Question 7 of 7: Problem 7 — Water resource management, water treatment and wastewater treatment

Question 7: Problem 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) — Water resource management for a contaminated shallow aquifer (5 marks)

Water resource management addresses this problem at the scale of the whole source–pathway–receptor system rather than at the wellhead, and its central instrument is source-water protection: preventing contamination from reaching the aquifer is invariably cheaper and more reliable than treating it afterwards, which is the multi-barrier principle that the Walkerton Inquiry made the foundation of Canadian drinking-water policy. The phrase “under the influence of surface water” is the technically decisive statement in the question. A groundwater source under the direct influence of surface water is classified in every Canadian jurisdiction as requiring surface-water-equivalent treatment, that is, filtration plus disinfection to the 3-log Giardia and 2-log virus standard, because the aquifer offers no reliable natural filtration barrier. It also means the contaminant pathway is fast and hydraulically connected, so tailings seepage will arrive at the wells with little attenuation and little warning.

The management response follows the source–pathway–receptor logic in that order. Delineate and characterise: build a conceptual hydrogeological model and a numerical flow and transport model, delineate the wellhead protection area and the capture zones on 2-year, 5-year and 25-year time-of-travel contours, and identify all significant threats within them — the tailings pond, but also septic systems, fuel storage and agriculture. Characterise the plume with a monitoring-well network between the pond and the wells, analysing for the mine's signature parameters: metals such as arsenic, cadmium, copper, lead, nickel and zinc, sulphate, cyanide where gold processing is involved, and low pH from acid rock drainage. Control at source, which is the priority: require the mine to eliminate the seepage under its provincial environmental compliance approval and the Metal and Diamond Mining Effluent Regulations — liner and cover upgrades, seepage collection ditches and toe drains returning water to the pond, dewatering and relocation of the tailings, water-cover or dry-stack tailings management to prevent acid generation, and full financial assurance for closure. Interrupt the pathway: where source control cannot act fast enough, install a hydraulic containment system of interception wells pumping to treatment, or a permeable reactive barrier of zero-valent iron or limestone for metals and acidity, or a low-permeability cut-off wall. Protect the receptor: establish a land-use planning overlay and prohibited-use zoning in the wellhead protection area under the provincial source protection legislation; relocate the production wells further from the plume or deeper into a confined unit if one exists; develop an alternative or blended supply and interconnections for redundancy; and, as the last barrier, upgrade the treatment plant to provide the filtration the GUDI classification demands together with metals removal. Govern and monitor: operate an early-warning sentinel-well network with trigger levels, apply the polluter-pays principle to fund the works, coordinate the municipality, the mine, the conservation authority, the provincial ministry and affected Indigenous communities through a watershed or source-protection committee, and maintain public reporting. This staged answer — prevent, contain, treat, plan — is what distinguishes water resource management from simply adding a treatment process at the plant.

Part (ii) — Disinfection in drinking water treatment, and why secondary disinfection is required (6 marks)

Example of primary disinfection. The standard Canadian example is free-chlorine primary disinfection in a chlorine contact chamber following filtration. Chlorine gas or sodium hypochlorite is dosed into the filtered water entering a baffled contact tank, where the objective is to achieve a specified CT, the product of the residual disinfectant concentration $C$ in mg/L and the contact time $T$ in minutes at the ten-percentile detention time $T_{10}$: $$CT = C \times T_{10}$$ The required CT is read from the Health Canada or USEPA tables as a function of pH and temperature for the target inactivation, typically 0.5-log Giardia and 2-log virus in a filtered supply where filtration supplies the balance of the 3-log Giardia requirement. For instance, a plant holding a 1.0 mg/L free chlorine residual through a baffled chamber with $T_{10}$ of 30 minutes achieves CT = 30 mg·min/L, comfortably above the roughly 15 mg·min/L needed for 0.5-log Giardia inactivation at 10 °C and pH 7. Ultraviolet disinfection is the common complement, because UV is highly effective against Cryptosporidium, which is essentially chlorine-resistant, while ozone is used where taste, odour and colour must also be addressed. Disinfection is always the last unit process, applied to already-clarified and filtered water, because turbidity shields organisms and organic matter exerts a demand that consumes the disinfectant and forms by-products.

Why secondary disinfection is required in a large municipality. Secondary disinfection means maintaining a measurable disinfectant residual — typically 0.2 mg/L free chlorine or, in a large system, 0.5 to 1.0 mg/L of the longer-lived monochloramine — throughout the distribution system to the furthest customer, with re-chlorination boosters at reservoirs and remote zones. It is required for four reasons, all of which intensify with system size. First, the distribution system is not sterile and is not sealed. A large network has hundreds of kilometres of main, storage reservoirs, thousands of joints, valves and hydrants, and continuous repair and new-main activity; contamination enters through main breaks, cross-connections and backflow from customer premises, and through intrusion at low or negative pressure during a transient or a hydrant operation. A residual is the only continuously acting barrier against that post-treatment contamination. Second, residence time in a large system is long — days to weeks in looped networks and oversized reservoirs — which is ample time for bacterial regrowth and for biofilm to establish on pipe walls; the residual suppresses regrowth, controls biofilm and limits nitrifying bacteria in chloraminated systems. Third, the residual is a sentinel. Its disappearance is a fast, cheap, continuously measurable indicator that something has gone wrong — an intrusion, a break, an excessive demand from corrosion products — and it gives operators warning long before a bacteriological sample could be collected, incubated and reported 24 to 48 hours later. Fourth, it is a legal requirement. Provincial drinking-water regulations mandate a minimum residual at the extremities of the distribution system; Ontario Regulation 170/03, for example, requires a free chlorine residual of at least 0.05 mg/L, and loss of residual is a reportable adverse water quality incident. The design trade-off is that maintaining the residual forms disinfection by-products — trihalomethanes and haloacetic acids, both regulated in the GCDWQ — which is exactly why large systems with long residence times commonly convert to chloramines, whose by-product formation is far lower and whose residual is far more persistent.

Part (iii) — Comparisons (3 + 3 + 3 marks)

(a) Coagulation and flocculation. Similarity: both are sequential stages of the same particle-aggregation train in a conventional drinking-water or advanced wastewater plant, both act on colloidal and fine suspended particles that will not settle unaided, and neither removes anything by itself — both merely prepare the particles for the sedimentation or filtration step that follows. Difference 1 — mechanism: coagulation is a chemical destabilisation, in which a hydrolysing metal salt such as alum or ferric chloride compresses the electrical double layer, neutralises the negative surface charge and provides hydroxide precipitate for sweep flocculation; flocculation is a physical transport process, in which gentle mixing promotes collisions among the already-destabilised particles so they grow into settleable flocs. Difference 2 — mixing energy and time: coagulation requires rapid, intense mixing at a velocity gradient $G$ of order 300 to 1000 s⁻¹ for a few seconds to tens of seconds, so the coagulant disperses before it hydrolyses; flocculation requires slow, tapered mixing at $G$ of order 20 to 70 s⁻¹ for 20 to 45 minutes, gentle enough not to shear the growing floc apart. Applying the wrong energy to either stage — slow mixing at coagulation, or vigorous mixing at flocculation — defeats the process.

(b) Osmosis and filtration. Similarity: both are separation processes that use a membrane or porous medium as a barrier between a feed and a product stream, and both are used in water treatment to remove constituents from water. Difference 1 — what is separated and by what mechanism: filtration, including microfiltration and ultrafiltration, separates suspended matter — particles, colloids, bacteria, protozoan cysts — by size exclusion, straining, interception and adsorption in a porous medium or membrane with discrete pores from about 0.01 to 100 µm; osmosis and its engineered form, reverse osmosis, separate dissolved matter — salts, metals, nitrate, small organics — through a dense non-porous polymer film by a solution–diffusion mechanism, in which water dissolves into the membrane and diffuses through it while solutes are rejected. Difference 2 — driving force and energetics: filtration is driven by a small hydraulic pressure difference, typically 0.1 to 3 bar, and natural osmosis is the spontaneous flow of water from dilute to concentrated across a semipermeable membrane driven by the chemical potential difference; reverse osmosis must apply a pressure exceeding the osmotic pressure to drive water the other way, some 15 to 25 bar for brackish water and 55 to 80 bar for seawater, making it one or two orders of magnitude more energy-intensive. Filtration produces a backwash waste and a modest head loss; reverse osmosis produces a concentrated brine reject of 15 to 50 % of the feed, whose disposal is often the governing design constraint.

(c) Aerobic and anoxic treatment. Similarity: both are biological processes carried out by heterotrophic bacteria that oxidise organic carbon for energy, and in a biological nutrient removal plant both occur in the same activated-sludge system, often in adjacent zones sharing the same mixed liquor and the same biomass — indeed many of the same facultative organisms perform both, switching according to the electron acceptor available. Difference 1 — terminal electron acceptor and the resulting product: aerobic treatment uses dissolved molecular oxygen, maintained at 1.5 to 2.0 mg/L by mechanical or diffused aeration, oxidising carbon to CO₂ and water and permitting autotrophic nitrification of ammonia to nitrate; anoxic treatment operates with dissolved oxygen essentially absent, below about 0.2 mg/L, but with nitrate or nitrite present as the electron acceptor, and denitrifying bacteria reduce that nitrate to nitrogen gas, which leaves the system to the atmosphere. Note that anoxic is distinct from anaerobic, where neither oxygen nor nitrate is available and sulphate reduction and methanogenesis take over. Difference 2 — purpose and energy balance: the aerobic zone removes BOD and converts ammonia to nitrate, but aeration is the single largest energy consumer in a wastewater plant, typically half of the total demand; the anoxic zone removes total nitrogen by converting nitrate to nitrogen gas, needs only gentle mixing rather than aeration, and actually recovers energy and alkalinity — denitrification uses influent organic carbon as the electron donor, which reduces the downstream aeration demand by roughly 2.86 g of oxygen equivalent per gram of nitrate-nitrogen reduced, and returns about half the alkalinity that nitrification consumed. This is why the modified Ludzack–Ettinger configuration places the anoxic zone first, upstream of the aerobic zone, and recycles nitrified mixed liquor back to it: the arrangement uses the influent's own carbon as the denitrification substrate and saves both energy and supplemental methanol.