Water & Wastewater Treatment
Have you ever wondered what happens to that water and waste after you flush? How about after you pull the plug on your tub? The modern wastewater-treatment plant employs basic physics and advanced technology to purify the dirtiest of water, so it can go back into the environment as a member in good standing of the water cycle.
Why Treat Wastewater?
We consider wastewater treatment as a water use because it is interconnected with the other uses of water. Much of the water used by homes, industries, and businesses must be treated before it is released back to the environment.
If the term "wastewater treatment" is confusing, you might think of it as "sewage treatment." Nature has an amazing ability to cope with small amounts of water wastes and pollution, but without treatment, it would be overwhelmed with the billions of gallons of wastewater and sewage produced every day before releasing it back to the environment. Treatment plants reduce pollutants in wastewater to a level nature can handle.
Wastewater also includes storm runoff. Although some people assume that the rain that runs down the street during a storm is fairly clean, it isn't. Harmful substances that wash off roads, parking lots, and rooftops can harm our rivers and lakes.
The Environmental Impact of Wastewater Treatment
When it comes to treating wastewater, it's a simple matter of caring for our environment and our own health. By helping keep our water clean, it can also help us with the following.
FISHERIES: Clean water is critical to plants and animals that live in water. This is important to the fishing industry, sport fishing enthusiasts, and the environment in general.
WILDLIFE HABITATS: Our rivers and ocean waters are teeming with life that depends on shorelines, beaches, and marshes. They are critical habitats for hundreds of species of fish and other aquatic life. Migratory water birds also use these areas for resting and feeding.
RECREATION & QUALITY OF LIFE: Water is a great playground for us all. Scenic and recreational values are the inspiration for many people to choose to live here. Visitors are often drawn to water activities such as swimming, fishing, boating, and picnicking.
HEALTH CONCERNS: If it is not properly cleaned, water can carry disease. Since we live, work, and play so close to water, harmful bacteria must be removed to make the water safe.
The Effects of Wastewater Pollutants
If wastewater is not properly treated, both our health and the environment can be negatively impacted. This can include harm to fish and wildlife populations, oxygen depletion, beach closures, and other restrictions on recreational water use, and more.
The Texas Commission on Environmental Quality has provided examples of the pollutants that can be found in wastewater, and the potentially harmful effects these substances can have on ecosystems and human health:
Decaying organic matter and debris can use up the dissolved oxygen in a lake so fish and other aquatic biotas cannot survive.
Excessive nutrients, such as phosphorus and nitrogen (including ammonia), can cause eutrophication, or over-fertilization of receiving waters, which can be toxic to aquatic organisms, promote excessive plant growth, reduce available oxygen, harm spawning grounds, alter habitat and lead to a decline in certain species.
Chlorine compounds and inorganic chloramines can be toxic to aquatic invertebrates, algae and fish.
Bacteria, viruses, and disease-causing pathogens can pollute beaches and contaminate shellfish populations, leading to restrictions on human recreation, drinking water consumption, and shellfish consumption.
Metals, such as mercury, lead, cadmium, chromium. and arsenic can have acute and chronic toxic effects on species.
Other substances, such as some pharmaceutical and personal care products, primarily entering the environment in wastewater effluents, may also pose threats to human health, aquatic life, and wildlife.
Water Rates vs. Wastewater Rates
When comparing the number of contaminants found between water and wastewater, you can begin to understand why more effort is required to purify everyday waste and sewage. We work hard to perfect this filtration process and keep rivers clean by preventing raw sewage from polluting the environment. So, while the project requires an upfront investment, the outcome is projected to make a monumental difference in the long run.
As it’s said, the major aim of wastewater treatment is to remove as much of the suspended solids as possible before the remaining water, known as "effluent”, is discharged back to the environment. As solid material decays, it uses up oxygen, which is needed by the plants and animals living in the water.
"Primary Treatment" removes about 60 percent of suspended solids from wastewater. This treatment also involves aerating (stirring up) the wastewater adding oxygen back in. The secondary treatment removes more than 90 percent of suspended solids. For more information, please review the Primary Treatment Process below.
The Primary Treatment Process
Wastewater entering the treatment plant includes items like wood, rocks, and even dead animals. Unless they are removed, they could cause problems later in the treatment process. Most of these materials are sent to a landfill.
The wastewater system relies on the force of gravity to move sewage from your home to the treatment plant. Due to this, wastewater treatment plants are located on low ground, often near a river into which treated water can be released. If the plant is built above the ground level, the wastewater must be pumped up to the aeration tanks. From here on, gravity takes over to move the wastewater through the treatment process.
To begin the process, the sewage is shaken in order to expose it to air. This causes some of the dissolved gases that taste and smell bad (such as hydrogen sulfide, which smells like rotten eggs) to be released from the water. Wastewater enters a series of long, parallel concrete tanks which are each divided into two sections. In the first section, air is pumped through the water.
As organic matter decays, it uses up oxygen. Aeration replenishes the oxygen. Bubbling oxygen through the water also keeps the organic material suspended while it forces 'grit' (coffee grounds, sand, and other small, dense particles) to settle out of the mixture. Grit is pumped out of the tanks and taken to landfills.
4. Removing Sludge
The wastewater then enters the second section or sedimentation tanks. Here, the sludge (the organic portion of the sewage) settles out of the wastewater and is pumped out of the tanks. Some of the water is removed in a step called thickening and then the sludge is processed in large tanks called digesters.
5. Removing Scum
As sludge is settling to the bottom of the sedimentation tanks, lighter materials float to the surface. This 'scum' includes grease, oils, plastics, and soap. Slow-moving rakes skim the scum off the surface of the wastewater. Scum is thickened and pumped to the digesters along with the sludge.
Many cities also use filtration in sewage treatment. After the solids are removed, the liquid sewage is filtered through a substance, usually sand, by the action of gravity. This method eliminates almost all bacteria, reduces turbidity and color, removes odors, reduces the amount of iron, and removes most other solid particles that remained in the water. Water is sometimes filtered through carbon particles, which removes organic particles. This is a common method used in some homes.
6. Killing Bacteria
Finally, the wastewater flows into a 'chlorine contact tank, where the chemical chlorine is added to kill bacteria that pose a health risk, just as is done in swimming pools. The chlorine is mostly eliminated as the bacteria are destroyed, but sometimes it must be neutralized by adding other chemicals. This protects fish and other marine organisms, which can be harmed by the smallest amounts of chlorine.
The treated water is then discharged to a local river or the ocean.
Another part of treating wastewater is dealing with solid waste material. These solids are kept for 20 to 30 days in large, heated, and enclosed tanks called 'digesters.' Here, bacteria break down (digest) the material, reducing its volume, odors, and getting rid of organisms that can cause disease. The finished product is mainly sent to landfills, but sometimes can be used as fertilizer.