18-Env-A4 Water and Wastewater Engineering · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Examination, December 2019 — 18-Env-A4 Water and Wastewater Engineering (3 hours). Question 1 is compulsory; this study resource answers all five questions in full. Reference texts: Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); MWH’s Water Treatment: Principles and Design (3rd ed.); Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Guidelines for Canadian Drinking Water Quality (GCDWQ).
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.
Aerobic digestion stabilizes primary and/or waste-activated sludge by continuing to aerate it, essentially in a long-SRT, unfed activated-sludge tank, so the microbial population exhausts its available substrate and enters endogenous respiration, oxidizing its own cell mass (and the remaining biodegradable volatile solids) to $CO_2$, water, and a smaller residual biomass. It operates at ambient temperature, requires no special containment beyond an aerated, mixed open or covered tank, and produces a well-stabilized, relatively low-odour biosolids product, but it consumes substantial blower power throughout the (typically 15–20 day) detention period and yields no useful by-product gas.
Anaerobic digestion stabilizes sludge in a sealed, heated, mixed reactor with no oxygen present, through a four-stage microbial sequence — hydrolysis (complex organics broken to soluble sugars, amino acids, fatty acids), acidogenesis (fermentation to volatile fatty acids, $CO_2$, $H_2$), acetogenesis (conversion to acetate, $H_2$, $CO_2$), and methanogenesis (strictly anaerobic archaea convert acetate and $H_2$/$CO_2$ to methane) — typically run mesophilic ($\approx35\,{}^{\circ}\text{C}$) or thermophilic ($\approx55\,{}^{\circ}\text{C}$), at SRTs on the order of 15–30 days. The process destroys a substantial fraction of the volatile solids while generating biogas (roughly 60–65% methane, the balance mainly $CO_2$) as a captured by-product.
Two major advantages of anaerobic over aerobic digestion: (1) Anaerobic digestion is a net energy producer — the captured methane can be combusted to heat the digester itself and, at larger plants, to generate electricity or pipeline-quality renewable natural gas, whereas aerobic digestion is a continuous net energy consumer, since the blowers must run for the entire, multi-week detention period with no offsetting energy recovery. (2) Anaerobic digestion achieves greater volatile-solids destruction and correspondingly lower net biosolids production for disposal, because a larger share of the influent organic mass leaves the process as biogas rather than remaining as residual cell/inert mass — directly reducing biosolids hauling and disposal cost, which scales with mass. (Aerobic digestion is nonetheless often preferred at smaller plants, where its simpler operation, lower capital cost, and avoidance of a heated/sealed digester outweigh these two advantages.)
Nitrogen in raw municipal wastewater is measured and tracked in several distinct forms. Organic nitrogen is nitrogen bound in proteins, amino acids, and urea, most of which rapidly hydrolyzes/ammonifies to ammonia in the collection system and treatment process. Ammonia nitrogen exists in equilibrium between the ionized ammonium ion ($NH_4^+$) and the un-ionized, dissolved-gas form ($NH_3$, "free ammonia," the toxic and regulatory form of concern, whose fraction of the total rises sharply with pH); the sum $NH_4^++NH_3$ is total ammonia nitrogen (TAN). Organic nitrogen plus TAN together make up Total Kjeldahl Nitrogen (TKN), the standard raw-wastewater nitrogen parameter. Further downstream, biological nitrification produces nitrite ($NO_2^-$, an unstable intermediate) and then nitrate ($NO_3^-$, the stable oxidized end product); nitrite plus nitrate is termed oxidized nitrogen, and TKN plus oxidized nitrogen is total nitrogen (TN).
The two key mechanisms of nitrogen removal in wastewater treatment are: (1) Biological nitrification–denitrification, the dominant mechanism in modern plants — nitrifying autotrophs ($Nitrosomonas$ oxidizing $NH_4^+\rightarrow NO_2^-$, then $Nitrobacter$/$Nitrospira$ oxidizing $NO_2^-\rightarrow NO_3^-$) convert ammonia to nitrate under aerobic conditions (long SRT, since nitrifiers grow slowly), and heterotrophic denitrifiers then reduce that nitrate to nitrogen gas ($N_2$, released harmlessly to the atmosphere) under anoxic conditions, using the nitrate as their terminal electron acceptor in place of oxygen and BOD as the electron donor/carbon source. (2) Assimilation into biomass — a portion of the influent nitrogen (organic and ammonia) is incorporated directly into new bacterial cell mass as the microorganisms grow, and is permanently removed from the liquid stream when that biomass is wasted as sludge; this pathway typically accounts for a smaller share of total nitrogen removal than nitrification–denitrification in a conventional plant, but requires no additional process step beyond the biological treatment already provided for BOD removal. (Physical–chemical alternatives — ammonia air stripping at elevated pH, breakpoint chlorination, and ion exchange — exist but are comparatively energy- or chemical-intensive and are reserved for niche applications rather than being the primary removal pathway.)
