Steam Stripping

Steam stripping, also known as steam distillation, is a process of removing temperature — sensitive compounds that cannot be separated by normal distillation due to decomposition at high sustained temperatures. It has been used to remove various organic contaminants from process plant waste water streams. Steam stripping can also be used to detoxify lignocellu — losic hydrolysates (Yu et al. 1987- . Leonard and Peterson — 1947) used steam stripping to remove inhibitory volatiles, such as furfural and acetic acids, from hydrolysates of maple and spruce. Maddox and Murray (1983) passed steam through hydrolysates of Pinus radiata to achieve a liquor temperature of 90°C for 15 minutes. Treatment of the hydrolysates by steam stripping followed by passage of the hydrolysates through activated carbon led to successful fermentation, but the treatment procedure caused about 30% sugar losses. The high stripping temperatures allow the removal of heavy and soluble organic compounds. The only waste generated in steam stripping is a small amount of concentrated organics. These organic wastes are easily processed by incineration, biological treatment, or recycling. However, steam strip­ping is not a good solution for hydrolysates that contain nonvolatile phenolics with high boiling points.

Evaporation

Evaporation when used as a detoxification method removes only volatile inhibitors. However, in previous studies, the volatile compounds did not affect either the enzymatic hydrolysis or

Detoxification Method Effectiveness

Physical

Steam stripping Remove volatile inhibitors or inhibiting end

products such as furfural and acetic acid

Evaporation

Solvent extraction Remove both volatile and nonvolatile

inhibitors pH dependently

Excellent biocompatibility; rapid mass transfer due to low-interfacial tension; remove both volatile and nonvolatile inhibitors; and potential for in situ extractive fermentation

Excellent biocompatibility; remove furans, phenolics, and aliphatic acids polarity dependently; high concentration factor of inhibitors

Alleviate the inhibitory effect of toxic compounds

Remove both volatile and nonvolatile inhibitors

The most commonly used method; precipitate a wide range of inhibitors

Extensive mediation and precipitation of inhibitors

Table 11.1. Continued. Detoxification Method Effectiveness Disadvantages References Reducing substances, NaHS03, Na2S205, KHS03, Na2S, etc. Overcoming unfavorable oxidation — reduction potentials in hydrolysates Less efficient than overliming Leonard and Flajny 1945; Larsson et al. 1999 Neutralization + zeolite Diatomaceous earth Activated carbon Wood charcoal Extensive removal of inhibitors Less efficient than overliming Sugar loss Need specially prepared wood charcoal and long treatment time Eken-Saragoglu and Arslan 2000 Ribeiro et al. 2001 Maddox and Murray 1983 Miyafuji et al. 2003 Polymeric adsorbents Mixed bed resin Ion-exchange resins Most efficient in inhibitors removal Costly and might cause sugar loss Weil et al. 2002 Tran and Chambers 1986 Florvath et al. 2005; Chandel et al. 2007 Biological Mutant S. cerevisiae Effective in utilizing acetic acid and keeping sugars intact Not effective in reducing other inhibitors Schneider 1996 Fugal isolate, Coniochaeta ligniaria NRRL 30616 Metabolizes furfural, HMF, as well as phenolics, aliphatic acids, and aldehydes Long treatment time Lopez et al. 2004; Nichols et al. 2008 Laccase and peroxidase from the white-rot fungus Highly efficient in reducing phenolics Not effective in reducing furans and aliphatic acids, etc.; long treatment time Jonsson et al. 1998; Larsson et al. 1999; Chandel et al. 2007 Adaptation Makes organisms more tolerant to the inhibitors Long treatment time and complicated manipulation Mussatto and Roberto 2004; Martfn et al. 2007; Agbogbo et al. 2008; Dinh et al. 2008

the fermentation significantly even at high concentrations. In contrast, the nonvolatile com­pounds severely affected both the hydrolysis and the fermentation (Palmqvist et al. 1996a, b). Palmqvist et al. (1996a) assessed the inhibitory effect of both the evaporation condensates and nonvolatile stillage by fermentation using S. cerevisiae. The most volatile fraction of a willow hemicellulose hydrolysate obtained by roto-evaporation (using a rotary evaporator) slightly decreased the ethanol productivity compared with a reference fermentation contain­ing no volatile fraction of hemicellulose hydrolysates. But in the nonvolatile fraction obtained by roto-evaporation, the ethanol yield decreased from 0.37g/g in the reference fermentation (glucose and nutrients) to 0.31 g/g in the treated lignocellulosic hydrolysates fermentation, and the average ethanol fermentation rate, r2h. decreased from 6.3 to 2.7 g/h. As a result, commercial application of evaporation for hydrolysates detoxification may be limited.