In eutrophic lakes and reservoirs the green – blue algae bloom contributes to increase the chlorophyll “a” concentration, deteriorates the water quality with anaerobic conditions in the deepest waters and provokes scum formation among other impacts. Due to the occurrence of irregular nitrification events in the waters of the polluted urban lake, A ude Velho which is located in Campina Grande, PB during the dry season, it was analyzed the performance of the electrolytic process in treating these waters. For the electrolysis treatment it was built, in laboratory scale, a 2 liters reactor where it was inserted an electrolytical assemblage made of steel and aluminum sheets. Samples were obtained in eight sampling points in the lake. The following physical, chemical and organic parameters were obtained: color, turbidity, pH, Alkalinity, calcium, chloride, nitrate, total phosphorus and chlorophyll “a”. After 15 minutes of electrolysis, it was applied 45 minutes of sedimentation to the effluent. The results showed that the alumen production by ion pairing with the water sulphate concentration provoked the water impurities flotation. It was also observed a total denitrification (100% nitrate removal) with N2 liberation to the environment. Phosphorus and chlorophyll “a” removal was also significant.