a wastewater inlet at the bottom of the reactor;

a first-stage reaction zone in fluid communication with and above the inlet;

at least one first-stage three-phase separator in the first-stage reaction zone;

a sludge inlet in fluid communication with the first-stage reaction zone;

a second-stage reaction zone in fluid communication with and above the first-stage reaction zone;

at least one second-stage three-phase separator in the second-stage reaction zone;

a separation zone comprising a gas/liquid separator above the second-stage reaction zone;

a first-stage upflow pipe in fluid communication with the upper portion of the first-stage reaction zone and the gas/liquid separator;

a second-stage upflow pipe in fluid communication with the upper portion of the second-stage reaction zone and the gas/liquid separator;

a gravity-fed downflow pipe in fluid communication with the bottom portion of the first-stage reaction zone and the gas/liquid separator;

a treated effluent outlet in the separation zone;

a pump in fluid communication with the gas/liquid separator and a water tank, the pump capable of providing continuous internal circulation of liquid in the reactor;

a biogas outlet in fluid communication with the gas/liquid separator;

biogas;

organics-laden wastewater within the inlet;

sludge; and

anaerobic biological organisms.