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Wastewater Treatment Process

WWTP 1st Pic

The objective of wastewater treatment and collection is to preserve quality water, our most valuable resource. Water is one of the greatest needs of any community. Without it, city life is impossible.

Almost every city in the United States has some kind of wastewater treatment plant or provisions for wastewater treatment through another provider. It is the law! We often take for granted those processes which ensure that the water we have used is clean when released back into streams, lakes, or ground water.

The process used in contemporary wastewater treatment plants is similar to the natural process by which water is cleaned while moving through a river. Early on, towns and communities pumped raw sewage from homes, businesses, and factories directly into rivers, streams, lakes, and oceans. As human populations grew, this practice degraded the water quality to the point of posing serious health hazards. Increasing growth and development created a demand for clean water that exceeded the rate at which it could occur naturally in streams and rivers. At one point many rivers and streams were so polluted that they could even burn; the Cuyahoga River caught fire in 1969 due to the oil and chemical wastes. Changes in national policies, such as the Clean Water Act, created legislation that led to the construction of many wastewater treatment plants and a national focus to improve our waters. Wastewater treatment today, in its various forms, still contains treatment processes that utilize soil and water microorganisms that convert the organic substances in wastewater into harmless material.

Preliminary treatment is the first step in the treatment process. Its purpose is to physically remove untreatable materials. Screenings and grit removal separate the sand and other inorganic material that may harm the pumps and other equipment in the treatment processes.

WWTP Primary Treatment


The simplest form of wastewater treatment called Primary Treatment WWTP Clarifierinvolves filtration and settling procedures. Forty-five to fifty percent of pollutants can be removed utilizing primary techniques. Sedimentation is the process by which solids and liquids are separated by gravitational forces. The thickened solids or sludge settle to the bottom of the clarifier, as seen to the right.

WWTP Trickling Filter

Secondary Treatment generally involves a biological process, which removes eighty-five to ninety percent of remaining pollutants. The two most common types of secondary treatment are attached growth processes and suspended growth processes.  Trickling filters (seen to the left) and activated sludge are examples of these two types of treatment.


Both processes create conditions favorable to the growth and reproduction of helpful microorganisms which consume most of the waste material. The microorganisms then settle out of the treated water in the secondary settling tank or clarifier.

Advanced or Tertiary Treatment removes small amounts of any remaining undesirable material in the treated water. These processes may include filtration, distillation, or flocculation to remove suspended particles, organic material, and sometimes specific chemicals such as nitrates, phosphates, and heavy metals. The level of treatment required prior to discharge in the receiving stream depends on the quality of the body of water and the types of reuse designated for the treated water. Both federal and state regulatory agencies, the U.S. Environmental Protection Agency (EPA) and Texas Commission on Environmental Quality (TCEQ), provide guidance and standards through permitting of allowable discharges.

The National Pollutant Discharge Elimination System (NPDES) and Texas Pollutant Discharge Elimination System (TPDES) are outcomes of the Clean Water Act, which collectively control direct discharges to lakes, streams, rivers and oceans within the United States.  Discharge parameters are determined based on the final uses of the receiving stream and water quality standards.

Discharge to the Elm Fork
of the Trinity River
WWTP Discharge
WWTP Disinfection


Disinfection is the final process prior to discharge or release of treated water. This step ensures any disease causing or pathogenic microorganisms are killed and the water released into the nearby waterway is safe for the environment and human life. This water is then picked up by the next downstream community for its needs.

Solid material removed during the treatment process from wastewater is called sludge.
Sludge Treatment WWTP Filter Pressinvolves the stabilizing of the organic material and the biologically active mass of microorganisms in the sludge. This process can be achieved through a number of processes which breakdown the sludge, biologically, chemically, and/or physically. In some cases the processed sludge or biosolids can be used as a fertilizer for crops and gardens. Some sludges are buried or burned. Wastewater treatment plants follow strict regulations and guidelines to ensure public health through the proper sludge handling and disposal. To the right, a filter press dewaters the sludge prior to final disposal.

For more information on wastewater treatment, please visit Water Environment Federation or call 972.219.3545.

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