COD (Chemical Oxygen Demand), ammonia nitrogen, total phosphorus, and total nitrogen are common pollutant indicators in wastewater treatment. These contaminants are removed through different biological, chemical, and physical processes in different treatment units. Here's an in-depth analysis of these contaminant removals:
1.COD (Chemical Oxygen Demand):
COD is a measure of the amount of organic matter in water. In wastewater treatment, COD removal is mainly achieved through biodegradation, adsorption, and chemical oxidation.
Biodegradation: Under aerobic conditions, bacteria and other microorganisms oxidize organic matter into CO2 and H2O, releasing energy. Under anaerobic conditions, microorganisms convert organic matter into CH4 and CO2 through fermentation and methanogenic processes.
Adsorption: Activated sludge and biofilm are able to adsorb organic matter in water and convert it into biomass.
Chemical oxidation: In an advanced oxidation process, chemical oxidants such as ozone, hydrogen peroxide, or potassium permanganate are used to oxidize organic matter.
2.Ammonia nitrogen:
Ammonia nitrogen refers to the sum of ammonia (NH3) and ammonium ions (NH4+) in water. The removal of ammonia nitrogen is mainly achieved through nitrification and denitrification processes.
Nitrification: Under aerobic conditions, ammonia nitrogen is first oxidized to nitrite nitrogen (NO2-) by ammonia-oxidizing bacteria (NOB), and then further oxidized to nitrate nitrogen (NO3-) by nitrite-oxidizing bacteria (NOB).
Denitrification: Under anoxic conditions, nitrate nitrogen is reduced to nitrogen (N2) by denitrifying bacteria, which is released into the atmosphere.
3.Total phosphorus:
Total phosphorus includes dissolved inorganic phosphorus, dissolved organic phosphorus, and particulate phosphorus in water. The removal of total phosphorus is mainly achieved by chemical precipitation and bioassimilation.
Chemical precipitation: In the process of sewage treatment, metal salts (such as aluminum salts or iron salts) are usually added to react with phosphate ions (PO4 3-) to form insoluble phosphate precipitates, which are removed by sludge discharge.
Bioassimilation: Microorganisms in activated sludge absorb phosphorus from water for cellular synthesis and energy metabolism, which is subsequently removed by sludge discharge.
4.Total Nitrogen:
Total nitrogen refers to the sum of ammonia, nitrite, nitrate, and organic nitrogen in water. The removal of total nitrogen is mainly achieved through nitrification, denitrification, and bioassimilation processes.
Nitrification and denitrification: As mentioned earlier, the nitrification and denitrification processes convert ammonia nitrogen to nitrate nitrogen and then nitrate nitrogen to nitrogen, respectively.
Bioassimilation: Microorganisms absorb nitrogen from water for the synthesis of proteins and other cellular components, which are subsequently removed by sludge discharge.
In the process of sewage treatment, in order to improve the removal efficiency of these pollutants, it is usually necessary to use a variety of treatment technologies and strictly control the process parameters, such as dissolved oxygen, pH value, temperature, sludge age, etc. At the same time, it is also necessary to regularly monitor the quality of the inlet and outlet water, and adjust the process operation strategy according to the monitoring results to ensure the stable operation and good treatment effect of the sewage treatment system.