18-Env-A4 Water and Wastewater Engineering · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams / EGBC — December 2016 — 04-ENV-A4 Water and Wastewater Engineering. Three-hour exam; Question 1 is compulsory (25 marks) and any three of the remaining four questions are required (25 marks each); all five are solved below for completeness. Closed book, one double-sided aid sheet permitted, approved calculator permitted.
Reference texts: Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — grit removal, the BOD test and azide modification, nitrogen speciation and removal, anaerobic digestion, sludge volume index; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — the Streeter–Phelps oxygen sag, indicator organisms, water intake structures, distribution-system layout; MWH's Water Treatment: Principles and Design (3rd ed.) — ion exchange, fluoridation/defluoridation.
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 adequate water distribution system must: deliver water at sufficient pressure (typically 275–550 kPa at the service connection, with a code-minimum floor even at peak/fire-flow demand) and quantity to meet the maximum-day plus peak-hour demand, and separately meet fire-flow requirements at an acceptable residual pressure; maintain water quality in transit (adequate disinfectant residual, no stagnation, no cross-connection/backflow risk); minimize water loss through leakage; be looped/interconnected wherever practical for reliability; be sized and valved so sections can be isolated for repair without shutting down large areas; and be economically constructed and operated over its design life.
A grid-iron (looped) system interconnects mains in a rectangular or radial grid so every point can normally be supplied from more than one direction. Advantages: no dead ends (water is always moving, so stagnation, taste/odour and disinfectant-residual decay are minimized), reliable service during a main break or repair (the isolated section is bypassed by the loop), and better fire flows (demand is met from two directions, reducing head loss and pressure drop at the hydrant). Disadvantages: higher capital cost (more pipe, more valves and connections) and more complex hydraulic design/analysis (a looped network needs iterative solution, e.g. Hardy-Cross, rather than a simple series calculation). A dead-end system instead branches out from the source with no loop closure. Advantages: lower capital cost and simpler design/analysis (straightforward branch-by-branch pipe sizing). Disadvantages: water at the dead-end tips stagnates (poor taste/odour, disinfectant decay, sediment accumulation, and the classic operational fix of periodic flushing), a break isolates every downstream customer with no alternate supply path, and fire flow at a dead end is limited to what that single branch can deliver.
Anaerobic digestion stabilizes primary and secondary (waste-activated) sludge in an oxygen-free reactor through a sequential microbial food chain of three stages. In hydrolysis, extracellular enzymes break complex particulate organics (proteins, lipids, carbohydrates/cellulose) into soluble monomers (amino acids, fatty acids, sugars) — usually the rate-limiting step for raw sludge. In acidogenesis/acetogenesis, fermentative and acetogenic bacteria convert those monomers into volatile fatty acids (VFAs), hydrogen and $CO_2$, and ultimately into acetate. In methanogenesis, strictly anaerobic archaea convert acetate ($CH_3COOH\rightarrow CH_4+CO_2$, roughly two-thirds of the methane produced) and $H_2/CO_2$ ($CO_2+4H_2\rightarrow CH_4+2H_2O$, the remaining third) into biogas, typically 60–65% $CH_4$ and 35–40% $CO_2$. Methanogens are the slowest-growing, most sensitive organisms in the chain, so digester stability is governed by keeping conditions within their narrow tolerance.
The key operating parameters follow directly from that sensitivity. Temperature is held steady either in the mesophilic range (30–38 °C, most common) or thermophilic range (50–57 °C, faster kinetics and better pathogen kill but less stable and more energy-intensive); methanogens tolerate only small, slow fluctuations. Solids/hydraulic retention time (typically 15–30 days mesophilic for a conventional completely-mixed digester, SRT=HRT without recycle) must be long enough for the slow-growing methanogens to be retained faster than they wash out. pH is maintained near neutral (6.8–7.2) — methanogenesis is severely inhibited below about pH 6.2 — buffered by the digester's own bicarbonate alkalinity, so the volatile-acids-to-alkalinity ratio (ideally $\lesssim0.3$–0.4) is tracked as the standard early-warning indicator of an overloaded/souring digester. Mixing keeps feed, biomass and heat uniformly distributed and prevents scum/grit buildup; the vessel must stay strictly anaerobic (oxygen intrusion inhibits or kills methanogens) and free of toxic loads (elevated ammonia, sulfide, heavy metals).
