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Applied Microbiology

Water Testing and Sewage Treatment

Water Testing

Water is tested for contamination with the use of indicator bacteria, most commonly fecal coliforms, and then examined for the presence of colony growth.

Water is vital to life, and it is also easily contaminated. Organisms such as E. coli and Legionella pneumophila (which causes Legionnaire’s Disease) can easily find their way into a water supply. Lack of access to clean drinking water is a problem affecting millions worldwide. Even in developed nations, water sources must be tested in order to ensure water is safe to drink. Water that is safe to drink is called potable water.

When testing water sources for contamination, it is often prohibitively expensive to test for every possible contaminant. Instead, tests for indicator bacteria are used. An indicator bacterium is a nonpathogenic bacterium used to estimate fecal contamination of water. Some common indicator bacteria are fecal coliforms, microbes found in the intestinal tracts of warm-blooded animals. While some of these are pathogenic, others are not. A common process for testing water for fecal coliforms is to filter the water and then apply the filter to a growth medium. After a few days, colonies are counted on the growth medium. For water to be considered potable, no colonies may be present. However, water used for other purposes, such as recreation, can have slightly higher counts. For example, in recreational bodies of water (such as lakes where people can swim), the federal standard in the United States says that 10% of samples taken over a 30-day period should not exceed 400 fecal coliforms per 100 milliliters of sample, with at least five samples taken during the 30-day period.

Filter Test for Water

In a water filtration test, potential microorganisms are isolated from water on a solid growth medium. After some time, bacterial colonies are analyzed and counted. For water to be potable, no bacteria should be present.
Water that is not potable can be made potable via various processes. These processes are used to make safe drinking water for cities and municipalities. Water is first screened to remove large debris, such as twigs and trash. If the source of the water is deep groundwater, this step is usually bypassed. The water is then treated to adjust the pH to 7, which is neutral. This reduces the chances that the water will corrode the pipes and tanks used in subsequent steps. Inorganic chemical coagulants are added at this point and cause the formation of aluminum and iron precipitates, solid particles that are removed from solution. As these compounds fall through the water, they absorb impurities, including silt, clay, and microorganisms such as algae and bacteria. This step is very important in removing waterborne pathogens, such as Giardia and Cryptosporidium. The coagulants cause chemical and physical reactions that draw suspended particles into clumps. The clumps then settle to the bottom of the tank in a process known as sedimentation. The water left on top is then filtered, usually through a sand filter. This filter often contains activated charcoal as a first filtration layer, which removes organic compounds that can affect the taste of the water. The water then passes through multiple layers of sand. The spaces between the grains of sand act as the pores of the filter, trapping particles while allowing the water molecules to pass through.

Sewage Treatment

Wastewater, or sewage, treatment involves three stages: the separation of heavy solids and liquids from water, the use of microorganisms to eliminate biological matter, and filtration and disinfection of the remaining water.

Water is commonly used to remove human and other waste from households and businesses. Wastewater of this type is called sewage, and it requires treatment before being released into the environment. Sewage does not encompass industrial wastewater, which must be treated prior to joining the sewage stream. Treatments for industrial wastewater vary according to the type of pollutants that exist in the water.

Sewage treatment is divided into primary, secondary, and tertiary treatment stages, which together make up a five-step process. Primary treatment involves steps 1 and 2, which entail filtering out large debris from the sewage and holding the sewage in a storage tank. There, heavy solids called sludge settle to the bottom of the tank while light materials such as oil float to the top. Sludge can be collected and treated for other uses, such as creating fertilizer. The remaining liquid is passed on to secondary treatment. Secondary treatment includes step 3 and involves the decomposition of biological matter, which is carried out by aerobic and anaerobic microorganisms. The microorganisms used for this process are indigenous to the region where the sewage treatment is carried out so that if they end up in the environment they do not cause damage. Because the secondary system relies on microorganisms, the treatment system is known as a bioreactor, an industrial system that relies on a biological process. Tertiary treatment includes steps 4 and 5 and represents any kind of treatment that follows secondary treatment, which varies by region and by the destination of the effluent (the treated water). This includes pH adjustment, disinfection, and desalination. The effluent is then released back into the environment.

Wastewater Treatment

Sewage treatment occurs in three stages: primary, in which biosolids are removed; secondary, in which microbes remove biological matter; and tertiary, in which final effluent is made ready to be reintroduced into the environment.
The solids generated in the primary treatment of sewage (sludge or biosolids) can contain pathogens and must be treated carefully. However, sludge can have benefits as well. In some locations, sludge is treated with microbes that use the sludge for anaerobic digestion, or the breakdown of biodegradable material by microorganisms without the use of oxygen. This converts the compounds in biosolids into a gaseous mixture of carbon dioxide and methane, called biogas. This biogas can replace natural gas in any application, such as a source of electrical energy or as heating.

In addition to sewage treatment, microbes can also be used to treat industrial wastewater. Some naturally occurring microbes, such as Alcanivorax borkumensis, have been found to break down oil. They are vital to cleanup efforts when an oil spill occurs. Genetic engineering has also produced microbes capable of breaking down plastics and other harmful compounds. These microorganisms provide a safe and effective means of keeping pollutants out of the environment.