Question 1 of 5: Water and Wastewater Terminology

Question 1: 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) Ortho, Poly and Organic Phosphates

Orthophosphate (PO₄³⁻ and its protonated forms HPO₄²⁻, H₂PO₄⁻, depending on pH) is the fully hydrolyzed, directly reactive form of phosphorus — the only fraction that reacts immediately with a colorimetric test reagent or with a metal coagulant. Polyphosphates are condensed, dehydrated forms (pyrophosphate, tripolyphosphate) used in detergents, cleaning agents and corrosion inhibitors; they hydrolyze slowly back to orthophosphate in the sewer and treatment process, so a sample tested immediately reads them as "unreactive" phosphorus. Organic phosphate is phosphorus bound within organic molecules of biological origin (food waste, human/animal excreta, microbial cell mass) and is released as orthophosphate only as that organic matter is biologically decomposed. Together, ortho + poly + organic phosphate sum to total phosphorus. The distinction is significant because phosphorus is usually the growth-limiting nutrient in freshwater eutrophication, so effluent phosphorus limits are written on total P; but process design must work with orthophosphate specifically, since chemical precipitation (alum or ferric salts) and biological luxury uptake both act on the orthophosphate ion, meaning poly- and organic phosphate must first hydrolyze or mineralize before they can be removed by either mechanism.

(ii) Seed and Blank in the BOD₅ Test

Seed is a source of viable, acclimated microorganisms — typically settled domestic sewage, or a commercial seed culture — added to the dilution water of a BOD bottle when the sample itself does not carry an adequate microbial population to exert its own oxygen demand (disinfected effluents, many industrial wastes). Blank is a companion bottle containing only seeded dilution water and no sample, incubated alongside the test bottles under identical conditions; its own dissolved-oxygen depletion measures the oxygen demand of the seed and dilution water themselves, independent of the sample. Significance: without subtracting the seed's own contribution (scaled to the amount of seed actually present in each sample bottle), the test would overstate the sample's true BOD by counting oxygen consumed by the seed culture as if it belonged to the sample; Standard Methods further requires the blank's own depletion stay below about 0.2 mg/L per day of incubation, or the dilution water itself is judged unsuitable and the whole test is invalid.

(iii) Oxygen Sag Curve in Stream Pollution

The oxygen sag curve is the dip-and-recovery profile of dissolved oxygen (DO) plotted against distance (or travel time) downstream of an organic (BOD-bearing) discharge, produced by two simultaneous, opposing first-order processes: deoxygenation, as bacteria oxidize the discharged BOD at rate $k_d$, and reaeration, as the stream re-absorbs oxygen from the atmosphere at rate $k_r$, combined in the Streeter–Phelps equation $\dfrac{dD}{dt}=k_dL-k_rD$ for the oxygen deficit $D$. Just below the outfall, deoxygenation dominates and DO falls; once the remaining BOD has thinned enough for reaeration to overtake it, DO recovers back toward saturation. The lowest point — the critical deficit $D_c$, at critical time $t_c$ where $dD/dt=0$ — is where the stream is under the greatest oxygen stress, and fish kills or septic conditions are most likely there. Significance: the sag curve is the quantitative basis for setting effluent BOD limits and a stream's assimilative (self-purification) capacity, since a discharge permit is normally sized so the predicted critical DO never falls below the water-quality objective (commonly 5–6 mg/L to protect aquatic life).

(iv) Indicator Organisms in Biological Testing of Water

Indicator organisms — most commonly total coliform, fecal coliform/E. coli, or enterococci — are organisms whose presence signals probable fecal contamination and, by extension, the possible presence of pathogens, without testing directly for every pathogen (impractical, since pathogens occur in low, sporadic numbers and many are slow, costly or hazardous to culture). A useful indicator originates from the same source as the pathogens (the intestinal tract of warm-blooded animals), occurs in much greater numbers than the pathogens themselves, survives in the environment at least as long as the pathogens without regrowing on its own, is easy and inexpensive to detect, and is itself non-pathogenic. Significance: indicator organisms are the practical, everyday basis for drinking-water and recreational-water quality standards (the Guidelines for Canadian Drinking Water Quality require zero total coliform and E. coli per 100 mL in treated water) and for verifying that a treatment plant's disinfection process is actually performing as designed.

(v) Turbidity in Water

Turbidity is an optical property of water — the extent to which suspended and colloidal matter (clay, silt, organic floc, algae, microorganisms, chemical precipitates) scatters and absorbs light rather than letting it pass in a straight line — measured by the nephelometric method (light scattered at 90° from an incident beam, compared to a formazin polymer standard) and reported in NTU. Significance is threefold: it is a first, real-time indicator of raw and treated water quality; it directly threatens disinfection performance, because turbidity-forming particles can physically shield embedded pathogens (particularly Cryptosporidium oocysts) from disinfectant contact or UV dose, reducing the achieved log-inactivation below what the nominal CT or dose would predict; and it is the standard, near-instantaneous surrogate used to monitor coagulation-flocculation and filtration performance between the much less frequent direct microbial tests — which is why regulators set stringent filtered-water turbidity targets (commonly ≤0.3 NTU, never exceeding 1.0 NTU) as a practical, continuous proxy for pathogen removal.