Question 7 of 18: Ten Definitions

Question 7: Ten Definitions (10 marks)

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7.1 BOD (biochemical oxygen demand). The mass of dissolved oxygen consumed by microorganisms as they biologically oxidize the degradable organic matter in a sample, measured over a standard incubation period at 20°C — conventionally 5 days (BOD₅). It is the most widely used measure of a wastewater's biodegradable organic strength.

7.2 COD (chemical oxygen demand). The oxygen equivalent of the organic matter in a sample that is oxidizable by a strong chemical oxidant (typically potassium dichromate in hot sulfuric acid). COD oxidizes both biodegradable and non-biodegradable organics, so COD ≥ BOD₅ for a given sample, and the test result is available in about 2–3 hours rather than 5 days.

7.3 TOC (total organic carbon). A direct instrumental measure of the mass of organic carbon in a sample, determined by combusting or chemically oxidizing the sample and measuring the CO₂ produced (typically by infrared detection). Unlike BOD or COD, TOC is independent of the oxidation state of the carbon and is fast enough (minutes) to support real-time process control.

7.4 TOD (total oxygen demand). The oxygen consumed when a sample is fully combusted at high temperature over a catalyst, oxidizing not only carbon but also nitrogen and sulfur species present. TOD is the most complete of the oxygen-demand parameters and, like TOC, is a fast instrumental test.

7.5 Zone settling. A settling regime (Type III/hindered settling) occurring at high particle concentrations, where particles are close enough together to settle collectively as a distinct blanket with a sharp interface against the clarified liquid above, rather than as discrete individual particles. It governs the settling behaviour of activated-sludge secondary clarifiers.

7.6 Specific resistance. A laboratory-measured property (units of m/kg) quantifying a sludge's resistance to dewatering by filtration, determined from a Buchner-funnel or similar filtration test. A higher specific resistance means a sludge is harder to dewater and requires more filter area or conditioning (e.g., polymer dosing) for a given throughput.

7.7 Freundlich isotherm. An empirical equation, $x/m = K\,C_e^{1/n}$, relating the mass of a contaminant adsorbed per unit mass of adsorbent ($x/m$) to the equilibrium concentration remaining in solution ($C_e$). It is the standard tool for characterizing adsorption capacity (e.g., onto activated carbon) and for sizing granular-activated-carbon treatment systems.

7.8 F/M (food-to-microorganism ratio). The ratio of the organic loading rate applied to an activated-sludge system (kg BOD₅ or COD per day) to the mass of active microorganisms present (kg MLVSS), expressed in units of kg BOD₅/kg MLVSS·day. F/M is a primary activated-sludge design and operating parameter, governing removal efficiency, sludge production, and settleability.

7.9 Priority pollutant. A chemical substance specifically identified by a regulatory agency (originating with the U.S. EPA's list under the Clean Water Act, and paralleled by Canadian federal/provincial toxic-substance lists) as posing particular concern for human health or the environment, subject to specific analytical monitoring and, often, numeric effluent limits distinct from conventional parameters like BOD₅ or SS.

7.10 Plug flow. An idealized reactor hydraulic regime in which fluid elements pass through the tank in the same sequence they entered, with no longitudinal (back) mixing between elements ahead of and behind any given element — every parcel of fluid has the same residence time, equal to the theoretical hydraulic retention time. It is the conceptual opposite of a completely-mixed (CSTR) reactor, in which an entering parcel is instantly diluted throughout the tank contents.