While lithium-ion batteries (LIBs) have been widely used for portable devices and electric vehicles, it is highly desirable to develop safer and less expensive batteries as alternative to LIBs. In this regard, zinc (Zn) batteries have attracted much attention because of their excellent safety and low cost. However, one of the challenges is to develop cost-effective and highly efficient cathode materials for Zn-ion batteries (ZIBs) based on transition metal oxides. It would be more economical to recycle transition metals in order to reduce the fabrication cost. Co-precipitation method is widely used for synthesis of LIBs cathode materials, and large amount of wastewater would be produced during co-precipitation and battery production process. In this presentation, we will report a facile and general process for fabrication of cathode materials for aqueous Zn-ion batteries (ZIBs) by reusing wastewater from co-precipitation method. We have selected manganese rich phases with different ratio of nickel to cobalt precursors from co-precipitation wastewater, followed by a simple ball milling process, resulting in metal-hydroxide cathode materials (Mn_0.6Ni_0.1Co_0.3O_xH_y, Mn_0.6Ni_0.2Co_0.2O_xH_y, and Mn_0.6Ni_0.3Co_0.1O_xH_y). Among them, the Mn_0.6Ni_0.1Co_0.3O_xH_y cathode (with mass loading of 15 mg cm^-2) exhibits an initial capacity of 263 mAh g^-1 at a current density of 0.1 A g^-1, as evaluated in an mixture electrolyte (2M ZnSO₄ and 0.1M MnSO₄). Furthermore, operando X-ray absorption spectroscopy analysis has revealed the role of each transition metal ions during insertion and desertion of Zn ions. It is found that the ratio of Ni to Co significantly influences ZIBs performances, providing important insight into rational design of more efficient cathode materials for aqueous Zn-ion batteries.