Wastewater is simply water that has been used. It usually contains various pollutants, depending on what it was used for. It is classified into two major categories, by source:
- Domestic or sanitary wastewater. This comes from residential sources including toilets, sinks, bathing, and laundry. It can contain body wastes containing intestinal disease organisms.
- Industrial wastewater. This is discharged by manufacturing processes and commercial enterprises. Process wastewater can contain rinse waters including such things as residual acids, plating metals, and toxic chemicals.
Wastewater is treated to remove pollutants (contaminants). Wastewater treatment is a process to improve and purify the water, removing some or all of the contaminants, making it fit for reuse or discharge back to the environment. Discharge may be to surface water, such as rivers or the ocean, or to groundwater that lies beneath the land surface of the earth. Properly treating wastewater assures that acceptable overall water quality is maintained.
In many parts of the world, including in the United States, health problems and diseases have often been caused by discharging untreated or inadequately treated wastewater. Such discharges are called water pollution, and result in the spreading of disease, fish kills, and destruction of other forms of aquatic life. The pollution of water has a serious impact on all living creatures, and can negatively affect the use of water for drinking, household needs, recreation, fishing, transportation, and commerce.
Objectives and Evolution of Wastewater Treatment
We cannot allow wastewater to be disposed of in a manner dangerous to human health and lesser life forms or damaging to the natural environment. Our planet has the remarkable ability to heal itself, but there is a limit to what it can do, and we must make it our goal to always stay within safe bounds. That limit is not always clear to scientists, and we must always take the safe approach to avoid it.
Basic wastewater treatment facilities reduce organic and suspended solids to limit pollution to the environment. Advancement in needs and technology have necessitated the evolving of treatment processes that remove dissolved matter and toxic substances. Currently, the advancement of scientific knowledge and moral awareness has led to a reduction of discharges through pollution prevention and recycling, with the noble goal of zero discharge of pollutants.
Treatment technology includes physical, biological, and chemical methods. Residual substances removed or created by treatment processes must be dealt with and reused or disposed of in a safe way. The purified water is discharged to surface water or ground water. Residuals, called sludges or biosolids, may be reused by carefully controlled composting or land application. Sometimes they are incinerated.
Since early in history, people have dumped sewage into waterways, relying on natural purification by dilution and by natural bacterial breakdown. Population increases resulted in greater volume of domestic and industrial wastewater, requiring that we give nature a helping hand. Some so-called advancements in cities such as Boston involved collecting sewage in tanks and releasing it to the ocean only on the outgoing tide. Sludge was barged out to sea so as to not cause complaint.
Until the early 1970s, in the United States, treatment mostly consisted of removal of suspended and floating material, treatment of biodegradable organics, and elimination of pathogenic organisms by disinfection. Standards were not uniformly applied throughout the country.
In the early 1970s until about 1980, aesthetic and environmental concerns were considered. Treatment was at a higher level, and nutrients such as nitrogen and phosphorus were removed in many localities.
Since 1980, focus on health concerns related to toxics has driven the development of new treatment technology. Water-quality standards were established by states and the federal government and had to be met as treatment objectives. Not just direct human health but aquatic-life parameters were considered in developing the standards.
Wastewater Treatment Types
Rural unsewered areas, for the most part, use septic systems. In these, a large tank, known as the septic tank, settles out and stores solids, which are partially decomposed by naturally occurring anaerobic bacteria. The solids have to be pumped out and hauled by tank truck to be disposed of separately. They often go to municipal wastewater treatment plants, or are reused as fertilizer in closely regulated land-application programs. Liquid wastes are dispersed through perforated pipes into soil fields around the septic tank.
Most urban areas with sewers first used a process called primary treatment, which was later upgraded to secondary treatment. Some areas, where needed, employ advanced or tertiary treatment. Common treatment schemes are presented in the following paragraphs.
Primary Treatment. In primary treatment, floating and suspended solids are settled and removed from sewage.
Flow from the sewers enters a screen/bar rack to remove large, floating material such as rags and sticks.
It then flows through a grit chamber where heavier inorganics such as sand and small stones are removed.
Grit removal is usually followed by a sedimentation tank/ clarifiers where inorganic and organic suspended solids are settled out.
To kill pathogenic bacteria, the final effluent from the treatment process is disinfected prior to discharge to a receiving water. Chlorine, in the form of a sodium hypochlorite solution, is normally used for disinfection. Since more chlorine is needed to provide adequate bacteria kills than would be safe for aquatic life in the stream, excess chlorine is removed by dechlorination. Alternate disinfection methods, such as ozone or ultraviolet light, are utilized by some treatment plants.
Sludge that settles to the bottom of the clarifier is pumped out and dewatered for use as fertilizer, disposed of in a landfill, or incinerated. Sludge that is free of heavy metals and other toxic contaminants is called Biosolids and can be safely and beneficially recycled as fertilizer, for example.
Secondary Treatment. Primary treatment provided a good start, but, with the exception of some ocean outfalls , it is inadequate to protect water quality as required by the Environmental Protection Agency (EPA).
With secondary treatment, the bacteria in sewage is used to further purify the sewage. Secondary treatment, a biological process, removes 85 percent or more of the organic matter in sewage compared with primary treatment, which removes about 50 percent.
The basic processes are variations of what is called the "activated sludge" process or "trickling filters," which provide a mechanism for bacteria, with air added for oxygen, to come in contact with the wastewater to purify it.
In the activated sludge process, flow from the sewer or primary clarifiers goes into an aeration tank, where compressed air is mixed with sludge that is recycled from secondary clarifiers which follow the aeration tanks. The recycled, or activated, sludge provides bacteria to consume the "food" provided by the new wastewater in the aeration tank, thus purifying it.
In a trickling filter the flow trickles over a bed of stones or synthetic media on which the purifying organisms grow and contact the wastewater, removing contaminants in the process. The flow, along with excess organisms that build up on the stones or media during the purification, then goes to a secondary clarifier. Air flows up through the media in the filters, to provide necessary oxygen for the bacteria organisms. Clarified effluent flows to the receiving water, typically a river or bog, after disinfection. Excess sludge is produced by the process and after collection from the bottom of the secondary clarifiers it is dewatered, sometimes after mixing with primary sludge, for use as fertilizer, disposed of in a landfill, or incinerated.
Advanced or Tertiary Treatment. As science advanced the knowledge of aquatic life mechanisms and human health effects, and the need for purer water was identified, technology developed to provide better treatment. Heavy metals, toxic chemicals and other pollutants can be removed from domestic and industrial wastewater to an increasing degree. Methods of advanced treatment include microfiltration, carbon adsorption, evaporation /distillation, and chemical precipitation.
Industrial Waste Treatment. Depending on the type of industry and the nature of its wastes, industries must utilize methods such as those used for advanced treatment of sewage to purify wastewater containing pollutants such as heavy metals and toxic chemicals before it can be discharged. Industries are permitted to discharge directly to receiving waters under the National Pollution Discharge Elimination System (NPDES) permit system or to municipal sewers under the Industrial Pretreatment Program. Pollution prevention programs are very effective in helping industries reduce discharged pollutants, by eliminating them at the source through recycling or through the substitution of safer materials. More and more industries are approaching or attaining zero discharge by cleaning and reusing their water over and over and over.
Combined Sewer Overflows
Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, or other water bodies. Some designs utilize an overflow at the treatment plant that diverts the excess flow to chlorination facilities for disinfection prior to discharge.
These overflows, called combined sewer overflows (CSOs), contain not only storm water but also untreated human and industrial waste, toxic materials, and debris. They are a major water pollution concern for the approximately 772 U.S. cities that have combined sewer systems.
CSO outfalls often result in violations of receiving stream-water quality standards and impairment to designated water uses. Violations can include aesthetics (including floatables, oil and grease, colors, and odor), solids, nutrients, harmful bacteria, metals, and reduced dissolved oxygen levels.
Historical and Regulatory Aspects
Environmental awareness and activism is not a present-day concept:
In the mid-1700s Benjamin Franklin and others petitioned the Pennsylvania Assembly to stop dumping waste and attempted to regulate waste disposal and water pollution. European countries were correlating sickness with lead and mercury in the late 1700s. In 1855, Chicago became the first U.S. city with a comprehensive sewer plan, and all U.S. towns with populations over 4,000 had city sewers by 1905.
In 1899 the Refuse Act prevented some obvious pollution of streams and placed the U.S. Army Corps of Engineers in charge of permits and regulation.
In 1914 U.S. government agencies began pollution surveys of streams and harbors. Reports filed by the early 1920s showed heavy damage from oil dumping, mine runoff, untreated sewage, and industrial wastes.
In 1924 the Oil Pollution Control Act prohibited discharge from any vessel within the three-mile limit, except by accident.
In 1948 the Federal Water Pollution Control Act and active House and Senate Public Works Committee in water pollution came about.
In 1956 Congress passed the Water Pollution Control Act, in 1961 the Clean Water Act, and in 1965 the Water Quality Act, setting standards for states.
In 1970 Congress and the president established the EPA.
In 1972 Congress passed the Federal Water Pollution Control Act (the "Clean Water Act").
In 1973 EPA issued the first NPDES permits.
In 1974 Congress passed the Safe Drinking Water Act.
The Clean Water Act of 1972. Said to be one of the most significant pieces of environmental regulations ever enacted, the federal Clean Water Act of 1972 was prompted by growing national concern for the environment in the late 1960s, fueled by such concerns as the burning Cuyahoga River in Ohio, an unfishable, unswimmable Potomac River, and a nearly dead Lake Erie.
National goals and objectives were established "to restore and maintain the chemical, physical, and biological integrity of the Nation's waters." There were two major goals:
- Eliminate the discharge of all pollutants into navigable waters of the United States; and
- Achieve an interim level of water quality that provides for the protection of fish, shellfish, and wildlife and recreation (the "fishable, swimmable" goal).
To help do this, the following were established:
A state grant program to support the construction of sewage treatment plants; the NPDES program, whose goal was to eliminate discharges to U.S. waters; and technological standards or discharge limits that had to be met, based on water-quality standards set by the states.
A minimum required percent removal of pollutants was added in 1985.
Secondary treatment was required, and limits were set for three major effluent parameters: biological oxygen demand, suspended solids, and pH.
The Water Quality Act of 1987 made several changes, addressing (1) excess toxic pollutants in some waters and (2) nonpoint source pollution . The construction grant program was phased out and replaced by financing projects with revolving fund, low-interest-rate loans. The amendments passed in 1987 also addressed storm-water controls and permits, regulation of toxics in sludge, and problems in estuaries. Penalties were added for permit violations. Also initiated were sludge-disposal regulations and funding for studies relative to nonpoint and toxic pollution sources.
The 1972 act has provided remarkable achievements, but there is still a long way to go. Forty percent of waters assessed by states still do not meet water-quality standards, mostly due to pollution from nonpoint sources. Other than from storm or combined storm sewer overflows, most of the remaining problem is not from pipes (point sources) but from sources such as farming and forestry runoff, construction sites, urban streets (storm water), automobiles, and atmospheric depositions, such as from power-plant air emissions (nonpoint sources). Current approaches to addressing nonpoint pollution include targeting and permitting by given watersheds and TMDL (total maximum daily load for a river stretch) assessments.
Many of the facilities funded by federal construction grants, which make up the wastewater collection and treatment infrastructure, are wearing out and are now undersized. Many, many dollars are needed to keep providing adequate treatment to maintain the status quo, let alone meet the needs of a growing populace.
Unfortunately, since the Industrial Revolution, most of Europe's rivers (not unlike in the United States) were utilized for transporting wastes to the sea, resulting in harm to human and aquatic health and causing coastal pollution. In earlier times, the rivers could handle the limited wastes discharged, through dilution and natural purification.
Significant progress has been made in treating the wastewater entering Europe's rivers, with measurable improvements in water quality. The agricultural sector (nonpoint pollution source) has not kept up, and nitrate levels are still high.
The fifteen-nation European Union's (EU) Urban Wastewater Treatment Directive has resulted in significant improvements in wastewater treatment capacity and methods. According to the European Environment Agency, increased treatment capacity has been realized in all EU countries except Sweden, Finland, and the Netherlands, where it is already efficient. The largest increase will be in southern Europe and Ireland. As a result, the EU's collection and treatment systems should be able to cope with all organic discharges from most member states by 2005. In Finland and Sweden most of the wastewater was being treated in tertiary plants in the 1980s.
Environmental Protection Agency, Office of Water. (1993). "Constructed Wetlands for Wastewater Treatment and Wildlife Habitat." Available from http://www.epa.gov/owow/wetlands/construct .
Ohio State University Extension, Food, Agricultural, and Biological Engineering. "Wastewater Treatment Principles and Regulations." Available from http://ohioline.osu.edu/aex-fact/0768.html .
Raymond Cushman and George Carlson
The liquid and solid material removed from domestic septic tanks is called septage. Most septage is hauled to municipal sewage treatment facilities and most septage haulers must be licensed.
A settling pond, usually man-made, collects and slows water flow so that suspended solids (sediments) have time to precipitate or settle out of the water. Some applications of settling ponds include capturing runoff from farms (agricultural waste), construction projects (soil sediment) and mines (sediment and toxic waste). Settling ponds eventually fill and must be dredged to remain in operation. Polluted water from abandoned mines is diverted to settling ponds to remove solids such as iron oxide. When dredged, these sediments must be treated as contaminated waste. Pilot projects are underway to recapture iron oxide for use in paint pigments.
Constructed wetlands are wetlands that are specially built for the purpose of wastewater treatment and are utilized in place of naturally occurring wetlands. They provide a greater degree of wastewater treatment than natural wetlands, as their hydraulic loadings can be managed as required. Because these wetlands are constructed specifically for wastewater treatment, they should not be included in the jurisdictional group, which avoids the regulatory and environmental entanglement associated with natural wetlands. This is in accordance with Environmental Protection Agency regulations. The treatment process can be either aerobic or anaerobic , depending on whether the wetlands are constructed with an exposed water surface or one with subsurface flow. These wetlands can also be used to remove nitrogen, which is usually not removed during the standard wastewater treatment process. Nitrogen removal is accomplished by the growth of cattails and reeds, which utilize the highly nutrient wastewater and consequently remove nitrogen in the process. Sometimes the cattails and reeds must be harvested to complete the removal process.