Cationic modified flocculant, synchronous flocculation of microalgae to recover phosphorus and preparation of biological fertilizer

The invention discloses a cationic modified flocculant. The cationic modified flocculant is obtained by modifying a polymeric flocculant with magnesium ions. The invention also discloses an application of the cation modified flocculant in flocculating microalgae and synchronously recovering phosphorus, a method for flocculating microalgae and synchronously recovering phosphorus by using the cation modified flocculant, the obtained flocculated microalgae, a microalgae biological fertilizer obtained by culturing the flocculated microalgae in a shaking table, and an application of the microalgae biological fertilizer. The cationic modified flocculant prepared by the invention has a strong flocculating settling effect on microalgae, the use amount of the flocculant is reduced, so that the production cost is reduced, the preparation method is simple in process, high in efficiency, short in time and free of environmental pollution, the nutrient content of soil can be increased, germination of pepper seeds and growt