Air/Water Pollution Abatement and TSS Loading: 23-Chem-B2 Environmental Engineering, Undated paper, Q1

Problem 1: Air/Water Pollution Abatement and TSS Loading (20 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.

(i) SO₂ concentration conversion and air monitoring

Given.

Quantity	Value
SO₂ concentration	0.60 ppm(v)
Molar mass of SO₂, M	64.0 g/mol
Molar volume of an ideal gas, V_m	24.45 L/mol

Find. SO₂ concentration in µg/m³.

Check — reference-condition assumption

The question states "STP" but gives no explicit temperature/pressure. True STP (0°C, 1 atm) gives V_m=22.41 L/mol; ambient air-quality practice (and the ppm→µg/m³ conversion tables published by environmental regulators) almost universally uses the 25°C, 1 atm reference state (V_m=24.45 L/mol), since ambient/stack monitoring is reported at that condition. That convention is adopted here. If true 0°C, 1 atm STP is intended instead, the same calculation gives 0.60×(64.0/22.41)×1000 ≈ 1,713 µg/m³.

Approach. The ppm(v)-to-mass-concentration conversion for an ideal gas scales the volumetric mixing ratio by the ratio of the pollutant's molar mass to the reference molar volume.

Apply the ppm→µg/m³ conversion. $C = \text{ppm} \times \frac{M}{V_m} \times 1000 = 0.60 \times \frac{64.0}{24.45} \times 1000$ $C = \boxed{1{,}571\ \mu\text{g/m}^3}$

Quantity	Value
SO₂ concentration	1,571 µg/m³ (at 0.60 ppm, 25°C/1 atm)

SO₂ air monitoring for abatement compliance. A continuous emission monitoring system (CEMS) — typically a UV-fluorescence or non-dispersive infrared (NDIR) analyzer sampling the stack gas — tracks the SO₂ mass emission rate at the source in near-real time, confirming that the installed abatement equipment (wet limestone flue-gas desulfurization, or a low-sulfur fuel switch) is holding its rated removal efficiency. A parallel network of fixed ambient SO₂ monitors (pulsed UV-fluorescence analyzers per the reference method) is sited at representative ground-level receptor locations, typically the point of predicted maximum impact from dispersion modelling, so that measured ambient levels can be compared directly against the applicable air-quality standard. Source (CEMS) and receptor (ambient) monitoring are complementary: stack compliance alone does not guarantee the receptor is protected once dispersion, terrain and multiple-source effects are accounted for, so both data streams are needed to close the loop between "the control equipment is working" and "the air people actually breathe meets the standard."

(ii) Regulatory vs. technology-based water pollution abatement

A common regulatory (effluent-standard) approach is a numeric discharge-permit limit (e.g. a maximum BOD₅/TSS concentration or mass-loading limit written into a facility's discharge permit); the comparable technology-based approach is a requirement to install and properly operate a prescribed treatment technology (e.g. secondary biological treatment) for that industry category, independent of the exact numeric effluent quality that results.

Aspect	Regulatory (numeric effluent standard)	Technology-based (prescribed process)
Advantage 1	High design flexibility — the discharger may choose any treatment train that reliably meets the numeric limit, encouraging least-cost, innovative compliance.	Straightforward and rapid to verify (inspect the installed equipment/operating parameters) even before enough effluent monitoring data exists to judge numeric compliance.
Advantage 2	Directly ties the permit to the receiving water's actual assimilative capacity, so limits can be tightened or loosened as receiving-water conditions change.	Gives every discharger in a sector a consistent minimum control floor, avoiding the "worst performer" outcome that a loosely enforced numeric limit can allow.
Limitation 1	Difficult to set correctly without good receiving-water assimilative-capacity data, and enforcement depends on catching an exceedance through periodic sampling.	Low design flexibility — the specific technology (or a demonstrated equivalent) is effectively mandated, which can lock in a particular process even where a cheaper alternative would achieve the same environmental outcome.
Limitation 2	A discharger can be numerically compliant on average while still causing short-term (e.g. storm-driven) spikes that a fixed limit does not capture well.	Verifying the technology is installed does not guarantee the resulting effluent quality, since operating practice, maintenance and influent variability still govern actual performance.

In practice regulators frequently combine both: a technology-based floor (ensuring every discharger installs at least a baseline treatment level) topped up with site-specific numeric limits where the receiving water's assimilative capacity requires tighter control than the base technology alone would achieve.

(iii) TSS loading reduction to control eutrophication

An effective treatment method is enhanced primary sedimentation with chemical coagulation: a metal-salt or polymer coagulant (alum, ferric chloride, or a cationic polymer) is dosed ahead of the primary clarifier to destabilize and flocculate the fine, otherwise poorly-settling colloidal solids into larger, faster-settling flocs, substantially increasing TSS (and associated particulate-phosphorus) removal above what plain gravity sedimentation alone achieves.

Coagulant dose control via jar testing. Because an under-dose leaves colloidal solids unflocculated (no TSS benefit) while an over-dose wastes chemical and can re-stabilize the floc, the dose is set (and periodically re-verified) by bench-scale jar testing against the actual influent, not a fixed literature value.
Surface overflow rate (SOR) and detention time control. The clarifier must be sized (and operated within) an SOR low enough, and a detention time long enough, for the now-larger chemical floc to settle before reaching the outlet weir; increasing flow beyond the design SOR (e.g. during wet-weather peaks) degrades TSS removal even with correct chemical dosing, so flow equalization or peak-flow bypass management is an important complementary operating control.

Topic: Air-quality unit conversion and CEMS/ambient monitoring; regulatory vs. technology-based water-pollution control; chemically-enhanced primary sedimentation for TSS/eutrophication control.

Key relations: C(µg/m³)=ppm×(M/V_m)×1000; coagulant dose from jar testing; surface overflow rate (SOR)=Q/A_surface as the sizing basis for a settling basin.

Why this works: Source and ambient SO₂ monitoring are complementary because stack compliance alone cannot guarantee receptor protection once dispersion and background are included; regulatory and technology-based approaches trade design flexibility against ease of enforcement; chemical coagulation converts otherwise non-settleable colloidal TSS into a settleable floc, so its benefit is realized only if the downstream clarifier is also correctly sized for the new floc's settling velocity.

Common pitfall: Using 22.4 L/mol (true 0°C STP) inconsistently with a 25°C ambient-monitoring dataset; assuming a technology-based requirement alone tells you the resulting effluent quality; dosing coagulant correctly but exceeding the clarifier's design SOR during wet weather, losing the TSS-removal benefit anyway.

Source: Cooper & Alley, Air Pollution Control, 4th ed., Ch. 1 (units and CEMS/ambient monitoring); Metcalf & Eddy, Wastewater Engineering, 4th ed., Ch. 5 (chemically enhanced primary treatment) and Ch. 8 (eutrophication/nutrient control).

Question 1 of 7: Air/Water Pollution Abatement and TSS Loading

Problem 1: Air/Water Pollution Abatement and TSS Loading (20 marks)

(i) SO₂ concentration conversion and air monitoring

(ii) Regulatory vs. technology-based water pollution abatement

(iii) TSS loading reduction to control eutrophication