18-Env-A4 Water and Wastewater Engineering · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2017 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved calculator permitted. Question 1 is compulsory; the paper instructs candidates to attempt any three of the remaining four questions — all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — oxygen sag/Streeter-Phelps, MLSS/MLVSS, population equivalent, primary clarifier design; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — turbidity, alkalinity chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — coagulation-flocculation, softening, disinfection by-products, pH; 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.
An aerated lagoon uses mechanical surface or diffused aerators to supply oxygen throughout the full water depth, keeping the entire basin aerobic; this lets the basin be sized much smaller (typical HRT 3–10 days) at the cost of continuous energy input for mechanical aeration and mixing. A facultative lagoon instead relies entirely on natural processes — atmospheric surface reaeration and algal photosynthesis in a shallow, sunlit upper aerobic zone, with a lower anaerobic zone where settled solids decompose — requiring no mechanical aeration but far more land and a much longer HRT (20–180 days) to achieve comparable BOD removal.
Orthophosphate ($PO_4^{3-}$ and its protonated forms) is the simple, fully hydrolyzed, directly bioavailable inorganic phosphate species measured colorimetrically without any digestion step. Polyphosphate is a condensed chain of two or more phosphate units linked by P–O–P bonds (common in synthetic detergents and some corrosion inhibitors) that slowly hydrolyzes back to orthophosphate in the environment or during wastewater treatment. Organic phosphorus is phosphorus chemically bound within biological molecules — DNA, phospholipids, ATP — released as orthophosphate only when that organic matter decomposes or is digested. Total phosphorus is the sum of all three; only orthophosphate is immediately available for algal uptake, which is why eutrophication assessments track it separately.
Total Nitrogen (TN) is the sum of every nitrogen form present — organic nitrogen, ammonia-nitrogen, nitrite-nitrogen and nitrate-nitrogen. Total Kjeldahl Nitrogen (TKN) is a laboratory-defined subset measured by the Kjeldahl digestion method: organic nitrogen plus ammonia-nitrogen only, explicitly excluding the already-oxidized nitrite and nitrate forms ($TKN = N_{org} + NH_3\text{-}N$). Ammonia-nitrogen ($NH_3\text{-}N$) is narrower still — the free ammonia/ammonium fraction alone, itself one component of TKN. In practice $TN = TKN + NO_2^{-}\text{-}N + NO_3^{-}\text{-}N$, and tracking the progression from TKN through nitrification to nitrate is exactly how a plant’s nitrogen-removal performance is monitored.
Hydraulic Retention Time (HRT), $HRT = V/Q$, is the average time a parcel of liquid spends in a reactor of volume $V$ at flow $Q$ — a purely hydraulic sizing parameter. Solids (or Sludge) Retention Time (SRT), also called mean cell residence time, $SRT = \dfrac{\text{mass of solids in the system}}{\text{mass of solids wasted per day}}$, is the average time biomass itself remains in the treatment system. In a conventional activated-sludge process with return activated sludge (RAS), SRT is deliberately decoupled from and made far longer than HRT (typically 5–20 days SRT vs. 4–8 hours HRT) because the biological solids are continuously recycled while the liquid passes through once; SRT is the parameter that actually controls biomass age and hence whether slow-growing nitrifiers can be retained, while HRT governs tank sizing and hydraulic capacity.
