**6. Potential of grass silage**

Several studies had been examined via grass/grass silage as feedstocks to produce biogas as a renewable energy; however, if grass is to be used as raw materials for AD for energy production, it should be converted to silage due to the presence of lignocellulosic materials [26]. Lehtomaki et al. [27] showed that AD of grass silage in batch leach bed processes has the highest methane potential when compared with other potential crops. Smyth et al. [26] compared the net energy of the grass in biomethane systems with other energy crops, and they found that grass has higher gross energy than rapeseed biodiesel and wheat ethanol systems [28]. The yields of dry matter in vetiver grass provided the yield of ethanol at 1091.84 L/ha/year, whereas the leaves of dwarf Napier grass given the maximum yield of 2720.55 L/ha/year (0.98 g/L or 0.12 g/g substrate equivalent to 30.60%) [26].

In numerous studies, grass silage has been recommended as an excellent substrate for biomethane production resulting from high-energy yields, low-energy input demand, long time storage, and usage of silage even for a whole year [29]. The higher potential of methane production from grass silage was confirmed both in batch and in semi-continuous experiments and batch leach bed processes [27]. In practice, grass silage is the most important substrate for agricultural biogas production following maize silage in Germany [30]. Though grass silage may be less energetically productive when compared to maize silage, it still offers a good energy balance and environmental advantages [31]. The key purpose of silage preparation is achieved by efficient preservation. It could keep high-energy content of a crop. And this is achieved by the combination of an anaerobic environment as well as the bacterial fermentation of sugar. The lactic acids formed in the latter progression lower the pH and avoid the proliferation of spoilage microorganisms.

Generally, the fermentation under farm conditions was not involved in a controlled process. The silage fermentation characteristics were depending on the nutrients that allow the growth of microorganisms. The fermentation is usually characterized by a low pH, high lactic acid content, and low concentrations of butyric acid and ammonia-N. Additionally, the ensiled energy is an entirely recoverable in a closed lactic acid-dominant fermentation. On the contrary, there is negligible loss of energy; the production of ethanol by yeast during fermentation is undesirable because no acidification occurs. Correspondingly, under suboptimal ensiling conditions, secondary clostridial fermentation may lead to considerable total solids and energy losses due to extensive production of CO2 and H2 from the fermentation of lactate and hexose sugars. If grass is to be used for energy, it must be harvested and stored, usually as silage. Silage is currently made for feeding livestock, and grass silage is mostly used as co-substrate in biogas plants based on cattle, pig, or chicken manure because of its inappropriate high nitrogen content [32, 33] of about 14% of total solids. The influence of ammonia on anaerobic digestion in terms of process inhibition was found in several literatures [34–36]. However, several authors proved that monodigestion of grass silage is possible, although both applied systems and experimental conditions differ occasionally significant.
