18-Env-A4 Water and Wastewater Engineering · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2017 — 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 questions — all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — oxygen sag/Streeter-Phelps, MLSS/MLVSS, population equivalent, primary clarifier design; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — turbidity, alkalinity chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — coagulation-flocculation, softening, disinfection by-products, pH; 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.
Population equivalent (PE) converts a non-domestic (industrial or institutional) wastewater load into an equivalent number of people producing the same organic loading, so that treatment plants sized around per-capita design flows can absorb mixed municipal/industrial catchments on one consistent basis. It is defined as the ratio of an industrial or institutional facility’s daily BOD5 load to the typical BOD5 contribution of one person per day (a design value on the order of 0.06–0.09 kg BOD5/capita·d is commonly used in North American practice): $PE = \dfrac{\text{Industrial } BOD_5 \text{ load (kg/d)}}{\text{Per-capita } BOD_5 \text{ contribution (kg/capita} \cdot \text{d)}}$. A cannery discharging 600 kg BOD5/d against a 0.075 kg/capita·d per-capita figure therefore contributes a PE of 8,000 — the treatment works must be sized as if 8,000 extra people were connected, even though no one actually lives at the plant. PE can equally be computed on flow or on other pollutant bases (TSS, TKN) when those are the design-controlling parameter.
The oxygen sag curve describes how dissolved oxygen (DO) in a receiving stream varies with distance (or travel time) downstream of an organic-waste (BOD) discharge. Immediately below the outfall, bacterial oxidation of the discharged organic matter consumes DO faster than atmospheric reaeration can replace it, so DO falls; further downstream the BOD load is progressively exerted and diminishes while the oxygen deficit itself drives faster reaeration, so DO recovers. The result is a characteristic sag-shaped DO-vs-distance profile with a minimum — the critical point $D_c$ at critical time $t_c$ — described by the Streeter–Phelps equation, $D = \dfrac{k_d L_0}{k_r - k_d}\left(e^{-k_d t} - e^{-k_r t}\right) + D_0 e^{-k_r t}$, where $k_d$ is the deoxygenation rate constant, $k_r$ the reaeration rate constant, $L_0$ the initial (ultimate) BOD, and $D_0$ the initial deficit. The critical deficit governs whether DO drops below the minimum needed to sustain fish and other aquatic life, and is the parameter regulators use to set effluent BOD limits for a given stream’s assimilative capacity.
Mixed Liquor Suspended Solids (MLSS) is the total concentration of suspended solids — biological floc plus inert inorganic material — in the aeration-tank mixed liquor of an activated-sludge process, typically 1,500–4,000 mg/L for conventional systems. Mixed Liquor Volatile Suspended Solids (MLVSS) is the organic (volatile, combustible-at-550 °C) fraction of MLSS, and is used as a practical surrogate for the concentration of active microbial biomass driving BOD removal, since the inert inorganic fraction of MLSS contributes no biological activity. The ratio MLVSS/MLSS, typically 0.7–0.85 for domestic wastewater, indicates the “biological activity” of the mixed liquor and is used directly in food-to-microorganism (F/M) ratio and sludge-age (SRT) calculations that govern process control.
Disinfection by-products (DBPs) are chemical compounds formed unintentionally when a chemical disinfectant — most commonly free chlorine — reacts with natural organic matter (humic and fulvic acids), bromide, or other precursors present in the water. The best-known families are trihalomethanes (THMs: chloroform, bromodichloromethane, etc.) and haloacetic acids (HAAs), both regulated under the Guidelines for Canadian Drinking Water Quality because of chronic health concerns (bladder-cancer association, developmental effects) associated with long-term exposure. DBP formation is minimized by removing precursor organics before the disinfectant is applied (enhanced coagulation, activated carbon), moving the chlorination point downstream of clarification/filtration where NOM is lower, or substituting an alternative disinfectant (chloramines, ozone, UV) — each of which carries its own, generally lower-risk, by-product profile (e.g. bromate from ozone of bromide-bearing waters, NDMA from chloramination).
Turbidity is an optical measure of water clarity caused by suspended and colloidal particles (clay, silt, organic detritus, microorganisms) that scatter and absorb light passing through the sample, measured in Nephelometric Turbidity Units (NTU) by comparing 90° scattered light against a formazin standard. Turbidity matters in water treatment both as a proxy for particulate/pathogen removal performance — particles can shield protozoan cysts (Giardia, Cryptosporidium) from disinfection — and as an aesthetic/regulatory parameter; the Guidelines for Canadian Drinking Water Quality set a treatment goal of ≤0.3 NTU in individual filter effluent and <1 NTU in combined filtered water at all times, well below the 30–50 NTU raw-water range that the coagulation-flocculation-sedimentation-filtration train in Question 2 is designed to reduce.
