Water resources are an absolute necessity for the success of manned planetary expeditions. Advanced water recycling technologies abound, but a low-tech, unpowered, proven system should be considered in case of crucial system failure in a remote environment. In this investigation, slow sand filters were developed utilizing local materials and Martian simulant. They were tested at the Mars Desert Research Station, and compared to its hydroponic wetland filtration system. The tested sand filters evolved as schmutzdecke biological films formed as evidenced by decrease in hydraulic conductivity, improving effluent quality as indicated by reductions in turbidity and bacteria, through both physical and chemical filtration. Matrix-water chemical interaction was indicated by dissolution of carbonates from the filtrant; modifying pH and increasing total dissolved solids, alkalinity, and hardness. The slow sand filters proved more effective than the wetland system, yet the optimal solution was determined a combination of both strengths; wetland roughing pre-treatment and denitrification, and bacterial removal from the sand filter.