Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) as biological and chemical pollutants of wastewater are renowned environmental problems. A study of the performance of BOD and COD degradation via adsorption was undertaken using Bio-derived ZnO nanopowder (ZnO-NP), synthesized using leaf extract of Spinacia oleracea, and Zn (NO3)2 at 500 0C, following a simple and green approach. XRD, SEM, FTIR, EDS and BET analysis were used to characterize the nano-adsorbent. EDS spectrum recorded elemental weight compositions of 67.33% and 32.67% for Zn and O, while the FTIR absorption peaks revealed the presence of Zn-O–H and Zn-O. Surface area analysis revealed the mesoporous structure of the ZnO-NPs. The reduction efficiency of the ZnO-NPs was evaluated in the presence of raw tannery wastewater by application of treatment time and adsorbent dosage as parametric factors; results obtained were compared to environmental regulatory limits (WHO and NESREA). Contact times reported removal efficiencies of 81.5-90.8 % and 12.8-50 % for BOD and COD, while adsorbent dosage-influenced BOD and COD removal at an optimum contact time of 30 minutes was found to be 79.2 - 97.5 % and 27.8 to 55.6 % respectively. This study reveals that ZnO nanopowder is better applied as BOD reductants over COD for spent tan liquor.