Washing Lime-Pretreated Rice Straw with Carbonated Water Facilitates Calcium Removal and Sugar Recovery in Subsequent Enzymatic Saccharification

Abstract

Generally, Ca(OH) 2 pretreatment of lignocellulosics for fermentable sugar recovery requires a subsequent washing step for calcium removal and pH control for optimized saccharification. However, washing Ca(OH) 2-pretreated feedstock with water is considered problematic because of the low solubility of Ca(OH) 2 and its adsorption to biomass. In this study, we estimated the availability of carbonated water for calcium removal from the slurry of Ca(OH) 2-pretreated rice straw (RS). We tested two kinds of countercurrent washing sequences, four washings exclusively with water (W4) and two washings with water and subsequent two washings with carbonated water (W2C2). The ratios of calcium removal from pretreatment slurry after washing were 64.2 % for the W4 process and 92.1 % for the W2C2 process. In the W2C2 process, 49 % of the initially added calcium was recovered as CaO by calcination. In enzymatic saccharification tests under a CO 2 atmosphere at 1.5 atm, in terms of recovery of both glucose and xylose, pretreated, feedstock washed through the W2C2 process surpassed that washed through the W4 process, which could be attributed to the pH difference during saccharification: 5.6 in the W2C2 process versus 6.3 in the W4 process. Additionally, under an unpressurized CO 2 atmosphere at 1 atm, the feedstock washed through the W2C2 process released 78.5 % of total glucose residues and 90.0 % of total xylose residues. Thus, efficient removal of calcium from pretreatment slurry would lead to not only the recovery of added calcium but also the proposal of a new, simple saccharification system to be used under an unpressurized CO 2 atmosphere condition.

Evaluation of the Enzymatic Saccharification Efficiency of an Energy Crop, <i>Erianthus arundinaceus</i>, Pretreated with Ca(OH)₂ Using both Countercurrent Washing System and pH Adjustment by Nonpressurized CO₂

“…To evaluate washing efficiency, the washing water (WW2-P4, WW3 * -P4, WW4-P4) was used for both model experiments #1 and #2. The calcium (Ca) residual ratio in the pretreated ER washed twice with water (W2R, 40 %) was lower than that of RS (48 %) reported in an earlier study 5) ( Fig. 2 ).…”

“…In our previous investigations, we have evaluated the removal efficiency of alkali from the pretreated RS by a washing step and the saccharification efficiency of washed substrates either under atmospheric pressure or using pressurized CO 2 gas. 5) In the present report, based on the results of the abovementioned research, we evaluated both the washing process and the enzymatic saccharification efficiency of Ca(OH) 2 -pretreated ER, which represents another strategic raw plant material. Furthermore, we discussed the overall comparative performance of the two types of raw material conversion processes that we have developed so far.…”

“…Web site). 5) Specifically, after the addition of Ca(OH) 2 and distilled water to ER, the plant material was packed in an aluminum package and stored at room temperature for one month. The basic washing process involved washing the pretreated ER twice with water and then twice with water infused with CO 2 .…”

“…1 (28.3 mM) was higher than that of RS (16.7 mM) in the past experiment. 5) The difference of lignin contents between ER and RS (25.2 % vs . 15.8 %) might affect the released acetic acid contents, and calcium washing efficiencies as a result.…”

“…The Ca residual rate of ER washed twice with water and twice with carbonated water (W2C2R) was 9 %, which was similar to that of rice straw (8 %) washed by the same method. 5) The recovery efficiency of total glucans (TG), mainly composed of cellulose and starch, is considered an important parameter in washing step since glucans represent substrates for the forthcoming saccharification process. Total xylans (TX), which are mainly composed of xylose, must also be efficiently recovered if xylose is available for microorganisms in the fermentation process.…”