The aim of this research was to investigate the use of drinking water treatment plant sludge (DWTS) industrial waste by-products, as alternative environmental additives in soil improvement projects. For this reason, in the first stage, the optimum mixing ratio of DWTS was determined. In the second stage, different H/D ratios were evaluated in the investigation of appropriate reinforcing thicknesses in the improvement of clay soil + DWTS mixtures. A geogrid was placed between the two soil layers for separation in this stage. In the third stage, consolidation settlements were investigated before and after improvement with DWTS. In the last stage, the reinforcement mechanism of clay soil + DWTS mixtures were investigated using scanning electron microscopy. As a result, the optimum mixing ratio of DWTS was found to be 10%. The bearing capacity of clay soil increased 1.69 times using DWTS. The optimum reinforcement thickness was determined as H/D = 2.25. We found that consolidation settlements decreased by up to 62%. Thus, the experimental results showed that the use of DWTS as an additive for soil improvement was an economical and environmentally friendly approach.