The researchers studied how changing the time wastewater stays in a treatment tank affects its cleaning process. They used a special method with low oxygen levels to treat wastewater from a specific source. Results showed that keeping the wastewater in the tank for 4 to 12 hours led to the best removal of pollutants like COD and TN. The system was able to clean the water to meet national standards, with a 99% removal rate for NH4+N and a 67% removal rate for TN. However, when the water stayed in the tank for only 4 hours, the removal rates dropped significantly.