The anoxic/aerobic process is a sewage treatment method that connects the front anoxic section with the rear aerobic section in series .
How does A/O works?
The A/O process hydrolyzes organic matter such as starch, fiber, and carbohydrates in sewage through heterotrophic bacteria in the anoxic section to improve its biodegradability. At the same time, pollutants such as protein and fat are ammonified to release ammonia. In the aerobic section, the nitrification of autotrophic bacteria oxidizes ammonia into nitrates. Through reflux control, these nitrates return to the anoxic section and are reduced to molecular nitrogen by the action of denitrifying bacteria, completing the cycle of C, N, and O in the ecosystem and achieving harmless treatment of sewage.
Specific process
Anoxic section (Section A): In the anoxic section, heterotrophic bacteria hydrolyze large molecular organic matter (such as starch, fiber, carbohydrates, etc.) in sewage into small molecular organic matter, improving the biodegradability of sewage. At the same time, heterotrophic bacteria ammonify pollutants such as protein and fat, releasing ammonia (NH3, NH4+).
Aerobic Section (O Section): In the aerobic section, autotrophic bacteria oxidize ammonia into nitrates (NO3-) through nitrification. These nitrates are returned to the anoxic section through reflux control.
Denitrification: Under anaerobic conditions, denitrifying bacteria use organic matter in sewage as electron donors to reduce the returned nitrate into nitrogen gas (N2), completing the nitrogen cycle and achieving denitrification.
What are the advantages of A/O ?
1) The denitrifying bacteria in the anoxic pool can be fully utilized, reducing the organic load in the aerobic pool;
2) Efficient degradation : The A/O process has a high removal efficiency for organic matter and ammonia nitrogen. The total residence time is greater than 54 hours, the COD value can be reduced to below 100 mg/L, and the total nitrogen removal rate is above 70%.
3) Simple process: This process uses organic matter in wastewater as the carbon source for denitrification. There is no need to add expensive carbon sources such as methanol, which saves investment and has low operating costs.
4) Strong load impact resistance: A/O process is adaptable to various environmental conditions and has strong load impact resistance.
What kind of effect can the A/O process achieve?
Highly efficient removal of organic pollutants: The A/O process decomposes high-concentration organic matter into small-molecule organic matter through the action of anaerobic microorganisms in the anaerobic section, and then further oxidizes it into carbon dioxide and water through aerobic microorganisms in the aerobic section. The chemical oxygen demand (COD) removal rate can generally reach more than 85%.
Effective denitrification: The A/O process releases ammonia nitrogen through the anaerobic stage, performs nitrification in the aerobic stage, and then reduces nitrates to nitrogen gas through denitrification. The ammonia nitrogen removal rate can usually reach 70%-90%, and the treated effluent can meet general emission standards or agricultural irrigation water requirements.
Partial phosphorus removal: The A/O process removes phosphorus by releasing phosphorus in the anaerobic stage and absorbing phosphorus in the aerobic stage, but its phosphorus removal efficiency is not as good as that of a dedicated phosphorus removal process, and the phosphorus removal efficiency is usually between 20%-30%.
What factors may affect the effect of A/O process ?
( 1 ) MLSS sludge concentration, its value should be greater than 3000 mg/L to ensure the denitrification effect;
( 2 ) DO value: the anoxic zone should be controlled below 2 mg/L, while the aerobic zone should be within the range of 2-4 mg/L;
( 3 ) TKN/MLSS load rate. In nitrification reaction, its value should not exceed 0.5gTKN/(gMLSS·d) to ensure stable operation of the process.
(4) BOD/MLSS load rate: This index should be controlled within 18kgBOD/(gMLSS·d) to ensure stable operation of the process.
(5) Mud-water mixture return ratio: The size of the return ratio R has a direct impact on the denitrification effect. The larger the R value, the better the denitrification effect, but at the same time, the operating power consumption will also increase accordingly.
(6) Anoxic tank BOD/N value: To ensure a good denitrification effect, the BOD/N value should be greater than 4, otherwise the denitrification rate will decrease rapidly.
(7) pH value: The optimal pH value range for nitrification reaction is 0-4, while the optimal pH value range for denitrification reaction is 5-5.
(8) Temperature: The nitrification reaction is best carried out at a temperature of 20-30°C. The reaction almost stops at a temperature below 5°C. The denitrification reaction is best carried out at a temperature range of 20-40°C. The reaction rate drops rapidly at a temperature below 15°C.
