Air/Water Pollution Abatement and Nutrient Removal: 23-Chem-B2 Environmental Engineering, December 2019, Q2

Problem 2: Air/Water Pollution Abatement and Nutrient Removal (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) NO₂ concentration conversion and air monitoring

Given.

Quantity	Value
NO₂ concentration	1.0 ppm(v)
Molar mass of NO₂, M	46.0 g/mol
Molar volume of an ideal gas, V_m	24.45 L/mol

Find. NO₂ concentration in µg/m³.

Check — reference-condition assumption

The question says "STP" but gives no explicit temperature/pressure. True STP (0°C, 1 atm) gives V_m=22.41 L/mol; however, 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.

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. $C = \text{ppm} \times \frac{M}{V_m} \times 1000 = 1.0 \times \frac{46.0}{24.45} \times 1000$ $C = \boxed{1{,}881\ \mu\text{g/m}^3}$

Quantity	Value
NO₂ concentration	1,881 µg/m³ (at 1.0 ppm, 25°C/1 atm)

NO₂ air monitoring for abatement compliance. A network of chemiluminescence (or, at lower-cost fixed sites, differential optical absorption spectroscopy) ambient NO₂ analyzers is sited around the source at representative receptor locations (typically the predicted maximum ground-level impact point from dispersion modelling, plus background/upwind stations), continuously logging concentration against the applicable ambient air-quality standard. At the source itself, a continuous emission monitoring system (CEMS) on the stack tracks the mass emission rate in near-real time so that any abatement equipment (low-NOₓ burner, SCR) can be confirmed to be holding its rated removal efficiency; the two data streams together (source CEMS + ambient network) close the loop between "is the control equipment working" and "is the receptor actually protected," which a stack-only or ambient-only monitoring program cannot do on its own.

(ii) Regulatory vs. technology-based (BATEA) 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 operate the Best Available Technology Economically Achievable (BATEA) for that industry category, regardless of what numeric effluent quality results.

Aspect	Regulatory (numeric effluent standard)	BATEA (technology-based)
Basis of compliance	Measured effluent concentration/mass loading against a fixed limit.	Installation and proper operation of a prescribed (or demonstrably equivalent) treatment technology, independent of the exact numeric result achieved.
Flexibility in design	High — the discharger may choose any treatment train that reliably meets the numeric limit, encouraging innovation and least-cost compliance.	Low — the specific technology (or an equivalent) is effectively mandated, which can lock in a particular process even if a cheaper alternative would meet the same environmental outcome.
Regulatory certainty and enforceability	Straightforward to enforce (sample, analyze, compare to the limit) but can be difficult to set correctly for a receiving water with variable assimilative capacity.	Easy to verify (inspect the installed equipment/operating parameters) even before enough effluent data exists, and gives all dischargers in a sector a consistent minimum control floor regardless of local receiving-water conditions.

(iii) Phosphorus removal to control eutrophication

An effective method is chemical precipitation of phosphorus using a metal salt (alum, Al₂(SO₄)₃, or ferric chloride, FeCl₃) dosed into the aeration basin or a tertiary clarification stage, forming an insoluble AlPO₄/FePO₄ floc that is removed with the wasted sludge.

Coagulant dose control. The metal-salt dose is set (and trimmed via jar testing or on-line phosphorus/turbidity feedback) to slightly exceed the stoichiometric Me:P molar ratio needed for precipitation, because competing reactions (alkalinity consumption, floc formation with other solids) reduce the effective phosphorus-removal yield below the ideal stoichiometric value.
Solids-liquid separation downstream of dosing. Because phosphorus removal here is only achieved once the precipitated floc is physically separated from the water (settling or filtration), the downstream clarifier/filter must be sized and operated (adequate detention time, low overflow rate) to capture the fine metal-phosphate floc, or the phosphorus simply re-enters suspension in the effluent solids.

Topic: Air-quality unit conversion and monitoring strategy; regulatory vs. technology-based water-pollution control; chemical phosphorus removal.

Key relations: C(µg/m³)=ppm×(M/V_m)×1000; metal-salt stoichiometric Me:P precipitation ratio.

Why this works: The ppm→mass conversion is simply the ideal-gas molar-volume relationship applied to a trace mixing ratio; source (CEMS) and receptor (ambient) monitoring are complementary because compliance at the stack does not guarantee protection at the receptor once dispersion, background and multiple-source effects are included; chemical P removal works because phosphate's solubility product with Al₃⁺/Fe₃⁺ is very low, so the precipitation reaction goes essentially to completion once the floc is captured.

Common pitfall: Using 22.4 L/mol (true 0°C STP) inconsistently with a 25°C ambient-monitoring dataset; assuming a technology-based (BATEA) requirement alone tells you the resulting effluent quality, when in fact it only guarantees the control method, not the numeric outcome.

Source: Cooper & Alley, Air Pollution Control, 4th ed., Ch. 1 (units and monitoring); Metcalf & Eddy, Wastewater Engineering, 4th ed., Ch. 8 (chemical phosphorus removal).