16-Civ-A3 Elementary Environmental Engineering · May 2014

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

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

Notes on this paper

Paper format. National Exams 98-Civ-A3 Environmental Engineering, May 2014 — 3 hours, closed book with one candidate-prepared double-sided aid sheet, approved Casio or Sharp calculator only. Seven questions are offered; any five constitute a complete paper (20 marks each, 100 marks maximum), and only the first five answers in the work book are marked. All seven are solved here, because the set is intended as a study resource rather than an examination script. Section marks are shown in brackets at the left margin of each part, and the marking scheme on page 6 confirms the split.

Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (5th ed.); Mihelcic & Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design (3rd ed.); Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); Crittenden et al., MWH’s Water Treatment: Principles and Design (3rd ed.). Canadian regulatory frame: the federal Impact Assessment Act (2019) and the Impact Assessment Agency of Canada, the Canadian Environmental Protection Act (CEPA 1999), CCME Canadian Environmental Quality Guidelines, and Health Canada’s Guidelines for Canadian Drinking Water Quality (GCDWQ).

Check: Henry’s law constant units in Question 1(i). The paper writes the constant as “0.30 (mol/atm)”, which is dimensionally incomplete — a Henry’s constant in the concentration/pressure form must carry a volume in the denominator. It is taken here as 0.30 mol/(L·atm), i.e. the aqueous-concentration form $C_{aq}=K_H\,p$. That reading is confirmed by the published value for ethyl acetate, $H \approx 1.3\times10^{-3}\ \text{atm}\cdot\text{m}^3/\text{mol}$, whose reciprocal is $\approx 0.77\ \text{mol}/(\text{L}\cdot\text{atm})$ — the same order of magnitude. Per NOTE 1 on page 1, this assumption is stated with the answer.

Question 7: Water Resource Management, Water Treatment 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.

Part (i) — Managing reservoir sedimentation (5 marks)

Reservoir sedimentation destroys hydroelectric capability in two ways: it consumes live storage, reducing the volume available to regulate flow and generate through dry periods, and it delivers abrasive sediment to turbine runners, causing wear that shortens overhaul intervals and reduces efficiency. Because the causes here are diffuse — stormwater and agricultural runoff — the response must be organised at the watershed scale, which is the governing principle of integrated water resources management: the reservoir is the downstream symptom, and the sediment is generated across the entire contributing catchment. The measures form a natural sequence from source to structure.

Source control in the catchment — the highest-value intervention. Sediment not eroded never has to be managed. On agricultural land this means conservation tillage or no-till, cover cropping to keep soil protected between crops, contour ploughing and terracing on slopes, grassed waterways in concentrated flow paths, and rotational grazing with livestock excluded from streams by fencing. In the urban and construction fraction of the catchment it means enforced erosion and sediment control — silt fence, sediment traps, stabilised entrances, prompt revegetation of disturbed ground — and low-impact development that reduces the runoff volume driving erosion in the first place. These measures are inexpensive relative to any downstream fix and deliver co-benefits in nutrient and pesticide reduction, which also addresses reservoir eutrophication. Their weakness is that they depend on the cooperation of many independent landowners, so they need an institutional vehicle: a watershed authority or conservation district with incentive payments, technical assistance and, where necessary, regulation.

Interception along the transport path. Sediment that does mobilise can be trapped before it reaches the reservoir through riparian buffer strips along every watercourse, restored wetlands acting as natural settling basins, stream-bank stabilisation and re-vegetation where bank erosion is a major contributor, and purpose-built check dams or sediment-detention basins on the principal tributaries, which are far cheaper to clean out than the reservoir itself.

Management at the reservoir and dam. Where hydrology permits, sediment sluicing and density-current venting pass turbid inflows through low-level outlets during flood events rather than allowing them to deposit — the most cost-effective structural measure when the dam has suitable low-level gates. Drawdown flushing during high flows can scour deposits from the reservoir bed, though it must be managed carefully because a sudden release of sediment-laden water harms downstream fish habitat. Where deposits are already established, dredging or hydrosuction is technically effective but expensive and creates a disposal problem, and the dredged material may be reusable as agricultural soil amendment if it is uncontaminated. The turbines themselves should be specified with erosion-resistant coatings, and desanding basins provided where the sediment load is unavoidable.

Monitoring and adaptive management underpins all of it: periodic bathymetric survey to track the loss of storage, continuous turbidity and suspended-sediment gauging at the main inflows to identify which sub-catchments dominate the load, and a sediment budget so that effort is directed where it yields the most. A reservoir designed with an explicit sediment-storage allocation, and operated with a sediment management plan reviewed on a fixed cycle, is what converts a fixed design life into an indefinite one.

Part (ii) — Coagulation and flocculation (6 marks)

Example. A conventional surface-water treatment plant drawing from a turbid river dosed with alum, $\text{Al}_2(\text{SO}_4)_3 \cdot 14\text{H}_2\text{O}$, at roughly 20–60 mg/L ahead of sedimentation and filtration — the standard first stage of the conventional treatment train and the one that makes filtration and disinfection work at all.

Theory. Clay colloids, natural organic matter, algae and bacteria in surface water are typically 0.01 to 10 µm in size and carry a negative surface charge arising from isomorphic substitution in the clay lattice and from ionised carboxyl and phenolic groups on organic matter. Two consequences follow. First, mutual electrostatic repulsion between the diffuse double layers surrounding the particles keeps them dispersed, so they never aggregate on their own. Second, they are so small that Stokes’ law gives settling velocities of the order of millimetres per day — a 1 µm clay particle would need weeks to settle through a metre of water, against a design detention time of two to four hours. Colloids are therefore stable, both electrostatically and hydrodynamically, and cannot be removed by settling or by filtration alone.

Coagulation destabilises them. When alum is added, the $\text{Al}^{3+}$ ion hydrolyses to a series of positively charged hydroxo complexes and, at typical doses, precipitates as amorphous aluminium hydroxide. Destabilisation proceeds by up to four mechanisms operating together: compression of the electrical double layer by the added ionic strength; charge neutralisation by adsorption of the positively charged hydrolysis products onto the negative colloid surface, reducing the zeta potential toward zero; sweep flocculation, in which the precipitating $\text{Al}(\text{OH})_3$ floc physically enmeshes colloids as it settles — the dominant mechanism at conventional doses; and interparticle bridging, where long-chain polymer molecules attach to more than one particle. The reaction consumes alkalinity and depresses pH, $$\text{Al}_2(\text{SO}_4)_3\cdot 14\text{H}_2\text{O} + 6\,\text{HCO}_3^- \rightarrow 2\,\text{Al(OH)}_3\!\downarrow + 6\,\text{CO}_2 + 3\,\text{SO}_4^{2-} + 14\,\text{H}_2\text{O}$$ so soft, low-alkalinity waters require supplemental lime or soda ash to keep the pH within the optimum window of roughly 6.0 to 7.8 for alum, outside which aluminium solubility rises and residual aluminium passes into the treated water.

Flocculation is the subsequent step of building the destabilised particles into settleable aggregates by gentle mixing, which increases the frequency of interparticle collisions without breaking the fragile floc apart. The two stages therefore demand opposite hydraulics, and this is the crux of the practice:

Rapid mix (coagulation): velocity gradient $G$ of 600–1000 s⁻¹ for 10–60 seconds. The hydrolysis and adsorption reactions occur within a fraction of a second, so the coagulant must be dispersed essentially instantaneously or part of the water is under-dosed while part is over-dosed.
Flocculation: $G$ of 20–70 s⁻¹ for 20–45 minutes, with the dimensionless product $Gt$ in the range 10⁴ to 10⁵. Tapered flocculation — three or more compartments at decreasing $G$ — grows large floc in the first stage and then protects it from shear in the later stages.

The velocity gradient is computed from the power dissipated, $$G = \sqrt{\frac{P}{\mu V}}$$ with $P$ the power input (W), $\mu$ the dynamic viscosity (Pa·s) and $V$ the basin volume (m³). Because $\mu$ rises as water cools, a Canadian plant must design for winter conditions, when both mixing and floc formation are slower.

Practice. The dose is not calculated from theory but determined empirically by jar testing — parallel beakers dosed across a range of coagulant concentrations and pH values, subjected to a programmed rapid-mix and slow-mix sequence, and assessed on settled turbidity, residual colour and organic removal. Testing is repeated seasonally, because raw-water character changes markedly with spring freshet, summer algal growth and winter cold. Operators supplement alum with a coagulant aid such as an anionic polymer to toughen the floc, or use ferric chloride where better organic removal or a wider effective pH range is needed. Modern plants monitor streaming current or zeta potential for real-time dose control. The same chemistry is applied in wastewater treatment for chemically enhanced primary treatment and, most commonly, for phosphorus removal, where ferric or alum precipitates phosphate to meet a discharge limit. The resulting sludge — voluminous, gelatinous and difficult to dewater — is the principal residual and must be thickened, conditioned and disposed of, which is a real part of the cost of the process.

Topic. Colloid stability, coagulation chemistry and flocculation hydraulics.

Key relations. DLVO theory — net interaction energy as the sum of double-layer repulsion and van der Waals attraction; zeta potential as the operational measure of stability; Stokes’ law $v_s = g(\rho_p-\rho_w)d^2/18\mu$; the Camp–Stein velocity gradient $G=\sqrt{P/\mu V}$ and the design parameter $Gt$; Smoluchowski perikinetic and orthokinetic flocculation kinetics; alum’s alkalinity consumption of about 0.50 mg/L as $\text{CaCO}_3$ per mg/L of alum.

Why this works. Stokes’ law shows settling velocity scales with the square of diameter, so aggregating particles from 1 µm to 1 mm increases settling velocity by a factor of a million and turns an impossible separation into a routine one. Coagulation removes the electrostatic barrier that prevents aggregation, and flocculation supplies the collisions that build the aggregate — two distinct physical problems, which is exactly why the process is split into two basins with velocity gradients differing by more than an order of magnitude. Every downstream barrier depends on this step: filters pass poorly coagulated water, and disinfectants are consumed and shielded by particles that should have been removed.

Common pitfall. Confusing the two stages and applying a single mixing intensity, which either fails to disperse the coagulant or shears the floc back into fragments — the classic cause of a plant that cannot make settled turbidity. A second pitfall is treating the dose as a fixed design value rather than a seasonally re-tested operating variable, and a third is ignoring the alkalinity consumption, which on a soft Canadian source water can drive pH below the effective range and leave residual aluminium in the finished water.

Source. Crittenden et al., MWH’s Water Treatment: Principles and Design, 3rd ed., Ch. 9 (Coagulation and Flocculation); Davis & Cornwell, Introduction to Environmental Engineering, 5th ed., Ch. 6; Metcalf & Eddy, 5th ed., Ch. 6.

Part (iii) — Definitions and distinctions (9 marks: 3 + 3 + 3)

(a) Facultative and aerobic (3 marks)

The terms describe how an organism — or by extension a treatment process — relates to molecular oxygen as a terminal electron acceptor. Aerobic organisms require dissolved oxygen and use it exclusively; without it they cannot respire and they die or go dormant. Aerobic metabolism yields the most energy per unit of substrate, so aerobic processes give rapid treatment, high-quality effluent and relatively odourless operation — at the cost of continuously supplying oxygen, which is typically the single largest energy consumer in a wastewater treatment plant, and of producing a large volume of biological sludge because high energy yield means high cell growth. Nitrifying bacteria are strict aerobes, which is why nitrification fails first when aeration is inadequate.

Facultative organisms are metabolically flexible: they use dissolved oxygen when it is available, because it is energetically preferable, and switch to an alternative electron acceptor — nitrate, sulphate, or fermentation of organic compounds — when it is not. This adaptability makes facultative organisms extremely robust in variable environments, and it is the basis of two important processes. A facultative lagoon is stratified by design: the upper layer is aerobic, oxygenated by surface reaeration and by algal photosynthesis, while the lower layer and benthic sludge are anaerobic, with a facultative zone between; the arrangement gives effective, very low-energy treatment for small communities and is widely used in rural Canada, at the cost of a large land area and reduced winter performance. In biological nitrogen removal, facultative heterotrophs in an anoxic zone respire nitrate in place of oxygen, reducing it to nitrogen gas — denitrification — which is only possible because those organisms are facultative rather than strictly aerobic. The contrasting term to keep clear is obligate anaerobe, an organism for which oxygen is actually toxic; facultative is the middle case, not a synonym for anaerobic.

(b) Colloidal and dissolved solids (3 marks)

The distinction is one of size, of phase, and consequently of how each can be removed. Dissolved solids are true solutions — individual ions and small molecules smaller than about 0.001 µm (1 nm), such as sodium, chloride, calcium, sulphate and small organic molecules. They are homogeneously distributed at the molecular level, form a single phase, do not scatter light, never settle no matter how long they are left, and pass through any conventional filter. They are quantified by evaporating a filtered sample to dryness as total dissolved solids, and estimated in the field from electrical conductivity. Removing them requires a process operating at the molecular scale: reverse osmosis, nanofiltration, ion exchange, electrodialysis or distillation — all comparatively energy-intensive.

Colloidal solids occupy the intermediate range of roughly 0.001 to 1 µm — clay particles, natural organic macromolecules, viruses, and precipitated metal hydroxides. They constitute a genuine second phase dispersed in the water, so the mixture is heterogeneous, but the particles are small enough that Brownian motion and mutual electrostatic repulsion keep them permanently suspended: they will not settle in any practical time, yet they are not truly dissolved. They scatter light, which is precisely what a turbidity meter detects and why colloids dominate the appearance of a turbid water. The operational definition in standard laboratory practice is the 0.45 µm filter — what passes is reported as dissolved, what is retained as suspended — which means a portion of the colloidal fraction is conventionally counted with the dissolved solids, an important caveat when interpreting a TDS result. Because colloids are stable but are a separate phase, they are removed neither by plain settling nor by molecular-scale processes but by destabilisation and aggregation — coagulation and flocculation followed by sedimentation and filtration, as described in part (ii), or directly by membrane microfiltration and ultrafiltration. Colloids matter disproportionately because they carry most of the surface area, adsorb metals and organic contaminants, harbour pathogens, and are the principal precursors of disinfection by-products.

(c) Secondary and tertiary wastewater treatment (3 marks)

Secondary treatment is biological treatment aimed at the soluble and colloidal organic matter that primary settling cannot remove. Microorganisms are cultivated under controlled conditions — suspended growth in an activated-sludge aeration basin, or attached growth on a trickling filter or rotating biological contactor — and metabolise the organic substrate into carbon dioxide, water and new cell mass, which is then separated in a secondary clarifier and partly returned to maintain the biomass inventory. Typical performance is 85 to 95 % removal of BOD and suspended solids, giving an effluent around 20–30 mg/L of each. This is the level defined as secondary treatment by Canada’s federal Wastewater Systems Effluent Regulations, which set a national standard of 25 mg/L carbonaceous BOD and 25 mg/L suspended solids, and it is the minimum acceptable for most municipal discharges to a receiving water of reasonable assimilative capacity. Secondary treatment addresses oxygen demand and solids; it does not reliably remove nutrients, pathogens, or trace contaminants.

Tertiary treatment is any process added after secondary treatment to meet a requirement that secondary cannot satisfy — hence it is defined by objective rather than by a single technology. The common objectives and their processes are: nutrient removal, comprising biological nitrogen removal through nitrification and denitrification and biological or chemical phosphorus removal to limits as low as 0.1 mg/L of phosphorus where the receiving water is eutrophication-sensitive, as in the Great Lakes basin; residual solids and turbidity polishing by granular media filtration or membrane filtration; disinfection by UV, chlorination with dechlorination, or ozone, to protect downstream drinking-water intakes and recreational and bivalve-harvest waters; and advanced contaminant removal by activated carbon, advanced oxidation or reverse osmosis where trace organics, pharmaceuticals or specific industrial contaminants must be addressed, or where the effluent is to be reused. Tertiary treatment is driven by the sensitivity of the receiving environment and by the intended end use, and it carries substantially higher capital, energy and operating cost than secondary — which is why it is applied where a specific water-quality objective demands it rather than as a matter of course.

Summary of distinctions — Question 7(iii)
Term pair	Basis of the distinction	Key consequence
Facultative vs aerobic	Requirement for molecular oxygen as terminal electron acceptor	Aerobes need continuous aeration (high energy, fast, low odour); facultative organisms switch acceptors, enabling lagoons and anoxic denitrification
Colloidal vs dissolved solids	Particle size and number of phases: 0.001–1 µm, two phases vs < 0.001 µm, one phase	Colloids need destabilisation (coagulation/flocculation) or membranes; dissolved solids need RO, ion exchange or distillation
Secondary vs tertiary treatment	Biological removal of organics vs objective-driven polishing beyond it	Secondary meets the 25/25 federal BOD/SS standard; tertiary targets nutrients, pathogens, turbidity or trace contaminants at higher cost

Topic. Microbial oxygen relationships, the solids-size spectrum, and the levels of wastewater treatment.

Key relations. Redox tower ordering terminal electron acceptors by energy yield ($\text{O}_2 > \text{NO}_3^- > \text{SO}_4^{2-} > \text{CO}_2$); the solids classification $TS = TSS + TDS$ with the 0.45 µm operational cut; Stokes’ law showing why colloids will not settle; the federal Wastewater Systems Effluent Regulations limits of 25 mg/L cBOD and 25 mg/L SS.

Why this works. All three pairs are the same kind of distinction — a spectrum with an operational boundary drawn where the engineering response changes. Microorganisms are classified by which electron acceptor they can use, because that determines whether oxygen must be supplied and therefore the energy cost of the process. Solids are classified by size, because size determines whether a particle settles, is filtered, or must be destabilised first. Treatment is classified by level, because each level removes a different class of constituent and each successive level costs substantially more. Knowing where a constituent or organism sits on its spectrum tells the engineer directly which unit process applies.

Common pitfall. Equating facultative with anaerobic — facultative organisms prefer oxygen and use it whenever it is present. Similarly, assuming a TDS result contains no colloidal material, when the 0.45 µm operational filter passes much of the colloidal range; and describing tertiary treatment as “a third stage” as though it were a single fixed process, when it is a family of processes selected against a specific objective.

Source. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery, 5th ed., Ch. 1, 2, 7 and 8; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed., Ch. 5 and 7; Wastewater Systems Effluent Regulations (SOR/2012-139) under the federal Fisheries Act.

← Question 6 Back to the paper →