Energy "powers" provides us with pleasant light and temperatures in our living spaces and working environments; it feeds manufacturing plants, urban infrastructure, and the multitude of electronic assistants we use in our daily lives; and it allows us to travel around the world almost indefinitely (Kumar et al. 2020; Zakari et al. 2022). The current global energy issues related to the lack of fossil fuels like crude oil, coal, and natural gas, along with excessive gas emissions from excessive utilization of fossil fuels have great concern worldwide (IEA 2020).
Global energy demand is increasing rapidly, rising from 14.00 BTOE (billion tonnes of oil equivalents) today to more than 18 BTOE by 2030. It's also worth noting that worldwide energy consumption increased by 5.3%, 1.5%, and 1.4%, respectively, for natural gas, oil, and coal (BP energy outlook 2019). By 2050, energy demand could be three times what it was in 2020, according to the International Energy Agency (IEA 2020).
Furthermore, fossil fuel combustion has resulted in air pollution, global warming, and climate change (Rogelj et al. 2018; Roy et al. 2020; Hajilary et al. 2020; Bhattacharya et al. 2020; Zhang et al. 2022). An environmental crisis resulting from rising CO2 emissions and decreasing fossil fuel availability leaves no other option but to find a new clean and sustainable energy source. There is a shift away from fossil fuels and toward renewable energy sources (Baena-Moreno et al. 2019; Dalmo et al. 2019; Dlamini et al. 2019; Ghosh et al. 2019; Oliveira et al. 2019; Sharma et al. 2019; Vaish et al. 2019).
Renewable energy sources such as biomass, hydro, wind, solar, geothermal, and tide provide about 17.8% of global energy consumption (WBA 2019). In terms of waste management and environmental impact reduction, biomass and organic wastes are superior renewable energy sources upon fossil fuels (Drożdż et al. 2022).
Animal manure (Chowdhury et al. 2020), agricultural residues (Tamburini et al. 2020), food wastes (Bedoi et al. 2020), sewage sludge (Ghosh et al. 2020), and other energy crops (O'Keeffe and Thrän 2020) are some of the wastes that can be used as feedstock. For animal manure, livestock accounts for roughly 40% of the global value of agricultural products (WHO 2017). Traditional scattered family-scale livestock farms have been steadily converted into centralized ones in recent years to meet an increased demand for dairy and meat products (Li et al. 2020a). These farms produce a lot of manure (cattle, swine, poultry, and sheep) that needs to be treated properly (Li et al. 2020b).
Cattle manure accounts for more than half of all manure produced (Scarlat et al. 2018) and is expected to increase to over 75% in the next decade (Meyer et al. 2018). Cattle manure contains a large amount of undigested lignocellulosic components such as cellulose, hemicellulose, and lignin (more than 50% of total solids) (Abbas et al. 2020; Li et al. 2020b; Tezel et al. 2011; Song et al. 2017; Sahota et al. 2018; Achinas et al. 2017; Zhou et al. 2016; Hu et al. 2018).
Anaerobic digestion (AD) is a complex biochemical process that uses the impact of microbes to convert organic waste into renewable energy in the form of methane (CH4)-enriched biogas and digestate in the absence of oxygen (Arif et al. 2018; Zhang and Zang 2019; Liew et al. 2022). Biogas is typically composed of 40–75% CH4 and 25–60% CO2, with trace amounts of other impurities such as H2O (5–10%) and H2S (1-10000 ppm) (Kadam and Panwar, 2017).
The heat value of biogas with a high CH4 component is in the range of 20–25 MJ/m3. Biogas is a good alternative to fossil fuels since it not only reduces the consumption of conventional energy sources but also reduces green gas emissions by roughly 80% (Arsova 2010). Furthermore, using digestate as a substitute to inorganic mineral fertilizer could minimize fossil fuel usage while also lowering the danger of pollution (Seman et al. 2019; Li et al. 2020c).
To make cattle manure available to anaerobic bacteria, several ways have been used. Co-digestion with other wastes (Jugal and Rao, 2019; Shen et al. 2019; Wei et al. 2019; Akyol 2020; Vijin et al. 2020), pretreatments (chemical, ultrasound, and thermal) (Fernandez et al. 2020; Wahid et al. 2018; Yuan et al. 2019), optimization of operating parameters and bioreactor design (Han et al. 2019; Chen et al. 2020; Farghali et al. 2020). Chemical pretreatment necessitates careful material selection to avoid harmful activities during the procedure (Kaur et al. 2020). Furthermore, ultrasonic and heat treatments can result in significant carbohydrate losses, lowering sugar levels. Physical pretreatment also takes a lot of energy to get the best particle size reduction (Biswal et al. 2020).
Organic additions, such as green biomass and enzymes (Angelidaki et al. 2018; Yuan et al. 2022), and inorganic additives (Romero-Güiza et al. 2016), are utilized to boost CH4 production in AD processes. Inorganic additives are classified into two types: macronutrients and micronutrients (Dang et al. 2016). To boost the system buffer capacity and maintain microorganism activity, macronutrients (i.e., P, N, and S) are added to the AD process substrate in the form of salts (Zhang et al. 2018). However, a high dose of bulk materials can cause anaerobic microbe toxicity; also, these materials may not biodegrade efficiently during digestion (Abdelsalam et al. 2017a). Micronutrients (Fe, Ni, and Co) are added to the AD feedstock in the form of salts, bulk materials, and, more recently, nanoparticles (NPs) (Garuti et al. 2018; Wandera et al. 2018).
NPs are particles with three dimensions ranging from 1 to 100 nanometers (Parisi et al. 2015). The chemical composition, dimensions, appearance, condition, and origin of NPs are all used to classify them (Gleiter, 2000; Hochella et al. 2015; Sharma et al. 2015; Wagner et al. 2014). This classification is also based on their size, which in at least one dimension varies from 1-100 nm (Saleh, 2020). In general, NPs have a high surface-to-volume ratio, a large number of particles per unit weight, and confinement or quantum effects, i.e. a small number of atoms per particle. These characteristics of NPs result in properties that are strikingly different from those associated with the same material in its bulk state (Saleh 2020).
In fact, most NPs can absorb inhibiting substances, such as heavy metals, and retain them on their surface (Lei et al. 2018; Zhang et al. 2019). NPs (e.g. Fe, Ni, Co, and metal oxides) stimulate the activation of microorganisms and key enzymes, resulting in more biogas production (Abdelwahab et al. 2020a and 2021c; Abdelsalam et al. 2016, 2017a and b).
Hence, this review aims to provide insights to the influences released by NPs (e.g. Fe, Ni, Co, and metal oxides) on cattle manure AD process in terms of biogas yield (CH4, CO2, and H2S), their influences on fundamental mechanisms such as pH, volatile fatty acids (VFAs) and total alkalinity (TA) production, total solids (TS) and volatile solids (VS) degradation, as well as their influences on the characterization of organic materials and chemical composition of the effluent. Finally, perspective on the required future trends and research on the application of NPs in the AD process are highlighted.