Question 1 of 5: Definitions — Water and Wastewater Terminology

Question 1: Definitions — Water and Wastewater Terminology (25 marks)

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.

i. Grit removal in wastewater treatment (6 marks)

Grit removal is the preliminary-treatment unit process that separates dense, mostly-inert inorganic solids — sand, gravel, coffee grounds, small food-waste particles, metal fragments — from the lighter, putrescible organic solids that the rest of the plant is designed to treat. It is placed immediately after screening and before primary sedimentation because uncontrolled grit abrades pump impellers and piping, accumulates in digesters and channels (reducing effective volume and requiring costly cleanout), and would otherwise settle preferentially with the organic sludge and complicate downstream sludge handling. Removal exploits the large difference in specific gravity between grit (roughly 2.65, quartz sand) and organic solids (close to 1.0, water) rather than particle size alone, so the unit is sized to a settling (or scour) velocity that lets the dense grit fall out while carrying the lighter organics through to the clarifiers. Common configurations are horizontal-flow (velocity-controlled) channels, aerated grit chambers, and vortex (centrifugal) grit removal units; all are typically sized for an influent design velocity near 0.3 m/s and a grit particle target of about 0.2 mm or larger, and the recovered grit is washed (to strip organics off its surface) before landfill disposal.

ii. Carbonaceous BOD5 and azide modification of BOD5 test (7 marks)

The standard 5-day, 20 °C BOD5 test measures total oxygen consumption by micro-organisms in a sample, which in a nitrifying sample includes both the carbonaceous demand (heterotrophic oxidation of organic carbon) and the nitrogenous demand (autotrophic oxidation of ammonia to nitrite/nitrate by Nitrosomonas/Nitrobacter). Carbonaceous BOD5 (cBOD5) isolates the carbon-oxidation share alone by suppressing nitrification — typically by adding a nitrification inhibitor (e.g. 2-chloro-6-(trichloromethyl)pyridine, TCMP/“nitrification inhibitor”) to the bottle before incubation — so the DO depletion reflects only organic-carbon oxidation, which is the parameter usually regulated in a discharge permit since it better isolates the stream-oxygen-demand impact of the effluent's carbon load from a separately-limited ammonia parameter.

The azide modification (Standard Methods 4500-O C) is a titrimetric procedure for measuring the dissolved oxygen at the start and end of the BOD incubation. Manganous sulfate and alkali-iodide-azide reagent are added to fix the DO as a manganese hydroxide floc (Winkler method); the sodium azide destroys nitrite interference (nitrite would otherwise falsely liberate iodine and bias the titration high), which matters because nitrite is commonly present in partially-nitrified BOD bottles and municipal samples. The floc is then dissolved in sulfuric acid, liberating iodine in an amount stoichiometrically equivalent to the original DO, and the iodine is titrated with sodium thiosulfate to a starch endpoint to back-calculate the DO concentration.

iii. Oxygen sag curve in stream pollution (6 marks)

The oxygen sag curve describes how dissolved oxygen (DO) in a receiving stream falls and then recovers downstream of an organic (BOD) discharge, the net result of two competing first-order processes: deoxygenation from bacterial BOD exertion (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 of the discharge 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 point and $D_0$ is the initial deficit. Immediately below the outfall, deoxygenation dominates and DO falls; as the discharged BOD is progressively consumed, reaeration takes over and DO recovers toward saturation. The curve's lowest point 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 the worst fish-kill/water-quality risk occurs and is the design basis for stream assimilative-capacity and effluent-limit calculations.

iv. Indicator organism in water quality analysis (6 marks)

An indicator organism is a micro-organism whose presence and concentration in a water sample is used as a surrogate signal for the likely presence of pathogens, rather than testing directly for every possible disease-causing organism (which would be slow, expensive and insensitive at low pathogen densities). The classic indicator group is the coliform bacteria, and specifically Escherichia coli (fecal coliform) — a normal, abundant inhabitant of the human/warm-blooded-animal gut that is not itself normally pathogenic but is excreted in huge numbers in fecal material, so its detection signals recent fecal contamination and therefore the possible presence of enteric pathogens (Salmonella, Shigella, pathogenic viruses, protozoan cysts) that are much harder and slower to culture directly. A useful indicator must satisfy several criteria: it should be present whenever the pathogens of concern are present, absent in unpolluted water, present in numbers that correlate with the degree of pollution, at least as persistent as the pathogens in the environment (and, ideally, no more resistant to treatment/disinfection than the target pathogens), easy and inexpensive to culture and enumerate, and harmless to the analyst. E. coli/fecal coliform meets these well for bacterial and much viral risk, though it under-represents chlorine-resistant protozoan cysts (Giardia, Cryptosporidium), which is why those are now regulated with their own direct indicators/surrogates (e.g. turbidity, spore-forming bacteria) in modern drinking-water practice.