18.1 — Significance of the 50th/90th percentile data. These are statistical characterizations of a waste stream that is known, from the survey, to vary considerably (flow alone ranges from 2,000 to 4,500 m³/d). The 50th percentile (median) represents the typical day-to-day condition — the value the plant will actually see roughly half the time — and is the appropriate basis for average-day design criteria such as expected long-term treatment performance, chemical/energy consumption, and O&M cost estimation. The 90th percentile represents a realistic near-peak condition (exceeded only 10% of the time) and is the appropriate basis for sizing critical hydraulic and structural capacity, peak organic/nutrient loading, and equalization-basin volume, so that the plant remains reliable and permit-compliant on its higher-loading days without being absurdly oversized for a true worst-case/extreme outlier. Providing both values — rather than a single average or a single maximum — lets the designer separate "size for average cost-efficiency" decisions from "size for peak reliability" decisions, and the gap between the two (here roughly 2.25× on flow) is itself the design signal for how much equalization capacity is warranted.
18.2 — Approach to WWTP design. A systematic design approach follows these stages:
Confirm/extend waste characterization — validate the given 50th/90th percentile data (and collect any additional parameters — oil & grease, specific toxics, metals — not covered by this table) via an in-plant survey per the methodology already discussed (Questions 3–6, 13).
Set effluent objectives — obtain the applicable discharge limits from the receiving municipal sewer-use bylaw (if discharging to sewer) or the direct-discharge permit (if to a receiving water), which fixes the removal efficiency each unit process must achieve.
Screen and select unit processes based on treatability: given BOD5/COD ≈ 2,000/4,500 mg/L (BOD5:COD ≈ 0.44, moderately biodegradable but far stronger than domestic sewage), a two-stage approach — equalization/primary treatment ahead of a robust biological process — is indicated; the elevated TKN (80–140 mg/L) and TP (20–40 mg/L) both signal that nutrient (N and P) removal will likely be needed to meet typical discharge limits, not just carbonaceous BOD removal.
Flow and load equalization — sized using the spread between the 50th and 90th percentile flow/load to dampen the swings identified in 18.1 before biological treatment.
Primary treatment — screening/grit removal and a primary clarifier to remove settleable solids and reduce the load reaching biological treatment.
Secondary (biological) treatment with nitrification/denitrification — sized on the 50th-percentile average load for normal performance and checked against the 90th-percentile peak load for reliability, given the elevated TKN indicates ammonia removal will be needed to meet typical effluent ammonia-N limits.
Tertiary treatment — chemical phosphorus removal (metal-salt precipitation) and filtration if the TP limit cannot be met biologically alone, given TP up to 40 mg/L is well above typical discharge limits.
Disinfection before final discharge, if required by the receiving-water permit.
Sludge handling and disposal — thickening and dewatering of primary and waste-activated sludge, with a disposal route (land application, landfill, or further stabilization) selected per its own characterization.
Pilot testing — given the unusually high strength of this waste relative to domestic sewage, pilot-scale testing of the proposed biological process is prudent before full-scale design.
Economic evaluation and phased/expandable design — compare capital and O&M costs among alternative trains, and design for future flow/load growth.
Figure: Generalized treatment train for this industrial wastewater — equalization and primary treatment ahead of a nitrifying biological process, tertiary chemical phosphorus removal and disinfection, with a parallel sludge thickening/dewatering train.
