Solid-liquid separation of the wastewater is very difficult because of high viscosity and high SSconcentration. In this study, the effectiveness of preozonation on improving the settleability of the solids in wheatand sweet potato wastewaters was investigated using a bench-scale system and pilot treatment system respectively.Results showed that solid-liquid separation in the wheat wastewater was greatly improved by the decanter in thesystem(SS reduction from 24100 mg/L to 100 mg/L). However, preozonation practice did not show a significanteffect on solid-liquid separation of the sweet potato wastewater. Effect of preozonation on solid-liquid separationbetween wheat and sweet potato wastewater showed different.