Wastewater Treatment Process

WWTP 1st Pic

The ultimate objective of wastewater treatment and collection is the preservation of good, 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 broad sweeping legislation that led to the construction of many wastewater treatment plants and a national focus to improve our national waters.

Collection systems were developed to transport the wastewater from homes, businesses and industries to wastewater treatment plants. 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 materials.

Preliminary treatment is the first step in the treatment process. Its purpose is to remove those materials that are untreatable and can be separated by physical means. Screenings and grit removal are used to separate the sand and other inorganic material that can be harmful to the pumps and other equipment in the treatment processes.

WWTP Primary Treatment


The simplest form of wastewater treatment called Primary Treatment involves filtration and WWTP Clarifiersettling 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 and activated sludge are examples of these two types of treatment.


Both are processes that 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.

Trickling Filter

Advanced or Tertiary Treatment are processes that remove small amounts of undesirable materials that still remain 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 to the receiving stream depends on the water 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 provided prior to discharge or release of treated water. This final step ensures that any disease causing or pathogenic microorganisms are killed and that the water released into the nearby waterway is safe to the environment and human life. After all, 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 involves 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 handling and disposal of sludge.


At right is a Filter Press which dewaters the sludge prior to final disposal.

WWTP Filter Press
For more information on wastewater treatment, please visit Water Environment Federation or call 972.219.3504.

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