Phosphorus Removal And Denitrification In Sewage Treatment Methods

Phosphorus removal and denitrification in sewage treatment methods:

Phosphorus removal: The main sources of phosphorus in urban wastewater are feces, detergents and certain industrial wastewater, which are dissolved in water in the form of orthophosphate, polyphosphate and organic phosphorus. Common phosphorus removal methods include chemical method and biological method. 

A. Chemical phosphorus removal: Phosphorus is removed from wastewater by reacting phosphate with iron salts, lime, aluminum salts, etc. to generate precipitates such as iron phosphate, calcium phosphate and aluminum phosphate. The characteristics of the chemical method are high phosphorus removal efficiency and stable treatment results. Sludge will not release phosphorus again during treatment and disposal to cause secondary pollution, but the sludge output is relatively large.

B. Biological phosphorus removal: Biological phosphorus removal is to remove phosphorus by using microorganisms to absorb excessive soluble phosphates in wastewater under aerobic conditions, and precipitate and separate. The entire treatment process is divided into two stages: anaerobic phosphorus release and aerobic phosphorus absorption.

After the wastewater containing excessive phosphorus and phosphorus-containing activated sludge enter the anaerobic state, the polyphosphate business in the activated sludge decomposes the accumulated polyphosphate into inorganic phosphorus and releases it back into the wastewater under anaerobic conditions. This is "anaerobic phosphorus release". 

In addition to a part of the energy generated by polyphosphate bacteria when decomposing polyphosphate for their own survival, the rest is used for polyphosphate bacteria to absorb organic matter in the wastewater, and is converted into acetic acid under the action of anaerobic fermentation acid-producing bacteria, and further converted into PHB (poly-short-butyric acid) and stored in the body. 

After entering the aerobic state, polyphosphate bacteria aerobically decompose the PHB stored in the body and release a large amount of energy, part of which is used for their own proliferation, and the other part is used for it to absorb phosphates in the wastewater and accumulate in the body in the form of polyphosphate. This is "aerobic phosphorus absorption". At this stage, the activated sludge continues to proliferate. In addition to a part of the phosphorus-containing activated sludge returning to the anaerobic tank, the rest is discharged from the system as residual sludge to achieve the purpose of phosphorus removal.

Denitrification: The proportion of various forms of nitrogen in domestic wastewater is relatively constant: organic nitrogen 50%~60%, ammonia nitrogen 40%~50%, and nitrogen in nitrite and nitrate 0~5%. They all come from the protein in people's food. There are two major methods of denitrification: chemical and biological. 

A. Chemical denitrification: including ammonia absorption and chlorination. 

a. Ammonia absorption: first adjust the pH value of the wastewater to above 10, and then desorb ammonia in the desorption tower

b. Chlorination: add chlorine to wastewater containing ammonia nitrogen. By properly controlling the amount of chlorine added, ammonia nitrogen in the water can be completely removed. In order to reduce the amount of chlorine added, this method is often used in conjunction with biological nitrification, first nitrification and then removing trace amounts of residual ammonia nitrogen. 

B. Biological denitrification: Biological denitrification is the process of converting organic nitrogen and ammonia nitrogen into nitrogen gas under the action of microorganisms, which includes two reaction processes: nitrification and denitrification. 

Nitrification is the process of converting ammonia nitrogen in wastewater into nitrite and nitrate by nitrifying bacteria (nitrite bacteria and nitrate bacteria) under aerobic conditions. Denitrification is the process of reducing nitrate nitrogen (N03-) and nitrite nitrogen (NH2-) to nitrogen gas by denitrifying bacteria under anaerobic conditions. Therefore, the entire denitrification process needs to go through two stages: aerobic and anoxic.