Question 1 of 5: Definitions and Distinctions in Water and Wastewater Practice

Question 1: Definitions and Distinctions in Water and Wastewater Practice (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.

Given. The question supplies no data, so one representative data set is carried through all five parts; each definition is closed with a numerical illustration drawn from it, which is what turns a definition into a markable engineering answer.

Find. For each of the five pairs, a definition, the physical distinction between the two members, and one worked number from the table above that demonstrates the distinction quantitatively.

(i) MLSS and MLVSS

Mixed liquor suspended solids (MLSS) is the total dry mass of suspended solids per unit volume of the aerated mixed liquor, measured by filtering a sample through a 1.2 µm glass-fibre disc and drying the residue at 103–105 °C. It counts everything the filter retains: living bacteria and protozoa, dead cell debris, extracellular polymer, and the inert mineral matter (grit, silt, precipitated metal hydroxides) that entered with the sewage and has been swept into the floc.

Mixed liquor volatile suspended solids (MLVSS) is the portion of that residue that burns off when the dried filter is ignited at 550 °C for 15 minutes. Because organic matter volatilises and mineral matter does not, MLVSS is the conventional surrogate for the organic — and by extension the biologically active — fraction of the sludge. The difference, MLSS − MLVSS, is the fixed or inert suspended solids.

The distinction matters because the two are used for different purposes. Solids-handling calculations (clarifier solids loading, thickener and dewatering duty, sludge tanker volumes) are done on MLSS, since the mineral fraction has to be settled, pumped and hauled just like the organic fraction. Kinetic calculations (food-to-microorganism ratio, specific substrate utilisation rate, specific oxygen uptake rate) are done on MLVSS, because only the organic solids approximate the biomass doing the work. Using MLSS in an F/M ratio understates the loading on the biomass; using MLVSS in a solids loading rate undersizes the clarifier.

For the representative sludge:

$$\text{MLVSS} = f_v \cdot \text{MLSS} = 0.75 \times 3\,500 = \boxed{2\,625\ \text{mg/L}}$$

so 875 mg/L is fixed inert solids. A volatile fraction of 0.70–0.80 is typical of a conventional activated-sludge plant; a value drifting down toward 0.55–0.60 signals either heavy inert loading from infiltration and grit carry-over, or a very long sludge age in which endogenous decay has mineralised much of the biomass.

(ii) BOD₅ and cBOD₅

BOD₅ is the total biochemical oxygen demand exerted in five days at 20 °C in the dark: the mass of dissolved oxygen per litre consumed by the mixed microbial population of the seeded, diluted sample. It is a total demand, and in a nitrifying sample it lumps together two chemically distinct oxygen sinks — heterotrophic oxidation of carbon, and autotrophic oxidation of reduced nitrogen to nitrite and nitrate.

cBOD₅ is the carbonaceous 5-day demand, measured on an identical bottle to which a nitrification inhibitor (typically 2-chloro-6-(trichloromethyl)pyridine, TCMP, or allylthiourea) has been added at about 10 mg/L. The inhibitor suppresses Nitrosomonas and Nitrobacter without affecting the heterotrophs, so the oxygen consumed is attributable to organic carbon alone. The nitrogenous demand is the difference, nBOD₅ = BOD₅ − cBOD₅.

The distinction is regulatory as much as scientific. Raw sewage rarely nitrifies within five days — the ammonia-oxidising population is slow-growing and largely washed out — so on an influent sample the two results are nearly identical. A well-nitrified secondary effluent, however, carries a large seed of ammonia oxidisers and very little organic carbon, so an uninhibited bottle can report a BOD₅ two or three times the true carbonaceous value. A plant achieving excellent nitrification would then appear to be violating its own permit. For that reason Canadian and US secondary-treatment permits are almost universally written on cBOD₅, and the Wastewater Systems Effluent Regulations under the federal Fisheries Act set the national standard as 25 mg/L cBOD₅.

Stoichiometrically, complete oxidation of ammonia requires 2 mol O₂ per mol N, that is $2 \times 32/14 = 4.57$ g O₂ per g N. For the representative sample:

$$\text{nBOD}_5 = 4.57\,N_{ox} = 4.57 \times 6.0 = 27.4\ \text{mg/L}$$

$$\text{cBOD}_5 = \text{BOD}_5 - \text{nBOD}_5 = 150 - 27.4 = \boxed{122.6\ \text{mg/L}}$$

The nitrogenous component is therefore 18 per cent of the reported total, which on an effluent sample would be more than enough to convert compliance into non-compliance.

(iii) Surface overflow rate and solids loading rate

Surface overflow rate (SOR), also called the overflow velocity or hydraulic loading rate, is the clarified flow divided by the plan area of the tank, $\text{SOR} = Q/A$, with units of m³/m²·d, which reduce to m/d — and that is the point. SOR is a velocity: it is the upward velocity of the liquid rising to the weirs, and in ideal-settling theory it is exactly the settling velocity of the slowest particle that is still completely removed. It governs clarification, the quality of the overflow.

Solids loading rate (SLR) is the mass of suspended solids applied per unit plan area per day, $\text{SLR} = (Q + Q_R)X/A$, in kg/m²·d. It governs thickening, the ability of the tank to compact the applied solids into an underflow concentrated enough to return to the aeration tank.

The single distinction most often examined is which flows enter each expression. Only the plant flow crosses the effluent weirs, because the return activated sludge leaves through the underflow and never reaches them — so the RAS flow is excluded from SOR. Every kilogram of solids entering the tank must be thickened and removed regardless of the route by which it arrived — so the RAS flow is included in SLR. Interchanging them is a large error in both directions: omitting the RAS from the solids loading understates the thickening duty, while adding it to the overflow rate oversizes the tank on the wrong criterion.

For the representative clarifier:

$$\text{SOR} = \frac{Q}{A} = \frac{6\,000}{500} = 12.0\ \text{m}^3/\text{m}^2\!\cdot\!\text{d}$$

$$\text{SLR} = \frac{(Q+Q_R)X}{A} = \frac{(6\,000+3\,000)(3\,000)}{500 \times 1\,000} = \boxed{54.0\ \text{kg}/\text{m}^2\!\cdot\!\text{d}}$$

Had the return flow been forgotten, the solids loading would have been reported as 36.0 kg/m²·d — a factor of 1.5 low, and comfortably inside a design ceiling the tank does not actually satisfy. Typical design limits for a conventional activated-sludge secondary clarifier at average flow are an SOR of 16–33 m/d and an SLR of 70–150 kg/m²·d, so this tank is conservatively loaded on both.

(iv) Combined and free residual chlorine

Free residual chlorine is the chlorine remaining in solution as hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻), together with any dissolved molecular Cl₂. The speciation is set by pH through $\mathrm{HOCl} \rightleftharpoons \mathrm{H}^{+} + \mathrm{OCl}^{-}$, with $pK_a = 7.5$ at 20 °C; HOCl is the far more potent biocide, being uncharged and able to diffuse through the cell envelope, so free-chlorine disinfection weakens sharply as pH rises above about 8.

Combined residual chlorine is chlorine bound to nitrogen as the chloramines — monochloramine NH₂Cl, dichloramine NHCl₂, and trichloramine NCl₃ — formed by the successive substitution reactions

$$\mathrm{NH_3} + \mathrm{HOCl} \rightarrow \mathrm{NH_2Cl} + \mathrm{H_2O}$$

$$\mathrm{NH_2Cl} + \mathrm{HOCl} \rightarrow \mathrm{NHCl_2} + \mathrm{H_2O}$$

Both are measured by the DPD colorimetric method: the first colour development gives the free residual, and addition of potassium iodide releases the combined fraction, so that total residual minus free residual gives combined residual.

The practical distinction is a trade between disinfecting power and persistence. Free chlorine is roughly 25 to 100 times more effective than monochloramine on the same CT basis and acts within minutes, but it decays quickly and reacts with natural organic matter to form trihalomethanes and haloacetic acids. Combined chlorine is a weak, slow disinfectant but is stable for days, produces far fewer regulated by-products, and penetrates biofilm well — which is why Canadian utilities commonly disinfect at the plant with free chlorine or ozone and then convert to a monochloramine secondary residual for the distribution system. The design rule is a Cl₂:NH₃-N mass ratio of about 4:1 to 5:1 to make monochloramine preferentially and avoid the taste-and-odour-producing di- and trichloramines.

For the representative sample:

$$C_{\text{comb}} = C_T - C_F = 2.0 - 0.5 = \boxed{1.5\ \text{mg/L as Cl}_2}$$

Only 25 per cent of the residual is free. Note also the breakpoint arithmetic that lies behind such a result: full oxidation of ammonia to nitrogen gas consumes $3\mathrm{Cl_2} : 2\mathrm{NH_3}$, that is $3 \times 71 / (2 \times 14) = 7.6$ g Cl₂ per g NH₃-N, so a water carrying 0.5 mg/L of ammonia nitrogen must absorb about 3.8 mg/L of chlorine before any free residual can appear.

(v) Hydroxyl and bicarbonate alkalinity

Alkalinity is the acid-neutralising capacity of a water, reported by convention as mg/L as CaCO₃, and in almost all natural waters it is carried by the three species in $\text{Alk} = [\mathrm{HCO_3^-}] + 2[\mathrm{CO_3^{2-}}] + [\mathrm{OH^-}] - [\mathrm{H^+}]$.

Bicarbonate alkalinity is the HCO₃⁻ contribution. It dominates the pH range 6.3–8.3 covered by virtually every raw and finished water, it originates in the dissolution of carbonate rock by carbon-dioxide-charged rainwater, and it is the component that gives a water its buffering against acid addition — the reason alum coagulation, nitrification and chlorination do not send plant pH into free fall.

Hydroxyl alkalinity is the free OH⁻ contribution. It is essentially absent below pH 8.3 and appears only when a water has been dosed with a strong base — excess lime in a softening plant, caustic soda for corrosion control — or in a highly alkaline industrial waste. It therefore signals a chemically conditioned water rather than a natural one, and it is aggressive: it drives scaling, interferes with coagulation and chlorination, and must be neutralised by recarbonation before filtration.

The two are separated in the laboratory by the two-titration procedure. Titrating to the phenolphthalein end point at pH 8.3 gives $P$, which measures all of the hydroxide and exactly half of the carbonate; continuing to the methyl-orange end point at pH 4.5 gives the total alkalinity $T$. Comparing $P$ with $T/2$ then fixes the split:

Hydroxide and bicarbonate are mutually exclusive: they cannot coexist in the same water, because free hydroxide converts bicarbonate to carbonate on contact.

For the representative softened water, $T/2 = 110$ and $P = 30$, so $P$ is less than $T/2$ and the second row governs:

$$\mathrm{OH^-} = 0, \quad \mathrm{CO_3^{2-}} = 2P = 60, \quad \mathrm{HCO_3^-} = T - 2P = \boxed{160\ \text{mg/L as CaCO}_3}$$

The water therefore carries no hydroxyl alkalinity at all, and the operator can conclude that excess-lime feed has been properly recarbonated.