Biological Water Treatment
Biological water treatment is used to clean wastewater containing organic material. In the decaying process of nature, bacteria and other microorganisms eat the organic materials as food. The biological water treatment plant speeds up the decaying process using microorganisms.
The 2 main biological treatment processes are the anaerobic and aerobic processes. Both use bacteria to consume and digest the waste contained in the water. Sludge is collected for disposal as solid wastes.
- Anaerobic Process
Anaerobic bacteria digest the organic matter in the absence of oxygen. Natural stagnant water or septic tanks use this method. The anaerobic fermentation produces gases like Carbon Dioxide, Hydrogen Sulfide, Methane, Ammonia, and Nitrogen.
- Aerobic Process
Aerobic bacteria digest the organic matter in Oxygen-rich water. Carbon Dioxide, Sulfur Dioxide, Nitrogen, and Ammonia gases are produced. To produce the Oxygen-rich water, air is pumped into aeration tanks. The aeration process increases the dissolved Oxygen content in the water.
When the aerobic process is working well, the bacteria multiply and increase their activity. New microorganisms and sludge floc is produced. The sludge is active.
Biochemical Characteristics
For all wastewater discharge, certain parameters have to be met. Temperature, pH value, suspended solids, heavy metals, and many others parameters have to comply with standards established on Environmental Regulations. For Biological wastewater, one of the most important parameters to be met is the Biochemical Oxygen Demand, or BOD.
Biochemical Oxygen Demand (BOD)
This is the most widely used parameter of organic pollution applied to waste water. It involves the measurement of the dissolved oxygen used by the microorganisms in the biochemical oxidation of organic matter. In that manner, the BOD is considered to be a measure of the organic content of the wastewater. (in mg / l)
The biochemical oxygen demand is determined after a 5-day period at 20-degree Centigrade when the oxidation is from 60 to 70 percent complete. The oxidation of 95 to 99 percent is considered complete after 20 days.
The value obtained for the BOD5 clearly indicates the concentration of oxygen in mg / l that has been consumed during 5 days and therefore indicates the organic content of the wastewater which has been consumed.
The higher the BOD5, the higher is the concentration in organic matter. Some regulations specify a 3-day period at 30-degree Centigrade for the test.
Chemical Oxygen Demand (COD)
The COD test also involves the measurement of dissolved oxygen used by the microorganisms in the chemical oxidation of organic matter. (in mg / l)
However, the oxygen equivalent of the organic matter that can be oxidized is measured by using a strong chemical-oxidizing agent. Therefore this test can be performed in 3 hours instead of 5 days of the BOD5.
The COD of wastewater is generally higher than the BOD because more compounds can be oxidized chemically than biologically.
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