Pollution control (2)

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. Physical treatment systems are processes which rely on physical forces to aid in the removal of pollutants. Physical processes used in water pollution control include screening, filtration, sedimentation, and flotation. Screening and filtration are similar methods which are 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.
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. 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 nutrients 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 include landfilling, composting, and incineration. Sanitary landfills are operated by spreading the solid waste in compact layers which are separated by a thin layer of soil. Aerobic and anaerobic microorganisms help to break down the biodegradable substances in the landfill and produce carbon dioxide and methane gas which is typically venter to the surface. Landfills also generate a strong wastewater called leachate which must be collected and treated to avoid groundwater contamination.