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.

Each pair below is answered in the same way: the two quantities are defined independently, the measurement or calculation that produces each one is stated, and the distinction that actually matters in design or operation is drawn out. The pairs are deliberately chosen so that the two members are easily confused; in every case one member is the broader or gross quantity and the other is the narrower quantity that the engineer actually controls.

Part (i) — MLSS and MLVSS (5 marks)

Mixed liquor suspended solids (MLSS) is the total concentration of suspended solids carried in the mixed liquor of an aeration tank, measured gravimetrically as the residue retained on a glass-fibre filter after drying at $105\,{}^{\circ}\text{C}$, and reported in mg/L. It counts everything in suspension: living bacteria and protozoa, dead cell debris, inert organic material that resists biodegradation, and the inorganic grit, silt and chemical precipitates that the plant has accumulated.

Mixed liquor volatile suspended solids (MLVSS) is the fraction of that same sample that is lost on ignition at $550\,{}^{\circ}\text{C}$ — the organic portion. It is obtained by re-firing the dried filter and taking the difference, so MLVSS is always the smaller number and the ratio MLVSS/MLSS is a dimensionless characterisation of the sludge. In a conventional municipal activated-sludge plant that ratio typically falls between 0.70 and 0.85; it drops when the collection system admits grit, industrial inerts, or metal-salt precipitate from chemical phosphorus removal.

The distinction is not academic. MLVSS is the closer surrogate for the active biomass that actually oxidises the substrate, so the food-to-microorganism ratio, the observed yield and the solids retention time are all more defensibly written on MLVSS. MLSS, by contrast, is the quantity that loads the secondary clarifier: the thickening duty and the solids loading rate depend on every kilogram of solids in suspension, inert or not. Designers therefore size the biological reactor on MLVSS and the clarifier on MLSS, and an operator who watches only MLSS while the volatile fraction quietly falls is losing treatment capacity without seeing it in the daily log.

Part (ii) — BOD₅ and COD (5 marks)

Five-day biochemical oxygen demand (BOD₅) is the mass of dissolved oxygen consumed by micro-organisms while they stabilise the biodegradable organic matter in a sample held in the dark at $20\,{}^{\circ}\text{C}$ for five days. It is a bioassay: a seeded, diluted sample is incubated in a sealed bottle and the depletion of dissolved oxygen is measured. Because nitrifiers exert their own demand, the carbonaceous value cBOD₅ is obtained by adding an inhibitor such as 2-chloro-6-(trichloromethyl)pyridine, and it is cBOD₅ that appears in Canadian effluent permits.

Chemical oxygen demand (COD) is the oxygen equivalent of the organic matter that can be oxidised by a strong chemical oxidant — acidified potassium dichromate under silver catalysis at $150\,{}^{\circ}\text{C}$ — with the residual dichromate titrated or read colorimetrically. The test takes two to three hours rather than five days.

The differences follow from the mechanism. COD is always the larger number, because the dichromate attacks biologically refractory material (lignin, many synthetic organics) that the bacteria in the BOD bottle leave alone, and because it also registers reduced inorganic species such as sulfide, sulfite and ferrous iron. BOD₅ is biologically meaningful — it is the demand the receiving water will actually see — but it is slow, imprecise (typically $\pm 15$ to 20 percent), and can be suppressed altogether by a toxic or metal-bearing industrial discharge. COD is fast, reproducible and therefore suited to process control, but it must be calibrated against BOD for a given wastewater before it can be used as a surrogate. For raw municipal sewage the ratio BOD₅/COD usually lies between 0.4 and 0.6; a ratio below about 0.3 signals a poorly biodegradable stream for which conventional biological treatment alone will not meet a permit.

Part (iii) — Surface overflow rate and solids loading rate (5 marks)

Surface overflow rate (SOR), also called the overflow velocity or hydraulic loading rate, is the clarified flow divided by the plan area of the settling tank:

Dimensionally it is a velocity, and that is exactly its physical meaning: in ideal (Type I) settling every particle whose terminal settling velocity exceeds the SOR is captured, and every slower particle escapes over the weir. The SOR therefore fixes the clarification duty — the size of the smallest particle the tank will hold. Crucially, only the flow that leaves over the weirs is counted, so the return activated sludge is excluded from the SOR of a secondary clarifier.

Solids loading rate (SLR) is the mass of suspended solids applied per unit plan area per unit time:

Here every kilogram of solids that crosses the tank floor counts, however it arrived, so the return sludge flow $Q_R$ is included. The SLR fixes the thickening duty — whether the sludge blanket can be compacted and withdrawn as fast as it is delivered.

A secondary clarifier must satisfy both criteria, and which one governs depends on the mixed-liquor concentration. At low MLSS the tank is clarification-limited and the SOR governs; at high MLSS and high recycle it becomes thickening-limited and the SLR governs. Typical Canadian design practice for activated sludge is an average SOR of roughly $16$–$24\ \text{m}^3\,\text{m}^{-2}\,\text{d}^{-1}$ and an average SLR of $70$–$120\ \text{kg}\,\text{m}^{-2}\,\text{d}^{-1}$. Confusing the two — in particular, computing the SLR on the plant flow alone — understates the thickening duty badly and is the classic cause of a blanket that rises and washes solids over the weir at peak flow.

Part (iv) — Total and residual chlorine (5 marks)

Total chlorine means the sum of all chlorine species present in the oxidation state available for disinfection: free available chlorine (aqueous $\mathrm{Cl_2}$, hypochlorous acid $\mathrm{HOCl}$ and hypochlorite ion $\mathrm{OCl^-}$) plus combined available chlorine (the mono-, di- and trichloramines formed when chlorine reacts with ammonia). It is measured as the full DPD colour development after adding potassium iodide.

Residual chlorine is the chlorine still present at a stated point and time after the water’s chlorine demand has been satisfied — that is, after the fast reactions with ferrous iron, manganese, sulfide, nitrite and oxidisable organic matter have run to completion. The bookkeeping identity is

so a residual only exists once the demand is met. A residual is itself reported as free or total: free residual is the disinfecting fraction, roughly 80 to 100 times more germicidal than the combined fraction, while total residual includes the weaker but longer-lived chloramines.

The practical distinction is therefore between a speciation (total versus free) and a state of the reaction (dose versus demand versus residual). Both are regulated. On the drinking-water side, a free residual of roughly $0.2\ \text{mg/L}$ is carried to the far end of the distribution system as the sentinel against regrowth and ingress, and $\mathrm{CT}$ credit for primary disinfection is computed on the free residual. On the wastewater side the concern is reversed: total residual chlorine is acutely toxic to fish at concentrations below $0.02\ \text{mg/L}$, so a chlorinated effluent must be dechlorinated with sulfur dioxide or sodium bisulfite before discharge, and it is the total residual that the permit limits.

Part (v) — Hydroxyl and bicarbonate alkalinity (5 marks)

Alkalinity is the acid-neutralising capacity of a water, conventionally reported as mg/L as $\mathrm{CaCO_3}$. In natural waters it is carried almost entirely by three species, and the two named here are the extreme members of that set.

Hydroxyl (caustic) alkalinity is the portion contributed by free hydroxide ion, $\mathrm{OH^-}$. It is essentially absent below about pH 9.5 and appears only in waters that have been dosed with lime or caustic soda — excess-lime softening, for example — or in some industrial wastes.

Bicarbonate alkalinity is the portion contributed by $\mathrm{HCO_3^-}$. It dominates the ordinary pH range of roughly 4.5 to 8.3, and it is what the weathering of carbonate rock puts into most Canadian surface and groundwaters. It is the buffer that keeps a nitrifying activated-sludge plant from crashing, since nitrification destroys about $7.14\ \text{mg}$ of alkalinity as $\mathrm{CaCO_3}$ per mg of ammonia nitrogen oxidised.

The two are separated operationally by a two-stage titration. Titrating to the phenolphthalein end point at pH 8.3 gives $P$; continuing to the total (methyl orange or inflection) end point at pH 4.5 gives $M$. Because carbonate is only half-neutralised at pH 8.3 while hydroxide is fully neutralised, the comparison of $P$ with $M/2$ resolves the speciation:

The table also makes the mutual-exclusion rule obvious: hydroxide and bicarbonate cannot coexist in appreciable amounts, because they react to form carbonate. That is the single most useful consequence of the distinction — a laboratory sheet reporting both hydroxyl and bicarbonate alkalinity in the same sample contains an error.