Due to the high nitrogen and phosphorus concentrations in wastewater from soilless cultivation of tomatoes, the sludge formed during wastewater treatment can be used as a source of nutrients in agriculture. The effect of electrical direct current (DC) density (J) and hydraulic retention time (HRT) in a rotating electrobiological contactor (REBDC) on the quantity and quality of sludge generated in the process of tomato soilless cultivation wastewater treatment was determined. The cathode consisted of discs immobilized with biomass, while the anode was an aluminum electrode. HRTs of 4 h, 8 h, 12 h, 24 h and (J) of 0.63, 1.25, 2.50, 5.00, and 10.00 A/m2 were applied. The study showed that the increase in (J) caused an increase in the amount of sludge generated. The increase of (J) contributed to increased concentrations of biogenes in the sludge, whereas extension of HRT at a given current density contributed to increased concentrations of nitrogen and phosphorus (up to 6.2% and 0.8% respectively) and to the reduced content of organic matter (up to 23.04%). The examined sludge was stabilized, sanitary, and safe, and may be an alternative to mineral fertilizers. This study showed that larger sludge amounts are produced in a DC mode aerobic REBDC than in an alternating current anaerobic one.