18-Env-A4 Water and Wastewater Engineering · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 04-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.) — population equivalent, oxygen sag/Streeter–Phelps, activated-sludge process control (RAS/WAS, HRT/SRT), secondary clarifier design; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — turbidity, alkalinity chemistry, digester fundamentals; MWH’s Water Treatment: Principles and Design (3rd ed.) — coagulation-flocculation mechanisms, ozonation, disinfection by-products, pH.
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) expresses a non-domestic (industrial or institutional) organic load on a treatment plant as the number of typical residents that would generate the same pollutant load, allowing a plant that serves a mixed municipal/industrial catchment to be sized on one consistent per-capita basis. It is calculated from the ratio of the facility’s daily BOD5 discharge to a standard per-capita BOD5 contribution (commonly 0.06–0.09 kg BOD5/capita·d): $PE = \dfrac{\text{Industrial/institutional } BOD_5 \text{ load (kg/d)}}{\text{Per-capita } BOD_5 \text{ contribution (kg/capita}\cdot\text{d)}}$. A dairy discharging 450 kg BOD5/d against a 0.075 kg/capita·d design figure therefore has a PE of 6,000 — the plant is designed and permitted as though 6,000 additional people were connected. PE can equally be computed on a flow or TSS basis when one of those, rather than BOD5, controls the design.
The oxygen sag curve is the characteristic dip-and-recovery profile of dissolved oxygen (DO) concentration with distance (or travel time) downstream of an organic (BOD-bearing) discharge into a stream. Just below the outfall, microbial oxidation of the discharged organic matter draws down DO faster than atmospheric reaeration can replace it, producing a falling limb; further downstream the exertable BOD is progressively consumed while the growing oxygen deficit itself accelerates reaeration, producing a recovery limb. The Streeter–Phelps equation, $D = \dfrac{k_d L_0}{k_r-k_d}\left(e^{-k_d t}-e^{-k_r t}\right)+D_0 e^{-k_r t}$, models the deficit $D$ as a function of the deoxygenation rate $k_d$, the reaeration rate $k_r$, the ultimate initial BOD $L_0$ and the initial deficit $D_0$, and locates the critical point (minimum DO, maximum deficit $D_c$) at the critical time $t_c$. Regulators use the predicted $D_c$ against the stream’s minimum DO standard for aquatic life to set the maximum permissible effluent BOD5 load for a given outfall.
In a conventional activated-sludge process, biomass settled in the secondary clarifier is split into two streams. Return activated sludge (RAS) is the larger fraction pumped back to the head of the aeration tank to maintain the mixed-liquor suspended-solids (MLSS) concentration needed to sustain the required food-to-microorganism ratio and treatment rate — without RAS the aeration tank would rapidly wash out its biomass. Waste activated sludge (WAS) is the smaller fraction deliberately removed from the system (either from the RAS line or directly from the aeration tank) to control the solids (sludge) retention time, SRT, and prevent the biomass inventory from growing indefinitely as new cells are produced through substrate oxidation. RAS flow is normally reported as a percentage of influent flow (commonly 25–100%, as in Question 5 here) and is a hydraulic/process-control decision made continuously, whereas the WAS rate is a mass-based decision made to hold SRT at its design value, directly setting the age and settleability of the biomass.
Disinfection by-products (DBPs) are compounds formed unintentionally when a chemical disinfectant — most often free chlorine — reacts with natural organic matter (humic/fulvic acids), bromide or other precursors present in the water being treated. The two best-characterized families are trihalomethanes (THMs, e.g. chloroform) and haloacetic acids (HAAs), both regulated under the Guidelines for Canadian Drinking Water Quality because of their association with chronic health risks (bladder-cancer risk, reproductive/developmental effects) at long-term exposure. DBP formation is minimized by removing precursor organic matter before disinfection (enhanced coagulation, granular/powdered activated carbon), moving the primary chlorination point downstream of clarification and filtration where NOM concentration is lower, or substituting a disinfectant with a different by-product profile (chloramines, ozone, UV) — each of which trades THM/HAA formation for its own by-product (bromate from ozonating bromide-bearing water, NDMA from chloramination).
Turbidity is an optical measure of water clarity: suspended and colloidal particles (clay, silt, organic detritus, microorganisms) scatter and absorb light passing through the sample, and the intensity of light scattered at 90° against a formazin reference standard is reported in Nephelometric Turbidity Units (NTU). Turbidity is tracked both as a treatment-performance surrogate — particles can shield protozoan cysts such as Giardia and Cryptosporidium from disinfection, so low turbidity is a precondition for reliable pathogen inactivation — and as a direct regulatory/aesthetic 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, far below the 10–100+ NTU range typical of raw surface water that the coagulation-flocculation-sedimentation-filtration train (Question 2) is designed to reduce.
