Cassava starch wastewater (CSW) poses a high polluting potential due to its high organic loading and cyanide (CN-) concentration, but this residue can be pretreated and reused. The present work proposes stabilizing the CSW pH and degrading CNto optimize biogas production. To control the acidity of the CSW we used natural oyster shells as source of CaCO3 , and the photocatalytic degradation of CNwas achieved with Degussa P25 TiO2 . Natural oyster shells raised pH from 4.5 to 6.2 over 6 h of reaction, efficiently controlling the effluent acidity. After pH stabilization, the TiO 2 photocatalyst tested in a degradation process under visible light was able to reduce CNconcentration by 73.02 %. After these pretreatments (pH stabilization and CNdegradation), the CSW was inoculated with sewage sludge (SS) to produce biogas. The pretreatments were proved to be efficient at favoring biogas production as this was heightened by 27.6 %. In addition, the pretreated CSW and digestate (anaerobic digestion) significantly reduced the toxicity of the effluent, assessed by investigating lettuce seeds (L. sativa) germination and root growth. Thus, pretreatments and reuse of residues may potentially provide socio-environmental and economic benefits. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.