Question 1 of 5: Water/Wastewater Parameter Definitions

Question 1: Water/Wastewater Parameter Definitions (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.

a. Carbonaceous BOD5 (5 marks)

Carbonaceous BOD5 (cBOD5) is the oxygen consumed over a standard 5-day, 20 °C incubation by heterotrophic micro-organisms oxidizing the biodegradable organic-carbon fraction of a sample, with the nitrogenous share of the demand deliberately suppressed. In an unsuppressed BOD5 test, nitrifying bacteria (Nitrosomonas, Nitrobacter) that are naturally present or seeded into an aged/biologically-treated sample begin oxidizing ammonia to nitrite and nitrate within the 5-day window, adding a second, nitrogenous oxygen demand on top of the carbon demand and inflating the result. cBOD5 isolates the carbon-only share by dosing a nitrification inhibitor (commonly TCMP, 2-chloro-6-(trichloromethyl)pyridine) into the bottle before incubation, which selectively suppresses the nitrifiers without affecting heterotrophic carbon oxidation. Because it removes the ambiguity of a variable, sample-dependent nitrification onset, cBOD5 is the parameter more commonly written into NPDES/provincial discharge permits for effluents where nitrification is likely (e.g. well-treated secondary effluent), whereas raw/primary influent BOD5 is usually reported unsuppressed since nitrifiers are not yet well established there.

b. Nitrification in wastewater treatment (5 marks)

Nitrification is the two-step, strictly aerobic, autotrophic biological oxidation of ammonia-nitrogen to nitrate: ammonia-oxidizing bacteria (Nitrosomonas) first convert $NH_4^+$ to nitrite, $NH_4^++1.5O_2\rightarrow NO_2^-+H_2O+2H^+$, and nitrite-oxidizing bacteria (Nitrobacter) then convert nitrite to nitrate, $NO_2^-+0.5O_2\rightarrow NO_3^-$. Both organism groups are slow-growing autotrophs (much slower than the heterotrophs that remove carbonaceous BOD), so a nitrifying activated-sludge plant must be operated at a long enough solids retention time (SRT, commonly 8–15 days at typical temperatures) to keep nitrifiers from washing out, and the process consumes roughly 4.6 g $O_2$ per gram of ammonia-N oxidized while also consuming alkalinity (about 7.14 g as $CaCO_3$ per gram N oxidized) and depressing pH. Nitrification is required wherever an effluent ammonia limit applies (ammonia is acutely toxic to fish and exerts its own oxygen demand in the receiving stream) and is usually followed, where a nitrate limit also applies, by a separate anoxic denitrification step.

c. Types of alkalinity in water or wastewater (5 marks)

Alkalinity is the water's acid-neutralizing capacity, contributed almost entirely by the carbonate system (bicarbonate $HCO_3^-$, carbonate $CO_3^{2-}$, hydroxide $OH^-$) plus, at typical natural pH, negligible contributions from borate/phosphate/silicate. Three operational types are distinguished by the pH endpoint used in a titration with standard acid: phenolphthalein (P) alkalinity is the acid consumed down to pH 8.3 (the phenolphthalein color-change point, where all carbonate has converted to bicarbonate and any hydroxide has been neutralized); total (T) alkalinity is the acid consumed down to pH 4.5 (the bicarbonate/methyl-orange or bromocresol-green endpoint, where essentially all $HCO_3^-$ has converted to carbonic acid/$CO_2$); and hydroxide/carbonate/bicarbonate alkalinity are the individual species concentrations recovered from P and T through the standard relationships ($P=0$: only $HCO_3^-$; $PT/2$: $CO_3^{2-}$ and $OH^-$; $P=T$: only $OH^-$). Most natural waters and domestic wastewaters sit in the bicarbonate-dominated regime ($PT/2$ (hydroxide plus carbonate present, no bicarbonate) signals a strongly basic condition, typically from lime treatment or an industrial caustic influence.

d. Sloughing of biomass in trickling filter (5 marks)

Sloughing is the periodic, natural detachment of the biofilm (the slime layer of bacteria, fungi and grazing organisms) that grows on a trickling filter's media surface, releasing chunks of biomass into the underdrain flow to be captured by a downstream secondary clarifier. As the biofilm thickens beyond the depth oxygen and substrate can diffuse through, the organisms at the media surface become starved and lose their attachment strength (endogenous respiration weakens the base layer), and hydraulic shear from the trickling flow — amplified periodically by dosing-arm rotation, recirculation-flow surges, or a deliberate flushing dose — then strips the loosened layer off. Sloughing is functionally necessary: it is how the filter sheds excess biomass and keeps the void spaces between media open (uncontrolled biofilm growth would otherwise cause ponding and filter flies), but heavy, unsteady sloughing events (common after a hydraulic surge or a cold-weather biofilm die-back) spike the effluent TSS and BOD load onto the secondary clarifier and can temporarily degrade final effluent quality if the clarifier is undersized for that transient solids loading.

e. Turbidity in water (5 marks)

Turbidity is an optical measure of water clarity — the light-scattering caused by suspended and colloidal particles (clay, silt, organic detritus, micro-organisms, precipitated floc) — reported in nephelometric turbidity units (NTU) from the intensity of light scattered at 90° to an incident beam passing through the sample. It is not a direct measure of particle mass or a specific contaminant, but it correlates strongly with the presence of pathogens and disinfection-demand-consuming material: turbid particles can shield micro-organisms from UV or chlorine disinfection, provide a hydrophobic surface for pathogen attachment, and consume oxidant demand that would otherwise be available for disinfection, so essentially every drinking-water regulation (including Health Canada's Guidelines for Canadian Drinking Water Quality) sets a turbidity performance target for filtered water (commonly ≤0.3 NTU, 95% of the time, for conventional/direct filtration) as an indirect but highly reliable proxy for effective pathogen (especially Giardia/Cryptosporidium) removal.