Questions & explanations
1. How does single fiber efficiency relate to the overall efficiency of a filter?
Single fiber efficiency is the probability that a particle passing near a single fiber will be captured. The overall filter efficiency depends on how many fibers the air passes through and how they are arranged. If each fiber captures a certain fraction of particles, the total fraction captured increases as air goes through many layers. The overall efficiency is calculated using the single fiber efficiency and the filter's physical properties like thickness and packing density. A higher single fiber efficiency means fewer layers are needed to achieve a given overall efficiency. This concept helps in designing filters with the right balance of pressure drop and capture.
2. Compare the Langmuir and Freundlich isotherms. When would you use each to model adsorption?
Both isotherms describe how much contaminant adsorbs onto a solid (like activated carbon) at a given concentration. The Langmuir isotherm assumes a single layer of molecules on a surface with identical sites, and it levels off at high concentrations. The Freundlich isotherm is an empirical equation that allows for multiple layers and heterogeneous surfaces. Use Langmuir when you think adsorption is limited to a monolayer, like for some metals on specific adsorbents. Use Freundlich when the surface is rough or when adsorption occurs in multiple layers, like for organic compounds on activated carbon. Langmuir has a maximum capacity, while Freundlich does not.
3. Explain the concept of 'granule size distribution' in an EGSB reactor. Why is it important for reactor performance?
Granule size distribution refers to the range of diameters of sludge granules in the reactor. In EGSB, granules are typically smaller (0.5–2 mm) than in UASB (1–3 mm) due to the higher shear forces. A uniform size distribution is important because it ensures consistent settling properties and prevents stratification. If granules are too large, they may settle too quickly and not expand properly; if too small, they may be washed out. The distribution affects the specific surface area for microbial activity and the hydraulic behavior. Operators monitor size distribution to detect changes in sludge health and adjust operating conditions accordingly.
4. Compare the scope of state EIA laws with the federal EPBC Act. What does each cover?
State EIA laws cover a wide range of local environmental issues, such as land use, pollution, and planning. The federal EPBC Act focuses only on matters of national environmental significance, like World Heritage properties, national heritage places, wetlands of international importance, listed threatened species and ecological communities, migratory species, Commonwealth marine areas, the Great Barrier Reef Marine Park, nuclear actions, and water resources in relation to coal seam gas and large coal mining. States handle most projects, but if a project affects a national matter, both state and federal assessments are needed.
5. What is an advanced oxidation process (AOP) and how does it differ from conventional chemical oxidation?
An advanced oxidation process (AOP) is a treatment method that generates highly reactive hydroxyl radicals (•OH) to break down organic pollutants. Unlike conventional oxidants like chlorine or hydrogen peroxide alone, hydroxyl radicals react very quickly and non-selectively, oxidizing even resistant compounds. AOPs can completely destroy pollutants into carbon dioxide and water, rather than just transferring them to another phase. Common AOPs include ozone (O₃) with hydrogen peroxide (H₂O₂), UV/H₂O₂, and Fenton's reagent. They are used for treating industrial wastewater, landfill leachate, and drinking water contaminants.
6. Explain the start-up procedure for a UASB reactor. Why is it important to gradually increase the organic load?
Start-up involves seeding the reactor with granular sludge from an existing UASB or with flocculent sludge, then feeding wastewater at a low organic load (e.g., 0.5 kg COD/m³·day). The load is gradually increased over weeks or months as the sludge develops into granules with good settling properties. Gradual increase is important because it allows the microbial community to adapt and grow without being overloaded. If the load is increased too fast, the sludge may become buoyant, wash out, or produce volatile fatty acids that inhibit methanogens. A successful start-up produces stable granules and high treatment efficiency.
7. Compare ozone (O₃) and UV/H₂O₂ as AOPs. Which one is more effective for removing pharmaceuticals from water?
Both ozone and UV/H₂O₂ generate hydroxyl radicals, but they work differently. Ozone directly oxidizes some compounds and also decomposes to form radicals, especially at high pH. UV/H₂O₂ uses ultraviolet light to split hydrogen peroxide into two hydroxyl radicals. For removing pharmaceuticals, UV/H₂O₂ is often more effective because it produces radicals more consistently and doesn't form bromate byproducts (a concern with ozone in bromide-containing water). However, ozone can be more energy-efficient for large flows if the water matrix is suitable. The choice depends on the specific pollutants, water chemistry, and cost.
8. Give an example of a real-world application of AOPs in wastewater treatment. What pollutant is targeted and why is AOP chosen?
A common application is treating landfill leachate, which contains high levels of refractory organic compounds like humic acids and ammonia. Conventional biological treatment often fails to degrade these pollutants. AOPs like ozone/H₂O₂ or Fenton are used to break down these recalcitrant organics, improving biodegradability and reducing color and odor. For instance, Fenton oxidation can reduce chemical oxygen demand (COD) by 60–80% in leachate. AOPs are also used to remove trace contaminants like pesticides and pharmaceuticals from drinking water sources. They are chosen when other methods are too slow or ineffective.
9. Explain why the Cunningham slip correction is more important for a 0.1 µm particle than for a 10 µm particle.
The Cunningham slip correction factor becomes significant when the particle size is comparable to the mean free path of air molecules (about 0.065 µm at room conditions). For a 0.1 µm particle, the particle is only about 1.5 times the mean free path, so slip effects are large. For a 10 µm particle, the particle is much larger than the mean free path, so the air behaves like a continuous fluid and slip is negligible. The correction factor for 0.1 µm might be around 2.9, meaning drag is less than half of Stokes drag. For 10 µm, the factor is close to 1. So ignoring slip for small particles leads to big errors.
10. What is Henry's law and how is it used in water treatment?
Henry's law says that the amount of a gas that dissolves in water is proportional to the pressure of that gas above the water. In water treatment, it helps predict how gases like oxygen, carbon dioxide, or chlorine will behave. For example, to add oxygen to water (aeration), higher pressure increases oxygen dissolution. Henry's law is used to design aeration systems for biological treatment or to remove dissolved gases like hydrogen sulfide by lowering pressure. The law is written as C = kH * P, where C is the gas concentration in water, P is the partial pressure, and kH is the Henry's constant for that gas.
11. Compare a catalytic oxidizer and a thermal oxidizer for VOC control.
A catalytic oxidizer uses a catalyst to oxidize VOCs at lower temperatures (300-500°C) than a thermal oxidizer (700-1000°C). This saves fuel and reduces operating costs. However, catalytic oxidizers are more sensitive to poisons and particulates, and the catalyst needs replacement every few years. Thermal oxidizers can handle a wider range of pollutants and are simpler but use more energy. For example, for a paint booth with low VOC concentration, a catalytic oxidizer might be chosen for energy savings. For a stream with chlorinated compounds, a thermal oxidizer may be better to avoid catalyst poisoning.
12. Compare impaction and interception: which mechanism becomes more important for larger particles?
Impaction becomes more important for larger particles because they have more inertia and tend to continue in a straight line when the air bends around a fiber. Interception is also more important for larger particles because they have a larger radius, so they come closer to the fiber surface as they follow the streamlines. Both mechanisms increase with particle size, but impaction increases more sharply at higher air velocities. For very large particles (above 1 µm), impaction dominates. For intermediate sizes (around 0.3-1 µm), interception is significant. Diffusion is dominant for very small particles.