NOVELTY - Two stage downflow anaerobic digester (100) comprises predigester (10) and anaerobic bioreactor (11) connected in series with predigester, where each of the predigester and the bioreactor comprise fluid tight vessel, the predigester having predigester inlet (50) and outlet (26); the anaerobic bioreactor having anaerobic bioreactor inlet (11a) and outlet (28) and high surface area porous media (111) suitable for growth of microorganisms; and each digester further comprise recirculation device, return circulation feed; and the fluid exits the digester at the anaerobic bioreactor outlet.    USE - For the treatment of fluid containing oxygen demanding organic material with microorganisms (claimed).    ADVANTAGE - The two stage downflow anaerobic digester provides highly efficient method for treatment of fluid containing high concentrations of organic matter or soluble Chemical Oxygen Demand (COD) levels. The COD level which is acted upon by bacteria or microbes to reduce the amount of or oxidize the organic contaminants. The two stage downflow anaerobic digester digester provides microbial population which maintains viability and is robust enough to withstand extended shutdown periods and has rapid restart capability.    DETAILED DESCRIPTION - Apparatus (100) comprises two stage downflow anaerobic digester comprising predigester (10) and at least one anaerobic bioreactor (11) connected in series with the predigester, where each of the predigester and the anaerobic bioreactor comprise fluid tight vessel, the predigester having at least one predigester inlet (50) and at least one predigester outlet (26) which are arranged so that the fluid enters the two stage anaerobic digester at the predigester inlet and flows in a downward direction through the predigester from the predigester inlet to the predigester outlet; the anaerobic bioreactor having at least one anaerobic bioreactor inlet (11a), at least one anaerobic bioreactor outlet (28) and high surface area porous media (111) suitable for growth of microorganisms intermediate the inlet and the outlet upon it filling at least a portion (11c) of the anaerobic bioreactor (11), where the inlet and outlet are arranged so that the fluid enters the anaerobic bioreactor at the anaerobic bioreactor inlet and flows in a downward direction through the anaerobic bioreactor and there engages with the microorganisms on the media; where at least one predigestor outlet is connected to at least one anaerobic bioreactor inlet; and where 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 (11d) and returning the withdrawn fluid to upper portion of each of the predigester and anaerobic bioreactor respectively (11b); where the downflow anaerobic digester further comprises at least one return circulation feed (25b) between anaerobic bioreactor outlet and predigester inlet; and the fluid exits the two stage anaerobic digester at the anaerobic bioreactor outlet (28, 20). An INDEPENDENT CLAIM is included for reducing the level of oxygen demanding material in fluid stream using two stage anaerobic digester comprising predigester and anaerobic bioreactor having high surface area porous media suitable for growth of microorganisms, involving introducing influent fluid stream to the two stage anaerobic digester to inlet on the predigester; flowing the fluid downwardly through the predigester to outlet on the predigester; flowing portion of the fluid from the outlet of the predigester into inlet on the anaerobic bioreactor, recirculating a further portion of the fluid from lower portion of the predigester to upper portion of the predigester and withdrawing a further portion of the fluid as an effluent stream from the outlet on the predigester; flowing the fluid from the inlet on the anaerobic bioreactor downwardly through the anaerobic bioreactor to outlet on the anaerobic bioreactor such that the fluid passes through the media and engages with the microorganisms so as to reduce the level of oxygen demanding material in the fluid relative to the level of oxygen demanding material in the influent fluid stream; recirculating a portion of the fluid from a lower portion of the anaerobic bioreactor to upper portion of the anaerobic bioreactor; recirculating a further portion of the fluid from the lower portion of the anaerobic bioreactor to the upper portion of the predigester; and withdrawing the remaining fluid as an effluent stream from the outlet on the anaerobic bioreactor.    DESCRIPTION OF DRAWING(S) - The figure shows schematic flow diagram of the two stage anaerobic digestion system.    Predigester (10)    Anaerobic bioreactor (11)    Anaerobic bioreactor inlet (11a)    Recirculation device (25, 25a, 27, 27a)    Return circulation feed (25b)    Predigester outlet (26)    Anaerobic bioreactor outlet (28)    Predigester inlet (50)    Two stage downflow anaerobic digester (100)    High surface area porous media (111)