The oyster shells produced at shucking sites contain various organic and inorganic impurities and are discharged without proper pretreatment. In the present study, we confirmed a process for removing organic/inorganic impurities and salts from abandoned shells using a wet type of trommel. A particle size analysis showed that the inorganic impurities in the studied shells were Si, Fe, Al, etc. derived from sediment, and their contents were relatively rich in fine particles. Dissolved ions (Cl−, SO42−, and NO3−) and organic matter (carbon, nitrogen, and phosphorus) can be removed concurrently for a short period (<5 min) by a trommel. The higher removal efficiencies for total organic carbon (TOC, ~93.4%), total nitrogen (TN, ~59.9%), and total phosphorous (TP, ~92.9%) in the washing wastewaters after filtration not only indicate increased organic matter in the fine particles derived from crushed shells during trommel operation, but also that a certain portion could be removed by precipitation. However, considering that a small amount of salt compounds (Na and Cl) still remains in the shell even after calcination followed by washing, the scope of shell use must be considered.