18-Env-A4 Water and Wastewater Engineering · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 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 (5th ed.) — BOD kinetics, activated-sludge clarifier design, anaerobic digestion; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — hardness, alkalinity, chlorination chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — 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.
A single-stage anaerobic digester is a heated, mixed, air-tight tank that stabilizes primary and/or waste-activated sludge in the absence of oxygen. Stabilization proceeds through three linked microbial stages: (1) hydrolysis, where extracellular enzymes break complex particulate organics (proteins, lipids, carbohydrates) into soluble monomers; (2) acidogenesis/acetogenesis, where fermentative bacteria convert those monomers to volatile fatty acids, hydrogen and acetate; and (3) methanogenesis, where strictly anaerobic methanogens convert acetate and H₂/CO₂ into methane and carbon dioxide (biogas). Because methanogens are slow-growing and highly sensitive to pH and toxicity, the whole process is normally the rate-limiting, and hence design-governing, step.
Key components: a digester tank (cylindrical or egg-shaped, with a conical/hoppered base for sludge withdrawal); a floating or fixed gas-holder cover that seals the tank from air and stores biogas at a small positive pressure; a heating system (hot-water coils or an external heat exchanger recirculating sludge) that holds the contents at a controlled temperature — typically mesophilic, ≈35°C, or thermophilic, ≈55°C; a mixing system (mechanical draft-tube mixer, recirculated-sludge pumping, or gas recirculation) that keeps the tank contents homogeneous, prevents scum and grit accumulation, and distributes heat and feed evenly; and supernatant/sludge draw-off piping at different elevations, since in a single-stage (unmixed-zone) digester the tank still stratifies into a scum layer, a clarified supernatant zone, an actively digesting sludge zone, and settled stabilized solids at the bottom. In operation, raw sludge is fed continuously or semi-continuously, an equal volume of well-digested sludge (and/or clarified supernatant) is withdrawn to hold a target solids retention time (typically 15–30 days for mesophilic single-stage digestion), and biogas (60–65% CH₄, balance mostly CO₂) is collected for use as plant fuel or flared.
Parameters governing/indicating VSS destruction efficiency: solids (hydraulic) retention time — longer SRT gives the slow-growing methanogens more time to act, up to a point of diminishing returns (typically 15–30 days mesophilic); temperature — must be held steady in the mesophilic (≈35°C) or thermophilic (≈55°C) range, since methanogens are highly temperature-sensitive and even a few degrees of swing per day measurably depresses activity; volatile solids loading rate (kg VS fed per m³ of digester volume per day) — overloading outpaces the methanogens' capacity and drives VS destruction down; mixing — ensures feed contacts active biomass and prevents localized souring or dead zones; and pH/alkalinity — methanogenesis is favoured near pH 6.8–7.4, and adequate buffering (bicarbonate alkalinity, typically 2000–3500 mg/L as CaCO₃) is needed to absorb the volatile-fatty-acid production from acidogenesis without the pH crashing. Percent VSS destruction is itself normally reported via the Van Kleeck formula, $\%\,\text{VS destroyed}=100\times\dfrac{\text{VS}_{in}-\text{VS}_{out}}{\text{VS}_{in}(1-\text{VS}_{out})}$ (VS fractions of TS in and out), which corrects for the concentrating effect of destroying volatile solids while inert (fixed) solids pass through unchanged.
Indications of operational instability almost all trace back to methanogens (slow-growing, pH-sensitive) falling behind acid-forming bacteria (fast-growing, more robust): a falling pH and rising volatile fatty acid (VFA) concentration as acids accumulate faster than methanogens can consume them; a declining VFA-to-alkalinity ratio problem in reverse — i.e. a RISING VFA/alkalinity ratio (a ratio above roughly 0.3–0.4 is a widely used early-warning threshold) signalling the buffer reserve is being consumed; declining or erratic biogas production combined with a falling methane content (and correspondingly rising CO₂ fraction) of the gas, since methanogenesis is the step actually consuming the intermediate acids to make CH₄; foaming or excessive scum buildup; and a visible increase in supernatant/effluent VS and COD as incompletely digested organics pass through. Root causes commonly include organic/hydraulic overloading, a toxic shock (heavy metals, excess ammonia from nitrogen-rich feed, or a sudden influx of industrial waste), or a temperature upset from a failed heating system — the operational response is to cut feed rate, restore temperature, and, if VFA/alkalinity has crashed the pH, add alkalinity (e.g. lime or sodium bicarbonate) while the biomass recovers.
