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1. A method of enzymatically hydrolyzing pretreated lignocellulosic biomass at high solids concentration, said method comprising: (a) providing pretreated lignocellulosic biomass containing cellulose; (b) introducing said pretreated lignocellulosic biomass to an enzymatic hydrolysis reactor under effective hydrolysis conditions and in the presence of enzymes including cellulases, to enzymatically hydrolyze said cellulose to glucose monomer and glucose oligomers, wherein said pretreated lignocellulosic biomass is present in said enzymatic hydrolysis reactor at a solids concentration of at least 20 wt %; and (c) circulating a liquid stream in a circulation line configured from an output of said enzymatic hydrolysis reactor back to an input of said enzymatic hydrolysis reactor, wherein said glucose monomer is removed from said circulation line, wherein said method further comprises a glucose oligomer hydrolysis step to generate a second amount of glucose monomer from said glucose oligomers, wherein said glucose oligomer hydrolysis step is integrated with step (c), and wherein said method further comprises introducing an external lignin-containing surfactant to said enzymatic hydrolysis reactor during step (b) or step (c), wherein said solids concentration is at least 25 wt %.
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