Oyster shell-based alkalinization and photocatalytic removal of cyanide as low-cost stabilization approaches for enhanced biogas production from cassava starch wastewater

[Display omitted] •A low-cost arduino board system was built to monitor wastewater pH and temperature.•Oyster shell waste was used as alkalinizing agent to increase wastewater pH.•Cyanide present in wastewater was 73.02 % degraded using TiO2 as photocatalyst.•The pretreated wastewater was subjected to anaerobic digestion, increasing methane production.•Phytotoxicity evaluation was performed using Lactuca sativa seeds. 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 CN- to optimize biogas production. To control the acidity of the CSW we used natural oyster shells as source of CaCO3, and the photocatalytic degradation of CN− was 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 TiO2 photocatalyst tested in a degradation process under visible light was able to reduce CN− concentration by 73.02 %. After these pretreatments (pH stabilization and CN- degradation), 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.