Two-stage CSTR–UASB digestion enables superior and alkali addition-free cheese whey treatment

• A two-stage bioprocess was studied for anaerobic digestion of diluted cheese whey.
• Acidifying biomass recirculation resulted in complete carbohydrate fermentation.
• Optimum methane recovery and COD removal was achieved compared to single stage.
• The methanogenic reactor was stable at organic loading rates up to 20 kg m−3 d−1.

Different process configurations for the anaerobic treatment of cheese whey were comparably investigated under continuous operation. Three systems consisting of (i) a single-stage UASB (upflow anaerobic sludge bed) reactor, (ii) a CSTR (continuous stirred-tank reactor) coupled to a sedimentation tank and a subsequent UASB reactor and (iii) the same CSTR–UASB reactor scheme, although with biomass recirculation taking place from the sedimentation tank to the CSTR, were tested during three successive experimental periods (i.e. periods I, II and III, respectively). Acidifying biomass recirculation resulted in complete carbohydrate fermentation in the CSTR, while partial acidification occurred in the absence of biomass recycling. Fluorescent in situ hybridization (FISH) analysis revealed that acidifying biomass was dominated by bacteria rather than yeasts. The OLR (organic loading rate) in the UASB reactor highly correlated with the specific biogas production rate (p < 0.01), although biogas composition differed significantly among the three periods. The enhanced acidification system was capable to ensure up to 90% COD (chemical oxygen demand) removal in the UASB reactor and optimum methane yield values (i.e. 0.37 Nm3 CH4 kg−1 CODrem). Moreover, recirculation of the acidifying biomass eliminated alkali addition under any OLR tested. Two-stage configurations has been proved superior in terms of process efficiency and methane recovery as compared to the single-stage system, while the enhanced acidification process resulted in optimum operational and performance features. A case study of a two-stage anaerobic digestion system for a small-to-medium size cheese production plant indicated a net profit of 0.94 € m−3 CW from biogas production and valorization.