What is biological wastewater treatment?

Biological wastewater treatment (BWT) is the use of microorganisms in the processing and cleansing of wastewater. Many microorganisms are able to metabolize a variety of organic and inorganic substances that are present in wastewater. BWT takes advantage of this property and supports it with various nutrients and aerators. The wastewater can come from a variety of sources, including factories, cities, or even small residential areas and usually contains solid biomasses, various objects, microorganisms, and other contaminants. The wastewater undergoes multiple treatments to remove these contaminants and neutralize any hazardous residuals.

What contaminants are present in wastewater?

Wastewater can contain many contaminants, both natural and artificial. The amount of contaminant is rated based on the biochemical oxygen demand, or BOD. BOD is the measure of the amount of oxygen required for microorganisms to process the waste in five days. This amount increases with contaminant density. Some common contaminants of wastewater are heavy metals and minerals. Organic compounds can also be found along with pathogens and inorganic chemicals. In residential wastewater, various items such as sand, gravel, food, sticks, and toys can be found. All of these contaminants must be removed before the wastewater is deemed safe.

What processes are involved in biological wastewater treatment?

There are three main processes involved in biological wastewater treatment. The first step is preliminary treatment. In this step, all large solids are filtered out of the wastewater using screens or other large filtration systems. If industrial wastewater is being processed from a plant, there may not be need for a preliminary screening step. Once the initial filtration is performed, primary treatment can begin. During the primary treatment, smaller solids are separated from the water. This step is usually performed in static tanks which allow solids to settle out of solution. These solids, known as sludge, are processed using vacuum filters, filter presses, and centrifuges in order to remove smell and decompose the matter. After all solids are removed, secondary treatment can begin. This is the step of major biological treatment. Microorganisms are introduced to the water and consume any remaining contaminants. After this step, the water is tested to determine its safety, and if all contaminants have been successfully removed, it is released or reused.

What is involved in the biological treatment stage?

The basic mechanism of biological treatment is the metabolism of organic material. Microorganisms introduced to the material metabolize it to a non-hazardous cellular mass which can be settled out and removed. The most important factor in biological treatment is oxygen supply. The oxygen supply must be efficient enough to support the microorganisms throughout the entire process. This is accomplished by aeration, or the addition of oxygen to the system. Luckily, microorganisms, specifically bacteria, are able to adapt to changing temperature and material compositions. This saves the effort of temperature control and material monitoring and manipulation. After the material is broken down, the residual cell material is removed and disposed of using a clarifier.

What types of biological wastewater treatment are available?

There are two main types of biological wastewater treatment, mechanical agitation and non-mechanical agitation. One method that employs mechanical agitation is the activated sludge process. In this process, two tanks are utilized. One tank is equipped with perforated pipes on the bottom and the other is a static tank. The pipes in the first tank are useful for both aeration and agitation. The bubbles offer sufficient oxygen for bacteria and also agitate the water, forcing more bacteria-waste interaction. After processing the wastewater in this tank, it is moved to the static tank where no agitation is involved. The cell masses will separate and some will be replaced in the first tank to seed the next process. The rest will be killed by deprivation of oxygen and distributed as a soil-like mass. This method is very efficient but somewhat costly. Substantial amounts of energy are required to power the pumps which supply air to the tanks.

Another method uses a trickling filter. The trickling filter is composed of a rotating arm which sprays wastewater over a large rock bed. The rock bed generally has an array of fungi and algae which consume the organic matter. Worms and insects may also be present, which aid in the removal of organic matter. The wastewater is passed over the filter bed several times before being considered clean and the microorganisms are regularly removed and disposed of. This method is exceptionally effective due to the fact that the water is applied in thin sheets and oxygen supply is not an issue. To further increase efficiency, plastic nets are being implemented, which reduce the equipment size and increase active surface area.

A third mechanically stimulated method uses biological contactors. These are large thin disks mounted on rods which are covered with bacteria. The disks are rotated partially submerged to allow for contact with the organic matter. This accomplishes both aeration and agitation. The costs of such an operation include the power consumption by the rotation motors. However, because the disks have such a large surface area, they are very efficient for the process.

Non-mechanical agitation systems employ the ecosystem for processing. Water is moved using gravity through natural areas with vegetation and other organisms where the organic matter is consumed by the inhabitants. This system uses little to no energy, suppresses odor, and can be incorporated in most landscapes without issue. Unfortunately, pathogens are still a threat and this process is not as fast as artificial systems. Also, this is not feasible in cold regions due to inhibited microbial growth.


References

http://ohioline.osu.edu/aex-fact/0768.html

http://www.water.siemens.com/SiteCollectionDocuments/Industries/Hydrocarbon_Processing/Brochures/CE_10-05_biological_trt.pdf

http://engineering.dartmouth.edu/~cushman/courses/engs37/BioTreatmentTypes.pdf

http://www.water.siemens.com/SiteCollectionDocuments/Industries/Hydrocarbon_Processing/Brochures/CE_10-05_biological_trt.pdf




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