A method of biological treatment is developed for completely removing the compost remaining after decomposing the dissolyed organic matter (COD) in wastewater by the bacteria or fungi. It comprises the steps of: adopting the algae and diatoms cultivated under the special conditions to treat wastewater containing excreta, applying Calcium Salt and Silica Salt to develop a competence cell in the algae and diatoms, removing insoluble composts after decomposing the dissolyed organic matter (COD), removing pollutants in the wastewater by flowing through a series of multi-stage filtering tanks consisting of 3.about.1,000 stages each 4.about.8 inches in width and sequentially treating the wastewater through the algae and diatoms cultivated under different conditions, improving the cultivation efficiency of the algae and diatoms by installing artificial illumination, insulating the cultivation place of the algae and diatoms to maintain optimum temperature, and stimulating the algae to photosynthesize with nitrate-gas without forcibly expelling to the air.