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Case Study: Water Treatment

This page will discuss the domestic water cycle including treatment before and after domestic use. After water is used domestically it is called wastewater due to contaminants such as shampoo, cleaning supplies, detergent, soap, and excrement. Wastewater treatment is the process of cleaning domestic wastewater so it can be either reused for irrigation, or safely put back into lakes, rivers, groundwater etc. Untreated wastewater is a threat to the health of humans and the environment. Treatment before domestic use is simply called water treatment which refers to the process that water from wells and other natural sources undergo in order to be potable for humans.

Introduction

The cycle of water in today's society takes the following path. (1) Water is taken from natural sources and undergoes Water Treatment. (2) It is then used domestically and produces two types of waste: greywater and blackwater. (3) These two types of wastes undergo Wastewater Treatment. (4) Water is then either Reused as irrigation on agricultural fields, golf courses, house lawns, etc. or it is discharged (returned) to natural systems such as lakes, rivers, and oceans. (5) Water is extracted from natural systems and the cycle begins again.  Let's begin by exploring wastewater and its components. 

Graywater

Graywater is the liquid waste that comes from showering, bathing, clothes washing, and kitchen water use including dish washing. Possible uses of this waste include irrigation of landscape and water used to flush toilets. Neither of these uses pose negative threats to the environment.

Blackwater

Blackwater is the other type of waste water in which Domestic Wastewater Treatment focuses on changing the most. It is the liquid waste from urine and feces that is flushed down toilets. Blackwater can also be referred to as sewage. This sewage carries bacteria that needs to be treated as soon as possible to prevent contamination in the event that te water ends up in a river or other body of water. Blackwater contains 90% of the nitrogen in wastewater and is hard to remove from water to make it drinkable. The process of treating this waste is what we will focus on in this section.

Summary

Graywater has fewer pathogens, less nitrogen, and takes less time to decompose than blackwater does. Because of graywater's relative innocuous nature, domestic wastewater treatment focuses mainly on blackwater.

After Domestic Use - Wastewater Treatment Process

Coagulation and Sedimentation

In the first step of water treatment, the solids and liquids of the blackwater are separated through coagulation and sedimentation. Coagulation is the process that removes suspended waste particles in water by binding them to a heavier substance, such as alum, so that the solid can fall to the bottom as a sediment clump during sedimentation. Sedimentation occurs in large tanks that allow for the solids to settle to the bottom, and the oils to rise to the top. The solid matter is known as sludge. This separation makes the process of removing the solids and oils much easier. The water that remains is then sent to stage two for filtration.

Biological Treatment 

The purpose of the Biological Treatment stage is to reduce the BOD (biological oxygen demand) content by having bacteria feed on organic waste. After bacteria metabolize the organic matter it can precipitate to the bottom of the tank, making the water much clearer and cleaner. Oxygen levels are important to monitor, yet temperature does not need to be modified because bacteria can thrive at a range of temperatures.

Filtration

For the filtration of water to occur, wastewater must pass through a filter in order to prevent the unwanted contaminants from remaining in the water. The filter type can vary depending on the kind of contaminant that must be removed. Some filters consist of sand, gravel, or charcoal because they capture smaller, unwanted particles in the water. The object of filtration is to completely remove all the particles in the water to clarify and aid in the process of disinfecting. 

Disinfection

After the solids and other particles have been eliminated from the wastewater, the resulting liquid must be disinfected to kill the microorganisms that remain in the water. To accomplish this, the water is disinfected using products such as chlorine. This step is used to ensure that the water is free from bacteria and is better to drink than it was in the previous step.

Storage

When the processes of water treatment is complete, the water is placed in a holding tank for disinfection to continue and is then sent to homes and businesses in the community for domestic use.

Click http://www.excelwater.com/eng/b2c/about_8.php and scroll down to the bottom for a diagram of this process.

Before Domestic Uses Described Above: Water Treatment

Water Treatment Plants

As described for After Domestic Use- Water Treatment, the process for water before domestic treatment is essentially the same but with a few exceptions. Below are the methods in which the water is treated to before domestic use.

Methods of Water Treatment

  1. Flocculation and Sedimentation: This process (as mentioned above) is used to combine the smaller unwanted particles with alum to form larger clumps of particles that are easier to remove after settling out of the water as sediment clumps.
  2. Filtration: Removes unwanted particles from the water by clarifying the water and improving the success of the disinfecting process.
  3. Ion Exchange: This is used to remove inorganic contaminants in hard water that remain after sedimentation and filtration. These contaminants include chromium, arsenic, fluoride, and nitrates.
  4. Adsorption: where activated carbon powder is added to the water to allow the organic contaminants, coloring, taste, and odor to "stick" to the carbon powder and be removed from the drinking water.
  5. Disinfection: Microorganisms cannot be left in drinking water and must be dealt with using chlorine and other products to destroy the bacteria. The biological contaminants are controlled and removed before the water can enter the distribution pipes and be delivered to the communities.

Reuse and Discharge

An important step in the wastewater treatment cycle is what is done with the water after it is treated. It can be either reused or discharged (returned) immediately to natural systems. Reusing the water includes irrigation of agricultural fields, golf courses, and domestic lawns. This water eventually makes its way to natural systems by evaporation or settling into the groundwater. It is important for human and environmental health that the wastewater be properly treated no matter what its final destination is.

Neglecting Water Treatment

Negative Effects

Water Pollution occurs when water is left untreated and circulates into fresh water or bodies of water. This can lead to devestation of life in water-based ecosystems. Ecosystems have been dramatically changed or destroyed due to contaminated water. Marine life is also greatly effected by the pH changes that foreign chemicals can contribute to their habitat. Coastal wetlands are extemely vulnerable to water pollution because these areas are home to highly diverse populations. Humans are also effected by water pollution because we can potentially consume the fish that swim in contaminated water and contract diseases from the harmful chemicals that the fish had come in contact with. Animals could drink water that has been polluted and become ill or die because of the chemicals. It is important that we as humans keep treating our water to ensure a safe and healthy community and environment.

Humans are suseptible to contaminants in drinking water if it has not been properly treated. When water treatment does not occur or is incorrectly completed, contaminants in drinking water are more likely to be found. These contaminants can cause chronic effects, or effect people over time, such as causing cancer, liver and kidney damage, immune system damage, nervous system disorders, and birth defects. Some examples of contaminants would be disease producing mircroorganisms, organics (which can be toxic chemicals), inorganics (such as toxic metals), and radioactive elements like Radon which can cause lung cancer.

Case Study: UC Davis Wastewater Treatment

What happens to water after it is used on the UC Davis campus? The University has its own wastewater treatment facility that processes the 1.6 million gallons of wastewater generate each day! Water is collected from all buildings, labroratories, dining kitchens etc. into a sanitary sewer and transported to the facility. Interesting, normal domestic waste is not separated from laboratory drainage systems and there is a potential for hazardous chemical contamination. After the water is treated it is released into Putah Creek and heavily regulated under State Regional Water Quality Control Board.

Case Studying: UC Davis Water Contamination

On the southern edge of the UC Davis campus chromium-6 exists in elevated levels, causing the EPA to declare it a federal Superfund Site. Chromium-6 is a human carcinogen and can leak into groundwater causing well water contamination. The source has been determined as "sewage sludge dumped in a landfill at the site decades ago appears to have begun a series of chemical reactions that eventually converted the naturally occurring, nontoxic chromium-3 in the soil into the carcinogen chromium-6" (Reference 7). Luckily, no wells have been reported as contaminated however, "over the past decade, chromium-6 has been detected at the site at levels up to 500 parts per billion, or 10 times the California standard for drinking water" (Reference 7). The current plan for clean-up is to inject the chromium-6 with calcium polysulfide to convert it back to nontoxic chromium-3.   

 

References

  1. Pettrucci, Ralph  H. General Chemistry: Principles and Modern Applications. 9th. Upper Saddle River: Pearson Prentice Hall, 2007
  2. Mara, Duncan. Domestic Wastewater Treatment in Developing Countries. Earthscan Publications Ltd. October 2004.
  3. Hammer, Mark J. Water and Wastewater Technology. Prhttp://www.excelwater.com/eng/b2c/about_8.phpentice Hall. June 2007.
  4. Lehr, J.H. D omestic Wa ter Treatment. Natl Water Well Assn. June 1988.
  5. Lindstrom, Carl. “Greywater irrigation - grey waste treatment.” Web. http://www.greywater.com. 21 Nov. 2010.
  6. "Overview Of The UC Davis Wastewater Collection And Treatment System." http://facilities.ucdavis.edu/util/w...osal/overview/. 25 April 2007.
  7. "Groundwater Clean Up at Superfund Site." UC Davis News and Information. http://news.ucdavis.edu/search/news_detail.lasso?id=9392. February 5, 2010.
  8. "Biological Wastewater Treatment." Dartmouth College. http://engineering.dartmouth.edu/~cushman/courses/engs37/BioTreatmentTypes.pdf

Problems

1. What are the key ways to treat water that is sent out to the community for domestic use and which do you think is most important? Why?

2. What are the two types of wastewater that can be formed after the domestic use of water?

3. At a water treatment plant, where is water taken to be disinfected?

4. What do ion exchange and adsorption contribute to the overall purity of water?

5. Based on what you read about the negative effects of untreated water, give one example of each effect and explain how it could have been prevented.

Contributors

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