In the methane fermentation process, sewage sludge is the single substrate or serves as a co-substrate with the addition of various waste products. After the treatment stable digestate is obtained, which consists of two phases solid and liquid. Liquid phase, called as a leachate, due to the high content of nutrients must be treated before they are discharged into the final receiver. Physical and chemical methods of leachate treatment are usually expensive and difficult to maintain. Application of biological methods seems to be promising in such applications, however number of papers focused on such issue is limited. The aim of the presented study was to determine the treatment possibility of leachate generated during co-fermentation of agricultural products (bovine slurry) and excessive activated sludge in the deammonification process. During the experiment dewatered digestate from the mesophilic co-fermentation of bovine slurry and excessive active sludge, were co-treated with synthetic wastewater in a 1:3 weight ratio in the sequencing bath reactor. In the final test period, the Superfloc C494VP polyelectrolyte (from Kemira) was dosed into the leachate in order to enhance solids removal. AUR, NPR and AA were calculated to evaluate the deammonification process. It turns out that it is possible to co-treat leachate from biogas plants in C/N ratio no more than 1. The test also noted that a better oxidation effect of NH4 - N was noted by adding polyelectrolyte to leachate. On the other hand, this negatively affected the viscosity of the granules and their sticking.