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Microbes and the Environment

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Scenarios 1: Characterize the differences among untreated, primary-treated, and secondary-treated sewage and compare and explain the effects of dumping each type on the eutrophication of a pond and a fast-moving river.

Answer 1:
The general idea of wastewater treatment plants is to decrease the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) in the runoff into the lakes and streams, and do so by meeting the standards of the state and federal government. The purpose of the treatment plants is to feed oxygen and organic waste to bacteria and other microorganisms (Cooke, n.d.).

The untreated sewage waters frequently contain high levels of organic matter from industrial, agricultural wastes and from human wastes. It is a necessity to get the organic matter removed by the process of waste water treatment. Pathogens like bacteria, protozoa and viruses are all in the untreated water (Untreated Sewage, 2014).

The primary sewage treatment removes 25-35%of the BOD that is present in particulate form and thus there will be a slight increase in dissolved oxygen. In this treatment, particulate material is reduced by screening, precipitation of small particulates, and settling in basins or tanks. The resulting solid material is usually called sludge (Tortura, 2013, p. 789). Secondary sewage treatment promotes the biological transformation of dissolved organic matter to microbial mass and carbon dioxide. They remove 75-95% of the BOD and many bacterial pathogens are removed by this process which will ultimately increase dissolved oxygen. Under some toxic conditions, the dissolved organic matter will be transformed into additional microbial biomass plus carbon dioxide. When the microbial growth is completed, under ideal conditions microbes will aggregate and form a stable structure which can then be removed (Cooke, n.d.).

When the untreated waste is dumped into a pond or fast-moving river, there is less dissolved oxygen and the waste content is high. This causes the rate of eutrophication from dumping untreated water to have high levels of nitrates and phosphates, and then the microbes can decompose the waste matter. When the primary treated water is dumped into the pond water, then the rate of eutrophication is less than the untreated water due to the increase in dissolved oxygen and decrease in waste matter (Cooke, n.d.).

The rate of eutrophication is lowest in the secondary treatment water due to a decrease in the waste matter. This then leads to the decrease in the availability of waste to be acted upon by microbes and the dissolved oxygen also increases (Muir, 2012).

When the dissolved oxygen is higher in fast-moving water as compared to that in pond water due to aeration which occurs when the river flows, this causes the rate of eutrophication to be higher in fast moving water (Muir, 2012).


Cooke, R. L. (n.d.). Lesson 1: Wastewater Treatment. Mountain Empire Community College. Retrieved from http://water.me.vccs.edu/courses/env108/Lesson1_print.htm

Muir, P. (2012, October 29). Eutrophication. Retrieved from http://people.oregonstate.edu/~muirp/eutrophi.htm

Tortora, G., Funke, B., Case, C. (2013). Microbiology: An Introduction. 11th Ed. Pearson. www.pearsonhighered.com Untreated Sewage. (2014). Untreated Sewage. Retrieved from http://www.worstpolluted.org/projects_reports/display/63

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