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.

(i) An engineered example of wastewater reuse (5 marks)

The example: a municipal water reclamation and purple-pipe distribution scheme. A secondary wastewater treatment plant serving a growing community is extended with a reclamation train, and the reclaimed water is distributed through a separate, purple-coloured non-potable network to irrigate parks, playing fields, golf courses and highway landscaping, to supply toilet flushing in nearby commercial buildings, and to provide cooling-tower makeup and construction dust suppression. Vancouver's Southeast False Creek and several Okanagan municipalities operate schemes of exactly this form, and irrigation is the natural target because it is the peak-day demand that sizes the potable system.

Four engineering principles govern the design.

Fit-for-purpose treatment. Water is treated to the quality the end use requires, and no further — the principle that makes reuse economic. Unrestricted public-access irrigation requires a train of secondary treatment, coagulation and tertiary granular or membrane filtration to a turbidity below 2 NTU, and disinfection by ultraviolet light or chlorine to a fecal coliform target of the order of 2.2 per 100 mL, together with a maintained residual in the distribution network. Restricted-access agricultural irrigation may be satisfied by secondary treatment and disinfection alone. The quality target is set by the exposure pathway, and the pathway is set by the use.

The multi-barrier principle and log-removal accounting. Because the source is by definition contaminated, protection cannot rest on a single unit process. The design assigns quantified log-removal credits for enteric virus, protozoa and bacteria across successive independent barriers — secondary treatment, filtration, disinfection — so that the failure of any one barrier does not produce a failure of the system, and specifies continuous online monitoring of turbidity, ultraviolet transmittance and disinfectant residual with automatic diversion to the outfall whenever a critical limit is breached.

Physical separation and cross-connection control. The reclaimed network is entirely separate from the potable network, colour-coded purple to AWWA convention, with tagged and distinctively shaped valves and fittings, no hose bibbs on the reclaimed side, and mandatory reduced-pressure-principle backflow preventers on every potable service in the served area. A programme of periodic cross-connection surveys and dye testing verifies the separation. This is the barrier that protects public health against human error rather than against pathogens, and it is where reuse schemes fail when they fail.

Storage to reconcile mismatched patterns. Wastewater is produced continuously and fairly uniformly; irrigation demand is seasonal, diurnal and concentrated into a few night-time hours. Balancing reservoirs and, in some schemes, aquifer storage and recovery bridge the mismatch, and the potable system remains available as a backup supply through an air-gapped connection.

The water-resource outcome. The scheme achieves a double benefit from a single work: potable demand falls by the reclaimed volume — typically 20 to 40 % of summer municipal demand, precisely the peak that drives source, treatment and transmission sizing — while the nutrient and organic load discharged to the receiving water falls by the same volume. Deferring a source expansion and a plant expansion simultaneously is what makes the economics work, and it is a clear illustration of the water-use cycle being closed rather than run as a straight line from source to outfall. In Canada the practice is governed by the CCME Canada-wide Strategy for the Management of Municipal Wastewater Effluent and the Guidelines for Canadian Reclaimed Water, with provincial instruments such as BC's Municipal Wastewater Regulation setting the enforceable quality and monitoring requirements.

(ii) Three key engineering principles in sand filter operation (6 marks)

Principle 1 — filtration is a depth process of transport and attachment, and it is chemically dependent. A rapid sand filter does not act as a sieve. The pore openings between grains of 0.5 mm effective size are of the order of 80 µm, while the particles to be removed — clay colloids, Giardia cysts, Cryptosporidium oocysts, bacteria — span roughly 1 to 15 µm and would pass straight through a sieve of that opening. Removal occurs instead throughout the depth of the bed in two sequential stages. Transport carries the particle from the bulk flow to the vicinity of a grain surface by interception, gravitational settling and, for the smallest particles, Brownian diffusion. Attachment then holds it there, and attachment depends entirely on surface chemistry: natural particles and quartz sand both carry a negative surface charge and repel one another, so an uncoagulated water passes through a sand filter almost unchanged. The practical consequence is the single most important operational fact about a rapid filter — it is only as good as the coagulation ahead of it. Proper coagulant dose control, verified by streaming-current or zeta-potential measurement and by jar testing against the raw-water temperature and alkalinity, is a filter-performance variable, not merely a clarifier variable. Slow sand filters are the partial exception: they operate at 0.1–0.4 m/h and rely on the biologically active schmutzdecke at the surface for both straining and predation, which is why they can function without coagulant.

Principle 2 — head loss develops as the bed loads, and the run ends at whichever of terminal head loss or turbidity breakthrough arrives first. A clean bed's head loss is predicted by the Carman–Kozeny or Rose equation, in which loss varies with the square of the flow velocity, inversely with the square of the grain size, and steeply with porosity — typically 0.3 to 0.6 m for a clean bed at a rapid filtration rate of 5 to 15 m/h. As deposits accumulate in the pores the porosity falls and the loss climbs, at first gradually and then sharply. Operation is designed around this curve. In constant-rate operation the effluent control valve opens progressively to hold the rate as the media resistance grows, so the available head is transferred from the valve to the bed until the terminal head loss, commonly 2.5 to 3 m, is reached. The filter must be backwashed at that point, and the depth of water above the media must be sufficient that the pressure never falls below atmospheric within the bed — the negative-head condition, which releases dissolved gases into the pores, air-binds the filter and causes a sudden loss of both capacity and quality. The run may equally be ended by turbidity breakthrough, when the shear from the increasing interstitial velocity detaches previously captured particles and the effluent turbidity begins to rise. A well-designed filter reaches both limits at about the same time; if breakthrough consistently arrives first, the problem is upstream in the coagulation, and if head loss always governs, the media may be too fine for the water.

Principle 3 — media specification and backwashing determine whether the bed is restored to its design condition. The media are specified by effective size $d_{10}$, the sieve size passing 10 % by mass, and by uniformity coefficient $d_{60}/d_{10}$, which is held below about 1.5 so that the bed does not stratify excessively during backwash. A common arrangement is dual media — coarse, light anthracite over fine, dense silica sand — which after backwash restratifies with the coarse layer on top, so that the water encounters progressively finer media as it descends. This gives depth utilisation and long runs, whereas a single sand medium restratifies fine-on-top and clogs at the surface. Backwashing reverses the flow at 35–50 m/h to fluidise the bed to 20–30 % expansion, and because fluidised grains rarely collide, hydraulic backwash alone is often insufficient; air scour or surface wash is added to provide the abrasion that detaches attached floc, without which mud balls form and the bed progressively loses capacity. Two further requirements complete the principle: the underdrain and wash-water troughs must distribute and collect uniformly, since a poorly distributed backwash cleans part of the bed while leaving the rest fouled, and the filter must be returned to service through a filter-to-waste or slow-start ripening period, because turbidity and pathogen passage are highest in the first 15 to 30 minutes after a wash. That initial spike is the single largest source of pathogen passage in a well-run plant, and controlling it is why filter-to-waste is required for Cryptosporidium log-removal credit.

(iii) Two main differences between the paired terms (9 marks)

(a) Coagulation and flocculation (3 marks)

First difference — the nature of the process. Coagulation is a chemical process: it destabilises colloidal particles by adding a hydrolysing metal salt such as alum or ferric chloride, which compresses the electrical double layer and neutralises the negative surface charge, or which precipitates as a hydroxide that enmeshes the colloids in sweep-floc. Nothing grows in size; the particles merely lose the electrostatic repulsion that keeps them apart. Flocculation is a physical process: gentle mixing induces velocity gradients that bring the now-destabilised particles into contact, and successive collisions aggregate them into settleable floc. Coagulation makes collisions effective; flocculation makes them happen.

Second difference — the mixing energy and the time scale, which are opposite in each case. Coagulation requires rapid, intense mixing to disperse the coagulant throughout the water before hydrolysis is complete — a velocity gradient G of 500 to 1000 s^-1 for a detention time of a few seconds to a minute. Flocculation requires the reverse: gentle mixing at G of 20 to 70 s^-1 for 20 to 30 minutes, tapered downward through successive stages, because too much shear tears the growing floc apart faster than it forms. The design parameter is the Camp number $Gt$, held in the range 10⁴ to 10⁵. Applying rapid-mix energy in the flocculation basin, or under-mixing at the coagulant injection point, are the two classic causes of poor settled-water quality, and they are opposite errors.

(b) Anoxic and aerobic treatment (3 marks)

First difference — the terminal electron acceptor. Aerobic treatment uses free dissolved molecular oxygen, maintained at 1.5 to 2.0 mg/L in the mixed liquor. Anoxic treatment operates in the deliberate absence of free dissolved oxygen — below about 0.2 to 0.5 mg/L — and the facultative bacteria present instead respire using the combined oxygen in nitrate and nitrite. Anoxic is emphatically not the same as anaerobic: anaerobic conditions have neither free nor combined oxygen and support fermentation, sulphate reduction and methanogenesis, which is a different biochemistry and a different set of organisms. Confusing the two is the single most common error on this term.

Second difference — the purpose, the products and the energy balance. The aerobic zone oxidises carbonaceous BOD to carbon dioxide and biomass and, at sufficient solids retention time, allows the autotrophic nitrifiers to convert ammonia to nitrate — a process that consumes 4.57 kg of oxygen and destroys 7.14 kg of alkalinity as CaCO₃ per kilogram of nitrogen oxidised, and which requires continuous aeration, typically half of a plant's entire electrical consumption. The anoxic zone reduces that nitrate to nitrogen gas, which leaves the system harmlessly to the atmosphere, using influent BOD as the electron donor. Its benefits compound: it removes total nitrogen, it recovers roughly half the destroyed alkalinity, it uses the nitrate's combined oxygen to oxidise BOD that would otherwise need blower air — so it reduces aeration energy — and, in the pre-anoxic Modified Ludzack–Ettinger configuration, it does all of this using the raw wastewater's own carbon rather than a purchased methanol supplement. This is why virtually every nitrogen-removing plant is arranged as an anoxic zone followed by an aerobic zone with internal mixed-liquor recycle, rather than the reverse.

(c) Municipal wastewater and sludge (3 marks)

First difference — solids concentration and physical character. Municipal wastewater is a dilute aqueous stream that is about 99.9 % water, carrying total suspended solids of roughly 100 to 350 mg/L, BOD₅ of 110 to 400 mg/L and total nitrogen of 20 to 70 mg/L. It behaves as a Newtonian fluid, flows in open channels and pipes under gravity, and is handled in volumes of megalitres per day. Sludge is the concentrated solids residual separated out of that stream: 0.5 to 1 % solids as waste activated sludge, 3 to 6 % after gravity or mechanical thickening, and 20 to 30 % as a dewatered cake that is handled by conveyor and truck rather than by pipe. Above roughly 2 % solids it is markedly non-Newtonian and pseudoplastic, so pumping design must use apparent viscosity and cannot rely on the Hazen–Williams or Darcy–Weisbach relations appropriate to the liquid stream.

Second difference — the treatment objective and the disproportionate share of plant cost. The liquid stream is treated to remove constituents so that the effluent may be discharged to a receiving water under a permit, using screening, grit removal, sedimentation, biological treatment and disinfection. The sludge stream is treated to reduce and stabilise a material that is not going to be discharged at all, using thickening to reduce volume, anaerobic or aerobic digestion to destroy volatile solids and pathogens, dewatering to reduce mass for transport, and finally beneficial use as biosolids on agricultural land, composting, or landfill. The engineering point that is easy to miss is the asymmetry of scale and cost: sludge represents only about 1 to 2 % of the influent volume, yet its handling routinely accounts for 30 to 50 % of a treatment plant's capital and operating cost, and it is the source of most of a plant's odour complaints and regulatory difficulty. Its end use is separately regulated — by the CCME Guidance Document for the Beneficial Use of Municipal Biosolids and, in British Columbia, by the Organic Matter Recycling Regulation, which set metals limits, pathogen classes and vector-attraction-reduction requirements that have no counterpart in the liquid-effluent permit.