Biological Oxygen Demand (BOD)
It is also known as Biochemical Oxygen Demand (BOD). It is defined as the amount of oxygen consumed during the process of degradation and eventual stabilization of unstable organic substances by the biochemical activities of aerobic and other microbes. This degradation of the chemical complex is a desirable process and the final product is called stabilized wastewater. The aerobic bacteria consume oxygen during the process of oxidization of the organic and other oxidizable inorganic substances. The immensity of biochemical degradation depends on the population of bacteria. An actively growing population of bacteria will consume more oxygen to quickly decompose unstable complexes. Biological/Biochemical Oxygen Demand (BOD) is reduced with the decrease in the quantity of these complexes in the wastewater. Therefore, it can be surmised that BOD is directly proportional to the level of degradable chemical complexes; high concentration of chemical substances will result in the high BOD.
The BOD is a very useful measure of the efficiency of methods of wastewater treatment. A method in which amount of BOD reduced quickly is considered as most effective and efficient method. Therefore, exactly stabilized effluent, when discharge in the body of water, does not cause reduction of oxygen in the water.
Wastewater Disposal Methods
It is a well-known fact that the wastewater should be treated properly and effectively before its disposal into receiving water bodies. Disposal of wastewater may be accomplished with or without treatment.
Disposal of Wastewater by Treatment Methods
There are different methods available for the removal of microorganisms and stabilized the putrescible organic and inorganic chemicals in the wastewater. These methods are known as wastewater treatment methods. It is a very interesting fact that usually microorganisms are used to reduce the large burden of wastewater, which is organic matter. With few exceptions, wastewater treatment plants are integrated with physical, chemical and microbiological methods to concern with the different problems related to wastewater.
According to distinct types of treatment, they are divided into four types. Each type of treatment process has a special purpose, targeting the removal of all sources of materials and reducing the burden of microorganisms from the wastewater.
Primary Treatment of Wastewater
This process is mainly designed to remove the total solids from the wastewater by sedimentation and render it adequately free from pathogenic bacteria by chlorination. Initially, large objects are removed by bar screens from the wastewater flow. It removes a significant amount of particulate matter. The collected objects are then put into the grinder and released back into the wastewater flow.
The wastewater is then allowed to flow to a series of large primary settling compartments in which most of the organic matters and dense inorganic particles such as grit and sands are removed. Usually, there are two types of settling compartment, (a) grit compartment and (b) sedimentation tank or quiescent settling compartment. In grit compartment, wastewater flows very slow which permits large and heavy particulate matter to settle out. In the next step, the municipal and industrial wastes (particulate organic matters) in wastewater are removed in the sedimentation tank. In sedimentation tank, wastewater is allowed to stay for 1 to 3 hours during which most of the suspended organic matter settles out. The sedimented material is in the form of a semi-solid mass called sludge. The efficiency of sludge formation can be increased by the addition of various chemicals to coagulate the suspended particles which enhanced the sedimentation rate. The sludge is not allowed to remain in the bottom of sedimentation tank for a long period because of anaerobic bacteria produce gases during metabolism that tend to resuspend the settled material and increased the odor. Therefore, the sedimentation tank is equipped with scrapper mechanisms that occasionally removes the bottom sludge to a collection hopper. The underflow sludge becomes a waste product of the process. The remaining liquid portion of the wastewater which leaves the tank is called effluent.
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