## 6 Strategies to Improve the Efficiency of the AD Process
### Microbiological Tools to Improve Biogas Production  
The use of selected microorganisms can improve biogas production in different ways. One of these applications is biological pre-treatment which is the most accepted pretreatment of lignocellulosic materials as it requires low energy consumption, it is high specific, cost-effective and environmentally-friendly. The microorganisms mostly used for pre-treatments of lignocellulosicmaterials are white-rot fungi (Coriolus versicolor, Aspergillus sp, Trichoderma sp, Phanerochaete chrysosporium and Penicillin camemberti), which are preferred to fermentative bacteria (Pseudomonas sp, Cellulomonas sp, Streptomyces sp, Bacillus sp and Actinobacteria) [152]. These microorganisms are the most efficient and extensive lignine degraders as they are able to decompose lignin with little consumption of cellulose component. Other microbiological approaches to increase the hydrolysis of the biopolymer complexes are focused on \"bioaugmentation\" of the AD microbial community through enrichment with specific functional components, taking advantage of the abilities of the inoculated community to encode either specific cellulolytic enzymes (e.g., cellulase, b-glucosidase, xylanase) or multienzyme complexes (e.g., cellulose) [153; 154]. Other useful techniques are based on the use of microorganisms coming from the rumen ecosystem. This substrate is first colonized by anaerobic ruminal fungi, characterized by highly hydrolytic capabilities. [PERSON] et al. [155] tested the use of bioaugmentation with anaerobic ruminal fungi and a pool of hydrogen-producing fermentating bacteria on wheat straw and mushroom spent straw with the aim of improving anaerobic digestion performance. Different process configurations were applied and the final results revealed that the two-stage configuration allowed an enhancement in CH\({}_{4}\) production. Furthermore, results on microbial community confirmed the efficiency of the bioaugmentation treatment, mainly related to the Methanosarcinales increase, mostly composed by Methanosaeta. Other studies [156] used brewery spent grain as model lignocellulosic substrate and some rumen bacteria (Pseudobutyvivibrio xylanivorans Mz5T, Fibrobacter succinogenes S85, Clostridium cellulovorans and Ruminococcus flavefaciens 007C) in addition to microbial biomass from biogas digester treating brewery wastewater. This study revealed that treatment of brewery spent grain with P. xylanivorans Mz5T, C. cellulovorans and F. succinogenes S85 promoted biogas production and significantly increased methane production, confirming that bioaugmentation is a promising method for increasing methane production from this substrate. Another microbiological method to enhance the biogas production is the use of animal manures, such as cow dung, pig dung, poultry droppings, etc. as source of various kinds of microorganisms. In particular, cow dung contains microorganisms capable of enhancing the degradation during AD [157]. Different sources, such as sewage water, rumen fluid, sago industry effluent and termite gut, can be used as carriers of aerobic and anaerobic bacteria [158; 159]. Cellulosic materials can also be used as source of specific microorganisms for AD [160]. The size of the inoculum is an important factor influencing the degradation of total organic materials, with an increment of methane yield when increasing substrate to inoculum's ratio size. For example, [PERSON] et al. [161] demonstrated that a concentration of 50% rumen fluid increased substrate degradation, including lignocellulosic materials as compared with lower concentrations. Recently, the use of selected microorganisms for substrate degradation is increased as a consequence of the higher efficiency of the microbes used in the process. Furthermore, the preparation of microbial consortium composed by selected microorganisms increased not only the degradation level, but also the competence of microorganisms for degradation.