18-Env-A4 Water and Wastewater Engineering · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2014 — 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; Davis & Cornwell, Introduction to Environmental Engineering; MWH's Water Treatment: Principles and Design; 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 (typically industrial or institutional) organic loading into the number of "equivalent people" that would generate the same pollutant load, almost always expressed on a BOD₅ basis: $\text{PE}=\dfrac{\text{BOD}_5\ \text{load discharged (kg/d)}}{\text{per-capita BOD}_5\ \text{contribution (kg/person}\cdot\text{d)}}$, using a standard per-capita contribution of roughly 0.07–0.09 kg BOD₅/person·d for domestic sewage. It lets a municipality express a brewery, dairy, or food-processing discharge in the same units used to size the treatment plant for its residential population, so combined design flow and load can be planned on one common basis. PE is also the standard basis for industrial sewer-use bylaws and surcharge tariffs: a discharger whose PE greatly exceeds its actual headcount (a common outcome for high-strength industrial wastes) is billed and regulated accordingly, and the concept extends to hydraulic loading (an "equivalent population" based on flow) as well as organic loading.
The oxygen sag curve is the characteristic dip-and-recovery profile of dissolved oxygen (DO) concentration measured along (or in time downstream of) a receiving stream after an organic (BOD-bearing) discharge. Two competing first-order processes drive it: deoxygenation, as bacteria oxidize the discharged BOD at rate $k_d$, and reaeration, as the stream re-absorbs oxygen from the atmosphere at rate $k_r$, described by the Streeter–Phelps equation $\dfrac{dD}{dt}=k_dL-k_rD$ for the oxygen deficit $D$. Immediately below the outfall, deoxygenation dominates and DO falls; once the remaining BOD has thinned enough that reaeration overtakes it, DO recovers — the minimum point (critical deficit, at the critical time $t_c=\frac{1}{k_r-k_d}\ln\!\left[\frac{k_r}{k_d}\left(1-\frac{D_0(k_r-k_d)}{k_dL_0}\right)\right]$) is where fish kills and septic conditions are most likely. Whipple's classical zonation (clean-water, decomposition/degradation, active decomposition or septic, recovery, clean-water) maps directly onto this curve and is the conceptual basis for setting BOD discharge limits and stream assimilative-capacity studies.
MLSS (mixed liquor suspended solids) is the total suspended-solids concentration of the aeration-tank contents in an activated-sludge system — living biomass plus inert organic debris plus inorganic (grit/ash) solids — typically maintained at 1,500–4,000 mg/L for conventional plants (higher for extended aeration or MBR). MLVSS (mixed liquor volatile suspended solids) is the organic (combustible, 550°C-volatile) fraction of the MLSS, which approximates — though does not exactly equal, since it also includes non-viable organic debris — the active biomass concentration; MLVSS is typically 70–85% of MLSS for domestic wastewater. The distinction matters operationally: F/M ratio, sludge age (SRT), and process control calculations are all conventionally based on MLVSS (the biologically relevant mass), while MLSS is the number more directly tied to clarifier solids loading and settleability.
Disinfection by-products (DBPs) form when a chemical disinfectant reacts with natural organic matter (humic and fulvic acids), bromide, or other precursors present in the source water. Chlorination produces trihalomethanes (THMs, e.g. chloroform) and haloacetic acids (HAA5) as the principal regulated families; chloramination shifts the profile toward different halogenated species; ozonation and chlorine dioxide instead risk bromate and chlorite/chlorate, respectively, when bromide is present. Several DBP classes are suspected or probable carcinogens, so Health Canada's Guidelines for Canadian Drinking Water Quality set maximum acceptable concentrations (e.g. THM4 — the sum of four common trihalomethanes — at 100 µg/L and HAA5 at 80 µg/L, running annual averages). Control strategies attack either side of the reaction: reduce the organic precursor before disinfection (enhanced coagulation, GAC, membrane pre-treatment) or reduce reliance on chlorine (UV disinfection, ozone with bromate control, optimized chloramination), while still meeting the microbial-inactivation CT requirement.
Turbidity is an optical measure of water clarity — the degree to which suspended and colloidal matter (clay, silt, organic floc, algae, microorganisms) scatters light passing through the sample — quantified by the nephelometric method (light scattered at 90° to the incident beam) and reported in NTU. It is one of the most important operational indicators in water treatment: it tracks coagulation-flocculation and filtration performance in near real time, and elevated turbidity physically shields pathogens (particularly Cryptosporidium oocysts) from disinfectant contact and UV dose, reducing effective inactivation even when the nominal CT target is met. The Guidelines for Canadian Drinking Water Quality set filtered-water turbidity targets of ≤0.3 NTU (conventional/direct filtration, 95% of samples each month) and require it never exceed 1.0 NTU, precisely because turbidity is used as the continuous surrogate for filter integrity and pathogen-removal performance between the infrequent direct microbial tests.
