Pollution Control

Pollution control is the process of reducing or eliminating the release of pollutants into the enviroment. It is regulated by various environmental agencies which establish pollutant discharge limits for air, water, and land.

Air pollution control strategies can be divided into two categories, the control of particulate emission and the control of gaseous emissions. There are many kinds of equipment which can be used to reduce particulate emissions. Physical separation of the particulate from the air using settling chambers, cyclone collectors, impingers, wet scrubbers, electrostatic precipitators, and filtration devices, are all processes that are typically employed.

Settling chambers use gravity separation to reduce particulate emissions. The air stream is directed through a settling chamber, which is relatively long and has a large cross section, causing the velocity of the air stream to be greatly decreased and allowing sufficient time for the settling of solid particles.

A cyclone collector is a cylindrical device with a conical bottom which is used to create a tornado-like air stream. A centrifugal force is thus imparted to the particles, causing them to cling to the wall and roll downward, while the cleaner air stream exits through the top of the device.

An impinger is a device which uses the inertia of the air stream to impinge mists and dry particles on a solid surface. Mists are collected on the impinger plate as liquid forms and then drips off, while dry particles tend to build up or reenter the air stream. It is for this reason that liquid sprays are used to wash the impinger surface as well, to improve the collection efficiency.

Wet scrubbers control particulate emissions by wetting the particles in order to enhance their removal from the air stream. Wet scrubbers typically operate against the current by a water spray contacting with the gas flow. The particulate matter becomes entrained in the water droplets, and it is then separated from the gas stream. Wet scrubbers such as packed bed, venturi, or plate scrubbers utilize initial impaction, and cyclone scrubbers use a centrifugal force.

Electrostatic precipitators are devices which use an electrostatic field to induce a charge on dust particles and collect them on grounded electrodes. Electrostatic precipitators are usually operated dry, but wet systems are also used, mainly by providing a water mist to aid in the process of cleaning the particles off the collection plate.

One of the oldest and most efficient methods of particulate control, however, is filtration. The most commonly-used filtration device is known as a baghouse and consists of fabric bags through which the air stream is directed. Particles become trapped in the fiber mesh on the fabric bags, as well as the filter cake which is subsequently formed.

Gaseous emissions are controlled by similar devices and typically can be used in conjunction with particulate control options. Such devices include scrubbers, absorption systems, condensers, flares, and incinerators.

Scrubbers utilize the phenomena of adsorption to remove gaseous pollutants from the air stream. There is a wide variety of scrubbers available for use, including spray towers, packed towers, and venturi scrubbers. A wide variety of solutions can be used in this process as absorbing agents. Lime, magnesium oxide, and sodium hydroxide are typically used.

Adsorption can also be used to control gaseous emissions. Activated carbon is commonly used as an adsorbent in configurations such as fixed bed and fluidized bed absorbers.

Condensers operate in a manner so as to condense vapors by either increasing the pressure or decreasing the temperature of the gas stream. Surface condensers are usually of the shell-and-tube type, and contact condensers provide physical contact between the vapors, coolant, and condensate inside the unit.

Flaring and incineration take advantage of the combustibility of a gaseous pollutant. In general, excess air is added to these processes to drive the combustion reaction to completion, forming carbon dioxide and water.

Another means of controlling both particulate and gaseous air pollutant emission can be accomplished by modifying the process which generates these pollutants. For example, modifications to process equipment or raw materials can provide effective source reduction. Also, employing fuel cleaning methods such as desulfurization and increasing fuel-burning efficiency can lessen air emissions.

Water pollution control methods can be subdivided into physical, chemical, and biological treatment systems. Most treatment systems use combinations of any of these three technologies. Additionally, water conservation is a beneficial means to reduce the volume of wastewater generated.

Physical treatment systems are processes that rely on physical forces to aid in the removal of pollutants. Physical processes which find frequent use in water pollution control include screening, filtration, sedimentation, and flotation. Screening and filtration are similar methods used to separate coarse solids from water. Suspended particles are also removed from water with the use of sedimentation processes. Just as in air pollution control, sedimentation devices utilize gravity to remove the heavier particles from the water stream. The wide array of sedimentation basins in use slow down the water velocity in the unit to allow time for the particles to drop to the bottom. Likewise, flotation uses differences in particle densities, which in this case are lower than water, to effect removal. Fine gas bubbles are often introduced to Equipment for the complete recovery and control of air, acids, and oxide emissions. Photograph by Tom Carroll. Phototake NYC. Reproduced by permission.

assist this process; they attach to the particulate matter, causing them to rise to the top of the unit where they are mechanically removed.

Chemical treatment systems in water pollution control are those processes which utilize chemical reactions to remove water pollutants or to form other, less toxic, compounds. Typical chemical treatment processes are chemical precipitation, adsorption, and disinfection reactions. Chemical precipitation processes utilize the addition of chemicals to the water in order to bring about the precipitation of dissolved solids. The solid is then removed by a physical process such as sedimentation or filtration. Chemical precipitation processes are often used for the removal of heavy metals and phosphorus from water streams. Adsorption processes are used to separate soluble substances from the water stream. Like air pollution adsorption processes, activated carbon is the most widely used adsorbent. Water may be passed through beds of granulated activated carbon (GAC), or powdered activated carbon (PAC) may be added in order to facilitate the removal of dissolved pollutants. Disinfection processes selectively destroy disease-causing organisms such as bacteria and viruses. Typical disinfection agents include chlorine, ozone, and ultraviolet radiation.

Biological water pollution control methods are those which utilize biological activity to remove pollutants from water streams. These methods are used for the control of biodegradable organic chemicals, as well as nutrient s such as nitrogen and phosphorus. In these systems, microorganisms consisting mainly of bacteria convert carbonaceous matter as well as cell tissue into gas. There are two main groups of microorganisms which are used in biological treatment, aerobic and anaerobic microorganisms. Each requires unique environmental conditions to do its job. Aerobic processes occur in the absence of oxygen. Both processes may be utilized whether the microorganisms exist in a suspension or are attached to a surface. These processes are termed suspended growth and fixed film processes, respectively.

Solid pollution control methods that are typically used include landfilling, composting, and incineration. Sanitary landfills are operated by spreading the solid waste in compact layers separated by a thin layer of soil. Aerobic and anaerobic microorganisms help break down the biodegradable substances in the landfill and produce carbon dioxide and methane gas, which is typically vented to the surface. Landfills also generate a strong wastewater called leachate that must be collected and treated to avoid groundwater contamination.

Composting of solid wastes is the microbiological biodegradation of organic matter under either aerobic or anaerobic conditions. This process is most applicable for readily biodegradable solids such as sewage sludge, paper, food waste, and household garbage, including garden waste and organic matter. This process can be carried out in static pile, agitated beds, or a variety of reactors.

In an incineration process, solids are burned in large furnaces thereby reducing the volume of solid wastes that enter landfills, as well as reducing the possibility of groundwater contamination. Incineration residue can also be used for metal reclamation. These systems are typically supplemented with air pollution control devices.