Air pollution causes the death of millions of people every year. It particularly affects the most vulnerable groups – children, senior citizens, pregnant women, and people suffering from chronic diseases. However, even relatively healthy ones can fall ill within a short period if the air pollution reaches high levels. The longer it affects the inhabitants of a particular region, the higher is the risk of developing health diseases such as chronic obstructive pulmonary disease, asthma, or even lung cancer.
Some types of air pollution can have just a mild influence on our body, while others can pose a serious danger. Among all, particulate matter seems the most harmful one.
How does particulate matter influence our health?
These particles are basically invisible and odorless. Their dimensions are specified in micrometers (µm) – a unit that also serves for measuring the size of bacteria or human cells. That illustrates how tiny they are and explains why we cannot really see them.
There are two main types of particulate matter detectable by the air pollution sensor – PM 2.5 and PM10. Both are dangerous to human health. However, the smaller ones pose a higher risk of getting to the blood through the lungs. Their size facilitates it. Once in the circulatory system, they can get to the organs and accumulate there. PM 10, on the other hand, irritates mucous membranes and accumulates in the lungs, leading to respiratory diseases.
The particulate matter from anthropogenic sources often contains carcinogenic substances such as dioxins, heavy metals, and polycyclic aromatic hydrocarbons. Thus, it can undoubtedly be considered the most dangerous type of air pollution. Note that the particulate matter of a natural source (desert or volcanic dust) doesn’t affect our health to that extent. However, it can still be very harmful to those who suffer from asthma or other chronic respiratory diseases.
How do toxic gases influence our health?
The inhabitants of the cities and industrial areas are usually exposed to both types of air pollution at the same time. The toxic gases – an effect of industrial activity and the release of exhaust fumes – can also affect human health. That’s why the air pollution sensor usually provides information on its levels. When using air pollution maps, such as Airly.org, you can check real-time data on toxic gases, too.
Nitrogen dioxide (NO2) can irritate the respiratory system, exacerbating asthma and other respiratory problems. Sulfur dioxide (SO2) is particularly harmful to plants, leading to their damage or death. It can also affect the human respiratory system, accumulating in the lungs. The very high concentration of carbon dioxides can cause hypoxia in the circulatory system and carbohydrate disorders.
If you want to protect yourself from the influence of particulate matter and toxic gases, start by controlling the levels of air pollution. You can do it through Airly.org, where you’ll find updated data from all over the world.
Air pollution control equipment for particulates
Particulate control equipment is available in different classes, including gravity settlers, cyclones, electrostatic precipitators, wet (venturi) scrubbers, and baghouse (fabric) filters. We briefly introduce each of the major types of particulate control equipment and discuss their advantages and disadvantages in this section.
1. Gravity Settlers
The simplest kind of air cleaning device, gravity settlers have long been used by industry for removing solid and liquid waste materials from gaseous streams. Simply constructed, a gravity settler provides nothing more than an enlarged chamber in which to slow horizontal gas velocity, allowing particles to settle out by gravity. To their advantage, gravity settlers have low initial cost and are relatively inexpensive to operate not much can go wrong. To their disadvantage, although they are simple in design, gravity settlers need a large space for installation and have relatively low efficiency, especially for removal of small particles (< 50 pm).
2. Cyclone Collectors
The collector of choice for removing particles greater than 10 pm in diameter, the cyclone (or centrifugal) collector removes particles by causing the entire gas stream to flow in a spiral pattern inside a tube. The larger particles move outward by centrifugal force, collide with the wall of the tube, then slide down the wall and fall to the bottom of the cone where they are collected for removal. The cleaned gas flows out of the cyclone’s top. Cyclones have the advantage of low construction cost and need relatively small space for installation. The cyclone’s overall particulate collection efficiency is low, however, especially on particles below 10 pm in size and they do not handle sticky materials well. The most serious problems encountered with cyclones concern a tendency to plug and possible problems with air flow equalization. Cyclones are used successfully in applications involving large quantities of gas containing relatively large particles such as at feed and grain’mills, cement plants, fertilizer plants, and petroleum refineries.
3. Electrostatic Precipitators
A widely used method, the electrostatic precipitator removes small (fine) particles from moving gas streams at high collection efficiencies. Popular for use in power plants for removing fly ash from gases prior to discharge, an electrostatic precipitator applies electrical force to separate particles from the gas stream. Electrostatic precipitators work to establish a high-voltage drop between electrodes, which allows particles passing through the resulting electrical field to acquire a charge. The charged particles collect on an oppositely charged plate, and the cleaned gas flows through the device. The plates are periodically cleaned by rapping to shake off the layer of accumulated dust, which is collected in hoppers at the bottom of the device. Electrostatic precipitators have the advantages of low operating costs, capability for operation at high temperature applications (to 1300°F), and low pressure drop. They can also attain extremely high particulate (coarse and fine) collection efficiencies. However, they also have high capital costs and space requirements.
4. Wet (Venturi) Scrubbers
In widespread use for cleaning contaminated gas streams (e.g., foundry dust emissions, acid mists, and furnace fumes), wet scrubbers (or collectors) effectively remove particulate and gaseous pollutants by trapping particulates through the technique of direct contact with an aerosol spray of water or some other liquid. Simple spray chamber wet scrubbers remove coarse particles, and high-efficiency systems (venturi-type scrubbers) remove fine particles. Spray chamber and venturi operations both employ basic principles of inertial impingement or impaction and interception of dust particles by droplets of water. Large, heavy water droplets easily separate from the gas by gravity, and the solid particles are independently separated from the water, or the water is somehow treated before reuse or discharge. Increasing the gas velocity or the liquid droplet velocity in a scrubber increases the efficiency, ensuring a greater number of collisions per unit time. The venturi scrubber is used in applications where high collection efficiency is desired. Most efficient for removing particulate matter in the size range of 0.5 to 5 pm, venturi scrubbers are especially effective for the removal of submicron particulates associated with smoke and fumes. The venturi operates at extremely high gas and liquid velocities with a very high pressure drop across the venturi throat.
Wet scrubbers require relatively small space for installation, have low capital cost, and can handle high-temperature, high-humidity gas streams. However, their relatively high power and maintenance costs, water disposal problems, and corrosion problems are more severe than in dry systems, and their final product is collected wet.
5. Baghouse (Fabric) Filters
The most commonly used air pollution control filtration system, bag- house filters (or fabric filters) are one of the most efficient devices for removing suspended particulates from a gas stream. In a method similar to the common vacuum cleaner, baghouse fabric filter material (capable of removing most particles as small as 0.5 pm and substantial quantities of particles as small as 0.1 pm) is formed into cylindrical or envelope bags and suspended in the baghouse. The particulate-laden gas stream is forced through the fabric, and particulates accumulate on the cloth, providing a cleaned air stream. The pressure drop increases as particulates build up on the inside surfaces of the bags. The bags must be relieved of some of the particulate layer before the pressure drop becomes too severe. Shaking or reversing the air flow periodically removes the particulates from the cloth.
Relatively simple to operate, fabric filters provide high overall collection efficiencies up to 99+% and are very effective in controlling submicrometer particles. They do have limitations, however, including relatively high capital costs, high maintenance requirements (bag replacement, etc.), high space requirements, and flammability hazards for some dusts.
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