19.3 Emissions Evaluation, Mitigation, and Remediation

19.3 Emissions Evaluation, Mitigation, and Remediation

The NCEES specification lists Environment (e.g., emissions evaluation, mitigation, remediation) as the third subtopic under Safety, Health, and Environment. It is the subtopic most often absent from PE Chemical preparation, and it is examined the same way as any other unit operation — quantify the stream, select a technology, size it.

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1. Classifying and Quantifying Emissions

Three quantification approaches recur:

1. Material balance. The most defensible method when the inventory is well measured: what enters minus what leaves in product, waste, and recycle is what was emitted. If a tank farm receives $500{,}000\text{ kg/yr}$ of toluene and $499{,}200\text{ kg/yr}$ is accounted for in product and shipped waste, the $800\text{ kg/yr}$ difference is the emission estimate. The weakness is precision: the emission is a small difference between two large numbers, so a $0.5\%$ metering error swamps the answer.
2. Emission factors. Published mass of pollutant per unit of activity (per $\text{m}^3$ processed, per tonne produced, per tank turnover). EPA's AP-42 compilation is the standard source. Fast, but factors carry wide uncertainty ratings.
3. Direct measurement. Most accurate, most expensive, and the basis of compliance demonstration for major point sources.

Storage tank emissions split into working losses (vapor expelled as liquid is pumped in) and breathing losses (vapor expelled as the vapor space heats and expands diurnally). Both are attacked structurally: a floating roof eliminates most of the vapor space and is the standard control for volatile liquids in large fixed-roof service; a nitrogen blanket with a vapor recovery unit handles smaller or higher-value streams.

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2. Air Emission Control Technologies

The Deutsch-Anderson equation for an electrostatic precipitator:

$$\eta = 1 - \exp\!\left( -\frac{w A}{Q} \right)$$

where (w) is the particle migration velocity, (A) the collecting plate area, and (Q) the gas volumetric flow.

Worked example. $w = 0.10\text{ m/s}$, $A = 3{,}000\text{ m}^2$, $Q = 50\text{ m}^3\text{/s}$:

$$\frac{wA}{Q} = \frac{(0.10)(3{,}000)}{50} = 6.00 \qquad \eta = 1 - e^{-6.00} = 1 - 0.00248 = 99.75\%$$

Now tighten the permit to $99.9\%$. Required $wA/Q = \ln(1/0.001) = 6.908$, so (A) must rise by (6.908/6.00 = 1.15), a $15\%$ area increase. But going from $99\%$ to $99.9\%$ requires (wA/Q) to rise from (4.605) to (6.908) — an additive increment of $\ln(10) = 2.303$. Every additional nine costs the same absolute increment of (wA/Q), so the marginal cost of capture is constant in "nines" and explosive in absolute terms. That structure is why permit limits expressed as $99.99\%$ are enormously more expensive than $99.9\%$, and it appears identically in adsorption bed depth and in scrubber transfer units.

Flares. 40 CFR 60.18 does not itself state a destruction efficiency; it sets the operating criteria a flare must meet — a continuously present pilot flame, exit velocity below the specified limit, and a net heating value of at least $300\text{ Btu/scf}$ for steam- or air-assisted flares ($200\text{ Btu/scf}$ for unassisted). EPA then credits a flare operated within those criteria with $98\%$ destruction efficiency for emission inventory and compliance purposes. Over-steaming a flare quenches the flame and destroys the destruction efficiency, which is why steam ratio is controlled rather than maximized.

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3. Water and Waste

Wastewater treatment is staged by contaminant class:

• Primary (physical): equalization, oil-water separation (API separator), dissolved air flotation, sedimentation.
• Secondary (biological): activated sludge, trickling filter, anaerobic digestion — for biodegradable organic load measured as BOD/COD.
• Tertiary (polishing): filtration, carbon adsorption, air stripping (Section 12.1), membranes (Section 12.3), advanced oxidation.

Stream segregation is the highest-leverage decision: mixing a small, concentrated, toxic stream into a large dilute one creates a large toxic stream and forfeits the option of treating the concentrate at its source.

The pollution prevention hierarchy, established by the Pollution Prevention Act of 1990, ranks responses:

   1. SOURCE REDUCTION      <- eliminate the stream (solvent substitution, leak-free seals,
                                improved conversion/selectivity, closed sampling)
   2. REUSE / RECYCLE       <- return it to the process or recover the material
   3. TREATMENT             <- destroy or convert what remains (oxidizer, biotreatment)
   4. DISPOSAL              <- landfill, deep well, incineration without recovery

Exam questions frequently offer a control device where a process change is available. If one option eliminates the stream and another captures it, the hierarchy says eliminate it.

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4. The Regulatory Framework

A licensed PE is expected to recognize which statute governs which stream.

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5. Remediation

Once a release has reached soil or groundwater, technology selection is driven by contaminant volatility, solubility, and biodegradability.

Two practical facts shape almost every remediation question. First, pump and treat is very good at containing a plume and very poor at cleaning one up: once the mobile dissolved mass is removed, contaminant desorbs slowly from soil and the effluent concentration plateaus, a phenomenon called tailing, so cleanup asymptotes rather than finishing. Second, dense non-aqueous phase liquids (DNAPLs) such as chlorinated solvents sink below the water table and pool on low-permeability layers, where they act as a long-term source that no groundwater extraction system can reach. Locating and removing the source zone matters more than pumping the plume.