An apparatus for the treatment of fluid containing oxygen demanding organic material with microorganisms comprising: a two stage downflow anaerobic digester comprising a predigester and at least one anaerobic bioreactor connected in series with the predigester, wherein each of the predigester and the at least one anaerobic bioreactor comprise a substantially fluid tight vessel, the predigester having at least one predigester inlet and at least one predigester outlet whereby the inlet and the outlet are arranged so that the fluid enters the two stage anaerobic digester at the at least one predigester inlet and flows in a downward direction through the predigester from the at least one predigester inlet to the at least one predigester outlet; the anaerobic bioreactor having at least one anaerobic bioreactor inlet (11 a), at least one anaerobic bioreactor outlet and a plurality of high surface area porous media suitable for growth of microorganisms intermediate the inlet and the outlet thereupon filling at least a portion of the anaerobic bioreactor, whereby the inlet (11 a) and outlet are arranged so that the fluid enters the anaerobic bioreactor at the at least one anaerobic bioreactor inlet and flows in a downward direction through the anaerobic bioreactor and thereby engages with the microorganisms on the media; wherein at least one predigester outlet is connected to at least one anaerobic bioreactor inlet (11 a); and wherein each of the predigester and anaerobic bioreactor further comprise at least one recirculation device (25, 25a, 27, 27a) withdrawing fluid from a lower portion of each of the predigester and anaerobic bioreactor and returning the withdrawn fluid to an upper portion of each of the predigester and anaerobic bioreactor respectively (11 b); wherein the downflow anaerobic digester further comprises at least one return circulation feed between at least one anaerobic bioreactor outlet and at least one predigester inlet; and the fluid exits the two stage anaerobic digester at the at least one anaerobic bioreactor outlet (28, 20), wherein the headspaces of the predigester and the downflow anaerobic bioreactor are linked by a conduit to ensure equal gas pressure between the headspaces of the predigester and the downflow anaerobic bioreactor, and wherein the plurality of high surface area porous media are ordered plastic media which are stacked in the centre of the downflow anaerobic bioreactor such that, in use, they are submerged in the liquid bulk of the bioreactor with vertical channels, and wherein there are liquid void spaces (11b, 11d) of approximately 1 m depth both above and below the media stack. a two stage downflow anaerobic digester comprising a predigester and at least one anaerobic bioreactor connected in series with the predigester, wherein each of the predigester and the at least one anaerobic bioreactor comprise a substantially fluid tight vessel, the predigester having at least one predigester inlet and at least one predigester outlet whereby the inlet and the outlet are arranged so that the fluid enters the two stage anaerobic digester at the at least one predigester inlet and flows in a downward direction through the predigester from the at least one predigester inlet to the at least one predigester outlet; the anaerobic bioreactor having at least one anaerobic bioreactor inlet (11 a), at least one anaerobic bioreactor outlet and a plurality of high surface area porous media suitable for growth of microorganisms intermediate the inlet and the outlet thereupon filling at least a portion of the anaerobic bioreactor, whereby the inlet (11 a) and outlet are arranged so that the fluid enters the anaerobic bioreactor at the at least one anaerobic bioreactor inlet and flows in a downward direction through the anaerobic bioreactor and thereby engages with the microorganisms on the media; wherein at least one predigester outlet is connected to at least one anaerobic bioreactor inlet (11 a); and wherein each of the predigester and anaerobic bioreactor further comprise at least one recirculation device (25, 25a, 27, 27a) withdrawing fluid from a lower portion of each of the predigester and anaerobic bioreactor and returning the withdrawn fluid to an upper portion of each of the predigester and anaerobic bioreactor respectively (11 b); wherein the downflow anaerobic digester further comprises at least one return circulation feed between at least one anaerobic bioreactor outlet and at least one predigester inlet; and the fluid exits the two stage anaerobic digester at the at least one anaerobic bioreactor outlet (28, 20), wherein the headspaces of the predigester and the downflow anaerobic bioreactor are linked by a conduit to ensure equal gas pressure between the headspaces of the predigester and the downflow anaerobic bioreactor, and wherein the plurality of high surface area porous media are ordered plastic media which are stacked in the centre of the downflow anaerobic bioreactor such that, in use, they are submerged in the liquid bulk of the bioreactor with vertical channels, and wherein there are liquid void spaces (11b, 11d) of approximately 1 m depth both above and below the media stack.