Anaerobic digestion (AD) is a microbial-mediated process which is widely used for the conversion of complex organic waste to renewable energy in the form of biogas [
1]. The synergistic catalysis of various microorganisms without oxygen determines the biological route of the AD process. The organic matter conversion to biogas follow four main conversion phases namely; hydrolysis, acidogenesis, acetogenesis, and methanogenesis [
2]. During the hydrolysis stage complex polymeric organic matter including carbohydrates, proteins, and fats transform into simple organic monomers by the action of hydrolytic bacteria. The monomers such as sugar, amino acids, and fatty acids are then converted into volatile fatty acids (VFAs) under the action of fermentative bacteria during the second stage called as acidogenesis. During the third phase, acetogenic bacteria transforms VFAs into acetic acid and hydrogen (H
2) gas. Methanogenic bacteria transform acetic acid and H
2 into methane (CH
4) and carbon dioxide (CO
2) [
3]. The quality of biogas in terms of composition varies depending on biomass, precursors, additives and the conversion process. In general, biogas contains 50–75% methane and 25–45% carbon dioxide, in addition to small amounts of other gases and typically has a calorific value of 21–24 MJ/m
3 [
4].
One of the potential feedstocks for biogas generation by AD process is algal biomass [
5]. Algae are unicellular or multicellular organisms. In comparison with other biomass, they possess many benefits such as they can grow in natural and artificial systems, they can grow in fresh and marine water [
6,
7]. In addition, they have high biomass yield and greater carbon dioxide capture. Algal biomass can offer numerous biofuels such as biohydrogen, methane, biodiesel, bioethanol, and biogas [
8]. The strong resistant algae cell wall is composed of three main components: biopolymers, cellulose, and hemicellulose. These components play a protective role in cells. Cellulose molecules are arranged regularly in the form of bundles. It also contains a small portion of pectin, protein, ash, and extracts, including soluble non-structural substances, non-structural sugars, nitrogen compounds, chlorophyll, and waxes [
9]. However, the inter- and intra-molecular hydrogen bonds have made the dissolution of cellulose a difficult process in common solvents. This hinders or limits the anaerobic digestion of algal biomass during the hydrolysis stage.
Numerous pretreatment methods for algae can be used including biological (enzymatic), chemical (acid or alkali), physical (ultrasound, microwave, or shear force) and thermal methods [
10]. However, selection of a pretreatment process is mainly reliant on its low capital cost, positive energy balance, and lesser operational cost to make AD process economically feasible [
11]. Microwave (MW) pretreatment is the transmission of electromagnetic energy in the frequency range of 0.3 to 300 GHz. MW pretreatment involves no contact amongst the source and the chemicals [
12]. Passos et al. [
13] studied the effect of MW pretreatment on algae from High Rate Algal Ponds (HRAP). Results showed that MW pretreatment enhanced biogas production rate (25–75%) and successfully improved the digestibility of algal biomass. Several studies discussed MW pretreatment applied to waste activated sludge [
14,
15,
16,
17,
18,
19]. Almost all the studies reported an enhancement in sludge solubilization and biogas generation. In our previous study [
20], optimization of MW pretreatment for an AD of
Enteromorpha was carried out using response surface methodology. Results showed that 24.4 mL biogas/g dry algae was produced at the optimized MW pretreatment conditions after AD.
The concerns about expansion in the bioenergy sector during the past decade have driven a number of scientists and researchers to pursue innovative solutions for its production. Nanotechnology is one of the emerging branches of science. It deals with dimensions less than 100 nm. It is the art of manipulating individual atoms. It is the most striking and fertile field which allows researchers to work at the molecular level [
21]. In the field of bioenergy, nanotechnology can be applied for feedstock modification and more efficient catalysis. Minerals are needed for microorganism development [
4]. Liu et al. [
22] reported that minerals deliver upright atmosphere for anaerobic bacteria inside a digester and enhance biogas and methane generation. In another study, Qiang et al. [
23] stated that in the presence of iron, cobalt, and nickel, methanogenic bacteria grow quickly during enzyme production. Heavy metal ions such as Co, Cu, Fe, Mo, Ni, and Zn have been documented as essential for several reactions during AD by Luna-deRisco et al. [
24]. Micronutrients such as Co, Ni, Fe, Mg, and Ca are crucial for a variety of chemical, biochemical, and microbiological reactions related to VFA utilization, biogas generation, and cell lysis [
25]. Nanoparticles (NPs) of micronutrients had an augmented effect on biogas production. Cascals et al. [
26] applied 100 ppm (100 mg/L) of Fe
3O
4 NPs (7 nm) to organic waste in an anaerobic digester under mesophilic conditions (37 °C) for 60 days. Results showed an enhancement of 180% in biogas and 234% increase in methane yield. The authors mentioned that Fe
2+ act as a unique source, which disintegrates the organic matter and increases biogas production in the anaerobic bacterial reactor. Suanon et al. [
27] studied the metal distribution conversion during AD of wastewater sludge under the presence of Fe
3O
4 NPs. Batch anaerobic system was used under mesophilic conditions (37 °C). Methane production increases by 1.25 and 0.9 times by 0.75 g and 1.5 g per 500 mL dose of Fe
3O
4 NPs, respectively. The addition of Fe
3O
4 NPs showed an improvement of metals stabilization in the digestate resulted in an enhancement of biogas and methane production. Abdelsalam et al. [
28] examined the influence of Fe
3O
4 NPs with different concentrations (5, 10, and 20 mg/L) on biogas and methane yield from the AD of cattle manure (CM) slurry. Anaerobic fermentation of CM was carried out batch-wise at operating temperature and mixing rate of 37 ± 0.3 °C and 90 rpm for a hydraulic retention time (HRT) of 50 days. The study indicated that the addition of 20 mg/L Fe
3O
4 NPs increases biogas production by 1.66 times and methane production by 1.96 times. Our previous work [
29] investigated the effect of Fe
3O
4 NPs on biogas yield from anaerobic digestion of green algae (
Enteromorpha). Results showed that the 10 mg/L of Fe
3O
4 NPs cumulative increase in biogas production was 28%. It was observed that during the less effective domain NPs had no additional effect as a controlled sample. However, approximately after 60 h of the digestion process, NPs showed the incremental effect on biogas production. It has been suggested that combining the pretreatment with NPs may result in an early dissolution of the algae cell wall and provide faster action by NPs on stimulation of microorganisms to achieve high cumulative biogas yield with positive energy balance. Therefore, the objective of the present study is to examine the effect of combining a microwave (MW) pretreatment of
Enteromorpha with Fe
3O
4 NPs. Energy ratio was calculated, and established prediction models are used to substantiate the experimental results of this work for the approximation of biogas generation during AD.