Note that oxidation occurs in the presence of organic matter, so nitrate acts an electron acceptor in the absence of free oxygen. The chemical reaction in this phase can be represented as:
The precipitation of ammonia in the form of MgNH4PHO4 (MAP) through the addition of phosphoric acid and magnesium oxide is called MAP process. In this process the pH value is critical in the precipitation of MAP. It has 90% efficiencies (Siegrist, 1996). This is a physico-chemical process.
When the pH of wastewater is maintained to 10 is easy to convert NH4+ to free ammonia. Then, an air stream is used to capture NH3 and it is absorbed in a solution with sulphuric acid to form (NH4)2SO4. It has 97% efficiency, (Siegrist, 1996). This is a physico-chemical process called air stripping process.
Finally, biological phosphorus removal technique offer advantages over chemical processes, reduced energy consummation and reduce sludge production (Park, 1999). This method is justified because phosphorus exists in several forms in wastewater: orthophosphate, polyphosphate and organic phosphates. Note that phosphorus comes from human excrement and detergents presents in domestic wastewater.
- There are some microorganisms that without oxygen and nitrates (anaerobic atmosphere), assimilate and store fermentation products produced by other facultative bacteria. The microorganisms derive energy for this assimilation from polyphosphates organisms, which are hydrolyzed to liberate energy. The free phosphorus that results from the hydrolysis reaction is released to the mixed liquor (Sankaramanchi, 2010).
- In the following anoxic or aerobic phase these same microorganisms, consume both phosphorus and oxygen to metabolize the previously stored substrate for energy production and cell synthesis. The organisms take up the phosphorus in excess.