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 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 purerwater 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.
