• Ozonation

Ozone, the strongest chemical oxidant after fluorine, can be utilised as pre-treatment for removing the lignin from the lignocellulose biomass and finally improving the enzymatic degradation. Compared with other conventional pre-treatment methods, no harm residue such as acids or mineral bases are produced after ozonation [41]. Unlike ultrasound pre-treatment that is carried out at extremely high temperatures and pressures, ozone reactions can occur even at room temperature and pressure. Besides, ozone could be generated in situ in the bioethanol plant, thus avoiding transportation and storage issues.

Although ozonation has been demonstrated to be an effective pre-treatment, it is regarded as uneconomical due to the assumed need for lignin mineralization [42]. In order to overcome this issue, ozonation has been applied to tannic acid to simulate the effects of ozonation process on biomass pre-treatment for bioethanol production [42]. A high concentration of tannic acid solution (60 g/dm3), as a lignin model, was treated by ozonation. Most of tannic acid was disappeared after 3.5 h treatment. As a negative result, tannic acid negatively affected cellulases activity. Despite the aforementioned doubt of widespread application to biomass as a pre-treatment, it was stated that a short-time ozonation, as an alternative, could reduce the pre-treatment energy costs from about ∼\$104.28 to ∼\$2.22 per ton of biomass and thus cut down the labor costs and operation times [42]. In spite of these advantages, to the best of our knowledge there are not available works on the use of ozonation as OP pre-treatment for bioethanol production.

• Steam explosion

Steam explosion, where the biomass is subjected to pressurised steam injected to a high temperature (180–240 ◦C) for a short time (from 10 s to several minutes) and ends with a sudden decompress of the system, is a widely used pre-treatment of biomass. The cellulose and lignin degraded by the high temperature whilst the tissue structure damaged during the rapid pressure release. In this way, the biomass is easier to be hydrolysed and fermented. A high d-xylose yield can be achieved especially when an acid catalyst is applied. The role of sulphuric acid (as catalyst) is to hydrolyse the hemicellulose into monomers without degrading them furfural and 5-hydroxy-methyl furfural (5-HMF). As a result, the addition of an acid catalyst can reduce the processing temperature to 150–200 ◦C, thus improving the subsequent enzymatic hydrolysis. Steam explosion has been applied to OP and these facts have been verified. In this way, it was reported the solubilisation of hemicellulose from olive tree pruning was improved by using steam explosion at temperatures between 190 ◦C and 240 ◦C [13]. Other acids have been used to improve the steam explosion of OP, such as phosphoric acid [43]. Furthermore, the steam explosion of olive tree pruning has been investigated at pilot scale to maximize the glucose yield in the subsequent enzymatic hydrolysis [44].

• Autohydrolysis or liquid hot water (LHW) pre-treatment

This technique is quite similar to the steam explosion and leads to similar results. LHW refers to the process of solubilisation of hemicellulose in pressurized water at temperatures ranging between 165 ◦C and 225 ◦C [45]. The hemicellulosic acetyl groups are rapidly released, thus hydrolyzing the rest of hemicellulose (autohydrolysis). To determine the effect of this pre-treatment, the severity parameter, *R*0, is calculated as follows:

$$R\_0 = \int\_0^t \exp^{\frac{T(t) - T\_R}{w}} dt$$

where *T*(*t*) is the temperature (◦C)–time (min) function, calculated graphically, and *T*<sup>R</sup> a reference temperature (100 ◦C) below which the autohydrolysis can be considered of scarce significance [46]. The value of 14.75 is the generally used for the parameter *w* in the autohydrolysis of olive pruning debris [47].

LHW pre-treatment has been applied to OP in the temperature range 150–210 ◦C for short reaction times (0–5 min) at the selected temperature (average *R*<sup>0</sup> = 3.54 ± 0.06), resulting in a complete solubilisation of hemicellulose [4,47]. Similarly to steam explosion, LHW leads to a hemicellulosic oligomers solution which requires a further acid hydrolysis to release the monomeric sugars, if they are intended to be fermented [48]. To avoid the application of two pre-treatments, it has been assayed the addition of dilute acid into the pressurized water reactor, thus performing simultaneously both autohydrolysis and dilute-acid hydrolysis. Although it is performed in pressurized water reactors, this combined technique is generally referred as dilute-acid hydrolysis in spite of using high pressures, achieving high sugars yields and complete hemicellulose solubilisation when applied to OP [4].
