**4.4 Combination of pretreatment methods**

It seems that a single pretreatment method cannot yield an expected result. The combination of pretreatment methods has been a conventional procedure

in producing bioethanol from lignocellulose. First, the biomass needs to be sizereduced by a mechanical process, such as milling, cutting, and crushing, optionally followed by steam explosion before being soaked in alkaline or acidic solution (alkaline pretreatment is more preferred). Second, the pretreated material is washed and neutralized. The final product of this step is always a puffing material, which is ready for further fermentation [30, 31]. In addition, the reusability of internal waste was also discussed to enhance the sustainability of the lignocellulosic bioethanol production and reduce chemical cost [32].
