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WHAT HAPPENS TO WASTEWATER? Objectives: To explain to students what wastewater is, what are the pollutants and what wastewater treatment involves. QCC: S.8.6 Time Requirement: 1-2 class periods Background Information:Wastewater includes substances such as human waste; food scraps; oils; soaps; chemicals; and water from sinks, showers, bathtubs, toilets, washing machines and dishwashers. Homes, businesses, storms and industries produce wastewater. Wastewater is a source of water pollution. The major types of water pollutants can be divided in to six categories.
In the past, wastewater collected in the sewers was dumped directly into water bodies like rivers, lakes and oceans. It was originally thought that a natural process of purification would eliminate the pollution. First the sheer volume of water in the water bodies would eliminate the pollution through dilution, and secondly bacteria and other micro-organisms in the water bodies would decompose (consume) the sewage and organic matter. This may have been true if the wastewater pollution was on a very small scale. However, given today's population and greater volume of domestic and industrial wastewater production, we cannot rely on natural processes to clean up wastewater in the quantities we produce. The wastewater we produce must, therefore, be treated before we release it into waterways. The primary function of wastewater treatment is to speed up the natural processes by which water is purified by providing an optimal environment for micro-organisms to thrive and break down the wastewater into clean water. Micro-organisms eat away the organic matter contained in the wastewater. There are three different stages in the wastewater treatment: primary, secondary, and tertiary. Primary Treatment:Primary treatment is a physical process. At the treatment plant, first, the wastewater received from the sewer system flows through a screen, which removes large floating objects that might clog pipes or damage equipment. The removed solid stuff is called rag. The rag is spin dried and then taken to a rubbish dump. After screening, the sewage passes into a grit chamber, where cinders, sand and small stones settle to the bottom. After screening and grit removal, wastewater still contains organic and inorganic matter along with minute suspended solids particles. These suspended solid particles in the sewage are allowed to sink to the bottom by the process of sedimentation in a sedimentation tank, where the sewage may remain for 5 to 6 hours. This mass of solid deposit is called raw primary biosolids or sludge. The biosolids are removed from the sedimentation tanks by pumping, and may be further treated for use as a fertilizer or disposed of in a landfill or incinerated. Primary treatment removes 35-40% of the suspended material from the raw wastewater. Secondary Treatment:Secondary treatment is a biological process and is the heart of the treatment process. Secondary treatment of wastewater comprises conversion of biodegradable organic material using aerobic (oxygen) bacteria. Bacteria use the organic material as a food source, converting it into carbon dioxide and water. Secondary treatment removes 85-95% of the organic matter in sewage by making use of bacteria. The principal techniques used in secondary treatment are the trickling filter and the activated sludge process. A trickling filter is simply a bed of stones, more recently interlocking pieces of corrugated plastic or other synthetic media, which is 3 to 6 feet deep through which sewage passes. Bacteria gather and multiply on these stones and consume the organic matter in the sewage, making it cleaner. However, at present there is increasing use of the activated sludge process, which speeds up the work of the bacteria by bringing air and sludge heavily laden with bacteria into close contact with sewage in an aeration tank. The partially treated sewage from aeration tank goes to the sedimentation tank for removal of excess bacteria. The secondary treatment is completed by disinfecting the effluent (water to be discharged into aquatic system) to destroy pathogens (disease causing bacteria) prior to finally discharging it into an aquatic system. Effluent disinfection is done by treating it with chlorine, ozone or ultraviolet rays. Tertiary Treatment:In this stage chemicals that are not biodegradable (that can be decomposed by bacteria) are removed. Tertiary treatment is extremely expensive. Therefore, in most water treatment facilities secondary treatment is the terminal or final event. Today's pollutants, such as heavy chemicals, chemical compounds, and toxic substances from industrial plants are difficult to remove. An approach to this problem is to require pretreatment of industrial wastes, i.e. remove the troublesome pollutants at the beginning rather than at the end. Assessment:After the lecture, discuss with students:
INTERNET RESOURCES: How Waste Water Treatment Works City of Superior's Waste Water Treatment Waste Water Treatment Plant, Richmond Waster Water Treatment Works Waste water treatment water use A visit to a wastewater-treatment plant Recycling & Compost Trivia Maze (Game) |
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