Due to the shortage of water resources in China, the state has implemented a series of rainwater harvesting projects. The safety of water quality cannot be guaranteed due to the lack of an effective construction, running, and management system. Slow filters are low-maintenance systems that do not require special equipment. In order to improve the performance of SSF in terms of the removal of bacteria and solid granules, e.g., the microorganisms attached to the surface of a single grain of the filtering material under a scanning electron microscope (50×) have been studied. Based on the improvements of conventional slow sand filtration (SSF), the bio-slow sand filtration method has effectively mitigated and helps to remove bacteria and other microbiological contaminants, as well as heavy metals, ammonia, nitrogen, organic material, and turbidity of the harvested rainwater. The removal efficiency of bioslow sand filtration was approximately 20-30% on particulate organic carbon, above 95% on ammonianitrogen, and better than 96%, 95%, 95%, 80%, 70%, and 60% on Cu²⁺, Cd²⁺, Fe²⁺, Zn²⁺, Mn²⁺, and Pb²⁺, respectively. The effluent quality meets the requirements of “standards for drinking water quality” in China. The result indicated the bio-slow sand filtration method could achieve better water quality results as an available water treatment technology.