In recent years, perennial herbaceous plants have obtained increased attention for use as feedstock in anaerobic digestion units for production of biogas, a mixture of gases largely made up of methane (CH4) and carbon dioxide (CO2) that is used as a renewable energy source [1]. The main benefits of utilising herbaceous plants, such as common reeds (Phragmites australis), silvergrass (Miscanthus spp.), switchgrass (Panicum virgatum) and grass silages, for biogas production are their wide distribution and natural growth with high biomass yields throughout the year. These characteristics mean that energy needed for cultivation can be saved, the use of fertilisers and pesticides can be avoided, and the plants can be cultivated in non-arable and marginal lands, so they do not compete with food and fodder crops for arable land [2–4].
Generally, the quality of herbaceous plants as a feedstock for anaerobic digestion is influenced significantly by the grass composition, which is diverse seasonally [5]. The vegetative mass of grass and their cell contents of proteins, lipids and sugars increase in the early stages of these plants’ growth (often in spring to early summer), then decreases with increasing maturity (late summer to end of winter) and is offset by an increase in the percentage of cell wall components made up of cellulose, hemicellulose and lignin [6]. However, to obtain high methane production, plant biomass needs to be rich in readily degradable contents such as carbohydrates, proteins, and lipids, as well as having low lignin content [7, 8]. Hence, the effectiveness of methane production from these plants is strongly dependent on harvesting time [5].
To minimize the recalcitrance to degradation of the plants’ lignocellulosic biomass during anaerobic digestion and increase methane production, pre-treatments have been suggested by many researchers [9–13]. Various avenues of pre-treatment have been identified to improve and accelerate the degradation of feedstocks in the anaerobic digestion process, encompassing physical, chemical, and biological treatments [14–16]. Among these pre-treatment techniques, physical methods are the most widely used to enhance biogas production from lignocellulosic biomass [17]. The main benefit of physical pre-treatment is the absence of the need to add external chemicals or microorganisms, instead mechanical chopping, milling, and grinding, microwave irradiation, centrifuging, ultrasound treatment and exposing the plant material to high temperatures and pressures can be effective [18].
Generally, most pre-treatment methods can increase biogas production from herbaceous plants by around 50% [19]. However, mechanical pre-treatment techniques can deliver approximately 60% increases in biogas production [20, 21]. The grinding process can be carried out using different types of mills, such as knife mills, ball mills, hammer mills, disk mills, vibratory mills, colloid mills and centrifugal mills [22, 23]. The choice of milling equipment depends mainly on the type of feedstocks, required final particle size, and biomass moisture content [24]. For example, colloid mills are suitable to use with wet biomass (moisture contents 15–25%), whilst knife and hammer mills are preferred for use with dry biomass (moisture contents 10–15%) [24]. The implementation of mechanical pre-treatment by grinding leads to a reduction in the particle size of the substrate; an increase in pore size and particle surface area – which leads to an increase in their accessibility for hydrolysis enzymes; and decreases in cellulose crystallinity and polymerisation grade by dissociation of the bonds between lignin, hemicellulose and cellulose, which leads to increased cellulose and hemicellulose availability to microorganisms, thus maximising the substrate degradation and optimising biogas production [25–27].
Besides mechanical pre-treatment, biogas production from lignocellulosic biomass can be further improved by co-digesting it with other substrates [28, 29]. Co-digestion of two or more substrates can provide the appropriate nutrient composition required by microorganisms, thus enhancing anaerobic digestion performance. The co-digestion of carbon-rich lignocellulosic substrates with nitrogen-rich substrates such as food waste and animal manure helps to achieve an appropriate carbon to nitrogen (C:N) ratio, which is one of the critical factors that control the stability of the anaerobic digestion process [30, 31]. The low nitrogen content at a high C:N ratio negatively affects microbial growth and consumption of volatile fatty acids (VFAs), thus affecting biogas production [32–34]. In contrast, low C:N ratios lead to increased ammonia/ammonium concentrations generated from nitrogen reduction resulting in toxic effects on microorganisms and thus system inhibition or even failure [35, 36].
As far as the authors are aware, the current study uniquely explores the influence of a broad range of process parameters, including pH, alkalinity, total solids, volatile solids, VFAs, ammonia, cellulose, lignin, crude carbohydrates, crude proteins, and crude lipids, on the anaerobic digestion performance and methane production from mono-digestion and co-digestion of P. australis with food waste. The specific aims of the present study were to investigate the effect of (1) different harvesting times on P. australis cell composition and biogas production and (2) mechanical pre-treatment (grinding) and co-digestion of P. australis with synthetic food waste on the organic matter removal efficiency and improving methane production. The investigation was performed through batch studies.