Technical Documentation: Principles and Influencing Factors of Aerobic Biological Treatment of Wastewater

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This post was last edited by hesonchang214 on 2009-11-29 23:48. The term “aerobic” refers to organisms that must have molecular oxygen (O2) present in order to carry out their normal physiological and biochemical reactions; this includes most microorganisms, animals, and humans； By “anaerobic” is meant organisms that are capable of carrying out normal physiological and biochemical reactions in the absence of molecular oxygen, such as anaerobic bacteria and yeasts. Biochemical reaction equations in the aerobic biological treatment process: ① Decomposition reaction (also known as oxidation reaction, catabolism): CHONS + O2 → CO2 + H2O + NH3 + SO42- + energy (elements constituting organic matter). ② Synthesis reaction (also known as anabolism): C, H, O, N, S + energy → C5H7NO2. ③ Endogenous respiration (also known as the auto-oxidation of cellular substances): C5H7NO2 + O2 → CO2 + H2O + NH3 + SO42- + energy. Under normal conditions, the composition of microbial cellular substances remains relatively stable, and it can generally be represented by the following empirical formula: Bacteria: C5H7NO2 ； Fungus: C16H17NO6 ； Algae: C5H8NO2 ； Protozoa: The interrelationship between the decomposition and synthesis of C7H14NO3: 1) The two are inseparable and interdependent ； a. The decomposition process provides energy and precursors for synthesis, while synthesis provides the material basis for decomposition ； b. The decomposition process is an energy-generating process, while the synthesis process is an energy-consuming process. 2) Both make significant contributions to the removal of organic matter ； 3) The amount of synthesis has a direct impact on the subsequent treatment of sludge (the cost of sludge treatment typically accounts for 40–50% of the total cost of an urban sewage treatment plant). The processes of biodegradation for different forms of organic matter also vary: on one hand, those with simple structures, small molecular sizes, and solubility can enter the cell wall directly ； Substances with complex structures, large molecular weights, or a colloidal or particulate form are first adsorbed by microorganisms; subsequently, under the action of extracellular enzymes, they are hydrolyzed and liquefied into small organic molecules, which then enter the cells. On the other hand: different chemical structures of organic compounds result in different degradation processes; for example, sugars ； Lipids ； Proteins II. Main factors affecting aerobic biological treatment ① Dissolved oxygen (DO): Approximately 1~2 mg/l ； ② Water temperature: It is one of the important factors; within a certain range, as the temperature rises, the rate of biochemical reactions increases, and so does the growth rate ； The components of cells, such as proteins and nucleic acids, are very sensitive to temperature; when the temperature rises or falls beyond certain limits, irreversible damage occurs ； Optimal temperature: 15~30°C ； Adverse effects occur at >40°C or < 10°C. ③ Nutrients: In cell composition, C, H, O, and N account for approximately 90–97% ； The remaining 3–10% are inorganic elements, with P being the main one ； Domestic wastewater generally does not require the addition of nutrients ； Some industrial wastewater requires this; generally, for aerobic biological treatment processes, N and P should be added in a ratio of BOD : N : P = 100 : 5 : 1 ； Other inorganic nutrients: K, Mg, Ca, S, Na, etc ； Trace elements: Fe, Cu, Mn, Mo, Si, boron, etc ； ④ pH value: The optimal pH for most aerobic microorganisms is generally between 6.5 and 8.5 ； When pH is below 4.5, fungi will become dominant, leading to sludge bulking ； On the other hand, microbial activity also affects the pH value of the mixture. ⑤ Toxic substances (inhibitors): Heavy metals ； Cyanide ； H2S ； Halogens and their compounds ； Phenols, alcohols, aldehydes, etc ； ⑥ Organic load rate: Organics in wastewater are originally food for microorganisms, but when there is too much of them, it can also be detrimental to these microorganisms ； ⑦ Redox potential: Aerobic bacteria: +300 ~ 400 mV, with a minimum requirement of +100 mV ； Anaerobic bacteria: a value of less than +100 mV is required; for strictly anaerobic bacteria, then