18-Env-A4 Water and Wastewater Engineering · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved Casio/Sharp calculator permitted. Question 1 is compulsory; the paper instructs candidates to attempt any three of the remaining four questions — all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — population equivalent, oxygen sag/Streeter–Phelps, activated-sludge process control (RAS/WAS, HRT/SRT), secondary clarifier design; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — turbidity, alkalinity chemistry, digester fundamentals; MWH’s Water Treatment: Principles and Design (3rd ed.) — coagulation-flocculation mechanisms, ozonation, disinfection by-products, pH.
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.
Raw-water turbidity and colour are carried largely by colloidal particles whose surfaces are negatively charged, so mutual electrostatic repulsion keeps them in stable suspension indefinitely; coagulation-flocculation destabilizes and aggregates those particles into settleable floc through several distinct, often simultaneous, mechanisms.
Inter-particle bridging occurs when a high-molecular-weight polymer (natural or synthetic, typically anionic or nonionic) adsorbs onto more than one colloidal particle at once along its extended molecular chain, physically bridging them into a loose, three-dimensional floc network. It works best at low-to-moderate polymer dose — overdosing saturates every particle’s surface with polymer and leaves no adsorption sites free to bridge to a neighbour, a condition called restabilization.
Sweep coagulation is produced by dosing a hydrolyzing metal salt (alum, ferric chloride) well above its solubility limit for the water’s pH, so that a voluminous, amorphous metal-hydroxide precipitate (Al(OH)3 or Fe(OH)3) forms throughout the bulk water; colloidal particles are physically enmeshed and swept down as the precipitate itself settles, rather than being individually charge-neutralized. Sweep floc is the dominant, most robust mechanism at the metal-salt doses typically used in conventional water-treatment practice.
Ionic layer (double-layer) compression destabilizes colloids purely by raising the ionic strength of the water — adding indifferent electrolyte ions compresses the diffuse counter-ion layer surrounding each charged particle, shortening the range over which electrostatic repulsion acts until van der Waals attraction dominates at close approach and particles can collide and stick. Unlike charge neutralization by an adsorbing coagulant, compression works regardless of the sign of the added ion and depends only on ionic strength (and, for multivalent ions, the Schulze–Hardy rule that coagulating power rises sharply with counter-ion valence); it is the least commonly exploited of the three mechanisms in engineered water treatment because it requires impractically large electrolyte doses at typical natural-water ionic strengths.
Ozone (O3) is generated on site (it is unstable and cannot be stored) by passing dry oxygen or air through a high-voltage electrical discharge, then bubble-diffused or injected into the water to be disinfected. It inactivates microorganisms primarily through direct oxidative attack on cell walls, enzymes and nucleic acids by molecular O3 itself and by the hydroxyl radicals (•OH) generated as ozone decomposes in water — both are far stronger oxidants than free chlorine, giving ozone the highest disinfection potency (lowest CT requirement) of the disinfectants in common use, including strong efficacy against protozoan cysts such as Cryptosporidium that resist chlorine.
Advantages over chlorination: (1) Ozone leaves essentially no chlorinated disinfection by-products (THMs, HAAs) because it contains no chlorine, and its own principal by-product, bromate, is only a concern in bromide-bearing source waters. (2) Its far stronger oxidizing power gives much faster, more complete inactivation of resistant pathogens (Cryptosporidium, viruses) at a lower CT, and it simultaneously improves taste, odour and colour by oxidizing the organic compounds responsible for them.
Disadvantages over chlorination: (1) Ozone provides no residual disinfectant in the distribution system — it decomposes within minutes of dosing — so a secondary disinfectant (usually chlorine or chloramine) is still required downstream to protect against regrowth and recontamination in the pipe network. (2) It is markedly more capital- and energy-intensive: on-site generation equipment, high electrical demand, and off-gas ozone-destruction systems raise both construction and operating cost well above a simple chlorine feed system, and ozone's short half-life means it must be generated and applied continuously rather than stored.
