What is Sewage strength?
The concentration of dissolved and suspended matter in sewage as measured by biochemical oxygen demand or suspended solids is referred to as sewage strength.
The two main properties that indicate the strength of sewage are Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD). Both parameters assess the wastewater's oxygen-demanding strength.
What is Chemical oxygen demand or COD?
The amount of oxygen required for the chemical oxidation of organic and inorganic chemicals present in wastewater using oxidizing agents such as potassium permanganate, potassium dichromate, and others is referred to as chemical oxygen demand (COD). The presence of COD allows for the rapid chemical oxidation of organic matter without the need for any additional equipment. This is the only method for determining the organic load in highly toxic sewage.
What is Biological oxygen demand or BOD?
BOD is defined as the amount of oxygen required by microorganisms in sewage for aerobic decomposition of biodegradable matter. This is the most commonly used parameter to assess the strength of municipal or organic wastewater quality. The standard BOD test determines the amount of oxygen required by microorganisms for the decomposition of biodegradable matter in a wastewater sample after 5 days of aerobic conditions at 20 degrees Celsius. It is measured in milligrams per liter.
Why are COD values greater than BOD values in wastewater treatment?
The higher the BOD/COD, the greater the oxygen stripping capacity of the discharged effluent when discharged into receiving waters (oxygen is used biologically/chemically to break down the organic matter) and the greater the potential for biological life damage in those waters.
COD is typically greater than BOD because more organic compounds can be chemically oxidized than biologically oxidized. This includes chemicals toxic to biological life, which can make COD tests very useful when testing industrial sewage because BOD testing will not detect them.
What causes high COD values in wastewater?
The BOD test takes a minimum of 5 days’ time and due to this it is not useful in the control of treatment processes.
An alternate test is the COD test which can be used to measure content of organic matter of both wastewater as well as natural water. COD can be determined only in 3 hours in contrast to 5 days BOD test. The COD test is specifically more suitable to measure organic matter present in industrial waste having compounds that are toxic to biological life. However COD results are generally higher than BOD values since the test will oxidize materials such as fats and lignin’s which are only slowly biodegradable.
COD rises as the concentration of organic matter rises!
It also rises in the presence of inorganic compounds susceptible to oxidation by the oxidant (typically dichromate). Water with a high COD level typically contains a high concentration of decaying plant matter, human waste, or industrial effluent.
How to reduce COD values in wastewater?
There are several tried-and-true methods for reducing COD in wastewater management. The following are two of the most common methods for removing COD from wastewater:
1. Separation of wastewater (coagulation and flocculation)
2. Microbial action.
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