The activated sludge process is the most used biological wastewater treatment method in the world. Sludge bulking is one of the most common problems relateing to activated sludge wastewater treatment, and this fact causes poor settling in secondary sedimentation tank. Sludge volume index(SVI) and the zone settling velocity(ZSV) are parameters to quantify the settleability of activated sludge . Assessment of influence of walnut shell as an agricultural waste, powdered activated carbon as a common adsorbent and clay as a mineral on the activated sludge settling properties was the principal aim of this research. Second phase of Shahinshahr Wastewater Treatment Plant was selected for wastewater sampling and conducting experiments of this research. SVI tests were done in a 1-Lit graduated cylinders and settling column used for determining of ZSV was with 200 cm height and 20 cm diameter. Average values of SVI for control sample was 823 mL/g, optimum dosages of walnut shell, clay and powdered activated carbon reduced SVI to 100 mL/g (88 percent decrease). Average values of ZSV for control sample was 1.222 m/h, different additives increased ZSV of control sample to 2-3 times. This increase in ZSV is important because hydraulic surface overflow rate(SOR) is also increased without loss of their efficiencies. Sludge return with these additives caused an improvement in sludge settleability. Jar tests revealed that these additives mainly act as ballasting agents rather than as flocculants and this mechanism is responsible for decreasing SVI. Walnut shell and clay, the same as activated carbon, improved the sludge settleability. Activated carbon is an expensive adsorbent and using this material is costly, while walnut shell and clay are cheep and natural. Therefore, walnut shell and clay can be used as alternatives of activated carbon.