11-CS-3 Engineering Management · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 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.
Turbidity: cloudiness of water from suspended particles (light scattering, NTU). BOD: dissolved oxygen consumed by microbes decomposing organic matter—a measure of organic pollution. Watershed: the land area draining to a common outlet. Embodied (virtual) water: total water used to produce a product. Water footprint: total freshwater used to produce the goods/services a person or entity consumes. Hydrologic cycle: the continuous movement of water through evaporation, condensation, precipitation, infiltration, and runoff.
In coagulation, a chemical coagulant (such as alum) is rapidly mixed into the water; its positive charge neutralizes the negative surface charge on tiny colloidal particles that otherwise repel one another and stay suspended. In flocculation, the water is gently stirred so the destabilized particles collide and aggregate into larger, heavier clumps called floc. In sedimentation, the water sits in a quiescent basin so the heavy floc settles out under gravity and is removed as sludge. These processes are important for microbiological quality because many bacteria, viruses, and especially protozoan cysts are associated with or similar in size to the colloidal particles; removing the particles removes a large fraction of the pathogens and, critically, reduces turbidity so that the subsequent disinfection can actually reach and inactivate the remaining micro-organisms. They are the essential pre-treatment that makes filtration and disinfection effective.
Chlorination: advantage—effective, inexpensive, and provides a lasting residual that protects water throughout the distribution system; disadvantage—forms potentially harmful disinfection by-products (trihalomethanes) with organic matter, and can impart taste/odour. Ultraviolet (UV) disinfection: advantage—effective against bacteria, viruses, and chlorine-resistant protozoa (Cryptosporidium) with no chemical by-products; disadvantage—leaves no residual protection in distribution, requires clear (low-turbidity) water, and needs a reliable power supply. (Ozone is a third option: powerful and by-product-light on taste, but costly and residual-free.)
Water table: the upper surface of the saturated zone, where groundwater pressure equals atmospheric pressure. Aquifer: a permeable geologic formation that stores and readily transmits usable quantities of groundwater. Aquitard: a low-permeability layer that restricts groundwater flow (confining or semi-confining a nearby aquifer). Vadose (unsaturated) zone: the region between the ground surface and the water table, where pores contain both air and water. Potentiometric surface: the level to which water would rise in wells tapping a confined aquifer (the hydraulic-head surface). Darcy's Law: the relation that groundwater flow rate is proportional to the hydraulic gradient and the hydraulic conductivity of the medium ($Q = -KA\,dh/dl$).
