Why can the C/N and C/P ratios for nitrogen and phosphorus removal not follow the nutrient ratio?

The last edit to this post was made by Gorō’s Eight Trigrams Staff on 2015-6-1 at 19:09. C:N:P=100:5:1 refers to the ratio of nutrients required by microorganisms; in other words, 100 points correspond to carbon (usually referring to BOD5, rather than organic carbon TOC). 75 points are used for the oxidation of carbon into carbon dioxide and water, as well as for the energy needed for bacterial growth and reproduction, while 25 points are utilized for the synthesis of new microorganisms through bacterial growth and reproduction； The cellular composition of microorganisms is C:N:P at 25:5:1, and the final ratio becomes C:N:P=100:5:1; in other words, to degrade 100 units of carbon, at least 5 units of nitrogen and 1 unit of phosphorus are required. If N and phosphorus are in excess, it has little impact on bacterial growth and reproduction, as well as on the degradation of organic matter to obtain energy ； However, excessive nitrogen leads to a too low C/N ratio, which deteriorates the sedimentation properties of the activated sludge and thus affects its proper functioning ； Furthermore, if the BOD5/TN ratio is less than 5, insufficient carbon sources hinder denitrification; similarly, a lack of carbon sources also hinders biological phosphorus removal ； Furthermore, excessive ammonia nitrogen and the resulting nitrate salts can act as toxins to the activated sludge, affecting its normal functioning.