The invention relates to a method for producing
biogas energy while preparing an
organic fertilizer or a compound
fertilizer by using an
inorganic fertilizer. The method comprises the following steps of inputting the
inorganic fertilizer into a photo-
bioreactor inoculated with microalgae to enable the microalgae to rapidly grow in the photo-
bioreactor under the action of the
inorganic fertilizer, separating the microalgae from the photo-
bioreactor and placing the microalgae in a
methane fermentor when the concentration of the microalgae reaches to 0.5g / L-50g / L, and performing
fermentation for 3-45 days to generate
methane,
methane liquid and methane residues, wherein the methane liquid flows back to the photo-bioreactor to be re-utilized, the methane is used as an
energy source, and the methane residues are used as the
organic fertilizer or the compound
fertilizer. Compared with the prior art, a whole
system of the method is very simple, easy to manufacture and install and can be popularized and used in families, especially in peasant households, and the cost of required materials is low. When the whole
system operates, except simply adding of the inorganic
fertilizer, manual intervention is not required, manually-added inorganic substance and organic substance (including any forms of biomasses containing
nutrient substance and derived fertilizers of the biomasses) are not required.