The global demand for clean and safe drinking water is growing exponentially. To meet this demand, a large amount of sludge is formed in water treatment plants during the treatment of surface water. Sludge from these drinking water treatment plants (DWTS) is generally disposed of as non-hazardous waste in landfills. However, DWTS needs to be managed in an economically sustainable and environmentally friendly manner. In line with this need, one of the disposal methods of waste is its use in soil improvement. Evaluation of DWTS, in this context, is appropriate both economically and environmentally. For this purpose, in this study, the usability of DWTS in soil improvement was investigated. The California Bearing Ratio (CBR) of the mixture was determined by adding 5%, 7.5%, 10%, 12.5% and 15% DWTS to the clay soil, respectively. Depending on the rate of DWTS added to the clay soil, up to 75% improvements were observed in CBR values. In addition, the optimum ratio of DWTS in terms of CBR in the improvement of a clay soil was determined as 12.5%. As a result, DWTS is considered to be an alternative waste that can be used for soil improvement in geotechnical engineering.