insoluble electron acceptor may be selected from the group consisting of glassy carbon, graphite plates, rods and carbon fibrous material such as felt, cloth and paper. 

GLYPH<3> [0038] Preferably, the anode electrode comprises a continuous insoluble electron acceptor. In the present context the term "continous" relates to a material which may be the insoluble electron acceptor or which may be a material coated with the insoluble electron acceptor, which is provided without interruption. Accordingly, it is not an aspect of the present invention to mix small grains of insoluble electron acceptors into the anaerobic envi- ronment. 

## Cathode electrode 

GLYPH<3> [0039] The materials selected to be used as a cathode electrode is selected so as to obtain a sufficient reduction reaction. In an embodiment of the present invention the cathode electrode may comprise a metal catalyst, non- metal catalyst or a combination thereof. Preferably, the metal catalyst is a noble metal catalyst, non- GLYPH<3> noble metal catalyst or a combination thereof, wherein the noble met- al catalyst may be Pt or other known noble metal catalyst and wherein the non- GLYPH<3> noble metal catalyst may be Co or Fe or other known non- GLYPH<3> noble metal catalysts. 

GLYPH<3> [0040] In a further embodiment of the present invention one side of a cathode electrode, which may be in contact with electrolyte permeable membrane, may comprises a metal catalyst, non- GLYPH<3> metal catalyst or a combination there- of in the case wherein oxygen is used as an oxidant. 

## Electrolyte permeable membrane 

GLYPH<3> [0041] Electrolyte permeable membrane may be a semi permeable membrane being impermeable to gas- ses as well as resistant to the reducing environment at the anode as well as the harsh oxidative environment in the cathode chamber. 

GLYPH<3> [0042] Electrolyte permeable membrane are well known to a person skilled in the art and may be composed of polymer membranes or from composite membranes where other materials are imbedded in a polymer matrix and. In an embodiment of the present invention the elec- trolyte permeable membrane may comprise a proton ex- change membrane, a cation exhange membrane or a combination hereof. 

GLYPH<3> [0043] In a further embodiment of the present invention the electrolyte membrane comprises at least 40%, e.g. at least 50%, such as at least 60%, e.g. at least 70%, such as at least 80%, e.g. at least 90%, such as 100% of the cathode chamber surface, such as in the range of 40-100%, e.g. in the range of 60-95%, such as in the range of 55-88%, eg. in the range of 30-80%, such as in the range of 20-45%, e.g. in the range of 10-30%. 

## Anaerobic environment 

GLYPH<3> [0044] In the present context the term "anaerobic en- 

vironment" refers to an environment absent in an oxidant, such as oxygen or to an environment comprising very low oxygen concentrations. The term "anaerobic envi- ronment" also covers the term "anoxic" indicating ab- sence or very low concentrations of electron acceptors such as nitrate or sulphate. 

GLYPH<3> [0045] In an embodiment of the present invention the anaerobic environment may either by a naturally occur- ring anaerobic environment, an artificially generated anaerobic environment or a combination thereof. Prefer- ably, the naturally occurring anaerobic environment may be selected from the group consisting of wetlands, sea sediments, fresh water sediments and any other natural environments containing reduced organic or inorganic matter, whereas the artificially occurring environment may selected from the group consisting of fermentation plants, manure plants, sludge plants, hydrolysate pro- ducing plants, food processing plants, and any other re- duced organic or inorganic waste plants. 

GLYPH<3> [0046] It may be preferable to mix a naturally occurring anaerobic environment with an artificially occurring anaerobic environment. 

GLYPH<3> [0047] It may be easily recognised by a person skilled in the art that the microbial fuel cell and/or the combined electrode of the present invention may be generating electrical energy from reduced organic or inorganic mat- ter present or added to the anaerobic environment 

GLYPH<3> [0048] In a further embodiment of the present invention the anaerobic environment may act as an anode elec- trode chamber and wherein the anode chamber may sur- round the cathode chamber. 

GLYPH<3> [0049] Preferably, at least 40% of the cathode cham- ber is surrounded by the anaerobic environment, such as at least 50%, e.g. at least 60%, such as at least 70%, e.g. at least 80%, such as at least 90%, e.g. 100% so as to increase the yield of electrical energy generated. Ac- cordingly it may be preferable that all parts of the anode electrode and the cathode chamber are surrounded by the anaerobic environment. 

## Combined electrode 

GLYPH<3> [0050] In an additional embodiment of the present in- vention, the MFC may be constructed as a combined electrode as illustrated in figure 3. Thus, the distance between the anode electrode and the cathode chamber is 0 cm as illustrated in figure 3 and 4. In a preferred embodiment, the combined electrode comprise: GLYPH<3> 

(i) an anode electrode, 

(ii) a cathode chamber, said cathode chamber com- prising an inlet through which an influent enters the cathode chamber, an outlet through which an efflu- ent depart the cathode chamber, a cathode electrode and an electrolyte permeable membrane, GLYPH<3> wherein both the anode electrode and the cathode chamber are to be submersed into an anaerobic en- 

