18-Env-B5 Industrial & Hazardous Waste Management · May 2017
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; Canadian Nuclear Safety Commission (CNSC) regulations on radioactive waste under the Nuclear Safety and Control Act; provincial hazardous waste regulations (e.g. BC's Environmental Management Act and Hazardous Waste Regulation).
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 of these five unit processes is sized and operated around a different governing variable set, even though several (SRT, HRT, loading rate) recur across all of them under different numeric ranges. The table below lists the variables an engineer would actually specify on a design basis of size for each process, grouped by process.
| Unit process | Key design variables | Typical design ranges |
|---|---|---|
| 2.1 Conventional activated sludge | MLSS/MLVSS concentration; food-to-microorganism ratio (F/M); solids retention time (SRT); hydraulic retention time (HRT); recycle ratio (Qr/Q); dissolved oxygen (DO); volumetric BOD loading; sludge volume index (SVI) | MLSS 1,500–3,000 mg/L; F/M 0.2–0.4 kg BOD5/kg MLVSS·d; SRT 5–15 d; HRT 4–8 h; DO ≥ 2 mg/L |
| 2.2 Secondary settling tank | Surface overflow rate (SOR); solids loading rate (SLR); weir overflow rate; side water depth; detention time; sludge withdrawal (RAS) rate | SOR 16–28 m3/m2·d (avg); SLR 4–6 kg/m2·h; depth 3.5–5 m; detention 2–4 h |
| 2.3 Membrane separator (MBR) | MLSS concentration; permeate flux; transmembrane pressure (TMP); membrane pore size; SRT; HRT; air scour rate for fouling control | MLSS 8,000–12,000 mg/L; flux 15–25 L/m2·h; TMP < 50 kPa; pore size 0.03–0.4 µm; SRT 15–30 d |
| 2.4 Aerobic digester | SRT; volatile solids (VS) reduction target; temperature; DO; solids loading rate | SRT 15–20 d (unheated) or 10–15 d (heated); VS reduction 38–50%; DO > 1–2 mg/L |
| 2.5 Anaerobic digester | SRT/HRT; temperature (mesophilic ≈35°C or thermophilic ≈55°C); VS loading rate; pH; alkalinity; volatile fatty acid (VFA)/alkalinity ratio; biogas production rate | SRT 15–30 d (mesophilic); VS loading 1.6–6.4 kg VS/m3·d; pH 6.8–7.4; VFA/alkalinity < 0.3–0.4 |
The common thread across all five is that each variable set exists to control the same underlying quantities — how much biomass/media is present, how long the material stays in the unit, and how the unit's hydraulics or membrane surface limit throughput — but the numeric targets differ by an order of magnitude in places (MLSS in an MBR runs 3–5 times higher than conventional activated sludge, for example) because the membrane, rather than gravity settling, is what caps solids concentration.
