In the study, we examined if anaerobic degradation of raw sludge (RS) with addition of whey produces more biogas then anaerobic degradation of RS. The RS is a mixture of primary and secondary sludge taken from a wastewater treatment plant Domžale -¬ Kamnik. We wanted to find out the method of dosing and the maximum amount of added whey to the anaerobic bioreactors to keep the process stable and without adversely affecting machine densification. We checked whether the increased biogas production at the expense of whey increases the proportion of methane present in biogas. The study was carried out with two pilot bioreactors, the volume of which was 197 L. We found that anaerobic decomposition of the whey was stable. In the experimental reactor (R2), the production of biogas increased in the first five periods of adding 1 L/day of whey by 172 % and in the next five periods of adding 2 L/day of whey by 200 %. With the increased production of biogas in the R2, the proportion of methane in biogas did not increase, but it even declined. When adding 1 L/day of whey, the methane content decreased by 2 % and added 2 L/day of whey it decreased by 11 %. Biogas productivity was thus found to be in the R2 while adding 1 L/day of whey 19 m3/m3/day and adding 2 L/day of whey 21 m3/m3/day while 14 m3/m3/day was produced in the control reactor. The results showed that the extraction of the biogas was higher in the R2 where we used whey. It has been found that the amount of dry matter and the proportion of organic matter does not differ between bioreactors and that whey has no influence on the efficiency of centrifugation of dehydrated sludge.