NOVELTY - Producing biogas comprises: e.g. (a) grinding an organic substrate (1) comprising solids to produce smaller solids particles, transferring the substrate with smaller solids particles to a hydrolysis tank (11), hydrolyzing the substrate in anaerobic condition in a hydrolysis tank comprising ferric particles, transferring effluent from the hydrolysis tank to a solids separator, and separating solids from the effluent; and (b) transferring the formed screened hydrolysis effluent (13) for period of time (t1) to a methanogenesis tank (m1) (21) and producing biogas and (m1) liquid effluent.    USE - The method is useful for producing biogas (claimed) which is used for energy and in landfilling waste.    ADVANTAGE - The method: exhibits increased rate and efficiency of anaerobic hydrolysis to provide substrates for methanogenesis by utilizing ferric iron in a hydrolysis reactor; ferments organic substrates to biogas more quickly (i.e. with improved speed) and more completely than conventional methods; and utilizes the foam trap which minimizes or eliminates corrosion and plugging of downstream gas units, preferably when foam and vapor condenses at cooler temperatures.    DETAILED DESCRIPTION - Method (p1) of producing biogas comprises: (a) grinding an organic substrate (1) comprising solids to produce smaller solids particles, transferring the substrate with smaller solids particles to a hydrolysis tank (11), hydrolyzing the substrate in anaerobic condition in a hydrolysis tank comprising ferric particles, transferring effluent from the hydrolysis tank to a solids separator, and separating solids from the hydrolysis effluent (12) to produce a solids fraction and a screened hydrolysis effluent (13); (b) transferring the screened hydrolysis effluent for period of time (t1) to a methanogenesis tank (m1) (21) comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank (m1) liquid effluent, transferring the methanogenesis tank (m1) liquid effluent to a methanogenesis tank (m2) comprising fixed film methanogens or granular methanogens during period of time (t1) and producing biogas and methanogenesis tank (m2) liquid effluent in methanogenesis tank (m2), and discharging methanogenesis tank (m2) liquid effluent during period of time (t1); and (c) switching of methanogenesis tanks (m1) and (m2), transferring the screened hydrolysis effluent for a period of time (t2) to the methanogenesis tank (m2) and producing biogas and methanogenesis tank (m2) liquid effluent, transferring the methanogenesis tank (m2) liquid effluent to the methanogenesis tank (m1) during period of time (t2) and producing biogas and methanogenesis tank (m1) liquid effluent in methanogenesis tank (m1), and discharging methanogenesis tank (m1) liquid effluent during period of time (t2). INDEPENDENT CLAIMS are also included for:    (1) a method (p2) of producing biogas comprising: transferring an organic substrate to a hydrolysis tank, and hydrolyzing the substrate in anaerobic condition in a hydrolysis tank to produce a hydrolysis liquid effluent; transferring the hydrolysis effluent for a period of time (t1) to a methanogenesis tank (t1) comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank (m1) liquid effluent, transferring the methanogenesis tank (m1) liquid effluent to a methanogenesis tank (m2) comprising fixed film methanogens or granular methanogens during period of time (t1) and producing biogas and methanogenesis tank (m2) liquid effluent in methanogenesis tank (m2), and discharging methanogenesis tank (m2) liquid effluent during period of time (t1); switching the tanks (m1) and (m2), transferring the hydrolysis effluent for a period of time (t2) to the methanogenesis tank (m2) and producing biogas and methanogenesis tank (m2) liquid effluent, transferring the methanogenesis tank (m2) liquid effluent to the methanogenesis tank (m1) during period of time (t2) and producing biogas and methanogenesis tank (m1) liquid effluent in methanogenesis tank (m1), and discharging methanogenesis tank (m1) liquid effluent during period of time (m2); passing the biogas produced in methanogenesis tanks (m1) and (m2) through a foam trap to trap foam and separate it from the biogas; and collecting the biogas downstream of the foam trap;    (2) a method (p3) of producing biogas comprising: transferring an organic substrate to a hydrolysis tank, and hydrolyzing the substrate in anaerobic condition in a hydrolysis tank to produce a hydrolysis liquid effluent; transferring the hydrolysis effluent for a period of time (t1) to a methanogenesis tank (m1) comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank (m1) liquid effluent, transferring biogas from the methanogenesis tank (m1) through a foam trap no trap foam and separate it from the biogas; and collecting the biogas downstream of the foam trap;    (3) a method (p4) of producing biogas;    (4) a mobile system for digesting organic matter and producing biogas;    (5) a method (p5) of producing biogas; and    (6) a method (p6) of producing biogas.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the components of a digester system and producing biogas.    Organic substrate (1)    Hydrolysis tank (11)    Hydrolysis effluent (12)    Screened hydrolysis effluent (13)    First methanogenesis tank (21)