Air and Noise Pollution Control

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This section covers the history of air pollution control in Nepal, the emissions-transport-receptor framework, units and standards, the effects of air pollution on human health, property and visibility, the governing laws and regulations, and the general practices used to control air pollution.

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Definition and the Emissions-Transport-Receptor Framework • Air pollution is the presence in the outdoor or indoor atmosphere of one or more contaminants in such quantity, of such characteristics and of such duration as to be injurious to human, plant or animal life, to property, or to interfere unreasonably with the comfortable enjoyment of life and property.

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Note that the definition requires a quantity and a duration — a substance is not a pollutant merely by being present, which is why every standard carries an averaging time. • The whole subject is organised around three stages, and every control strategy acts on one of them: • 1.

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Emissions — the release of the pollutant from the source.

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Action here is the only action that actually reduces the mass of pollutant entering the atmosphere. • 2.

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Transport — dispersion, dilution and chemical transformation in the atmosphere between source and receptor.

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Action here, principally a taller stack, redistributes the pollutant but does not reduce it; this is the point on which the standard examination question turns. • 3.

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Receptor — the person, plant, animal or material affected.

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Action here means protecting the receptor by masks, air cleaning indoors, or moving people away. • Pollutants are classed as primary if they are emitted directly from a source — sulphur dioxide, nitric oxide, carbon monoxide, lead, primary particulate — and secondary if they are formed in the atmosphere from precursors — ozone, sulphate and nitrate particles, peroxyacetyl nitrate, much of the fine particulate mass. • Sources are classed as point (a stack), line (a road), area (a city or a cluster of brick kilns) and mobile, and the dispersion treatment of each differs.

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The Criteria Pollutants Pollutant Principal sources Principal effect PM10 and PM2.5 Combustion, vehicles, brick kilns, road dust, construction Respiratory and cardiovascular disease; visibility loss Sulphur dioxide, SO2 Coal and fuel-oil combustion, smelting Bronchoconstriction; acid deposition; sulphate particles Nitrogen oxides, NOx All high-temperature combustion, vehicles Respiratory irritation; ozone and nitrate precursor Carbon monoxide, CO Incomplete combustion, petrol vehicles, biomass stoves Binds haemoglobin; reduces oxygen transport Ozone, O3 Secondary, from NOx and VOCs in sunlight Powerful oxidant; lung function loss; crop damage Lead, Pb Formerly petrol additive; smelting, battery works Neurotoxic, especially to children Benzene and VOCs Petrol evaporation, solvents, incomplete combustion Carcinogenic; ozone precursor Units and Standards • Gaseous pollutants are reported either as a volume ratio (ppm or ppb) or as a mass concentration (μg/m³ or mg/m³), and the conversion between the two must be known: • μg/m³ = ppm × M × 1000 / 24.45 at 25 °C and 1 atmosphere, where M is the molecular mass in g/mol and 24.45 litres is the molar volume.

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At 0 °C the molar volume is 22.41 litres, so the reference temperature must always be stated. • The volume ratio is independent of temperature and pressure; the mass concentration is not, which is why emission limits are specified at stated conditions and usually at a stated reference oxygen content. • Particulate matter is always reported as a mass concentration, because it has no single molecular mass. • Standards are of two kinds and the distinction is frequently examined: ambient standards fix the permissible concentration in the air that people breathe, while emission standards fix what may be discharged from a source.

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An emission standard is a means; an ambient standard is the end. • Every ambient standard carries an averaging time — one hour, eight hours, twenty-four hours or a year — because the health effect depends on the duration of exposure as much as on the concentration.

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Pollutant Averaging time Nepal NAAQS (μg/m³) TSP 24 hours 230 PM10 24 hours 120 PM2.5 24 hours 40 Sulphur dioxide 24 hours 70 Nitrogen dioxide 24 hours 80 Carbon monoxide 8 hours 10 000 Ozone 8 hours 157 Lead Annual 0.5 Benzene Annual 20 These are the values of the National Ambient Air Quality Standards commonly cited for Nepal, first notified in 2003 and revised in 2012.

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They are periodically reviewed, so the current gazette should be consulted for professional work.

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The World Health Organization guideline values are considerably stricter, and a national standard that a country can realistically enforce is deliberately set above the guideline.

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History of Air Pollution Control in Nepal • The Kathmandu Valley is a bowl roughly 25 km across, enclosed by hills rising 400 to 1000 m above the floor, with a single significant outlet at Chobhar.

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This topography, combined with a strong night-time temperature inversion in the dry winter months, traps emissions close to the ground and is the root physical cause of the valley's air quality problem — a point worth stating in any descriptive answer. • Himal Cement at Chobhar was for decades the single most visible industrial source in the valley and was closed in 2002 on environmental and economic grounds. • Diesel three-wheeler tempos were banned from the Kathmandu Valley in 1999 and replaced by the battery-electric Safa Tempo, which remains the most often cited Nepali example of a successful source-substitution measure. • Leaded petrol was phased out, and vehicle mass emission standards were introduced from 2000, with subsequent tightening towards the Euro and Bharat stage norms for imported vehicles. • Brick kilns are the characteristic seasonal source of the valley.

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Following the 2015 earthquake the traditional fixed-chimney Bull's Trench kilns were largely rebuilt as induced-draught zigzag kilns, which burn more completely and substantially reduce particulate and black carbon emission per brick. • An ambient monitoring network was established in 2002 with assistance from the Danish government, lapsed after a few years for want of operating funds, and was re-established from 2016 as a network of continuous stations under the Department of Environment, reporting an air quality index publicly. • Indoor air pollution from biomass cooking remains the largest single air-related health burden in rural Nepal, addressed through improved cooking stoves and biogas rather than through ambient regulation.

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Effects of Air Pollution • On human health.

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PM2.5 is the pollutant responsible for the greatest burden of disease, because particles below 2.5 μm penetrate to the alveoli and the finest fraction crosses into the bloodstream, producing cardiovascular as well as respiratory effects.

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Carbon monoxide binds to haemoglobin with roughly 210 to 240 times the affinity of oxygen to form carboxyhaemoglobin, so a small concentration produces a large reduction in oxygen-carrying capacity.

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Sulphur dioxide is highly soluble and is absorbed in the upper airway, where it causes bronchoconstriction, particularly in asthmatics.

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Nitrogen dioxide is less soluble, so it penetrates deeper and acts on the lower airway.

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Ozone is a powerful oxidant that reduces lung function even in healthy adults during exercise.

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Lead is a cumulative neurotoxin with no safe threshold, and children are far more susceptible than adults. • The London smog of December 1952, which killed some four thousand people in a few days through a combination of a stagnant anticyclone, coal smoke and sulphur dioxide, is the standard historical illustration and led directly to the first Clean Air Act of 1956. • On property.

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Sulphur dioxide and its acid products convert the calcium carbonate of limestone and marble to gypsum, which is bulkier and soluble, so the stone blisters and spalls — the mechanism of damage to monuments.

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Acid gases corrode metals, ozone cracks rubber and perishes textiles, and particulate matter soils building surfaces and fabrics, imposing a recurrent cleaning cost. • On visibility.

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Visibility is reduced by the scattering and absorption of light by particles, and scattering is most efficient when the particle diameter is comparable with the wavelength of visible light, that is in the range about 0.1 to 1 μm.

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This is why fine particles, and not the coarse dust that is far more obvious, dominate the loss of visibility.

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The visual range is given by the Koschmieder relation, Lv = 3.912 / bext, where bext is the extinction coefficient in reciprocal metres. • On the global and regional scale, air pollutants also drive acid deposition, stratospheric ozone depletion by chlorofluorocarbons, climate forcing by carbon dioxide and by black carbon, and the regional atmospheric brown cloud over South Asia.

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Laws, Regulations and Control Practices • The Environment Protection Act 2076 (2019) and the Environment Protection Rules 2077 (2020) are the principal statutory instruments, replacing the Act of 2053 (1997).

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They provide for the Brief Environmental Study, the Initial Environmental Examination and the Environmental Impact Assessment, for the prescription of standards, and for the polluter-pays principle. • The National Ambient Air Quality Standards, the Nepal Vehicle Mass Emission Standards, the generator and brick kiln emission standards and the indoor air quality guidelines are notified under this framework, with the Department of Environment as the monitoring agency. • The four general control practices, in the order in which they should be considered, are: • 1.

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Change or treat the input — use a low-sulphur fuel, a cleaner fuel, or a less volatile solvent.

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This prevents the pollutant from being created at all and is always first choice. • 2.

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Change the process — a zigzag kiln rather than a Bull's Trench kiln, a low-NOx burner rather than a conventional one, electric rather than diesel traction. • 3.

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Treat the exhaust gas — cyclone, precipitator, filter, scrubber, oxidiser.

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This removes the pollutant but transfers it to a solid or liquid stream and costs money to run. • 4.

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Disperse — a taller stack, or relocation of the plant.

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This is not control; it is dilution, and it is the last resort.

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This section covers the alternatives open to the designer, resource recovery from pollutants, the ultimate fate of what is collected, the design of control systems and equipment, typical fluid velocities, the minimising of volumetric flow rate and pressure drop, efficiency, penetration and the language of nines, calculation on an inert basis, the correction of volumetric flow rates, the acid dew point, and catalysts.

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Alternatives and the Ultimate Fate of Pollutants • Faced with an emission, the engineer has four alternatives, and the examination expects them in order: improve the dispersion; change the process or the raw material so that the pollutant is not formed; collect and treat the pollutant; or cease the activity. • The ultimate fate of a captured pollutant is the central honest point of the subject: a control device does not destroy matter, it transfers it.

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The dust captured by a precipitator becomes a solid waste, the sulphur captured in a wet scrubber becomes a sludge or a gypsum, and the solvent captured on carbon becomes a liquid to be recovered or burnt.

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Air pollution control is therefore very often a cross-media transfer, and a design is not complete until the destination of the captured material is settled. • The only genuine exceptions are the destructive devices — thermal and catalytic oxidisers, which convert hydrocarbons to carbon dioxide and water, and selective catalytic reduction, which converts nitrogen oxides to nitrogen and water.

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These destroy rather than transfer, which is their great advantage. • Resource recovery turns the cross-media problem into a product: sulphur dioxide converted to elemental sulphur by the Claus process or to gypsum by limestone scrubbing, solvent recovered from activated carbon by steam stripping, fly ash used in cement and in blocks, and heat recovered from an oxidiser.

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Recovery rarely pays for the control by itself, but it reduces the net cost and removes the disposal liability.

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Efficiency, Penetration and Nines • Collection efficiency is η = (mass in − mass out) / mass in, and penetration is p = 1 − η, the fraction that gets through. • Penetration is the more useful quantity whenever devices are placed in series, because penetrations multiply while efficiencies do not: poverall = p1 × p2 × …, so ηoverall = 1 − (1 − η1)(1 − η2). • Worked illustration.

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A cyclone at 90 per cent followed by a precipitator at 98 per cent. p = 0.10 × 0.02 = 0.002, so η = 99.8 per cent.

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The efficiencies do not add to 188 per cent, and the cyclone's contribution is to reduce the loading on the precipitator rather than to add to its efficiency. • The language of nines expresses high efficiencies compactly:

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99.9 per cent is three nines, 99.99 per cent is four nines.

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Each additional nine divides the penetration by ten, so going from three nines to four nines removes a further ninety per cent of what was escaping, and the cost of each successive nine rises steeply.

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This is why a requirement is written as an emission limit rather than as an efficiency. • Fractional or grade efficiency is the efficiency as a function of particle size, and it is far more informative than a single overall figure, because a device can show ninety-five per cent by mass while passing almost all of the particles by number, the mass being dominated by a few coarse grains. • Homogeneous pollutants are uniformly distributed in the gas and behave as a single substance — the gases.

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Non-homogeneous pollutants vary in size and composition across the stream — the particulates — and must be treated with a size distribution rather than a single concentration.

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Flow Rate, Pressure Drop and Velocity • The running cost of a control system is dominated by the fan power, and fan power is proportional to the product of the volumetric flow rate and the pressure drop:

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Both terms must therefore be minimised, and this governs the economics of the whole installation. • Minimising the volumetric flow rate means above all excluding dilution air.

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Air drawn in through leaks, hoods that are larger than necessary, or deliberate tempering air enlarges every item of equipment without removing a single additional gram of pollutant.

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Since the emission limit is a concentration, dilution appears to reduce the concentration while leaving the mass emission unchanged — which is why limits are corrected to a reference oxygen content. • Calculations are therefore based on the inert flow rate — the flow of the non-reacting component, usually nitrogen or dry air — because that quantity is conserved through combustion, dilution, drying and absorption, while the total flow is not.

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This is the same device as the dry-gas basis in combustion calculations and as the inert-liquid basis in absorption. • Volumetric flow rates must be corrected between conditions by the gas law:

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Q2 = Q1 (T2/T1) (P1/P2), with absolute temperatures.

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Normal conditions (Nm³) are 0 °C and 1 atm; standard conditions (Sm³) are commonly 25 °C and 1 atm; and a flue gas at 150 °C occupies about 1.55 times the volume it occupies at 0 °C. • A further correction to a reference oxygen is applied to combustion sources:

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Ccorrected = Cmeasured × (21 − O2,reference) / (21 − O2,measured), which removes the effect of excess air and prevents compliance by dilution.

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Location Typical velocity Reason Duct carrying dust-laden gas 15 to 25 m/s Must exceed the saltation velocity or dust settles and plugs the duct Duct carrying clean gas 8 to 15 m/s No settling risk, so velocity is set by pressure drop economics Cyclone inlet 15 to 20 m/s Centrifugal force rises with the square of velocity Electrostatic precipitator 1 to 2 m/s Slow so that gas residence time is adequate and dust is not re-entrained Fabric filter (air-to-cloth) 0.01 to 0.04 m/s Filtration velocity; higher values blind the fabric and raise pressure drop Venturi scrubber throat 60 to 120 m/s High relative velocity atomises the liquid and drives impaction Settling chamber 0.3 to 3 m/s Low enough for coarse particles to settle under gravity Acid Dew Point and Catalysts • A small fraction of the sulphur dioxide in a flue gas, typically one to three per cent, is oxidised to sulphur trioxide, which combines with water vapour to form sulphuric acid.

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The acid dew point is the temperature at which that acid begins to condense, and it is far above the water dew point — typically 120 to 160 °C depending on the sulphur trioxide and moisture contents, against perhaps 50 °C for water. • The consequence governs the design of every flue gas path: surfaces downstream of the boiler must be kept above the acid dew point, which sets a floor on the stack gas temperature and therefore limits how much heat can be recovered from the flue gas.

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Cooling below it produces rapid low-temperature corrosion of economisers, air heaters, ducts and stack liners, and in a wet stack a visible acid plume. • Catalysts are used in air pollution control for three duties: • 1.

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Selective catalytic reduction (SCR) of nitrogen oxides, with ammonia or urea injected ahead of a vanadium pentoxide and tungsten oxide on titania catalyst, operating at 300 to 400 °C and achieving 80 to 95 per cent removal.

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The products are nitrogen and water. • 2.

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Catalytic oxidation of volatile organic compounds and carbon monoxide, over platinum or palladium, which allows destruction at 320 to 540 °C instead of the 760 to 870 °C needed for thermal oxidation, saving a great deal of fuel. • 3.

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The three-way catalyst on a petrol vehicle, over platinum, palladium and rhodium, which oxidises carbon monoxide and hydrocarbons and simultaneously reduces nitrogen oxides, but only within a narrow window about the stoichiometric air-fuel ratio, which is why it requires a lambda sensor and closed-loop fuelling. • Catalysts are poisoned by lead, phosphorus, arsenic and sulphur, and are deactivated by sintering if overheated and by fouling with particulate.

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The requirement for unleaded petrol followed from the three-way catalyst, not the other way round — a commonly examined point of history.

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This section covers primary and secondary particulates, settling velocity and drag forces, particle size distribution functions, the behaviour of particles in our bodies and in the atmosphere, secondary fine particles, and the control of primary particulates, volatile organic compounds, sulphur oxides, nitrogen oxides and the other criteria pollutants.

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Primary and Secondary Particulates • Primary particulates are emitted directly as particles — fly ash, soot and black carbon, road and construction dust, brick kiln emissions, pollen, sea salt, wind-blown soil. • Secondary particulates are formed in the atmosphere from gaseous precursors, and in most urban atmospheres they are the larger part of the fine particulate mass: • Sulphate from the oxidation of sulphur dioxide, forming sulphuric acid and then ammonium sulphate. • Nitrate from the oxidation of nitrogen oxides, forming nitric acid and then ammonium nitrate. • Secondary organic aerosol from the oxidation of volatile organic compounds to lower-volatility products that condense. • This matters practically: a city that controls only its primary dust will find that its fine particulate concentration barely falls, because the precursor gases continue to manufacture particles downwind.

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Control of PM2.5 therefore requires control of sulphur dioxide, nitrogen oxides, ammonia and volatile organic compounds as well. • The atmospheric size distribution is tri-modal and the modes should be known by name: the nucleation or ultrafine mode below 0.1 μm, formed by condensation and combustion and very numerous but of negligible mass; the accumulation mode from 0.1 to about 2 μm, formed by coagulation and growth of the nucleation mode and by secondary formation; and the coarse mode above 2 μm, formed mechanically by grinding, abrasion and wind.

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Settling Velocity and Drag • A particle falling in still air accelerates until the drag force balances the net gravitational force, after which it falls at a constant terminal settling velocity. • The drag force is FD = CD A ρ v² / 2, where A is the projected area and ρ the gas density. • In the Stokes regime, where the particle Reynolds number is below about one, CD = 24 / Re, and substituting gives Stokes' law for the terminal velocity: • vt = (ρp − ρ) d² g / (18 μ) ≈ ρp d² g / (18 μ) for a solid particle in a gas, since the gas density is negligible. • The square dependence on diameter is the governing fact of the whole subject.

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Halving the diameter divides the settling velocity by four, so a 100 μm grain settles ten thousand times faster than a 1 μm particle.

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This is why gravity and inertial devices work well on coarse dust and fail completely on fine dust. • Worked illustration.

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A particle of density 2000 kg/m³ and diameter 10 μm in air of viscosity 1.8 × 10−5 Pa·s: v = 2000 × (10 × 10−6)² × 9.81 / (18 × 1.8 × 10−5) = 2000 × 10−10 × 9.81 / 3.24 × 10−4 ≈ 0.006 m/s, that is about 6 mm per second.

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A 1 μm particle of the same density settles at about 0.06 mm/s and would take some five hours to fall one metre, which is why it is effectively permanently suspended. • Below about 1 μm the particle becomes comparable in size with the mean free path of the gas molecules, the gas can no longer be treated as a continuum, and the particle slips between molecules.

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The Cunningham slip correction factor C, which is greater than one and increases as the particle gets smaller, is applied as v = C ρp d² g / 18μ.

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Omitting it under-estimates the settling velocity of submicron particles substantially. • Above a Reynolds number of about one Stokes' law ceases to apply, and the intermediate and Newton regimes must be used, with the Newton regime giving v proportional to the square root of diameter rather than to its square.

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Particle Size Distribution Functions • Atmospheric and industrial dusts are almost always well described by the log-normal distribution, that is the logarithm of the diameter is normally distributed.

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Plotted on log-probability paper the cumulative distribution becomes a straight line, which is why that paper is used. • Two parameters then describe the dust completely: the geometric mean or median diameter, and the geometric standard deviation σg, obtained as the ratio of the 84.1 percentile diameter to the 50 percentile diameter.

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A monodisperse dust has σg = 1; most industrial dusts lie between 2 and 5. • Distributions may be expressed by number, by surface area or by mass, and the three are very different for the same dust: the number distribution is dominated by the smallest particles and the mass distribution by the largest, since mass goes as the cube of diameter.

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Confusing them is the commonest error in this topic.

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The mass median diameter is always larger than the count median diameter. • The cut diameter d50 is the diameter collected with fifty per cent efficiency and is the standard single-figure description of a collector's performance. • Regulatory size cuts are defined by what the human body does with the particle:

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PM10 is the thoracic fraction, which passes the larynx;

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PM2.5 is the respirable fraction, which reaches the alveoli;

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PM1 and the ultrafines can cross the alveolar wall into the blood.

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Note that PM10 means particles with a 50 per cent cut at 10 μm aerodynamic diameter, not all particles smaller than 10 μm. • The aerodynamic diameter is the diameter of a sphere of density 1000 kg/m³ having the same settling velocity as the particle in question, and it is used because it is the settling behaviour rather than the geometric size that matters for both sampling and deposition.

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Particles in Our Bodies and in the Atmosphere • Three deposition mechanisms act in the respiratory tract, each dominant in a different size range: • Inertial impaction removes particles above about 5 to 10 μm in the nose, throat and large bronchi, where the airflow changes direction sharply. • Gravitational sedimentation removes particles of about 1 to 5 μm in the smaller bronchi and bronchioles, where the velocity is low and the residence time long. • Brownian diffusion removes particles below about 0.1 μm in the alveoli. • Deposition is therefore at a minimum near 0.3 to 0.5 μm, where the particle is too small to impact or settle and too large to diffuse appreciably, and a large fraction of particles of that size is simply breathed back out.

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The same size range is the hardest for a filter to capture, for exactly the same reasons, and is the basis of the HEPA most-penetrating particle size. • In the atmosphere the accumulation mode, 0.1 to 2 μm, has the longest residence time — days to weeks against minutes to hours for the coarse mode — because it is removed neither efficiently by settling nor efficiently by diffusion and coagulation.

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This removal minimum is called the Greenfield gap, and it explains why fine particles travel hundreds of kilometres and why their control is a regional and not a local problem. • Removal from the atmosphere occurs by dry deposition (settling and impaction on surfaces) and by wet deposition (rainout, where the particle acts as a condensation nucleus within the cloud, and washout, where falling rain scavenges particles below the cloud).

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This is why air quality improves dramatically with the onset of the monsoon.

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Control of Gaseous Criteria Pollutants Pollutant Control measures Volatile organic compounds Thermal oxidation at 760 to 870 °C; catalytic oxidation at 320 to 540 °C; adsorption on activated carbon with steam regeneration; condensation; biofiltration for dilute odorous streams; vapour recovery at loading racks; leak detection and repair; substitution of low-volatility solvents Sulphur oxides Low-sulphur fuel; fuel desulphurisation; wet limestone scrubbing to gypsum, 90 to 98 per cent; spray dryer absorber with lime, 70 to 90 per cent; dry sorbent injection, 40 to 60 per cent; fluidised bed combustion with limestone in the bed;

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Claus recovery of elemental sulphur Nitrogen oxides Low-NOx burners; staged and air-lean combustion; flue gas recirculation; water or steam injection; selective non-catalytic reduction at 850 to 1100 °C, 30 to 70 per cent; selective catalytic reduction at 300 to 400 °C, 80 to 95 per cent Carbon monoxide Complete combustion with adequate oxygen, temperature, time and turbulence; oxidation catalyst; good burner maintenance Lead Removal from petrol; bag filters on smelters and battery works; enclosure of handling operations • Nitrogen oxides form by three routes, and the distinction determines the control: • Thermal NOx — from atmospheric nitrogen and oxygen at high flame temperature, by the Zeldovich mechanism.

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It becomes significant above about 1300 °C and rises very steeply with temperature, so it is controlled by lowering the peak flame temperature. • Fuel NOx — from nitrogen chemically bound in the fuel, important for coal and heavy oil and insensitive to flame temperature; controlled by staging the combustion so that the fuel nitrogen is released into an oxygen-deficient zone. • Prompt NOx — formed rapidly in the flame front by hydrocarbon radicals attacking nitrogen; a small contribution in most equipment. • Because thermal NOx depends on peak temperature and not on average temperature, measures that spread the heat release — staging, recirculation, water injection and low-NOx burner design — are far more effective than measures that reduce the total heat released.

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This section covers the taking of a representative sample, its transport to the detector, the determination of concentration, averaging, the standard analytical methods, the determination of pollutant flow rates, isokinetic sampling, emission factors and the assessment of visible emissions.

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A Representative Sample and Its Transport • A measurement is worth no more than the sample on which it is made, and three decisions determine whether the sample is representative: where, when and for how long. • For ambient monitoring, the station must not be unduly influenced by a single nearby source unless that is its purpose.

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Stations are classified as background, residential, kerbside or industrial, and results from the different classes are not comparable.

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Conventional siting requires the inlet 3 to 10 m above ground, clear of obstructions and trees, and well away from the exhaust of the shelter itself. • For stack sampling, the sampling plane must lie in a straight run, conventionally at least eight duct diameters downstream and two upstream of any bend, damper or change of section, so that the velocity profile is settled and swirl has decayed.

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Because neither velocity nor dust loading is uniform across a duct, the cross-section is divided into equal-area segments and a traverse is made, sampling at the centroid of each. • Transport of the sample to the detector can destroy the measurement if it is mishandled: • The sample line must be heated above the dew point, otherwise water condenses and soluble gases such as sulphur dioxide, ammonia and hydrogen chloride are absorbed and lost. • Lines must be short and of inert material — PTFE or stainless steel — because reactive species adsorb on, and then slowly desorb from, ordinary surfaces, which smears a changing signal. • Particulate must be removed before the analyser but after the point at which it is itself to be measured, and a filter placed before the point of particulate measurement destroys the measurement. • The common collection media are the filter for particulate, the impinger containing an absorbing solution for soluble gases, the sorbent tube of activated carbon or Tenax for volatile organics, the evacuated canister for whole-air samples and the passive diffusion tube for long-period low-cost surveys.

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Concentration, Averaging and Standard Methods • The gravimetric determination is the primary method for particulate:

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C = (m2 − m1) / V, the gain in filter mass divided by the volume of air drawn, with the filter conditioned and weighed at controlled temperature and humidity before and after, and the volume corrected to stated conditions. • Averaging times follow the standard being assessed, and the arithmetic must match the standard.

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A twenty-four hour standard is assessed on a twenty-four hour mean, and an eight-hour standard on a rolling eight-hour mean, so that the worst eight-hour period of the day is captured rather than three fixed blocks. • Percentile and exceedance forms are common: a standard may permit a stated number of exceedances in a year, which recognises that extreme meteorology cannot be engineered away. • Data capture matters: an annual mean computed from less than about seventy-five per cent valid data is not normally accepted, because instrument downtime is rarely random with respect to pollution episodes. • The Air Quality Index converts the concentration of each pollutant to a common dimensionless scale by a piecewise linear transformation, and the index reported is the worst of the individual sub-indices, not their average — a frequently examined point.

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The pollutant producing it is reported as the responsible pollutant.

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Pollutant Standard analytical method PM10 and PM2.5 Gravimetric with a sized inlet (high- or low-volume sampler) as reference; beta attenuation and tapered element oscillating microbalance as continuous equivalents Sulphur dioxide Pulsed ultraviolet fluorescence (continuous);

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West-Gaeke pararosaniline colorimetry (manual reference) Nitrogen dioxide Chemiluminescence with a molybdenum converter, measuring NO and NOx and taking the difference;

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Jacobs-Hochheiser and Griess-Saltzman colorimetry (manual) Carbon monoxide Non-dispersive infrared absorption Ozone Ultraviolet photometric absorption at 254 nm; ethylene chemiluminescence Lead and metals Collection on filter, digestion, then atomic absorption spectrophotometry or ICP Volatile organics Sorbent or canister collection, then gas chromatography with FID or MS Opacity and smoke Transmissometer in the stack;

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Ringelmann chart by trained observer Pollutant Flow Rates and Isokinetic Sampling • The emission rate is the concentration multiplied by the volumetric flow rate, both at the same conditions:

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Mixing a concentration at stack conditions with a flow at standard conditions is the classic blunder, and it can introduce an error of fifty per cent or more. • The duct gas velocity is obtained with a pitot tube, conventionally the S-type for dirty gas, from v = Cp √(2 Δp / ρ), with the gas density evaluated from the measured temperature, pressure and molecular mass.

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The velocity is traversed over equal-area segments and averaged. • Isokinetic sampling means drawing the sample into the nozzle at exactly the local gas velocity, so that the streamlines enter the nozzle undisturbed.

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This is required only for particulate; gases, having negligible inertia, follow the streamlines whatever the sampling rate. • The direction of the error must be known: • Sub-isokinetic sampling, where the sample is drawn too slowly: gas spills around the nozzle, but the heavier particles cannot follow the curved streamlines and continue into the nozzle by inertia.

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Coarse particles are therefore over-represented and the dust loading is over-estimated. • Super-isokinetic sampling, where the sample is drawn too quickly: extra gas is sucked in from the sides, but the heavier particles in it do not follow.

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Coarse particles are under-represented and the loading is under-estimated. • Isokineticity is normally required to be within 90 to 110 per cent, and the nozzle must face directly into the flow.

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The whole procedure is codified as United States EPA Method 5 and its equivalents. • Worked illustration.

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A stack of 1.2 m diameter carries gas at 12 m/s with a measured loading of 150 mg/Nm³ at 180 °C.

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Area = π × 0.6² = 1.13 m², so Q = 1.13 × 12 = 13.6 m³/s at stack conditions.

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13.6 × 273/453 = 8.2 Nm³/s.

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E = 150 mg/Nm³ × 8.2 Nm³/s = 1230 mg/s ≈ 4.4 kg/h.

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Emission Factors and Visible Emissions • An emission factor is the mass of pollutant released per unit of activity — per tonne of cement produced, per thousand bricks fired, per vehicle-kilometre, per terajoule of fuel burnt.

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It allows an inventory to be built where measurement is impossible, which is the usual case for area and mobile sources. • The working equation is E = A × EF × (1 − ER/100), where A is the activity rate, EF the uncontrolled emission factor and ER the overall efficiency of the control in per cent. • The standard compilation is the United States EPA AP-42, whose factors carry a quality rating from A to E.

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An emission factor is a population average and may be badly wrong for an individual plant, so it should never be used in place of a measurement where the measurement can be made — a limitation that is regularly examined. • Visible emissions are assessed by opacity, the percentage of light obscured by the plume, measured continuously by a transmissometer or estimated by a trained and certified observer. • The Ringelmann chart is a set of grids numbered 0 to 5 which the observer compares with the plume, each number corresponding to twenty per cent opacity:

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Ringelmann 1 is twenty per cent, Ringelmann 2 is forty per cent, and Ringelmann 5 is completely black.

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Readings are taken with the sun behind the observer and against a contrasting background. • Opacity is a useful compliance screen because it needs no instrument and responds immediately to a failure of control equipment, but it responds principally to fine particles, is affected by the presence of a condensed water plume, and tells nothing about gaseous pollutants.

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A steam plume is white and dissipates as it mixes with ambient air; a particulate plume persists.

1

This section covers the control devices themselves — settling chambers, cyclones, electrostatic precipitators, fabric filters, wet scrubbers, absorbers, adsorbers and oxidisers — together with the basis on which a device is selected, and the treatment of stack height and atmospheric dispersion.

2

The syllabus repeats under this heading the particulate topics already listed under 8.3.

3

Those are covered there; this section is devoted to the control equipment and to dispersion, which is where the remaining examinable material of the chapter lies.

4

Mechanical Collectors • The gravity settling chamber is an enlarged duct in which the gas velocity falls to 0.3 to 3 m/s so that coarse particles settle out.

5

The minimum particle collected is d = √(18 μ vgas H / ρp g L), and efficiency improves with a longer, lower chamber, which is the principle of the Howard multi-tray chamber.

6

It is effective only above about 50 μm, but its pressure drop is negligible and it is maintenance-free, so it is used as a pre-cleaner to protect downstream equipment from abrasion. • The cyclone replaces gravity with centrifugal force, which may be hundreds of times greater, by setting the gas spinning in a vortex.

7

The cut diameter is d50 = √(9 μ W / 2π N vi ρp), where W is the inlet width, N the effective number of turns (typically 5) and vi the inlet velocity. • Three consequences follow from that expression and are regularly examined: efficiency improves as the inlet velocity rises, but so does the pressure drop, which goes as the square of velocity; efficiency improves as the body diameter is reduced, which is why many small cyclones in parallel — a multiclone — outperform one large one of the same capacity; and collection falls away rapidly below about 5 to 10 μm. • A cyclone typically gives 70 to 90 per cent on total dust at a pressure drop of 500 to 2000 Pa, has no moving parts, tolerates high temperature and is cheap.

8

It is the standard pre-cleaner, not a final collector.

9

The Electrostatic Precipitator • The precipitator charges the particles in a corona discharge from a high-voltage wire or rigid discharge electrode, drives them under the electric field to an earthed collecting plate, and removes the collected layer by rapping. • The governing relation is the Deutsch-Anderson equation: η = 1 − exp(−A w / Q), where A is the total collecting plate area, w the effective migration velocity of the particle (typically 0.02 to 0.2 m/s) and Q the volumetric gas flow. • The exponential form has an important practical meaning: efficiency approaches but never reaches one hundred per cent, and each successive increment of efficiency requires a disproportionately larger plate area.

10

Doubling the area does not double the efficiency; it squares the penetration. • Worked illustration.

11

If A w / Q = 3, then η = 1 − e−3 = 0.950.

12

Doubling the plate area gives Aw/Q = 6 and η = 1 − e−6 = 0.9975, that is the penetration falls from 5 per cent to 0.25 per cent. • Dust resistivity controls precipitator performance and is the most examined operational point.

13

The workable window is roughly 104 to 1011 ohm-cm.

14

Below it the particle loses its charge on touching the plate, is re-entrained and is blown off again.

15

Above it the collected layer holds its charge, the voltage across the layer rises until it breaks down, and a back corona of positive ions is emitted which neutralises the incoming particles and collapses the efficiency. • High resistivity is characteristic of low-sulphur coal ash, and it is conditioned by injecting sulphur trioxide, ammonia or moisture, or by operating hotter or cooler to move out of the resistivity peak, which occurs near 150 °C. • The precipitator gives 99 to 99.9 per cent including the fine fraction, handles very large flows at high temperature, and has a very low pressure drop of 100 to 300 Pa, which is its chief economic advantage.

16

Against this it has a high capital cost, is large, requires a high-voltage supply, and performs poorly on dusts of unfavourable resistivity.

17

The Fabric Filter and the Wet Scrubber • The fabric filter or baghouse passes the gas through a woven or felted fabric.

18

Collection is by interception, impaction and diffusion, and after a short time by the dust cake itself, which is the real filtering medium — so a baghouse becomes more efficient, and more resistant, as the cake builds. • The air-to-cloth ratio, that is the gas flow divided by the cloth area, is the key design parameter, typically 0.5 to 2 m/min for shaker and reverse-air cleaning and 1 to 4 m/min for pulse-jet.

19

Too high a ratio forces dust into the fabric, blinds it, raises the pressure drop and shortens bag life. • Cleaning is by mechanical shaking, by reverse air flow, or by a pulse of compressed air, the last allowing continuous on-line cleaning and the highest ratios. • Performance is 99.9 per cent and better, effective down to submicron sizes, and nearly independent of the dust loading and the dust's electrical properties — its great advantage over the precipitator.

20

The limits are the fabric temperature (cotton 80 °C, polyester 135 °C, Nomex 200 °C, glass 260 °C, PTFE 260 °C), the risk of blinding if the gas falls below its dew point, the unsuitability for sticky or hygroscopic dust, and a pressure drop of 1 to 2 kPa. • Wet scrubbers collect by impaction of particles on liquid droplets and simultaneously absorb soluble gases, which is their unique advantage.

21

In the venturi scrubber the gas is accelerated to 60 to 120 m/s in a throat where the liquid is injected and atomised, and the very high relative velocity gives collection down to submicron sizes. • The contacting-power principle states that for a given scrubber the efficiency depends chiefly on the energy dissipated, so higher efficiency is bought with higher pressure drop — 2.5 to 10 kPa for a venturi against 0.2 to 0.5 kPa for a simple spray tower. • Scrubbers handle hot, sticky, corrosive, explosive and flammable dusts safely and can be compact, but they consume a great deal of fan power, produce a contaminated wastewater that must itself be treated, give a visible condensed plume, and are liable to corrosion and to freezing.

22

Device Effective size Efficiency Pressure drop Chief limitation Settling chamber Above 50 μm Below 50 per cent Negligible Very bulky; coarse dust only Cyclone 5 to 10 μm 70 to 90 per cent 500 to 2000 Pa Poor on fine dust Electrostatic precipitator Below 1 μm 99 to 99.9 per cent 100 to 300 Pa Resistivity-sensitive; costly Fabric filter Below 1 μm Above 99.9 per cent 1 to 2 kPa Temperature and moisture limits Venturi scrubber Below 1 μm 95 to 99 per cent 2.5 to 10 kPa High power; wastewater Packed absorber Gases only 90 to 99 per cent 0.5 to 2 kPa Gas must be soluble or reactive Carbon adsorber Gases only 95 to 99 per cent 0.5 to 2 kPa Spent carbon; humidity sensitive Thermal oxidiser Gases only 98 to 99.9 per cent 0.5 to 1 kPa Fuel cost; may form NOx Gas Treatment Devices • Absorption transfers a soluble gas into a liquid in a packed, plate or spray tower.

23

The driving force is the difference between the partial pressure in the gas and the equilibrium partial pressure over the liquid, given by Henry's law, p = H x, so solubility is improved by low temperature, high pressure and a liquid that reacts with the solute — alkali for acid gases, acid for ammonia.

24

Countercurrent operation is used because it keeps the driving force large over the whole height. • Adsorption holds the molecule on the surface of a solid, almost always activated carbon for organics, with silica gel, alumina and zeolites used for polar species.

25

It suits dilute streams of valuable or odorous vapours, and two beds are provided so that one adsorbs while the other is regenerated with steam or hot gas.

26

Performance falls sharply at high humidity because water competes for the sites, and beds of organics can self-heat and ignite. • Oxidation destroys organics and carbon monoxide.

27

Thermal oxidation requires 760 to 870 °C with 0.5 to 1 second residence and good turbulence; catalytic oxidation achieves the same at 320 to 540 °C but is vulnerable to poisoning.

28

Recuperative or regenerative heat exchange recovers most of the heat and is what makes oxidation affordable. • Condensation recovers vapours from concentrated streams by cooling below the dew point, and is normally a pre-treatment ahead of another device rather than a final control. • Biofiltration passes a humidified dilute stream through a bed of compost, peat or bark on which micro-organisms oxidise the contaminant.

29

It is cheap to run, excellent for odours and dilute organics, but needs a large footprint, a controlled moisture content and a long residence time.

30

Stack Height and Atmospheric Dispersion • The Gaussian plume model represents the concentration downwind as a normal distribution about the plume centreline in both the horizontal and the vertical, with standard deviations σy and σz that grow with distance and depend on the atmospheric stability. • For a ground-level receptor on the plume centreline the maximum concentration is proportional to Q / (u H²), where Q is the emission rate, u the wind speed and H the effective stack height.

31

The inverse square dependence on effective height is the reason tall stacks are built, and the inverse dependence on wind speed is why calm conditions are the worst case. • The effective stack height H = hs + Δh, the physical height plus the plume rise, and plume rise comes from the momentum of the efflux and from its buoyancy.

32

A high exit velocity, conventionally at least 1.5 times the wind speed to avoid downwash into the lee eddy of the stack, and a high exit temperature both add to plume rise.

33

Plume rise is estimated by the Holland or Briggs formulae. • Atmospheric stability is set by the comparison of the environmental lapse rate with the dry adiabatic lapse rate of 9.8 °C per kilometre: superadiabatic conditions are unstable and disperse well; an isothermal or inverted profile is stable and suppresses mixing.

34

The Pasquill-Gifford classes run from A, extremely unstable, through D, neutral, to F, moderately stable. • The plume shapes follow directly from the profile and are a favourite examination item: • Looping — strongly unstable, superadiabatic; the plume meanders violently and can bring high concentrations to ground close to the stack. • Coning — neutral or slightly stable, typical of windy or overcast conditions; the classic cone-shaped Gaussian plume. • Fanning — a stable layer or inversion throughout; the plume spreads horizontally but not vertically and travels far without reaching the ground. • Lofting — unstable above, stable below; the plume disperses upward only.

35

This is the most favourable condition for ground-level concentration. • Fumigation — stable above, unstable below, as occurs when the morning sun breaks up a nocturnal inversion from the ground upward; pollutant accumulated aloft overnight is mixed rapidly to the ground.

36

This is the most hazardous condition and produces the highest short-term ground concentrations. • The mixing height is the depth of the layer within which vertical mixing occurs, and a low mixing height combined with light winds — the standard Kathmandu winter morning — gives the worst air quality of the year.

1

This section covers the nature of noise and the decibel scale, the arithmetic of combining and attenuating sound levels, weighting networks and the noise indices, the effects of noise on people, measurement, control at source, along the path and at the receiver, and the Nepali standards and guidelines.

2

The Decibel Scale and Its Arithmetic • Noise is unwanted sound, which makes it partly a subjective judgement: the same sound may be music to one person and noise to another, and this is why annoyance criteria differ from hearing-damage criteria. • The audible range is about 20 Hz to 20 kHz, and the ear is most sensitive between about 1 and 5 kHz.

3

The range of audible sound pressures spans about a million to one, which is why a logarithmic scale is used. • Sound pressure level:

4

Lp = 20 log10(p / p0), with the reference p0 = 20 μPa, the nominal threshold of hearing at 1 kHz.

5

The factor is 20 because sound intensity is proportional to the square of pressure. • Sound power level:

6

LW = 10 log10(W / W0), with W0 = 10−12 W; sound intensity level uses the reference 10−12 W/m². • Sound power is a property of the source alone; sound pressure depends on the distance and on the room, and confusing the two is the commonest error in the subject. • Decibels do not add arithmetically.

7

Levels are combined by converting back to energy:

8

Ltotal = 10 log Σ 10Li/10. • The results worth memorising: two equal sources give +3 dB; four equal sources give +6 dB; ten equal sources give +10 dB.

9

Two machines of 80 dB each give 83 dB, not 160 dB. • Where two levels differ by 10 dB or more the lower may be neglected, since it contributes less than 0.5 dB — which means that silencing the quieter of two machines achieves nothing at all, and the loudest source must always be dealt with first. • Attenuation with distance in free field: a point source falls by 6 dB for each doubling of distance (inverse square law), and a line source such as a stream of traffic falls by only 3 dB for each doubling.

10

This is why road noise persists so far from the road, and the distinction is regularly examined. • Worked illustration.

11

A point source gives 90 dB at 10 m.

12

At 20 m it gives 84 dB, at 40 m 78 dB and at 80 m 72 dB.

13

A line source at 90 dB at 10 m would give 87, 84 and 81 dB at the same distances.

14

Weighting Networks and Noise Indices • The ear is far less sensitive to low frequencies than to mid frequencies at ordinary levels, so a measured sound pressure level does not correspond to loudness.

15

Weighting networks correct for this. • The A-weighting network discriminates strongly against low frequencies and approximates the response of the ear at moderate levels.

16

It correlates well with annoyance and with hearing damage and is used in virtually every standard; results are written dB(A). • C-weighting is nearly flat and is used for peak and impulsive levels;

17

B and D weightings are largely obsolete, D having been used for aircraft noise. • Because environmental noise fluctuates, single-figure indices are used: • Leq — the equivalent continuous level, the steady level that contains the same energy as the actual fluctuating noise over the period.

18

It is the basic index of all modern standards. • L10 and L90 — the levels exceeded for 10 and 90 per cent of the time.

19

L10 indexes the intrusive peaks and is used for traffic noise;

20

L90 is taken as the residual background level. • Ldn or DNL — the day-night level, a twenty-four hour Leq with a 10 dB penalty added to night-time (22:00 to 07:00) levels to reflect the greater disturbance then.

21

Lden adds a further evening penalty of 5 dB. • SEL — the sound exposure level, the energy of a single event normalised to one second, used to compare individual aircraft or train passages. • Lmax and Lpeak — used for impulsive noise and for hearing-damage risk from single events.

22

Effects of Noise on People Level, dB(A) Typical source Effect 0 Threshold of hearing Reference level 30 to 40 Quiet bedroom, library Sleep undisturbed;

23

WHO indoor night guideline about 30 55 to 60 Normal conversation, busy office Speech interference begins;

24

WHO outdoor daytime guideline 55 70 to 80 Busy traffic, vacuum cleaner Annoyance, stress; prolonged exposure begins to matter 85 Heavy traffic, workshop Occupational limit for eight hours; damage risk threshold 100 to 110 Pneumatic drill, chainsaw, discotheque Rapid hearing damage; minutes of exposure suffice 120 to 140 Jet engine at close range, gunshot Threshold of pain; immediate and permanent damage • Auditory effects.

25

A temporary threshold shift is the dulling of hearing after exposure, which recovers with rest.

26

Repeated exposure converts it into a permanent threshold shift, which does not recover.

27

Noise-induced hearing loss characteristically appears first as a notch in the audiogram at 4 kHz, and because it begins at frequencies above those of conversation it is not noticed by the sufferer until it is well advanced and irreversible — the single most important public health point about noise.

28

Acoustic trauma is immediate damage from a single very loud impulse such as an explosion or gunshot. • The occupational criterion is 85 dB(A) for an eight-hour working day, with an exchange rate of 3 dB in the ISO convention — so 88 dB(A) is permitted for four hours, 91 dB(A) for two and 94 dB(A) for one.

29

The 3 dB rule follows from equal energy; the older OSHA 5 dB rule is more permissive and is being abandoned. • Non-auditory effects are now regarded as the larger part of the population burden: annoyance, interference with speech and with learning in schools, sleep disturbance with consequent fatigue, and raised blood pressure and ischaemic heart disease associated with long-term exposure to road and aircraft noise.

30

Sleep is disturbed by events rather than by average level, which is why Lmax matters at night as well as Leq.

31

Measurement • The sound level meter consists of a microphone, a preamplifier, weighting networks, a root-mean-square detector with selectable time weighting and a display.

32

Time weightings are Fast (125 ms), Slow (1 s) and Impulse (35 ms). • Class 1 instruments are for precision and reference work and Class 2 for general survey work.

33

An integrating meter computes Leq directly; a dosimeter is worn by a worker to record the personal daily dose. • The meter must be field-calibrated with an acoustic calibrator before and after every set of measurements, and a drift of more than about 0.5 dB invalidates the readings. • Practical requirements: use a windscreen, and do not measure in winds above about 5 m/s; hold the microphone at least 1.0 to 1.5 m from the body and about 1.2 to 1.5 m above ground; keep 3.5 m from reflecting surfaces other than the ground; record the background level and discard any measurement less than 3 dB above it; and record the weather and the operating condition of the source. • Octave and one-third octave band analysis is used when the noise is to be controlled rather than merely assessed, because the choice of absorber, barrier or silencer depends entirely on the frequency content, and a single A-weighted figure conceals it.

34

Source, Path and Receiver • Control is always attempted in the order source, path, receiver, and hearing protection is the last resort rather than the first. • At the source — select quieter machinery and specify sound power in the purchase order; maintain equipment, since wear, imbalance and loose parts are a common cause of increased noise; reduce driving forces and impacts; balance rotating parts; replace gears with belts and impact processes with gradual ones; isolate machines on resilient mounts so that vibration is not transmitted into the structure; damp panels that radiate sound; fit mufflers and silencers to exhausts and intakes; and enclose the machine.

35

A complete enclosure, sealed and lined with absorbent, can give 20 to 40 dB, but any unsealed gap destroys it: a one per cent open area limits the insertion loss to about 20 dB. • Along the path — increase distance, exploiting the 6 dB per doubling from a point source; interpose a barrier; and add absorbent to reverberant surfaces. • A barrier works only if it breaks the line of sight between source and receiver, and its performance is governed by the Fresnel number, which depends on the extra path length the sound must travel over the top relative to the wavelength.

36

Because the extra path is compared with the wavelength, barriers are effective against high frequencies and poor against low ones.

37

A practical roadside barrier gives 5 to 15 dB; the barrier must be imperforate and heavy enough that sound does not pass through it, and a hedge or a line of trees gives almost no attenuation, perhaps 1 to 3 dB, whatever the public belief. • Transmission loss through a partition follows the mass law: doubling the surface mass, or doubling the frequency, raises the transmission loss by about 6 dB.

38

Hence a heavy partition insulates better, and the weak point is always a gap, a duct or a window rather than the wall itself. • Absorption and insulation must not be confused.

39

An absorbent material such as mineral wool reduces reverberation within the space but transmits sound readily; a dense imperforate barrier insulates between spaces but reflects sound back into the first room.

40

Absorption is rated by the noise reduction coefficient and insulation by the sound transmission class or weighted sound reduction index. • At the receiver — ear plugs give about 15 to 25 dB and ear muffs about 20 to 30 dB in laboratory tests, and much less in practice because of poor fit and intermittent use; administrative rotation limits the exposure time; a sound-insulated control cabin removes the worker from the field; and audiometric surveillance detects loss before it becomes disabling. • Land-use planning is the cheapest of all measures over the long term: separating residential areas from highways, airports and industrial zones, orienting buildings so that bedrooms face away from the road, and designating silence zones around hospitals and schools.

41

Standards and Guidelines Area Day, dB(A) Night, dB(A) Industrial area 75 70 Commercial area 65 55 Mixed residential and commercial 63 55 Urban residential area 55 50 Rural residential area 45 40 Silence zone: hospital, school, court 50 40 These are the values of the National Noise Standards commonly cited for Nepal, with day taken as 06:00 to 22:00 and night as 22:00 to 06:00.

42

The World Health Organization community noise guideline is 55 dB(A) outdoors by day to prevent serious annoyance and about 45 dB(A) outdoors at night, corresponding to roughly 30 dB(A) inside a bedroom.

43

Standards are revised periodically and the current gazette should be consulted.