Seaweed industries are producing solid waste composting of about 60-70% w/w fiber, which is rich in cellulose. Conversion of cellulose-rich waste into sugar and ethanol can provide a zero-waste industry. The objectives of this study were to characterize the solid waste biomass (SWBS) of agar processing industries and convert it into sugar and ethanol. The chemical composition and morphology of SWBS samples from three agar extraction plants (SWBS EP1, SWBS EP2, and SWBS EP3) were examined. Results showed that the SWBS had 3.01-16.4% cellulose, hemicellulose of 3.41-11.53%, lignin of 5.49-6.45%, agar of 7.28-17.03%, and ash content of 59.18-78.75%. X-ray diffraction (XRD) analysis supported the Fourier transform infrared ( FTIR) result while revealing that the filter aid of the SWBS EP3 sample was not the same with the two others. SWBS EP3 sample contained more biomass and less filter aid, and it was selected to convert into sugar and ethanol in a 250-L tank. The concentration biosugar reached 21.45 g/L with the addition of the cellulase after the agarase (A24S24), and 15 g/L when the cellulase was mixed together with the agarase (AS24). The highest ethanol production was obtained when agarase and cellulase were added simultaneously for 24-h saccharification followed by yeast fermentation.