**7.1 Approaches to achieving circular bioeconomy**

Circular bioeconomy requires input from all stakeholders, including those that risk their well-being to put out wildfires and rescue flood victims. CO2 is a major GHG tied to energy use, and agriculture is a major GHG and aerosol generator, via inorganic fertilizer use. Thus, decarbonization of energy systems and another fertilizer source are necessary. Approaches advocated here obligate renewable energy and the concept of integrated regenerative agriculture (IRA). Renewable energy [85], encompasses a broad gamut of energy carrier types, technologies, and names such as bioenergy [86], biofuels [87], biogas [88], biomass [89], biomethane [90], hydro [91], solar [92], wind [93], and several others. The work presented here highlights anaerobic digestion (AD) as the purveyor of renewable energy. This is because AD is perhaps the only current technology capable of addressing both issues of energy systems decarbonization, and substitution of ammonia-based fertilizer. In addition, AD will simultaneously address another issue threatening planet earth: waste management.

#### **7.2 Anaerobic digestion (AD) technology**

AD is a biochemical process that decomposes organic matter into flammable gas and nutritious watery sludge. The process occurs in a near oxygen-free environment with natural microorganisms. AD was probably recognized in a scientific sense about 392 years ago in 1630, when Jan Baptist van Helmont (1580–1644), observed that decomposing organic matter produced combustible gas; and during 1804–1808 John Dalton and Humphrey Davy established that the combustibility of the gas was due to the presence of methane [94, 95]. Today, AD is carried out within artificial environments named digesters, which are designed to optimize the process [96–98]. The combustible gas is called biogas, and the watery sludge, digestate. Biogas is composed of methane (40–75%), carbon dioxide (25–40%), and trace contaminants (≈ 0.1–3%). Digestate is rich in plant growth macronutrients; with their quantities and micronutrients contents related to the quality of the feedstock [98, 99]. Digestate attributes, AD's technical feasibilities, types of feedstocks utilized (virtually all organic matter), limitations, advantages, benefits, applications, and much more have been presented in the literature [83, 96, 98, 100–102]. AD may be now considered a mature technology that could help humanity decarbonize industries, supplant inorganic fertilizers, and address waste management shortcomings.

#### **7.3 Integrated regenerative agriculture (IRA)**

IRA system combines food, feed, bioenergy, and livestock production in the farm space. IRA practices minimize the fuel/energy versus food mindset competition. The "this or that" image of pitting food production against energy production could be abrogated, thereby promoting synergy and coexistence of fuel, feed, and food cropping; enhancing carbon sequestration, and fostering sustainable ecological intensification. As of 2018, it was estimated that about 163 million farms in 100 countries, engaged 453 million hectares of agricultural land in some form of IRA [103]. The origin/history, definitions, lessons, modifications, and various aspects of IRA have been explored in published literature [104–107]; tried, and/or implemented in several countries such as Colombia [108]; Italy [109]; Egypt [110]; Bangladesh [111]; USA [112]; Finland [113]; Chile and Vietnam [114]. Advocates argue that IRA would also enhance biodiversity, create wealth, support rural communities, improve the environment, increase equity, and mitigate injustice [84, 103, 115–118]. Perhaps these reasons could invoke concern globally to justify a more stringent climate policy [119].
