Water Pollution Control Technology in the United States
Water Pollution Control Technology in Environmental Law
Techniques used to restore wastewater after use. Both groundwater and surface water are impacted by pollution.
Surface water is protected through the National Pollutant Discharge Elimination System, the massive permit program established by the Clean Water Act. Groundwater is protected indirectly. If groundwater is the water supply for a public system, it cannot exceed established levels of contamination. Injection of waste into the ground [see underground injection control] is also regulated by the Safe Drinking Water Act. Disposal of waste on land and safeguards to prevent underground storage tanks from leaking fall under the Resource Conservation and Recovery Act. When groundwater is contaminated by an abandoned hazardous waste site at which the Environmental Protection Agency is ordering a cleanup, those activities fall under the Comprehensive Environmental Response, Compensation, and Liability Act.
Water, an essential ingredient of life, comes in limited quantities. It makes the circuit through precipitation, percolation, runoff, and evaporation. Water supply is impacted by all organisms, but humans have been far more inventive about the use of water than other species. People pump groundwater and use streams and rivers for industry, drinking water, cooking, sewage disposal, and generally end up with polluted water.
In this entry, wastewater refers to surface water that has been used in an industrial process or for sewage before it is treated. Groundwater is water that flows underground. The direction of the flow and its velocity depends on the geological formations in the ground. Soil porosity, gradient of structural formations, fissures in formations, and water contamination all have an influence on the flow of groundwater.
In the absence of treatment, the demand for and recycling of water causes water quality to deteriorate rapidly Water pollution treatment methods are often old; human beings learned long ago that they had to have clean water to survive. Treatment of water began with surface water, and only some of the procedures have been adapted to groundwater treatment. This entry examines the most common water treatment technologies and how they relate to environmental law.
Neither environmental statutes nor the regulations dictate specific techniques that must be used to treat wastewater. Instead, publicly owned treatment works and industrial treatment plants must obtain permits that set limits on the concentration of each pollutant discharged. The permit then becomes the standard the permittee must meet.
The reason results rather than technology are emphasized is simple: technology changes constantly, and the government wants to benefit from improvements. It can do that only if it allows permittees to choose whatever method works best. In most cases, treating wastewater requires more than one type of technology. Three general categories of wastewater treatment exist: mechanical, chemical, and biological. Some procedures combine more than one category in a single step.
Mechanical Water Treatment
Aeration adding air to water can be done by spraying water in the air and letting it fall back to the surface. This technique reduces odors and tastes. It also softens the water and eliminates some iron and manganese.
Sedimentation is used to separate solids from liquids. Large tanks hold the waste water, and floating wastes are skimmed off. Settled wastes are pumped out of the bottom for disposal.
Water may also be filtered to capture suspended materials. Filtration often follows sedimentation, coagulation, and micros training (to remove algae and other particles). The water must have low turbidity during filtration. It seeps through sand to a layer of gravel. Suspended matter gets trapped, resulting in cleaner water.
Reverse osmosis is related to filtration, in that contaminants are captured by a membrane. Hydrostatic pressure is used to push a solution through a substance that will not allow pollutants through. They are caught on the membrane while the cleaned water continues to flow.
A number of chemicals are used to restore wastewater. One of the processes is called coagulation or flocculation. Chemicals are added to wastewater, causing color, minerals, and other contaminants to clump together and settle. This is accomplished in two stages. The first involves rapid mixing and the second, extended slow mixing. The floe settles by gravity.
The process reduces the bacteria in the water but may cause hardness and corrosivity.
The process reduces the bacteria in the water but may cause hardness and corrosivity.
Water softening requires the removal of calcium and magnesium. Either chemical precipitation or ion exchange will achieve the removal. The most common method utilizes soda ash and lime or lime alone. If ion exchange is done, the water is sent through beds of ion exchange resins. Water may have to be stabilized after softening.
Disinfection of wastewater can be done several ways. One is addition of chlorine, but ozone or ultraviolet radiation can also be used. Regardless of the method, disinfection is important. It not only destroys bacteria and inactivates viruses but also reduces faint odors and tastes. However, if chlorine is used for this treatment, it may combine with other organics and result in distasteful water.
Other chemicals used in treatment include hydroxide, used to precipitate metals and other inorganics, and copper sulfate, which controls algae. For wastewater treatment, chemicals have not been widely used until recently. Biological and physical methods were favored, primarily due to cost. However, as water pollution restrictions have strengthened, the relative cost of chemical treatment is no longer disproportionately high.
Microorganisms have been used to clean wastewater for more than a century. In a natural ecosystem with normal demands on water, normal processes gradually clean the water: solids will precipitate due to gravity, aeration will occur naturally, and bacteria will biodegrade organic materials. These naturally occurring processes are so important to water quality that three conditions in wastewater treatment systems are watched very closely: biochemical oxygen demand, dissolved oxygen, and total suspended solids. Fecal coliform and alkalinity/acidity are also monitored.
Biochemical oxygen demand is a measurement of how much dissolved oxygen wastewater will need to break down organic material. The higher the demand, the more polluted the water is. Dissolved oxygen is desirable; it enables aerobic activity to go forward. Total suspended solids are particles that float within water. They are not beneficial additions to water, because they will eventually interfere with oxygen intake, obstruct sunlight, and affect the taste, appearance, and odor of the water.
Metropolitan growth and industrialization have accelerated the demand for water and increased the load of contaminants entering the waste stream. As a result, wastewater treatment through unassisted natural processes is no longer effective. Assistance can be provided by bringing the bacteria in contact with the wastewater, ensuring that the bacteria remain healthy and active by providing nutrients, and allowing the bacteria the time it needs to work. Some microbes need oxygen to work; they are called aerobic microbes. Those that do not are designated anaerobic microbes. Either type of bacteria can be used for wastewater treatment, depending on the type of system.
Trickling filters are common components in wastewater treatment plants. They are not filters in the normal sense but are large shallow concrete tanks, filled with medium sized stones that are covered with bacteria. Settled wastewater trickles through the stones, and the bacteria breaks down organic components in the wastewater.
Sludge may be activated by injecting compressed air into a tank containing wastes and circulating sludge to encourage biological activity. This method is used for raw sewage.
Digestion is a slight variation on activated sludge. The sludge is placed in a closed tank and heated, enhancing the speed and effectiveness of microbes.
The obvious problem with ground water treatment is the location of the water. To deal with that issue, groundwater treatment systems generally pump the water to the surface, treat it, and return it to the source. Contamination in groundwater presents unique difficulties, though, because pollutants sink to the bottom, float on the top, or are suspended within the liquid. Suspended contaminants and those that float are considerably easier to remove than the “sinkers.” In environmental parlance, sinkers are referred to as dense nonaqueous phase liquids or DNAPLs.
Before it is treated, groundwater must be analyzed to determine what the contaminants are. That is accomplished by drilling a well from which samples can be taken. Then pump and treat systems are designed for the particular sites where they operate. The designer will have to determine where to place extraction wells and how to create the correct hydraulic gradient so that the water flows in the direction of treatment. The object of the design phase is to get the contaminated water to the extraction wells in a short time with little dilution of the contaminants. A pilot system will actually test the accuracy of the design.
Treatment will vary, depending on the pollutants involved. Once the water is at the surface, many traditional types of wastewater treatment can be used on the groundwater.
Air stripping can be used to eliminate hydrocarbons from groundwater. The process is similar to aeration, but it involves building a tower, pumping the water up it, and then exposing it to air. Hydrocarbons are freed by this method, and they go into the air unless they are captured and destroyed.
Activated carbon is used in many pump and treat systems. It is combined with air stripping and other techniques of removing volatile organic compounds so that they do not escape. It can also be used to trap contaminants while they are still in liquid.
For removal of inorganic compounds, chemicals may be added to cause them to precipitate. Mechanical methods are then used to separate the particles from the treated water, and it may be necessary to restabilize the groundwater before returning it to the source.
Although groundwater is most commonly treated at the surface with pump and treat systems, biological methods may occasionally be used without removing the water from the source. Treatment procedures in which the contaminated groundwater remains in place are called in situ treatment. This technique can be used to destroy some volatile organic compounds. Nutrients and oxygen must be pumped into the aquifer to allow the bacteria to grow and break down the pollutants.
Selection of the type of technology to be used for groundwater cleanup follows extensive testing involving the water and subsurface conditions. The federal government will be involved in groundwater remediation when a federally listed abandoned hazardous waste site is determined to have groundwater contamination.
Federal water law does not protect groundwater in the same way that many state laws do. Surface waters fall under the Clean Water Act, but the definition of “waters of the United States” in that statute does not include groundwater. The indirect federal approach to preventing groundwater pollution stops some activities that could impact groundwater. States that depend heavily on their aquifers are more stringent with their regulations. See also cleanup technologies; maximum contaminant levels; secondary treatment.
Based on “Environment and the Law. A Dictionary”.