18-Env-B5 Industrial & Hazardous Waste Management · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Nemerow & Dasgupta, Industrial and Hazardous Waste Treatment, 2nd ed.; Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery, 5th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 6th ed.; LaGrega, Buckingham & Evans, Hazardous Waste Management, 2nd ed.; CCME, Guidelines for the Management of Biomedical Waste in Canada (1992); Canadian Environmental Protection Act (CEPA), 1999; provincial Environmental Protection / Hazardous Waste Regulations (e.g. BC's Hazardous Waste Regulation, O.Reg. 347 in Ontario); Montgomery & Runger, Applied Statistics and Probability for Engineers (for Q1–Q5's basic-statistics content).
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.
Every unit process is sized around a small set of governing design variables that control its removal performance; the table below identifies the primary variable(s) for each of the ten listed technologies and why each matters.
| Technology | Key design variable(s) | Why it matters |
|---|---|---|
| Primary settling tank | Surface overflow rate (SOR, m3/m2·d); detention time | SOR must be below the settling velocity of the target particle size or solids escape before they can settle — it is the single parameter that sets tank surface area for a given flow. |
| Aerobic digester | Solids retention time (SRT); volatile-solids loading rate; dissolved oxygen | SRT must be long enough (typically 15–20+ days) for the biomass to undergo endogenous decay and achieve stabilization (VS reduction), which is the entire purpose of the process. |
| Membrane bioreactor (MBR) | Membrane flux (L/m2·h); mixed-liquor suspended solids (MLSS); transmembrane pressure | Flux sets membrane area (and hence capital cost) for a given flow, while MLSS and transmembrane pressure govern fouling rate, which drives cleaning frequency and operating cost. |
| Conventional activated sludge | Food-to-microorganism ratio (F/M); solids retention time (SRT); mixed-liquor suspended solids (MLSS) | F/M and SRT together govern removal efficiency, sludge settleability and whether nitrification occurs — the process cannot be sized without fixing these two interdependent parameters. |
| Anaerobic digester | Solids/hydraulic retention time; organic loading rate; temperature (mesophilic/thermophilic) | Retention time must exceed the minimum needed for methanogenic bacteria (the slowest-growing, rate-limiting organisms) to avoid washout, and temperature governs their growth rate directly. |
| Biological rotating contactor (RBC) | Hydraulic loading rate; organic loading rate per unit disc area; rotational speed | Loading rate per unit disc area sets the required disc surface area (and hence the number of stages/units), since the attached biofilm's treatment capacity is fixed per unit area. |
| Advanced oxidation (photocatalytic) reactor | UV dose / catalyst (e.g. TiO2) surface area; oxidant (e.g. H2O2/O3) dose; hydraulic retention time | UV dose and oxidant concentration together control hydroxyl-radical generation, the actual oxidizing species, so under-dosing either leaves the target contaminant incompletely destroyed. |
| Granular activated carbon (GAC) columns | Empty-bed contact time (EBCT); carbon usage rate; breakthrough curve | EBCT must be long enough for the target compound to diffuse into the carbon's pore structure and adsorb, and the breakthrough curve determines when the bed must be regenerated or replaced. |
| Granular media filter | Filtration (hydraulic loading) rate; media size/depth; headloss/run length | Filtration rate sets the required filter surface area, while media size governs the trade-off between effluent clarity (finer media) and run length before backwashing (coarser media). |
| Dissolved air flotation (DAF) | Air-to-solids (A/S) ratio; hydraulic/solids loading rate; recycle ratio | The A/S ratio must be high enough to attach sufficient microbubbles to each solids/oil particle to float it, and it is the parameter that directly controls the required saturator/recycle system sizing. |
Across all ten, the pattern is consistent: each technology's governing variable is the parameter that controls the rate-limiting physical or biological mechanism (settling velocity, microbial growth rate, adsorption kinetics, mass-transfer rate) specific to that process, which is why a generic "detention time" alone is never sufficient to size any of them — the process-specific mechanism has to be identified first.
