18-Env-A4 Water and Wastewater Engineering · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2014 — 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 (100 marks total); all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery; Davis & Cornwell, Introduction to Environmental Engineering; MWH's Water Treatment: Principles and Design; 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 facultative lagoon is a shallow (roughly 1.2–2.5 m) earthen pond that relies entirely on natural oxygenation: an upper aerobic zone kept oxygenated by photosynthetic algae during daylight and wind-driven surface reaeration, a lower anaerobic zone where settled solids undergo anaerobic digestion, and a middle facultative zone whose dissolved oxygen swings between the two. It needs no mechanical equipment, so capital and energy costs are minimal, but retention times run to weeks, the land footprint is large, and the effluent typically carries a high algal solids concentration that itself must be addressed. An aerated lagoon is deeper (2–6 m) and uses mechanical surface or diffused aerators to supply oxygen throughout the water column, allowing much higher organic loading and shorter retention (days, not weeks) on a smaller footprint; the trade-off is continuous energy input and O&M, and because biomass is not wasted/recycled the way it is in activated sludge, aerated lagoons are normally followed by a separate quiescent settling pond to clarify the effluent.
Orthophosphate ($\text{PO}_4^{3-}$, $\text{HPO}_4^{2-}$, $\text{H}_2\text{PO}_4^-$) is the dissolved, inorganic, immediately bioavailable form — the species algae and plants actually assimilate, and the direct driver of eutrophication; it is measured colorimetrically (molybdenum-blue method) without any pre-digestion. Polyphosphate is a condensed inorganic form built from chains of orthophosphate units linked by P–O–P bonds (common in detergents, corrosion inhibitors and some food additives); it is not directly bioavailable until it slowly hydrolyzes back to orthophosphate in the environment or during treatment. Organic phosphorus is phosphorus bound within biological molecules — nucleic acids, phospholipids, ATP — and is released as orthophosphate only when that organic matter is microbially decomposed (mineralized). Total phosphorus is approximately the sum of the three ($\text{TP}\approx\text{Ortho-P}+\text{Poly-P}+\text{Organic-P}$), and because only the ortho form is immediately reactive, a sample's ortho-to-total ratio indicates how much of its phosphorus load is available for algal growth right now versus released only gradually.
Ammonia-nitrogen ($\text{NH}_3\text{-N}/\text{NH}_4^+\text{-N}$) is the already-mineralized, reduced inorganic nitrogen present as free ammonia or ammonium, with the split governed by the pH- and temperature-dependent equilibrium $\text{NH}_3+\text{H}_2\text{O}\rightleftharpoons\text{NH}_4^++\text{OH}^-$. TKN (Total Kjeldahl Nitrogen) is measured by digesting the sample and equals organic nitrogen (protein-, urea- and amino-acid-bound N) plus ammonia-nitrogen — it does not capture nitrite or nitrate, the oxidized forms: $\text{TKN}=\text{Organic-N}+\text{NH}_3\text{-N}$. Total Nitrogen is the true sum of every species present, adding the oxidized forms back in: $\text{TN}=\text{TKN}+\text{NO}_2^-\text{-N}+\text{NO}_3^-\text{-N}$. The three are nested rather than independent: ammonia-N is a subset of TKN, and TKN is in turn a subset of TN, which is why raw-wastewater loading is usually characterized by TKN (dominated by organic and ammonia-N, essentially zero NOx before nitrification) while an effluent permit limit is more often written on Total Nitrogen, once nitrification has converted much of that ammonia to nitrate.
Hydraulic retention time (HRT) is the average time the liquid itself spends in a reactor, $\text{HRT}=V/Q$ (tank volume over flow rate) — a purely volumetric/flow quantity. Solids retention time (SRT), also called sludge age or mean cell residence time ($\theta_c$), is the average time the biomass (solids) itself spends in the system, $\text{SRT}=\dfrac{\text{mass of MLVSS in the system}}{\text{mass of MLVSS wasted per day}}$. In a simple lagoon with no solids recycle, SRT and HRT are essentially equal because the biomass leaves with the same water it grew in. Conventional activated sludge deliberately breaks that equality: return activated sludge (RAS) recycles settled biomass back to the aeration tank while the liquid continues on to the clarifier and out, so SRT can be held at 5–15+ days even though HRT in the aeration tank is only a few hours. This SRT/HRT decoupling is exactly what makes activated sludge far more compact than a lagoon for the same treatment performance, and SRT (not HRT) is the primary design/control lever for sludge age, degree of nitrification, and F/M ratio, while HRT mainly sets tank sizing for a given flow.
