The overall nutrient removal mechanisms including organic carbon, nitrogen, and phosphorous removals are shown schematically in Figure 6. The carbonaceous nutrient and dissolved oxygen are absorbed by the microorganisms in the aerobic reactor, which grow to produce more biomass and release carbon dioxide (Figure 6, top left corner). Nitrogen removal involves two sequential steps (Figure 6, middle section in aerobic block and right section in anoxic block). The aerobic growth of autotrophic biomass consumes soluble carbon, ammonia, and dissolved oxygen to produce extra biomass and nitrates in solution. Occasionally, this step is further divided into two, one producing nitrite, and the second further oxidizing the nitrites to nitrates. The second major step is the anoxic growth of heterotrophic biomass, which uses the nitrites as a source of oxygen and produces extra biomass and nitrogen gas. The rest of Figure 6 (anaerobic block, bottom section of aerobic block and left section of anoxic block) depicts the mechanisms of phosphorus removal. The following four basic mechanisms for phosphorus removal are indicated in Figure 6:
• Fermentation of fermentable COD (SF) to VFA (SA), which can be utilized by the PAO microorganisms to store carbon as PHA (XPHA).
• Phosphorus release from PP (XPP) into solution at the same time as the VFA is converted to PHA.
Pretreatment and primary treatment
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