18-Env-A4 Water and Wastewater Engineering · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved calculator permitted. Question 1 is compulsory; the paper instructs candidates to attempt any three of the remaining four (100 marks total); all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — BOD kinetics, activated-sludge clarifier design, anaerobic digestion; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — hardness, alkalinity, chlorination chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — rapid sand filtration; Guidelines for Canadian Drinking Water Quality (Health Canada).
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.
A rapid (gravity) sand filter removes residual floc, turbidity and pathogens that survive coagulation-flocculation-sedimentation by passing settled water downward through a bed of graded sand at a comparatively high hydraulic loading (typically 5–15 m/h, versus <0.4 m/h for a slow sand filter). Removal occurs mainly by mechanical straining at the surface plus, more importantly, depth filtration through the bed — particles are captured throughout the sand's depth by physical adsorption/attachment onto sand-grain surfaces (interception, sedimentation on grains, and Brownian diffusion of the smallest particles) rather than being screened out purely by pore size.
As filtration proceeds, captured floc accumulates in the sand's pore spaces, progressively raising headloss across the bed (from a clean-bed value of ≈0.3 m up to a terminal headloss of 2–3 m) and, eventually, degrading effluent quality as the bed's capture sites saturate ("breakthrough"). At that point the filter is taken offline and cleaned by backwashing: water (often with an air scour) is pumped UP through the underdrain at a rate sufficient to fluidize and expand the sand bed (typically 40–50% bed expansion), scouring captured floc off the grains and carrying it out through wash-water troughs to waste; the bed then resettles by gravity (coarser/denser grains first) ready for the next filtration run. A well-run filter cycle is therefore a repeating sequence of filtration (hours to a day or two, governed by headloss or turbidity breakthrough) followed by a short (10–15 minute) backwash.
When chlorine is added to water containing ammonia, it first reacts to form chloramines (combined residual) while free residual stays near zero; as more chlorine is added the combined residual initially rises, peaks, and then falls as chlorine oxidizes the chloramines to nitrogen gas and other end products — this destruction of combined chlorine drives total and combined residual down to a minimum, the break point. Any chlorine dosed beyond the break point appears almost entirely as free available chlorine, which is the strong, fast disinfectant species. Break-point chlorination is the practice of deliberately dosing past this minimum so that the finished water carries a free (not combined) residual — it destroys ammonia, most taste-and-odour-causing compounds, and gives far more reliable, faster-acting disinfection credit than operating on the combined-residual side of the curve.
Superchlorination is dosing chlorine well in excess of the break point (and in excess of normal disinfection demand) — a much heavier dose used for a specific purpose: rapid, aggressive disinfection during an emergency (e.g. after a contamination event or main break), oxidation of taste-and-odour compounds, or iron/manganese/sulphide oxidation. Because the resulting free residual is far above the level acceptable for distribution (and above taste thresholds), superchlorination is normally followed by dechlorination (e.g. with sulphur dioxide or activated carbon) before the water is placed into service or discharged.
