Question 4 of 19: Waste Constituents Requiring Removal Before Discharge

Question 4: Waste Constituents Requiring Removal Before Discharge (9 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.

Nine constituents commonly regulated in a wastewater discharge, and the reason each must be controlled, are as follows. Biochemical oxygen demand (BOD) is removed because its biodegradation consumes dissolved oxygen in the receiving water, potentially causing fish kills and anaerobic conditions. Suspended solids (SS) cause sludge-bank deposition near the outfall, increase turbidity (reducing light penetration for aquatic plants) and can smother benthic habitat. Pathogens (bacteria, viruses, protozoa) pose a direct public-health risk to downstream drinking-water intakes and recreational users. Nutrients (nitrogen and phosphorus) drive eutrophication — excessive algal growth that subsequently depletes oxygen when it decomposes. Toxic heavy metals (Cu, Pb, Hg, Cd, Cr) are acutely or chronically toxic to aquatic life and can bioaccumulate up the food chain to levels unsafe for human consumption. Oil and grease forms a surface film that blocks re-aeration and light penetration, coats gills and feathers, and interferes with downstream treatment processes. Extremes of pH are directly toxic to aquatic organisms and can mobilize metals otherwise bound in sediment. Colour (from dyes or lignin, common in textile and pulp-mill effluent) reduces light penetration and photosynthesis and is an aesthetic nuisance even at concentrations too low to be otherwise harmful. Elevated temperature (thermal discharge) reduces the saturation concentration of dissolved oxygen, raises the metabolic rate and oxygen demand of aquatic organisms, and can exclude cold-water species entirely from an otherwise suitable reach.