18-Env-A4 Water and Wastewater Engineering · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Examination, December 2019 — 18-Env-A4 Water and Wastewater Engineering (3 hours). Question 1 is compulsory; this study resource answers all five questions in full. Reference texts: Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); MWH’s Water Treatment: Principles and Design (3rd ed.); Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Guidelines for Canadian Drinking Water Quality (GCDWQ).
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.
Grit is the heavy, inert inorganic fraction of the solids carried in a sewer — sand, gravel, cinders, coffee grounds, and metal filings, with a specific gravity around 2.65 — as distinct from the much lighter, putrescible organic solids (specific gravity roughly 1.02–1.2) that make up most of the raw wastewater solids load. Grit enters primarily through stormwater inflow/infiltration in combined and separate sewers. Grit removal is a dedicated unit process, placed immediately after coarse screening and ahead of primary clarification, whose entire purpose is to settle out this heavy inert fraction while keeping the light organic solids in suspension for later removal in the primary clarifier. It is designed on a settling-velocity/particle-size basis (typically targeting removal of particles ≥ 0.2 mm at the design peak flow) rather than a simple detention-time basis, because grit is defined operationally by how fast it settles, not by what it is made of. Design forms include horizontal-flow (velocity-controlled) channels, aerated grit chambers (a diffused-air spiral roll gives adjustable, flow-independent grit capture), and vortex-type units. Removing grit protects downstream mechanical equipment (pumps, scraper mechanisms, centrifuges) from abrasive wear, prevents grit deposition and blockage in pipes and channels, and keeps inert material out of the digesters, where it would otherwise occupy volume without contributing any volatile solids destruction or biogas.
When an oxygen-demanding effluent (BOD load) enters a receiving stream, two processes act on the stream’s dissolved oxygen (DO) simultaneously and in opposite directions: deoxygenation, as bacteria oxidize the organic (BOD) load at first-order rate $k_d$, and reaeration, as the stream absorbs oxygen from the atmosphere at first-order rate $k_r$. The Streeter–Phelps model tracks the resulting oxygen deficit $D=DO_{sat}-DO$ as a function of travel time $t$ downstream of the outfall:
Immediately downstream of the outfall, the BOD load is still largely unoxidized, so deoxygenation outpaces reaeration and DO falls — the deoxygenation (degradation) zone. As the BOD is progressively consumed, the deoxygenation rate itself falls (it is proportional to the remaining BOD), while the reaeration rate, driven by the deficit itself, keeps rising; the two rates become equal at the critical point, the point of minimum dissolved oxygen $DO_c$ at travel time $t_c$ — the single most important point on the curve, since it is where a fish-kill or a violation of a minimum-DO water-quality standard is most likely to occur. Beyond $t_c$, reaeration exceeds the (now much reduced) deoxygenation demand and DO recovers back toward saturation — the recovery zone. The curve is the standard tool for setting an effluent BOD (and ammonia, which exerts its own nitrogenous oxygen demand) discharge limit: a permit writer computes $DO_c$ for a proposed effluent load at critical (low-flow, high-temperature) stream conditions and checks it against the applicable minimum-DO water-quality objective before the discharge is approved.
An indicator organism is a microorganism whose presence, absence, or concentration in a water sample is used as a proxy for the likely presence of pathogens, rather than testing directly for every pathogen of concern — a practical necessity, since waterborne pathogens (bacterial, viral, and protozoan) are numerous, are frequently present at very low and hard-to-culture concentrations, and some (viruses, Giardia, Cryptosporidium) are slow, difficult, and expensive to assay directly. A useful indicator organism should originate from the same source as the pathogens of concern (the intestinal tract of warm-blooded animals, for waterborne enteric disease), occur in numbers roughly proportional to the degree of fecal contamination, survive in the environment at least as long as the pathogens without regrowing, and be detectable by a simple, rapid, inexpensive, reliable test, while itself being non-pathogenic and safe to handle. Total coliforms, fecal coliforms, and particularly Escherichia coli (fecal-specific) and enterococci (favoured for marine/recreational waters) are the standard bacterial indicators used in drinking-water and receiving-water monitoring; their presence signals fecal contamination and therefore the plausible presence of the actual disease-causing organisms, even though the indicator itself is essentially harmless.
Hydraulic retention time (HRT, $\theta$) is the average time a parcel of liquid spends in a tank or reactor, $\theta=V/Q$ (tank volume divided by the influent flow rate). Solids retention time (SRT, $\theta_c$, also called sludge age) is the average time a unit of biomass spends in the biological process before being wasted from the system, $\theta_c\approx\dfrac{V\,X}{Q_w X_w+Q_e X_e}$ — the mass of solids held in the system divided by the mass of solids wasted (in the waste-activated-sludge stream plus any solids escaping in the effluent) per day. In a conventional activated-sludge plant with return activated sludge (RAS), SRT and HRT are decoupled: settled biomass is continuously recycled from the secondary clarifier back to the aeration tank rather than flowing straight through with the liquid, so $\theta_c$ is normally days while $\theta$ is normally only a few hours. This decoupling is the entire point of the RAS/WAS arrangement — a long SRT sustains a large, slow-growing biomass population (essential for nitrifiers, whose growth rate is far slower than heterotrophs) without needing an impractically large tank, while the short HRT keeps the tank volume, and therefore capital cost, economical.
Standard $BOD_5$ measures the total oxygen consumed by a sample over 5 days of incubation at $20\,{}^{\circ}\text{C}$; if nitrifying bacteria are present in (or seeded into) the sample, that consumption includes both carbonaceous (organic-carbon) oxidation and nitrogenous oxidation (ammonia oxidized through nitrite to nitrate). $cBOD_5$ is the identical 5-day test run with a nitrification inhibitor added (commonly TCMP) to suppress the nitrifying organisms, so the result isolates the carbonaceous oxygen demand alone. $cBOD_5$ is the parameter almost always specified in modern effluent permits, because nitrogenous oxygen demand is normally regulated and tracked separately through an explicit ammonia-nitrogen limit; folding it back into an uninhibited $BOD_5$ result would double-count the same nitrogen load under two permit parameters, and would also make trends in organic loading harder to read, since nitrification onset in an uninhibited bottle test is erratic and strongly age/temperature dependent.
