## 3 Purification Routes Applied to Lignocellulosic Acid Hydrolysates
### Alkalitization/Overlining  
Alkali treatment is suitable for dilute acid hydrolysates because of the high quantity of base required to adjust the pH. It leads to great improvement in fermentability of the hydrolysates by chemically converting fermentation inhibitors [64] or precipitating and removing them by centrifugation or filtration [3; 70; 71]. Sodium hydroxide, potassium hydroxide, calcium hydroxide, and ammonia have already been tested for neutralization or alkalinization of lignocellulosic acid hydrolysate [64]. The best conditions to use sodium hydroxide are high temperatures in combination with moderate pHs or moderate temperatures in combination with high pHs. For instance, a pH adjustment to nine at 80 \({}^{\circ}\)C were the best conditions, with an increase of 110% in ethanol production from a lignocellulosic dilute acid hydrolysate compared to the value of the reference [70]. However, under similar conditions, sodium hydroxide treatment has so far been less efficient than overlining [70]. Overlining, i.e., the addition of calcium hydroxide, is considered tobe one of the most efficient processes for the removal of fermentation inhibitors [1; 70]. In the overlining process, the hydrolysate is detoxified by the addition of calcium hydroxide to adjust the pH to 9-10, leading to the precipitation of some of the furans and phenolic compounds, which are recovered by centrifugation or filtration [68; 72]. The resulting hydrolysate is readjusted to pH 5.5 with dilute sulfuric acid in order to carry out fermentation. An overlining treatment (pH adjustment to 10.5, 90 \({}^{\circ}\)C, 30 min) on an acid and enzyme treated rice hull hydrolysate, not only increased the maximum yield of ethanol, but also reduced the duration required for the maximum production of ethanol in the case of a simultaneous saccharification and fermentation process [51]. However, a study showed that overlining led to a drastic reduction of potential fermentable sugars, both at pH 5.5 or 10, with 28 and 47% sugar elimination, respectively [73]. This degradation of fermentable sugars resulting in moderate yields of ethanol cannot be acceptable for industrial implementation, as well as the formation of large amounts of gypsum [1; 70; 73]. In the NREL technical report mentioned in the previous section, it was reported that a significant amount of sugars (as much as 13%) could be lost because of side reactions occurring at high pH or it is pressed out with the wet gypsum [19]. Alkalinization with ammonia is an alternative to overlining. It presents the advantage of using milder conditions (pH 9 and 55 \({}^{\circ}\)C) and reducing the amount of precipitate formed [70]. Besides, detoxification with ammonia was found to be more efficient than with sodium hydroxide and calcium hydroxide regarding the yield and productivity of ethanol [64; 65]. Detoxification with ammonia was taken as a reference by NREL in their technical report (2011) [19]. In this process, ammonia was added to the lignocellulosic acid hydrolysate to raise its pH from about one to five before enzymatic hydrolysis. No precipitation occurred, and fermentation studies have indicated that there was no benefit to over-conditioning at high pH when using ammonia, so the hydrolysate was simply adjusted to the pH of the enzymatic hydrolysis in one step. Ammonia is more expensive than lime, but they claimed that the economic benefits of reduced sugar loss and reduced capital cost make ammonia the most economical alternative. It can also possibly reduce nitrogen requirements during the fermentation step, but this has not been demonstrated yet. Despite the optimization realized, alkalinization processes present some drawbacks including the large amount of base required, the impossibility to recycle the acid catalyst for the hydrolysis step, their lower efficiency compared to other purification techniques, and the acetic acid remaining fully in the hydrolyses [71; 72].