The efficiency of simultaneous digestion of sewage sludge and by-product of refining sugar beets (molasses) was investigated. The study was conducted for 28 days under mesophilic conditions. 0.5%, 1%, 1.5%, 2% and 3% (m/m) of molasses was added to the mixture of sludge. The result of the study showed that addition of molasses had positive effect the biogas production. The biggest biogas yield was achieved in sample with 0.5% of molasses (95.69 mL/g VS). In this sample biogas production

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... by 21% in comparison with reference sample (without molasses). The biggest methane content (73%) was also observed in the sample with 0.5% of molasses. For comparison in reference sample was produced biogas with 70% content of methane. The dose over 0.5% of molasses caused inhibition of fermentation process. The minimal degree (38%) of degradation of organic matter was achieved in reference sample (38.53%) and in sample with 0.5% of molasses (39.71%) but in other samples was in the range of 35.61-36.76 % (from 3% to 1%, respectively). Digestion process have adverse effect on dewatering properties of sludge. Before co-digestion capillary suction time was from 31 s to 55 s, and after process increased from 36 s to 556 s (from 0% to 3% of molasses, respectively).