To proceed with the paper, it is first necessary to obtain an understanding of the common types of filtration media as well as the range of their applications. According to Imbabi and Peacock (2004), the most common materials for air filtration used today are cellular-based filters, such as foam or sponge, membranes, fibrous materials, and pulp. These media are used for the filtering of particles that range from atomic radius to fine sand, effectively covering the entire range.
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The most viable solution, however, is fiber since it has several key advantages, including low cost, utility, availability, and low flow velocity (Imbabi & Peacock 2004). The ventilated air is cleared of the harmful particulate matter (PM) while passing through the fiber filter. Thus, it can be expected that a higher ventilation rate will result in more efficient removal of the pollutants from indoor spaces.
The mechanism of fiber filter operation is best described with the help of a single fiber theory. In the most basic terms, it allows calculating the flow field around a certain fiber by taking into account the combined effect of the nearby fibers (Imbabi & Peacock 2004).
Aside from the atomic and electrostatic force, several key mechanisms are involved in the process that results in the trapping of a particle in a filter. The first is the capturing of a particle via direct contact with the fiber while following a streamline, known as a direct interception (Taylor, Webster & Imbabi 1998). Second, a particle may be captured by fiber as a result of its inertial deviation from a streamline, termed inertial deviation. Third, a particle can come in contact with fiber as a result of moving in Brownian motion, known as diffusion deposition (Taylor, Webster & Imbabi 1998).
By extension, the straining or sieving of the particle, i.e., the situation where it is held by more than one fiber, is less likely in the air-filters with higher porosity. As can be seen, most of the particles’ behaviors within the filter can be relatively accurately described using the single fiber theory, since it offers the possibility to calculate the most important parameters of an effective filter, such as the size of fibers and particles, particle density, optimal porosity, and the efficiency of single fiber collection (Imbabi & Peacock 2004).
Interestingly, in the dynamic insulation, the efficiency of particle collection has an inverse relationship with the air velocity. According to Taylor, Webster, and Imbabi (1998), lower air velocity increases the number of trapped particles less than one μm in diameter. Also, particles traveling at a distance that exceeds the radius of the fiber become captured. In other words, the efficiency of diffusional deposition increases greatly for sub-micron particles at low airflows (Imbabi & Peacock 2004). The size of the most penetrating particle for dynamic insulation is also three times bigger than that of a conventional filter.
A multi-layered Pm filtration model, which applies only to the fiber-based materials, has been theoretically developed and calibrated. This model was expected to have a longer period before becoming clogged without compromising the efficiency in the areas of air flow rate and particle distribution. The important predictors of such a filter’s efficiency are the single fiber efficiency and the total thickness of the filter. The fibers inside the model are oriented differently in relation to each other.
Since such construction is expected to capture an equal proportion of the particles at each successive layer, the overall performance of such a filter would be determined by the filter’s size. It can be increased to the necessary level (Imbabi & Peacock 2004). The concentration of particles leaving such filter divided by the concentration of particles entering it would constitute a percentage penetration and could be used as a variable describing its efficiency.
The primary reason for the superiority of the dynamic insulation is the significantly lower face velocity. For practical application, this means that a wall insulated using the said technology can outperform high-efficiency particulate air filters without compromising the useful life of a building (Imbabi, Campbell & Lafougere 2002). However, It leaves the possibility of clogging in the case of excessively polluted air or improperly selected materials.
The model incorporates filter media in the form of several sequential fibrous layers of equal thickness. The parameters, such as fiber diameter and density of media, are adjustable. From the purely theoretical perspective, such setup would result in equal distribution of the particulate matter over the entire volume of the filter. However, the internal structure of such media is subject to gradual change due to the formation of branch-like structures from the particles within the filter (Imbabi & Peacock 2004). This process has low predictability due to its complexity. Still, it is highly likely that its overall effect is comparable to the increase of media density and fiber diameter early in the filter’s life span and will lead to clogging.
According to Imbabi and Peacock (2004), the hypothesized efficiency of a dynamic insulation filter was confirmed experimentally, with a 99.8% filtration efficiency rate for 10 and 25-micron fibers, with a drop to 99.4% within 60 years. These results open up the possibilities for designing energy-efficient buildings that ensure the absence of pollutants indoors.
Policies and Regulations
The issue of particulate matter concentration in the UK is controlled by several national agreements, as well as several international initiatives. One of the more prominent ones is the country’s participation in the UNECE Convention on Long-Range Transboundary Air Pollution, an organization whose scope extends beside the European Union and takes a long-term approach (UNECE 2016).
Numerous regulations on the matters of PM pollution originate in Europe. One of the recent examples is the 2008 Ambient Air Quality Directive. This initiative contains binding limits that determine an acceptable concentration of the air pollutants in outdoor air, with PM 10 and PM 2.5 listed as particles that significantly impact public health (European Commission 2016b). The directive incorporates most of the previous European air quality control laws. It is currently considered the law in the United Kingdom as a part of the Air Quality Standards Regulations 2010 alongside the 4th Air Quality Daughter Directive that contains requirements on the acceptable level of certain pollutants in the air, such as polycyclic aromatic hydrocarbons and toxic heavy metals (European Commission 2016a). Northern Ireland, Scotland, and Wales contain similar regulations.
The responsibility to meet the air quality standards defined by the regulations discussed above is distributed differently across Europe. In the United Kingdom, it is devolved to separate administrations in Northern Ireland, Wales, and Scotland. In England, these tasks are run by the Secretary of State for Environment, Food, and Rural Affairs. The combined effort of the four said departments are coordinated by the Department for Environment, Food, and Rural Affairs in the UK on the whole (DEFRA 2007).
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The Government of the United Kingdom is required to collaborate with the devolved administrations to produce a national air quality strategy. The latest edition of the strategy was published in 2007. The strategy contains objectives intended to attain the necessary level of air quality. It outlines the necessity of action on several levels, including national, regional, and local, to reach the desired resolution of the air quality issue and, by extension, improve public health.
On a local level, the air quality in the UK is regulated by the system of local air quality management (LAQM), introduced through a combination of the Environmental Order 2002 of Northern Ireland and the Environmental Act 1995.
The LAQM requires local authorities to review air quality in the area and assess the efficiency of the initiatives directed at its improvement. If any of the areas display an insufficient improvement rate, the authorities are then required to designate air quality management areas (AQMA) and concentrate efforts and resources on meeting the said objectives. The local authorities are supported by the UK government through a grant program, with similar mechanisms present in devolved administrations.
The air quality strategy outlined by DEFRA contains two objectives involving particulate matter pollutants. The first, known as a 24-hourly objective, covers safeguarding citizens from short-term exposure to the high concentration of harmful particulate matter (Air Quality expert Group 2005). Such exposure may occur in the areas with episodic increases in PM concentration, primarily due to weather conditions associated with high pollution levels. The second annual objective targets more prolonged exposure and involves average annual measurements of particles in the air over a year. Its goal is the protection of the population from PM on a large time frame (Air Quality expert Group 2005).
Air Quality Expert Group 2005, Particulate matter in the United Kingdom. Web.
European Commission 2016a, Air quality – existing legislation. Web.
European Commission 2016b, New air quality directive. Web.
Imbabi, M S, Campbell, J & Lafougere, S 2002, Multi-layer dynamic insulation panels for natural ventilation and filtration of urban air pollution. Web.
Imbabi, M S & Peacock, A D 2004, ‘Allowing buildings to breathe’, Renewable Energy, vol. 2004, pp. 85-95.
Taylor, B J, Webster, R & Imbabi, M S 1998, ‘The building envelope as an air filter’, Building and Environment, vol. 34, no. 3, pp. 353-361.
UNECE 2016, Workplan 2016-2017. Web.