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.
Given.
| Quantity | Symbol | Value |
|---|---|---|
| Flow rate | $Q$ | 20,000 m³/d |
| BOD₅ removed | $\Delta S$ | 120 mg/L |
| Net VSS production (from B2-I) | $P_x$ | 1,560 kg VSS/d |
| TKN in primary effluent | TKN | 25 mg/L |
| BOD₅/BODₖ ratio | $f$ | 0.68 (typical) |
Find. The actual oxygen requirement (AOR) and the corresponding air requirement per day.
Approach. Use the standard carbonaceous-plus-nitrogenous oxygen demand equation — carbonaceous demand from the BOD removed (scaled to ultimate BOD), less an oxygen credit for the fraction of that demand retained in new cell mass, plus the stoichiometric demand for nitrifying whatever ammonia is not itself used for cell synthesis — then convert the oxygen mass to an air volume via a transfer efficiency.
| Quantity | Value |
|---|---|
| Carbonaceous O₂ demand | 3,529 kg O₂/d |
| Credit for cell synthesis | 2,215 kg O₂/d |
| Nitrogenous O₂ demand | 1,429 kg O₂/d |
| Actual oxygen required (AOR) | 2,744 kg O₂/d |
| Air requirement | ≈123,200 m³/d (≈85.5 m³/min) |
A single-stage anaerobic digester is a single covered, heated, continuously (or semi-continuously) fed tank in which raw primary sludge and waste activated sludge are stabilized by anaerobic microorganisms in the absence of oxygen, in one combined vessel that provides both active digestion and, because it is not actively mixed at all times, some settling/thickening and gas separation in the same tank — unlike a two-stage system, where a first, heated/mixed tank does the active digestion and a second, unheated/unmixed tank handles supernatant separation and further storage. Digestion proceeds through three sequential microbial stages: hydrolysis (complex particulate organics broken into soluble sugars, amino acids, fatty acids), acidogenesis/acetogenesis (fermentation of those products into volatile fatty acids, then further to acetate, H₂ and CO₂), and methanogenesis (strictly anaerobic methanogens convert acetate and H₂/CO₂ into methane and carbon dioxide biogas, typically ≈60–70% CH₄). Because methanogens are slow-growing and highly sensitive to environmental upset, they are effectively the rate-limiting, most fragile step in the whole process and dictate the operating conditions the digester must maintain. As the figure shows, the tank naturally stratifies into a floating scum layer, a clarified supernatant layer (periodically drawn off and recycled to the head of the plant), the actively digesting sludge zone (heated and recirculated), and a stabilized sludge zone at the bottom that is periodically withdrawn for dewatering and disposal; the floating gas-holder cover rises and falls to accommodate varying biogas production without letting air infiltrate the anaerobic headspace.
Two key parameters defining digester efficiency. (1) Volatile solids (VS) reduction — the percentage decrease in volatile (organic) solids content between the raw feed sludge and the digested sludge, the direct measure of how much organic matter has actually been stabilized/destroyed (well-operated single-stage mesophilic digesters typically achieve 40–60% VS reduction). (2) Solids retention time / hydraulic retention time (SRT/HRT) — in a single-stage digester without recycle these are equal, and this is the time available for the slow methanogenic population to develop and stabilize the feed; a design/operating HRT below about 10–15 days risks washing out methanogens faster than they can reproduce, causing process failure (volatile acid accumulation, pH crash), while the typical design range for a heated, well-mixed single-stage mesophilic digester is roughly 15–30 days.
Operating parameters that dictate efficiency. Temperature must be held stable (mesophilic ≈35°C, or thermophilic ≈55°C) because methanogen growth rate is strongly temperature-dependent and swings of even a few degrees per day can upset the population balance. pH must stay near neutral (≈6.8–7.2), buffered by the system's own bicarbonate alkalinity, because methanogens are far more pH-sensitive than the acid-forming bacteria upstream — an overloaded digester where acid production outruns methane consumption drives pH down and can cascade into complete process failure ("sour" digester). Organic (volatile solids) loading rate must stay within the digester's assimilative capacity; overloading raises volatile-fatty-acid production faster than methanogens can consume it. Mixing (mechanical, gas recirculation, or pumped recirculation) keeps the feed in contact with the active biomass and prevents scum/grit accumulation and thermal stratification. Toxicity/inhibition (heavy metals, excess ammonia, sulfide) must be avoided, since methanogens are inhibited at much lower concentrations of many industrial contaminants than the acid formers. Together, these parameters explain why the two efficiency metrics above (VS reduction, SRT) are themselves outcomes of how well temperature, pH, loading and mixing are controlled.
