Lactic acid production from paper sludge by SSF with thermotolerant Rhizopus sp.

Background Rhizopus fungi is suitable for the production of lactic acid, which is the backbone material of polylactic acid used as green plastic from lignocellulosic biomass, since it can grow and ferment in simple medium with various carbon sources such as starch and cellulose. Although paper sludge (PS) contains a lot of cellulosic fibers and in general was incinerated for volume reduction and heat recovery, other efficient utilizations have hardly been developed. In effective production of lactic acid from PS, the research of the extraction of cellulosic fiber from raw PS to obtain effectively fermentable sugars by cellulase and the selection of lactic acid microorganism are necessary. In this study, the PS pretreatment method with NaOH and HCl and the optimization of cellulase reagent were achieved, and also a desirable thermotolerant Rhizopus was selected. Finally, the production of lactic acid from the treated PS at 40 °C by simultaneous saccharification and fermentation (SSF) with the strain and an optimized cellulase cocktail was investigated. Results Rhizopus oryzae NBRC 5384 was selected for thermotolerant lactic acid production from Rhizopus library because of its heat tolerance up to 40 °C and high lactic acid production of 80 g/L. The strain can ferment to lactic acid from hexose, pentose, sugar alcohol, disaccharide and starch. The soaking of raw PS in NaOH and HCl was able to reduce effectively inorganic materials and other reagents for repulping, and the content of Al and Ca per PS dry matter was mainly decreased from 32.9 and 30.8 to 14.1 and 1.66 %, respectively. SSF of the treated PS of 50 g/L with optimized cellulase cocktail and 5384 at 40 °C resulted in lactic acid production of 9.33 g/L for 96 h. Conclusion The thermotolerant Rhizopus fungus was found based on its high performance in lactic acid production at high temperature from not only glucose, but also other various carbon sources including polysaccharides and the secretion of amylases and cellulases. The treatment of raw PS by NaOH and subsequent HCl was able to remove a large amount of inorganic materials with decrease of hydrophobicity. In SSF of the treated PS with the strain and the optimized cellulase cocktail, lactic acid was able to be produced. However, the increase of initial PS concentration in SSF led to the decrease of the yield with ethanol production, because of limited aeration due to increase of density. An appropriate oxygen supply to the strain is necessary to improve lactic acid production.