Explanation: The retention time considered for the anaerobic zone for the AO process for removing phosphorus is 0.5-1.5 hrs. This process involves aerobic oxidation followed by anaerobic digestion. This is a very effective method to remove phosphorus compounds.

Explanation: The retention time considered for the aerobic zone for the AO process for removing phosphorus is 1-3 hrs. This process involves aerobic oxidation followed by anaerobic digestion. This is a very effective method to remove phosphorus compounds.

Explanation: The retention time considered for the anaerobic zone for the A2O process for removing phosphorus is 0.5-1.5 hrs. This process is a modification process of the AO process. This is a very effective method to remove phosphorus compounds.

Explanation: The retention time considered for the aerobic zone for the A2O process for removing phosphorus is 4-8 hrs. This process is a modification process of the AO process. This is a very effective method to remove phosphorus compounds.

Explanation: The retention time considered for the anoxic zone for the A2O process for removing phosphorus is 0.5-1 hrs. This process is a modification process of the AO process. This is a very effective method to remove phosphorus compounds.

Explanation: The retention time considered for the anoxic zone for the University of Cape Town treatment process for removing phosphorus is 2-4 hrs. The MLSS level maintained is 3000-4000 mg/L. The internal recirculation considered is 200-400%.

Explanation: The retention time considered for the anaerobic zone for the University of Cape Town treatment process for removing phosphorus is 1-2 hrs. The MLSS level maintained is 3000-4000 mg/L. The internal recirculation considered is 200-400%.

Explanation: The retention time considered for the aerobic zone for the University of Cape Town treatment process for removing phosphorus is 4-12 hrs. The MLSS level maintained is 3000-4000 mg/L. The internal recirculation considered is 200-400%.

Explanation: The retention time considered for the aerobic zone for the Virginia Initiative Plant treatment process for removing phosphorus is 4-6 hrs. The MLSS level maintained is 2000-4000 mg/L. The internal recirculation considered is 100-200% for the anoxic tank.

Explanation: The retention time considered for the anaerobic zone for the Virginia Initiative Plant treatment process for removing phosphorus is 1-2 hrs. The MLSS level maintained is 2000-4000 mg/L. The internal recirculation considered is 100-200% for the anoxic tank.