11-CS-3 Engineering Management · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2015 — 11-CS-3 Sustainability, Engineering and the Environment. Closed book; approved calculator permitted. Any four questions constitute a complete paper; all questions are of equal value (25 marks each).
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.
Suspended solids: undissolved particles suspended in water, removable by filtration/settling (contribute to turbidity). BOD: dissolved oxygen consumed by microbes decomposing organic matter—a measure of organic pollution. Embodied (virtual) water: total water used to produce a product. Hydrologic cycle: the continuous movement of water via evaporation, condensation, precipitation, infiltration, and runoff. Water table: the top of the saturated zone (pressure = atmospheric). Aquitard: a low-permeability layer restricting groundwater flow. Vadose zone: the unsaturated zone between the surface and the water table. Potentiometric surface: the level to which water rises in wells tapping a confined aquifer.
The correct order is coagulation-flocculation → sedimentation → filtration. In coagulation-flocculation, a coagulant (alum) is rapidly mixed to neutralize the charge on colloidal particles, then gentle mixing aggregates them into settleable floc. In sedimentation, the heavy floc settles out under gravity in a quiescent basin and is removed as sludge, taking out most suspended solids and turbidity. In filtration, the clarified water passes through sand/multimedia beds that remove the remaining fine particles and protozoan cysts, polishing it before disinfection. Each step removes progressively finer material, and the sequence prepares the water for effective disinfection.
After four days about 75 cell/mL remain viable.
Chlorination: advantage—effective, cheap, and provides a lasting residual protecting distribution; disadvantage—forms disinfection by-products (THMs) and can affect taste. Ultraviolet (UV): advantage—no chemical by-products and effective against chlorine-resistant protozoa; disadvantage—no residual, requires clear water and power. Ozonation: advantage—a very powerful oxidant/disinfectant that also improves taste and odour; disadvantage—expensive, energy-intensive, and leaves no residual (and can form bromate). Each trades effectiveness against by-products, residual protection, and cost.
