BACKGROUND: Raw cheese whey originating from white cheese production results in a strong and complex wastewater excessively rich in organic matter (chemical oxygen demand, COD=28-65 g L-1), fatty matter (14-24.5 g L-1) and acidity (3.9-6.1 g L-1). It was treated in a three-stage configuration consisting of a pre-acidification (PA) tank and sequential upflow anaerobic sludge bed reactors (UASBRs) at 2.8-7 g COD L-1 day(-1) organic loading rates, during which the effects of effluent recycling at low rates and promoted SRB activity were investigated. Acidification, volatile fatty acids (VFA), COD and fatty matter removal and volatile solids were monitored throughout the system during the study. RESULTS: Recycling of the effluent promoted VFA and COD removal as well as pH stability in both stages of the UASBRs and the effluent where high alkalinity levels were recovered reducing alkali requirement to 0.05 g OH g(-1) CODapplied. Higher removal rates of 71-100 and 50-92% for VFA and COD were obtained by use of recycling. Fatty matter was removed at 63-89% throughout the study. Volatile solids build-up was significant in the inlet zones of the UASBRs. CONCLUSIONS: The system produced efficient acidification in the PA tank, balanced pH levels and an effluent high in alkalinity and BOD/COD ratio. Efficient VFA removal and solids immobilization was obtained in both stages up to the highest loading rate. Recycling improved the system performance under high fatty matter loading conditions. A major advantage of the sequential system was that the second stage UASBR compensated for reduced performance in the first stage. (C) 2010 Society of Chemical Industry