NOVELTY - Production of liquid fuel involves cultivating aquatic plants (12) in an aquatic medium (11) exposed to light in a photosynthetic culture chamber (10); transferring organic material (32) of the plants to an anaerobic digester (30); fermenting the organic material in the digester to produce a biogas comprising methane and carbon dioxide; converting at least a portion of the biogas to synthesis gas comprising carbon monoxide and hydrogen; and contacting at least a portion of the synthesis gas with a catalyst to produce a liquid fuel.    USE - For producing liquid fuel i.e. alcohol or diesel fuel (claimed).    ADVANTAGE - The step of fermenting organic material to a biogas involves feeding the organic material into a vessel, fermenting and mixing the organic material in anaerobic conditions in the vessel to form the biogas, discontinuing the mixing to allow particulate unfermented organic material to settle in the vessel resulting in the formation of a low-suspended-solid supernatant, decanting the supernatant from the vessel, and repeating at least the feeding and fermenting steps. This method improves the efficiency of gasification of the organic material, as compared to a two-vessel system (as described in US5185079). By recirculating the effluent (i.e. typically rich in nutrients, including nitrogen, phosphorus, and potassium that are essential for the photosynthetic organisms) from the anaerobic digester to the photosynthetic culture chamber, the method reduces or eliminates the need to supplement the aquatic medium of the photosynthetic culture chamber with certain nutrients. Also by recirculating effluent from the anaerobic digester to the photosynthetic culture chamber, the whole system for producing biogas or liquid fuel is almost completely self-contained with very little input or waste product. The nutrients needed for both the aquatic plants and the microorganisms in the anaerobic digester are recycled between the photosynthetic culture chamber and the anaerobic digester.    DETAILED DESCRIPTION - Production of liquid fuel involves cultivating aquatic plants (12) in an aquatic medium (11) exposed to light in a photosynthetic culture chamber (10); transferring organic material (32) of the plants to an anaerobic digester (30); fermenting the organic material in the digester to produce a biogas comprising methane and carbon dioxide; converting at least a portion of the biogas to synthesis gas comprising carbon monoxide and hydrogen; and contacting at least a portion of the synthesis gas with a catalyst to produce a liquid fuel. The step of fermenting produces an effluent (34); and the method further involves transferring at least a portion of the effluent from the digester to the culture chamber. The aquatic plants are planktonic and the step of transferring organic material of the aquatic plants to an anaerobic digester involves transferring the aquatic medium containing the aquatic plants to the digester, without separating the aquatic plants from the aquatic medium. INDEPENDENT CLAIMS are included for the following:    (1) production of biogas involving the step of cultivating aquatic plants; the step of transferring organic material; the step of fermenting the organic material; and transferring carbon dioxide to the aquatic medium in the culture chamber by (i) transferring at least a portion of the liquid effluent from the digester to the culture chamber, where the liquid effluent has a concentration of dissolved carbon dioxide of at least 3 mM when contacted with the aquatic medium in the photosynthetic culture chamber, or (ii) contacting at least a portion of the biogas with an aqueous volume outside of the anaerobic digester and the photosynthetic culture chamber to dissolve carbon dioxide from the biogas in the aqueous volume and generate carbon dioxide-depleted biogas, and mixing the aqueous volume with the aquatic medium in the photosynthetic culture chamber, or (iii) contacting at least a portion of the biogas with the aquatic medium in the photosynthetic culture chamber to dissolve carbon dioxide from the biogas in the aquatic medium and generate a carbon dioxide-depleted biogas;    (2) a system (80) for producing liquid fuel comprising: the photosynthetic culture chamber functionally coupled to the anaerobic digester which is functionally coupled to device for producing synthesis gas and comprising a steam reformer, carbon dioxide reformer and/or partial oxidation unit; and a catalyst for converting synthesis gas to a liquid fuel, and is functionally coupled to the device for producing synthesis gas; and    (3) a system for producing biogas comprising the photosynthetic culture chamber; anaerobic digester; conduit functionally coupled to the chamber and digester, and adapted for transferring liquid effluent from the digester to the aquatic medium in the culture chamber without equilibration with air before the liquid effluent is mixed with the aquatic medium in the photosynthetic culture chamber.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a system for producing biogas or liquid fuel.    Photosynthetic culture chamber (10)    Aquatic medium (11)    Aquatic plants (12)    Conduit (20)    Anaerobic digester (30)    Organic material (32)    Effluent (34)    Sun (71)    System (80)