Great overview. Of course the nutrient-rich digestate which is also produced in the biogas system can be used as a renewable fertiliser. This reduces the carbon footprint of farming, as compared to using chemical fertilisers. See my introduction to digestate and potential use in horticulture: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-q5-Pm_1R4Wc.html