18-Env-A4 Water and Wastewater Engineering · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams / EGBC — December 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.) — grit removal, the BOD test and azide modification, nitrogen speciation and removal, anaerobic digestion, sludge volume index; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — the Streeter–Phelps oxygen sag, indicator organisms, water intake structures, distribution-system layout; MWH's Water Treatment: Principles and Design (3rd ed.) — ion exchange, fluoridation/defluoridation.
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.
Total Kjeldahl nitrogen (TKN) in raw municipal wastewater is split between organic nitrogen (bound in proteins, urea and cell material, both particulate and soluble) and ammonia nitrogen ($NH_3/NH_4^+$, the dominant dissolved inorganic species, released as organic nitrogen is hydrolyzed and deaminated). As the wastewater is biologically treated, ammonia is progressively oxidized through nitrite ($NO_2^-$, an unstable intermediate rarely present in large amounts) to nitrate ($NO_3^-$, the stable, fully-oxidized end product); a small amount of gaseous nitrogen gas ($N_2$) appears as the final removal product once denitrification occurs. Total nitrogen in an effluent is therefore the sum of organic-N, ammonia-N, nitrite-N and nitrate-N.
The two key removal mechanisms both rely on this speciation sequence. The first is biological nitrification–denitrification: nitrifying autotrophs (Nitrosomonas, then Nitrobacter) oxidize ammonia to nitrate in an aerobic zone ($NH_4^++2O_2\rightarrow NO_3^-+2H^++H_2O$), consuming oxygen and alkalinity, and heterotrophic denitrifiers then reduce that nitrate to nitrogen gas in an anoxic zone using an available carbon source as electron donor ($NO_3^-\rightarrow NO_2^-\rightarrow N_2\uparrow$), which is stripped from the liquid to atmosphere — this is the dominant mechanism in modern activated-sludge plants (e.g. Modified Ludzack–Ettinger/Bardenpho configurations) because it can achieve high (>80–90%) total-nitrogen removal at moderate cost. The second is ammonia stripping, a physical–chemical method that raises pH (to shift the $NH_4^+/NH_3$ equilibrium toward the volatile un-ionized $NH_3$ form, roughly pH ≥ 10.5–11) and contacts the wastewater with a large air (or steam) flow in a packed tower, driving dissolved $NH_3$ into the gas phase; it avoids a biological process but needs substantial lime dosing (with associated sludge and cost) and can freeze or scale in cold climates, so it is used mainly where nitrification is impractical or where a very high, targeted ammonia load must be pulled out quickly (e.g. sidestream treatment of digester dewatering liquor).
Free chlorine ($HOCl/OCl^-$) is a strong, non-selective oxidant, so it reacts with both organic matter and ammonia before it is available to disinfect. Ammonia reacts along the breakpoint curve to form progressively weaker combined-chlorine species (chloramines): $NH_3+HOCl\rightarrow NH_2Cl+H_2O$, then $NHCl_2$, then $NCl_3$; these are roughly 1/25–1/100 as effective a disinfectant per unit residual as free chlorine, so any ammonia present must first be satisfied — the dose driven past the breakpoint — before an effective free-residual is achieved, substantially raising chlorine demand. Organic matter (natural organic matter, humic/fulvic acids) both exerts its own chlorine demand and, more importantly, reacts with chlorine to form regulated disinfection by-products (DBPs) — trihalomethanes and haloacetic acids — so a high-organics source both raises the required dose and increases DBP formation risk.
UV disinfection inactivates pathogens by damaging nucleic acids (forming pyrimidine dimers that block replication) rather than by chemical oxidation. Two advantages over chlorination: (1) it forms no chlorinated DBPs and leaves no chemical taste/odour; (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. Two disadvantages: (1) UV leaves no residual disinfectant, so it provides no ongoing protection against regrowth/recontamination in the distribution system, unlike chlorine's persistent residual; (2) its effectiveness depends strongly on water clarity/UV transmittance — turbidity and particulates shield organisms from the dose, requiring good pretreatment that chlorination does not strictly need.
