Methods and systems for treatment of organic waste by means of hydrothermal carbonization include a mixing tank for receiving organic waste. A first batch of mixed wet waste is fed from the mixing tank to a first thermal reactor to undergo thermal hydrolysis. A second batch of mixed wet waste is fed from the mixing tank to a second thermal reactor to undergo thermal hydrolysis. Bio-char sludge is fed in an alternating manner from the first and second thermal reactors to a bio-char cooler. To save energy, hot and pressurized water from the first thermal reactor is subsequently supplied to the second thermal reactor or from the second thermal reactor to the first thermal reactor in an alternating manner for the respective hydrolysis processes.