18-Env-A4 Water and Wastewater Engineering · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2018 — 18-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved Casio/Sharp calculator permitted. Question 1 is compulsory; the paper instructs candidates to attempt any three of the remaining four questions — all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — trickling filters, activated-sludge SRT/yield design, nitrogen speciation; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — discrete particle settling theory, water-quality parameters; MWH’s Water Treatment: Principles and Design (3rd ed.) — coagulation-flocculation, adsorption, chlorine chemistry, water treatment plant process design; Guidelines for Canadian Drinking Water Quality (Health Canada/GCDWQ) — sulfate, nitrate and chloride aesthetic/health-based limits.
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.
| Parameter | Range | Treatment implication |
|---|---|---|
| TSS | 100–500 mg/L | Moderate-to-high; needs coagulation/sedimentation ahead of filtration |
| Turbidity | 30–100 NTU | Consistent with conventional (not direct) filtration train |
| Hardness (as CaCO₃) | 50–100 mg/L | Soft to moderately hard (Sawyer scale, 0–60 soft / 61–120 mod. hard) — below the threshold where dedicated softening is normally justified |
| Taste & odour | Seasonal | Needs an intermittent PAC (or oxidant) dosing point, not a permanent GAC contactor |
| pH | 6.5–7.0 | Slightly acidic of the 7.0–7.5 optimum coagulation/corrosion-control band; needs alkalinity/pH adjustment |
Find. A labelled process schematic (unit processes, streams, chemical injection points) for a conventional surface-water treatment plant sized to this raw water.
Approach. Select a conventional coagulation–flocculation–sedimentation–filtration–disinfection train (the TSS/turbidity range is squarely in conventional-filtration territory, not low enough for direct filtration nor high enough to force presedimentation), add the two chemical-injection points the raw water quality specifically calls for (PAC ahead of coagulation for seasonal T&O, and a pH/alkalinity adjustment point), and route the two waste streams (clarifier sludge, filter backwash) to solids handling.
Each unit process and injection point above is chosen directly from the raw-water characteristics: (1) the 100–500 mg/L TSS and 30–100 NTU turbidity place this water in conventional-filtration territory — light enough that presedimentation is not mandatory, but too turbid for direct filtration (which is normally limited to source turbidity below roughly 10–20 NTU with low, stable TSS) — so a full coagulation–flocculation–sedimentation train precedes the filters to keep particulate loading on the filter beds manageable and to protect filter run length. (2) A metal-salt coagulant (alum or ferric chloride) is dosed at rapid mix to destabilize the turbidity-causing colloids; because coagulant hydrolysis consumes alkalinity and depresses pH, and because the raw water is already slightly acidic (pH 6.5–7.0, below the roughly 7.0–7.5 band that gives the fastest, most complete alum/iron floc formation and also below the pH range that minimizes corrosion in the finished-water distribution system), a lime or soda-ash feed is co-located at rapid mix (or immediately downstream) to hold pH in the optimum coagulation band and to leave adequate finished-water alkalinity for corrosion control. (3) Seasonal taste-and-odour episodes (typically algal metabolites such as geosmin/2-MIB, or seasonal turnover-driven organics) are handled by an intermittent powdered activated carbon (PAC) feed at the same rapid-mix point — PAC is preferred over a fixed granular activated carbon (GAC) contactor here specifically because the problem is seasonal rather than continuous, so PAC can be dosed only when needed and is captured in the same downstream sedimentation/filtration steps rather than requiring a dedicated permanent unit. (4) Hardness of only 50–100 mg/L as CaCO₃ is soft to moderately hard on the standard classification scale (below roughly 120–180 mg/L, where lime-soda softening typically becomes economically justified for a municipal supply), so no dedicated softening train is included; the coagulant/lime dosing already in the process removes a small incidental fraction of hardness, which is judged sufficient. (5) Post-filtration chlorination (rather than pre-chlorination ahead of clarification) is used to minimize contact time between chlorine and the natural organic matter still present in the raw/settled water, limiting disinfection by-product (THM/HAA) formation, with the clearwell providing the CT (concentration × time) contact volume required for regulatory pathogen inactivation credit before the water reaches the distribution system.
