Sludge originating from e.g. wastewater treatment plants, manure or biomass waste streams can be serially treated by anaerobic digestion in at least three anaerobic reactors,wherein a major part of the influent sludge stream is fed to a first reactor, the major part of the liquid effluent of the first reactor is fed to a second reactor, the major part of the liquid effluent of the second reactor is fed to a last reactor, and the major part of the liquid effluent of the last reactor is disposed and/or optionally further treated, and wherein a controlled variable part of the effluent of the last reactor is fed to the first reactor, and a controlled variable part of the influent sludge stream is fed to the second reactor; the level (volume per time unit) of the variable parts being controlled by means the pH and/or redox values in the reactors possibly combined with flow and/or composition values of the gas produced in the reactors.
