BACKGROUND The inhibitory effect of a moisture-absorbent sodium polyacrylate (SPA), that is commonly used in disposable diapers, on the bio-hydrogen (H-2) generation from waste diaper-like material (WDM) was assessed in dark fermentation studies. Three types of treatments were evaluated in batch bioreactors at 37 degrees C: mixture of bond paper and filter paper (WDM, no SPA), SPA alone (only SPA or control), and WDM spiked with SPA [paper plus sodium polyacrylate hydrogel (WDM-SPA)]. The WDM simulated the cellulosic fraction of diaper composition. RESULTS The units with only SPA did not produce H-2. The WDM-SPA units exhibited H(2)production 25% lower than those of bioreactors loaded with only WDM but no SPA. Cumulative H-2 production was 0.03, 4.22 and 5.50 mmolH(2) g(TS)(-1) for the SPA, WDM-SPA and WDM reactors, respectively (0.04, 5.90 and 8.77 mmolH(2) g(VS)(-1); 0.89, 132.19 and 196.45 NmLH(2) g(VS)(-1)). BioH(2) generation was related to predominance of the fermentation route that produces 1 mol L-1 of butyric acid (HBu) plus 2 mol L-1 H-2 in the WDM and WDM-SPA units, as suggested by the ratio A:B (acetic: butyric acids; A:B < 0.79 mg HAc-COD mg HBu-COD-1 in both cases). CONCLUSION It seems that this is the first documented result on the inhibitory effect of SPA on the bioH2 fermentation of cellulosic wastes. However, further research should be carried out at different levels of SPA to determine, for instance, the half-inhibition value of SPA, that is, the concentration of SPA that would cause a 50% drop in H-2 generation of the non-SPA treatment. (c) 2019 Society of Chemical Industry