16-Civ-A3 Elementary Environmental Engineering · December 2014

Question 7 of 7: Water Resource Management, Water and Wastewater Treatment

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. National Exams, December 2014 — 98-Civ-A3 Environmental Engineering. Three hours, closed book with one candidate-prepared double-sided aid sheet. Seven problems, each worth 20 marks; any five constitute a complete paper (maximum 100 marks), and only the first five answers in the work book are marked. All seven problems are solved below, because the set is a study resource rather than an exam attempt.

Reference texts. Davis & Cornwell, Introduction to Environmental Engineering; Mihelcic & Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design; Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery; Crittenden et al. (MWH), Water Treatment: Principles and Design; CCME Canadian Environmental Quality Guidelines; Impact Assessment Agency of Canada, Impact Assessment Act guidance.

Check: two source inconsistencies are carried through deliberately. (1) Problem 1(i) prints the dipropylene glycol formula as C₆H₁₄O₂ (118.2 g/mol); the actual compound is C₆H₁₄O₃ (134.2 g/mol). (2) The same sentence states the dose as “76 kg (1000 mol)”, which implies a molar mass of 76 g/mol and matches neither formula — 1000 mol of the real compound is 134 kg. The mole quantity is the load-bearing datum for a closed-system balance, so 1000 mol is adopted and both molar masses are reported where a mass concentration is asked for. NOTE 1 on page 1 expressly invites this kind of stated assumption.

Question 7: Water Resource Management, Water and Wastewater Treatment (20 marks)

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.

7 (i) — Protecting a shallow aquifer under the influence of surface water (5 marks)

Given. A shallow unconfined aquifer serving as a large town's principal supply, hydraulically connected to surface water and receiving agricultural runoff — principally nitrate, phosphorus, pesticides and pathogens.

Find. A water-resource management strategy that minimises contamination of the aquifer.

The controlling fact is the phrase “under the direct influence of surface water”. It means the aquifer offers little natural filtration or travel time, so pathogens and dissolved contaminants can reach the well within days rather than the decades a confined aquifer would provide. Under the drinking-water regulations of every Canadian province such a source is treated as a surface water and requires full treatment, but treatment alone is the wrong first answer. A multi-barrier approach applied from the source outward is both cheaper and more reliable, and the following measures are given in the order they should be implemented.

Delineate the wellhead protection area. Map the aquifer, its recharge zone and the capture zone of the well field using a calibrated groundwater flow model, and define time-of-travel zones — commonly 100-day, one-year, five-year and ten-year captures. Everything that follows is targeted at these zones, and without them protection effort is scattered across land where it does nothing.
Control land use within those zones by regulation. Zoning bylaws, restrictive covenants and, on the highest-risk land nearest the wells, outright acquisition of the property or of a conservation easement. Prohibit the highest-risk activities — manure storage and land application, fuel storage, septic systems, feedlots — within the inner zone. This is the single most effective and most durable measure available.
Implement agricultural beneficial management practices in the wider recharge area. Nutrient management plans that match fertiliser and manure application to crop uptake and soil test results, so that nitrate is not applied in excess of what the crop can take up; setbacks and vegetated riparian buffer strips along watercourses; cover crops and conservation tillage to reduce runoff and erosion; controlled livestock access to watercourses; integrated pest management to reduce pesticide loading; and engineered manure storage with adequate capacity so that spreading is never done on frozen or saturated ground.
Establish monitoring and an early-warning network. Sentinel monitoring wells between the agricultural land and the production wells, sampled for nitrate as the leading indicator together with pesticides and microbial indicators; continuous turbidity and conductivity on the raw water to detect surface-water breakthrough after storms.
Protect the wells themselves and manage abstraction. Proper surface sealing and grouting of the well annulus, secured wellheads, and decommissioning of abandoned wells in the capture area — an unsealed abandoned well is a direct conduit past every natural barrier. Manage pumping rates and well spacing to limit the induced infiltration of surface water, and where feasible relocate or deepen the intake to increase travel time.
Provide treatment as the final barrier, not the first. Filtration and disinfection appropriate to a groundwater under direct influence, with nitrate removal by ion exchange or reverse osmosis if source control cannot hold nitrate below the 10 mg/L as N guideline.
Support all of the above with governance. A watershed or aquifer management plan developed with the agricultural community, cost-sharing or stewardship incentives for the practice changes being asked of farmers, and public education. Regulation without incentive and engagement is poorly complied with, and the landowners are the people who actually control the recharge area.

The economic case is decisive: preventing nitrate from entering a shallow aquifer costs a small fraction of removing it afterwards, and once an aquifer is contaminated the residence time means the problem persists for decades even after the source is eliminated.

7 (ii) — Chlorination and dechlorination in wastewater treatment (6 marks)

Example of use. The standard application is final effluent disinfection. After secondary treatment — say activated sludge followed by secondary clarification — the effluent still carries on the order of 10⁵ to 10⁶ fecal coliforms per 100 mL. Sodium hypochlorite is dosed into a serpentine, baffled chlorine contact tank designed to approach plug flow, at a typical dose of 5 to 20 mg/L as Cl₂ with 15 to 30 minutes of contact at peak flow. The design target is a residual–time product, and the objective is a permit limit of the order of 200 fecal coliforms per 100 mL. Chlorine is also used elsewhere in a plant for odour control, for controlling filamentous bulking in activated sludge, and for slime control in pipework, but effluent disinfection is the principal use.

Chemically, hypochlorite hydrolyses to hypochlorous acid, $\mathrm{HOCl} \rightleftharpoons \mathrm{H^+} + \mathrm{OCl^-}$, and it is the un-ionised HOCl — roughly eighty times more germicidal than the hypochlorite ion — that does most of the work, which is why disinfection efficiency falls sharply above about pH 7.5. In wastewater the ammonia present converts much of the applied chlorine to chloramines, so the residual is largely combined rather than free, and combined residual is both slower acting and considerably more persistent in the receiving water.

Why dechlorination is required. Four reasons, of which the first is the operative one.

Residual chlorine is acutely toxic to aquatic life at concentrations far below the disinfecting dose. Salmonids and the invertebrates they feed on are affected in the range of 2 to 20 µg/L, and the CCME guideline for the protection of freshwater aquatic life is 0.5 µg/L of total residual chlorine. An effluent leaving the contact tank with 1 mg/L residual is therefore two to three orders of magnitude above the guideline, and discharging it would create a toxic mixing zone that can eliminate fish habitat for a considerable distance downstream. Effluent chlorine residual is one of the parameters that makes an effluent acutely lethal under the federal Wastewater Systems Effluent Regulations.
Chloramines are persistent. The combined residual formed with effluent ammonia decays slowly, so it does not conveniently dissipate before reaching sensitive habitat, unlike free chlorine.
Disinfection by-products. Chlorine reacts with the residual organic matter in effluent to form trihalomethanes, haloacetic acids and, in some effluents, chlorinated organics of greater concern; removing the residual limits continued formation in the receiving water.
Regulatory requirement. Canadian discharge permits routinely specify total residual chlorine at or below 0.02 mg/L, a limit that cannot be met without deliberate dechlorination.

Dechlorination is normally achieved with sulphur dioxide or sodium bisulphite dosed immediately downstream of the contact tank, reacting essentially instantaneously; approximately 1 mg of SO₂ is required per mg of chlorine residual, with a small excess. Activated carbon is an alternative where flows are small. The paired chlorination–dechlorination arrangement is precisely why many Canadian plants have moved to ultraviolet disinfection instead, which leaves no residual, forms no by-products and eliminates both the chlorine and the sulphite chemical handling hazards — though it requires low turbidity and provides no persistent residual of its own.

7 (iii) — Two differences between each pair (9 marks)

(a) Sedimentation versus sand filtration. First, the separation mechanism differs. Sedimentation is gravity settling in a quiescent basin: particles denser than water fall at their Stokes velocity and are removed on the floor, with no medium involved and removal governed by the overflow rate $v_o = Q/A$. Sand filtration is a fixed-bed process in which water passes through a granular medium and particles are captured on and within the grains by straining, interception, sedimentation onto grain surfaces and Brownian diffusion, followed by attachment — a surface-chemistry step that requires prior destabilisation. Second, the size range and position in the train differ. Sedimentation removes the coarse and heavy fraction, practically anything above roughly 20 to 50 µm, and is placed early as a bulk-removal step ahead of filtration; filtration is the polishing step that removes the fine and colloidal fraction sedimentation cannot reach, achieving turbidity below 0.1 NTU. A third practical difference worth noting is operating mode: sedimentation is continuous with periodic sludge withdrawal, whereas filtration is cyclical and must be taken offline and backwashed as headloss accumulates.

(b) Turbidity meters versus particle counters. First, the measurement principle and the nature of the output. A turbidimeter is a nephelometer measuring the intensity of light scattered at 90° from a beam, and reports one aggregate, relative index in NTU calibrated against a formazin standard — it says nothing about how many particles there are or how big they are. A particle counter passes the sample through a narrow flow cell across a laser beam and registers each individual particle as a discrete pulse whose amplitude scales with size, reporting an absolute count per millilitre resolved into size channels. Second, sensitivity and diagnostic value. Because scattered intensity is strongly size-dependent and dominated by the abundant sub-micron fraction, turbidity is nearly blind to a small number of large particles; a filtered water at a steady 0.05 NTU may still be passing pathogen-sized particles. A particle counter resolves the 3 to 10 µm range directly, detects filter breakthrough one to two orders of magnitude earlier, and shows which size fraction is failing, which points the operator at the cause. The trade-off is that turbidimeters are cheap, robust, standardised and written into regulation, whereas particle counters are expensive, need frequent calibration, and are not directly comparable between instrument makes — so in practice turbidity remains the compliance parameter and particle counting is the optimisation and early-warning tool.

(c) Colloidal versus settleable solids. First, size and settling behaviour. Settleable solids are the fraction larger than roughly 20 to 100 µm that settles under gravity in a reasonable detention time and is measured directly in an Imhoff cone after one hour. Colloidal solids span roughly 0.001 to 1 µm; because Stokes velocity scales with the square of diameter, their settling velocities are of the order of centimetres per day or slower, and Brownian motion and convection keep them permanently suspended, so they are non-settleable for any practical basin. Second, stability and the treatment required. Settleable solids are removed by plain sedimentation alone, with no chemical addition. Colloids carry a net negative surface charge whose electrical double layer creates a repulsive energy barrier that prevents aggregation, so they are thermodynamically stable and cannot be removed without first being destabilised — coagulation to neutralise the charge, then flocculation to grow the aggregates, then sedimentation or filtration. It is this stability, rather than size alone, that makes colloids the expensive fraction to remove, and it is why coagulant chemistry rather than basin size is the controlling design variable in a surface-water plant.

Final results — Problem 7 summary
Part	Key outcome
7(i) Aquifer protection	Multi-barrier source protection: delineate capture zones, regulate land use, agricultural beneficial management practices, sentinel monitoring, wellhead integrity, treatment as the last barrier
7(ii) Chlorination	Final effluent disinfection, 5–20 mg/L Cl₂, 15–30 min contact, target ~200 fecal coliforms/100 mL
7(ii) Dechlorination	Required because residual chlorine is toxic to aquatic life at µg/L levels (CCME guideline 0.5 µg/L); SO₂ or bisulphite at ~1 mg per mg Cl₂
7(iii)(a)	Gravity settling versus granular-media capture; coarse bulk removal versus fine polishing
7(iii)(b)	Aggregate scattered-light index versus discrete size-resolved count; blind to large particles versus early breakthrough detection
7(iii)(c)	Settleable > 20–100 µm and removed by gravity alone; colloidal 0.001–1 µm, charge-stabilised, needs coagulation first

Topic. The multi-barrier approach to drinking water, disinfection chemistry and its ecological cost, and the physical basis of solid–liquid separation.

Key relations. Stokes' law $v_s = g(\rho_p - \rho_w)d^2/18\mu$, which underlies the whole settleable-versus-colloidal distinction; the clarifier overflow rate $v_o = Q/A$ as the design criterion for sedimentation; the Chick–Watson disinfection relation $\ln(N/N_0) = -k C^n t$, from which the regulatory $Ct$ concept derives; and the hypochlorous acid dissociation $\mathrm{HOCl} \rightleftharpoons \mathrm{H^+} + \mathrm{OCl^-}$ with $\mathrm{p}K_a \approx 7.5$, which explains the pH sensitivity of chlorination.

Why this works. The multi-barrier philosophy is a reliability argument rather than a chemical one: independent barriers fail independently, so the probability that all of them fail simultaneously is the product of small numbers. That is why source protection is not made redundant by treatment — it is the barrier that keeps working when the plant does not. The chlorination and dechlorination pairing is a clean illustration of a recurring theme in environmental engineering: a measure that solves one problem creates another, and the complete design must address both. Recognising the trade-off is what distinguishes a designer from a process operator.

Common pitfall. Answering the aquifer question with treatment alone and omitting land-use and source control, which is where the effective protection actually lies; and explaining dechlorination as being about human health, when the driver is aquatic toxicity in the receiving water at concentrations three orders of magnitude below anything of human-health concern.

Source. Davis & Cornwell, Introduction to Environmental Engineering, Ch. 3 (hydrology and groundwater), Ch. 6 (coagulation, sedimentation, filtration) and Ch. 7 (disinfection); Crittenden et al. (MWH), Water Treatment: Principles and Design, Ch. 13; Metcalf & Eddy, Wastewater Engineering, Ch. 12 (disinfection); CCME Canadian Water Quality Guidelines for the Protection of Aquatic Life (chlorine); Health Canada Guidelines for Canadian Drinking Water Quality.

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