18-Env-A4 Water and Wastewater Engineering · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams / EGBC — May 2015 — 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.) — nitrogen and solids characterization, BOD test theory, nitrification/alkalinity, disinfection chemistry, activated-sludge and sludge-processing design; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — the Streeter–Phelps oxygen sag, pH and coagulation–flocculation chemistry, jar testing; MWH's Water Treatment: Principles and Design (3rd ed.) — granular filtration (headloss, backwash) and chemical phosphorus removal.
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) is the sum of organic nitrogen and ammonia nitrogen, $\text{TKN}=N_\text{org}+\text{TAN}$, measured by the Kjeldahl method: the sample is digested with hot concentrated $H_2SO_4$ (with a catalyst) to convert organic nitrogen to ammonium sulfate, the ammonia is then distilled off after alkalization and quantified by titration or colorimetry. TKN excludes the oxidized nitrogen species (nitrite and nitrate), so total nitrogen $=\text{TKN}+NO_2^--N+NO_3^--N$. Total Ammonia Nitrogen (TAN) is the sum of the un-ionized (free) ammonia and the ionized ammonium ion, $\text{TAN}=[NH_3]+[NH_4^+]$, which exist in a pH- and temperature-dependent equilibrium ($NH_4^+\rightleftharpoons NH_3+H^+$); the un-ionized fraction rises sharply above pH 8–9 and is the toxic species to aquatic life, so TAN and pH are both needed to assess ammonia toxicity or stripping potential.
Total Suspended Solids (TSS) is the mass of solids retained on a glass-fibre filter after drying a well-mixed sample at 103–105 °C, i.e. all the non-filterable (particulate) matter. Volatile Suspended Solids (VSS) is the fraction of that residue lost on ignition at 550 °C — the organic, combustible portion, taken as a practical surrogate for the biodegradable and biomass content of the solids. Inert (fixed) Suspended Solids is the non-volatile remainder, $\text{ISS}=\text{TSS}-\text{VSS}$, mostly mineral/grit material that is not degraded biologically. In activated-sludge design, MLVSS (the VSS fraction of mixed-liquor suspended solids) is used as the working measure of active biomass for food-to-microorganism ratio and sludge-age calculations, since MLSS alone includes inert solids that contribute no biological activity.
The oxygen sag curve describes how dissolved oxygen (DO) in a receiving stream falls and then recovers downstream of an organic (BOD) discharge, the result of two competing first-order processes: deoxygenation from BOD exertion by the discharged organics (rate constant $k_d$) and reaeration of the stream from the atmosphere (rate constant $k_r$). The Streeter–Phelps equation gives the DO deficit $D$ (saturation DO minus actual DO) at travel time $t$ downstream as $$D=\frac{k_dL_0}{k_r-k_d}\left(e^{-k_dt}-e^{-k_rt}\right)+D_0e^{-k_rt},$$ where $L_0$ is the ultimate BOD of the mixed stream at the discharge and $D_0$ the initial deficit. Immediately downstream, deoxygenation dominates and DO falls; as the upstream BOD is consumed, reaeration takes over and DO recovers. The lowest point on the curve is the critical deficit $D_c$ at the critical time $t_c=\dfrac{1}{k_r-k_d}\ln\!\left[\dfrac{k_r}{k_d}\left(1-\dfrac{D_0(k_r-k_d)}{k_dL_0}\right)\right]$ (found by setting $dD/dt=0$); this is where fish kills and worst water-quality impacts are expected, and is the design point for assimilative-capacity/effluent-limit calculations.
Free available chlorine is the disinfecting chlorine remaining in solution as hypochlorous acid ($HOCl$) and hypochlorite ion ($OCl^-$) once the immediate chlorine demand of the water has been satisfied; it is a fast, strong oxidant/disinfectant (a decade or more more effective per unit dose than combined chlorine). Combined residual chlorine is chlorine tied up as chloramines ($NH_2Cl$, $NHCl_2$, $NCl_3$), formed when free chlorine reacts with ammonia present in the water before the breakpoint is reached; chloramines are much weaker, slower-acting disinfectants but are more persistent (less volatile, fewer disinfection by-products), which is why they are sometimes deliberately used for distribution-system residual maintenance (chloramination) even though breakpoint chlorination is required to guarantee a strong free residual for primary disinfection.
Mercaptans (thiols, general formula $R\text{-}SH$) are organic sulfur compounds produced by the anaerobic microbial decomposition of sulfur-containing amino acids and proteins under septic (low- or no-oxygen) conditions in sewers, wet wells, or sludge-handling processes. They, together with hydrogen sulfide ($H_2S$), are the principal contributors to the offensive, garlic/skunk-like odours associated with septic wastewater; their odour threshold is extremely low (parts-per-billion range), so even trace concentrations are noticeable. Their presence is used operationally as an indicator of septicity/insufficient aeration or excessive detention time in collection and treatment systems, prompting odour-control measures (aeration, chemical oxidation, activated-carbon scrubbing).
