Lignocellulosic biomass (LCB) is the most abundant source of low-cost, renewable, and energy-dense feedstocks that are often underutilized. Liquid biofuels and biogas are the main products obtained from LCB through conversion processes, such as anaerobic digestion (AD) and fermentation [1], turning LCB into alternative energy sources that can efficiently change human dependence on fossil fuels [2].

LCB can be found in a variety of sources including energy crops, agricultural and forestry residues, aquatic plants and organic fractions of municipal waste [3]. In Cuba, a significant quantity of lignocellulosic residue is generated from agroindustrial processing of rice, such as residues from the drying process (RD), composed by rice straw, rice husks and broken grains (0.04 tons for each ton of paddy rice processed) [4].

Anaerobic digestion of this substrate was studied by Contreras et al. [5], who obtained methane (CH4) yields of 168 and 196 L kg VS−1, concluding that RD is suitable for conversion to energy via AD. However, the lignocelluloses content is a problem for this process, as it forms a very stable compound that limits the growth of microorganisms and prevents the microbial community from degrading the substrate, thus limiting the hydrolysis stage [6,7]. For these reasons, substrate preparation or a pretreatment stage is usually required to improve its digestibility [8].

From the literature consulted, only a few studies were found on pretreatment for RD. For example, López González and Heiermann [9] studied the effect of pressurized hot water on methane yield, obtaining the best performance at 180 °C for 20 min, with an increase of 63 % over the untreated material. Although hydrothermal pretreatments have proven to be efficient in the solubilization of hemicellulose and low in contaminants, the high consumption of water and electricity are reported drawbacks [10].

In other recent research RD was biologically pretreated with a mixture of three microorganisms (Bacillus subtilis B/2345-10 Nato (54 000 colony-forming units (CFU) mL−1Lactobacillus bulgaricus B/103-4-1 (36 000 CFU mL−1), and Saccharomyces cerevisiae L-25-7-12 (2230000 CFU mL−1)). The mixture was diluted 1/100 in distilled water and applied in a ratio of 18 mL MC gVS−1 substrate for 48 h, with an increase of 9.9 % on methane yield respect untreated substrate [4]. The use of biological pretreatments adapted to the biomass is an attractive alternative since the low-cost hydrolysis of lignin and hemicellulose is achieved, and the cellulose structure is altered without the formation of inhibitory compounds. However, to assess the effectiveness of the process, several factors need to be considered, such as the type and complexity of microorganism, pretreatment time and temperature, and substrate composition [4].

The microorganisms used in pretreatment can be bacteria, fungi or combinations of these. Several studies have shown that the use of these microbial consortia produce enzyme complexes with higher lignocellulosic activity than pure strains due to their greater stability against environmental conditions (pH, temperature, inhibitors, etc.) and their greater redundancy [[11], [12], [13]].

Microbial consortia are obtained from environmental samples (natural consortia) or by combining microbial isolates (engineered consortia) [14]. Natural consortia designated for use in biogas production can be isolated from various ecological niches, including soil, agricultural residues, manure, or the rumen of animals, where they often form specialized consortia, capable of degrading lignocellulose [15]. Pretreatment with microbial consortia has shown a positive effect on improving AD of many LCB, such as rice straw, maize silage and alfalfa [12,15,16].

Therefore, this research aims to evaluate the effect of pretreatment on anaerobic digestion of residues from drying of rice in terms of methane yield by using an enriched microbial consortium.