling cannot be described by a single Stokes' law calculation; instead it is characterized empirically with a column settling test that reports percent removal as a function of both depth and detention time, and basin design uses those removal curves rather than a single computed velocity.
Hydraulic Retention Time (HRT) is the average time the liquid (and anything moving with it) spends in a reactor, $\text{HRT}=V/Q$, set purely by the tank volume and the flow rate through it. Solids (Sludge) Retention Time (SRT), or sludge age, is the average time the biological solids (the microorganisms) spend in the system, $\text{SRT}=\dfrac{V\,X}{Q_wX_w+Q_eX_e}$ (mass of solids in the system divided by the mass wasted per day). In a suspended-growth system with solids recycle (activated sludge), returning settled biomass from the secondary clarifier back to the aeration basin lets SRT be decoupled from and made much longer than HRT — this is essential because slow-growing organisms such as nitrifiers need a minimum SRT (often several days, longer in cold weather, per Q2a) far in excess of the few hours of HRT that would otherwise be needed just to hold the liquid. This decoupling is what allows the compact, short-HRT activated-sludge process to still achieve nitrification and good effluent quality, unlike a once-through (no-recycle) system where SRT and HRT are forced to be equal.
