The excess of fluoride ions (F−) in water for human supply is a serious public health. The recommended concentration of F− ions by the World Health Organization (WHO) is 1.5 mg L-1. Several groundwater sources around the world contain high F− concentrations, and require treatment before human consumption. It was developed an eco-friendly and low-cost strategy for groundwater defluorination, i.e., adsorption onto calcinated sludge. This strategy was efficient at pH of 5.5 and using 5 g L-1 of calcinated sludge. The groundwater attained the WHO standard within 60 min. The kinetic model of pseudo-second-order obtained a better adjustment to the experimental data. The equilibrium curve at 25 °C was better represented by the Tóth model. The maximum adsorption capacity was 2.04 mg g-1. Therefore, adsorption using calcinated sludge can be considered as an eco-friendly and low-cost strategy for groundwater defluorination.