18-Env-A4 Water and Wastewater Engineering · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2013 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved calculator permitted. The paper instructs candidates to attempt any two questions from Part A and any two from Part B (100 marks); all six are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — activated-sludge kinetics, nitrification, aeration, anaerobic digestion; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — discrete settling theory, indicator organisms, coagulation chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — process selection, softening, rapid sand filtration; 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.
Total Suspended Solids (TSS) is the mass of solid material retained on a standard glass-fibre filter after drying a known sample volume at 103–105°C. Firing that same dried residue in a muffle furnace at 550°C volatilizes (burns off) the organic fraction; the mass lost on ignition is the Volatile Suspended Solids (VSS), an approximation of the organic/biological portion of the solids, while the inorganic ash remaining behind is the Fixed Suspended Solids (FSS) — grit, silt and mineral matter that does not combust. By definition $\text{TSS}=\text{VSS}+\text{FSS}$. VSS is the more useful process-control number in biological treatment (it approximates active/degradable biomass, e.g. MLVSS), whereas FSS/TSS alone also carries inert mineral solids of no biological significance.
Chemical Oxygen Demand (COD) measures the total oxygen-equivalent of everything in a sample that can be chemically oxidized by a strong oxidant (typically potassium dichromate under acid reflux), including biodegradable AND non-biodegradable organic matter, in a lab procedure lasting a few hours. Biochemical Oxygen Demand (BOD) measures only the oxygen consumed by living microorganisms as they biologically degrade the biodegradable fraction of that organic matter, typically over a standardized 5-day incubation at 20°C (BOD₅). Because COD oxidizes both biodegradable and non-biodegradable material chemically while BOD only captures what organisms actually consume biologically, $\text{COD}\ge\text{BOD}_u$ (ultimate BOD) always, and the ratio COD/BOD₅ is a common indicator of a wastewater's biodegradability (a low ratio, near 1.5–2.5, indicates highly biodegradable domestic sewage; a high ratio indicates a large refractory/industrial fraction).
Orthophosphates ($\text{PO}_4^{3-}$, $\text{HPO}_4^{2-}$, $\text{H}_2\text{PO}_4^-$, depending on pH) are the simplest, fully soluble, immediately bioavailable form of phosphorus — directly usable by algae and bacteria without further breakdown, and the form measured directly by the standard colorimetric (molybdate blue) test. Polyphosphates are condensed chains/rings of phosphate units (commonly from synthetic detergents and some industrial/food-processing sources) that are not directly bioavailable until they hydrolyze (slowly, or via enzymatic/bacterial action) back to orthophosphate in the water or during wastewater treatment. Organic phosphates are phosphorus bound within organic molecules (as in living cells, decaying biomass, and biologically incorporated phosphorus in sludge) and become bioavailable only after microbial decomposition releases the phosphate group. Total phosphorus is the sum of all three fractions; distinguishing them matters for treatment design because only ortho-P is directly available for algal growth in a receiving water, while poly- and organic-P represent a "delayed-release" eutrophication load as they slowly convert to orthophosphate.
Return Activated Sludge (RAS) is the settled biomass withdrawn from the underflow of the secondary (final) clarifier and pumped back to the head of the aeration tank, to maintain the mixed-liquor suspended solids (MLSS) concentration needed to keep the biological process running at its design food-to-microorganism ratio. Waste Activated Sludge (WAS) is the portion of that same clarifier-underflow biomass that is instead deliberately removed from the system (wasted) rather than returned, in order to control the solids retention time (sludge age, SRT) of the process and to prevent the biomass inventory from growing without bound as new cells are continuously produced through substrate utilization. RAS and WAS are drawn from the identical physical stream (the clarifier underflow); the split between "how much goes back" (RAS) and "how much leaves the system" (WAS) is the primary operational lever by which a plant operator controls SRT and therefore effluent quality, sludge production and (as in Question B2/B3) whether the system achieves consistent nitrification.
