18-Env-A4 Water and Wastewater Engineering · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams / EGBC — May 2016 — 04-ENV-A4 Water and Wastewater Engineering. Three-hour exam; Question 1 is compulsory (25 marks) and any three of the remaining four questions are required (25 marks each); all five are solved below for completeness. Closed book, one double-sided aid sheet permitted, approved calculator permitted.
Reference texts: Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — nitrification, BOD test theory, alkalinity/anaerobic digestion, phosphorus removal, disinfection chemistry; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — the Streeter–Phelps oxygen sag, pH and coagulation–flocculation chemistry, water hardness; MWH's Water Treatment: Principles and Design (3rd ed.) — granular filtration (headloss, backwash) and ion exchange.
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.
Municipal wastewater carries phosphorus in three broad forms. Orthophosphate ($PO_4^{3-}$, $HPO_4^{2-}$, $H_2PO_4^-$, depending on pH) is the dissolved, immediately bioavailable inorganic form and the only species that reacts directly with a metal salt or is taken up across a cell membrane. Condensed (poly-) phosphates — from detergents and some industrial sources — are chains/rings of phosphate units (e.g. $P_2O_7^{4-}$, $P_3O_{10}^{5-}$) that slowly hydrolyze back to orthophosphate in the collection system and treatment plant. Organic phosphorus is bound in cell material, nucleic acids and phospholipids, entering the plant as particulate and soluble organic matter and released as orthophosphate on cell lysis or biological decay. Total P is the sum of all three, typically 60–70% ortho-P in raw municipal wastewater.
The two key removal mechanisms are chemical precipitation and biological (enhanced) phosphorus removal. Chemical precipitation doses a metal salt — alum ($Al_2(SO_4)_3$) or ferric chloride ($FeCl_3$) — that reacts with ortho-P to form an insoluble metal phosphate floc ($FeCl_3+PO_4^{3-}\rightarrow FePO_4\downarrow+3Cl^-$), which is then removed with the sludge in a clarifier or filter; it is fast, reliable and independent of biology, but consumes alkalinity, adds chemical sludge, and cannot remove condensed/organic P until it hydrolyzes. Biological removal (EBPR) exploits phosphorus-accumulating organisms (PAOs), which are cycled through an anaerobic zone (where they release stored P while taking up volatile fatty acids as internal carbon reserves, PHA) followed by an aerobic (or anoxic) zone (where they oxidize the PHA and take up phosphorus in excess of normal growth needs, storing it as polyphosphate); the P is then removed from the system in the wasted biomass. EBPR avoids chemical cost and sludge but is sensitive to influent VFA/COD availability, nitrate carry-over into the anaerobic zone (which lets denitrifiers out-compete PAOs for the VFA), and temperature.
Free chlorine ($HOCl/OCl^-$) is a strong oxidant and reacts readily with both organic compounds and ammonia before it can act as a disinfectant. With ammonia, chlorine forms chloramines along the breakpoint curve ($NH_3+HOCl\rightarrow NH_2Cl+H_2O$, then $NHCl_2$, $NCl_3$) — these combined-chlorine species are far weaker (roughly 1/25–1/100 as effective) disinfectants than free chlorine per unit residual, so any ammonia present must be satisfied (driven past the breakpoint) before an effective free-residual disinfectant dose is achieved, substantially raising the chlorine demand. Organic compounds (natural organic matter, humic/fulvic acids) consume chlorine directly (exerting a chlorine demand of their own) and, more importantly, react with it to form disinfection by-products (DBPs) — trihalomethanes (THMs) and haloacetic acids (HAAs) — which are regulated carcinogens/health concerns, so a high-organics source both raises chlorine demand and drives DBP formation.
UV disinfection inactivates pathogens by damaging their nucleic acids (forming pyrimidine dimers that block replication) rather than by chemical oxidation, giving two clear advantages over chlorination: (1) it forms no chlorinated DBPs and adds no chemical residual/taste-odour concern, and (2) it is highly effective against chlorine-resistant protozoan cysts/oocysts (Cryptosporidium, Giardia), which is why UV is now the standard barrier for those organisms in Canadian drinking-water practice. Its two disadvantages: (1) UV leaves no residual disinfectant, so it provides no protection against microbial regrowth or recontamination in the distribution system (chlorine's persistent residual does); and (2) UV effectiveness depends strongly on water clarity/UV transmittance — turbidity and particulate matter shield organisms from the dose and can cause under-disinfection, requiring pretreatment (filtration) that chlorination does not strictly need.
