Valorisation of cellulosic rejections from wastewater treatment plants through sugar production.

The widespread use of wipes and other sanitary products made of nonwoven fibres has led to an enormous problem in wastewater treatment systems that has been underestimated for some time. To date, there are no practical alternatives for recycling and valorisation. In this study, cellulosic rejections recovered from a wastewater treatment plant in Barcelona (Spain) were characterised and treated using hydrothermal and enzymatic methods to obtain free sugars. Steam explosion and autoclave pre-treatments were performed at different temperatures (120, 130, or 150 °C) and residence times (10-40 min) under neutral, acidic or basic conditions. The solids obtained after the pre-treatment, as well as the untreated material, were subjected to enzymatic hydrolysis using commercial enzymes. The untreated substrate reached the highest sugar production: 29 g glucose and xylose per 100 g of the cellulosic rejections, equivalent to 86% of the sugars contained in the initial material. These sugars can subsequently be transformed into biofuels or bioproducts within a biorefinery approach.

Biorefinery based on olive biomass. State of the art and future trends.

With currently more than nine million hectares, olive tree cultivation has spread worldwide, table olives and olive oil as the main products. Moreover, a number of by-products and residues derived from both tree cultivation and the process of industrial olive oil production, most having no practical applications, are obtained yearly. This paper reviews the research regarding these by-products, namely biomass from olive tree pruning, olive stones, olive pomace and wastewaters obtained from the process of olive oil production. Furthermore, a wide range of compounds has been identified and can be produced using a broad definition of the term biorefinery based on olive tree biomass. As an example, this paper reviews ethanol production as one of the main proposed applications, as well as research on other value-added products. Finally, this paper also assesses recent technological advances, future perspectives and challenges in each stage of the process.

Sugar production from barley straw biomass pretreated by combined alkali and enzymatic extrusion.

A pretreatment that combines a thermo-mechanical process (extrusion) with chemical and biological catalysts to produce fermentable sugars from barley straw (BS) biomass was investigated. BS was firstly extruded with alkali and then, the pretreated material (extrudate) was submitted to extrusion with hydrolytic enzymes (bioextrusion). The bioextrudate was found to have 35% (w/w dwb) of total solids in soluble form, partly coming from carbohydrate hydrolysis during bioextrusion. About 48% of soluble solids dry weight is comprised by sugars, mostly glucose and xylose. Further enzymatic hydrolysis of bioextrudate could be successfully carried out at high solid loading level of 30% (w/v), with sugar production yield of 32 g glucose and 18 g xylose/100g bioextrudate at 72 h incubation (equivalent to 96 and 52 g/l concentration, respectively). These results, together with the high level of integration of the process, indicate a great potential of this pretreatment technology for sugar production from lignocellulosic substrates.

Ethanol production from glucose and xylose obtained from steam exploded water-extracted olive tree pruning using phosphoric acid as catalyst.

In this work, the effect of phosphoric acid (1% w/w) in steam explosion pretreatment of water extracted olive tree pruning at 175°C and 195°C was evaluated. The objective is to produce ethanol from all sugars (mainly glucose and xylose) contained in the pretreated material. The water insoluble fraction obtained after pretreatment was used as substrate in a simultaneous saccharification and fermentation (SSF) process by a commercial strain of Saccharomyces cerevisiae. The liquid fraction, containing mainly xylose, was detoxified by alkali and ion-exchange resin and then fermented by the xylose fermenting yeast Scheffersomyces stipitis. Ethanol yields reached in a SSF process were close to 80% when using 15% (w/w) substrate consistency and about 70% of theoretical when using prehydrolysates detoxified by ion-exchange resins. Considering sugars recovery and ethanol yields about 160g of ethanol from kg of water extracted olive tree pruning could be obtained.

Effect of water extraction on sugars recovery from steam exploded olive tree pruning.

Biomass of olive tree pruning can be considered a suitable raw material for the production of ethanol due to its high content of potentially fermentable carbohydrates. However its high extractives content could cause condensation reactions between extractives and acid insoluble lignin during pretreatment, hindering the enzymatic hydrolysis of pretreated material. In this work, the effect of extractives removal before steam explosion of olive tree pruning was evaluated. The objectives are to recover as much glucose as possible in the extraction stage and to avoid the condensation reactions. The effect of temperature and time of water extracted material on sugars recovery was studied using a response surface method according to a central composite design. Extractive removal previous to steam explosion resulted in 20% more total sugars recovery in comparison to a material without water extraction stage.

Effect of endoxylanase and α-L-arabinofuranosidase supplementation on the enzymatic hydrolysis of steam exploded wheat straw.

The cost and hydrolytic efficiency of enzymes are major factors that restrict the commercialization of the bioethanol production process from lignocellulosic biomass. Hemicellulases and other accessory enzymes are becoming crucial to increase enzymatic hydrolysis (EH) yields at low cellulase dosages. The aim of this work was to evaluate the effect of two recombinant hemicellulolytic enzymes on the EH of steam pretreated wheat straw. Pretreatments at two severity conditions were performed and the whole slurry obtained after steam explosion pretreatment was employed as substrate. An endoxylanase (Xln C) from Aspergillus nidulans and an α-L-arabinofuranosidase (AF) from Aspergillus niger, have been applied in combination with cellulase enzymes. A degree of synergism of 29.5% and increases up to 10% in the EH yields were obtained, showing the potential of accessory activities to improve the EH step and make the whole process more effective.

Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review.

Biofuel produced from lignocellulosic materials, so-called second generation bioethanol shows energetic, economic and environmental advantages in comparison to bioethanol from starch or sugar. However, physical and chemical barriers caused by the close association of the main components of lignocellulosic biomass, hinder the hydrolysis of cellulose and hemicellulose to fermentable sugars. The main goal of pretreatment is to increase the enzyme accessibility improving digestibility of cellulose. Each pretreatment has a specific effect on the cellulose, hemicellulose and lignin fraction thus, different pretreatment methods and conditions should be chosen according to the process configuration selected for the subsequent hydrolysis and fermentation steps. This paper reviews the most interesting technologies for ethanol production from lignocellulose and it points out several key properties that should be targeted for low-cost and advanced pretreatment processes.

Effect of different cellulase dosages on cell viability and ethanol production by Kluyveromyces marxianus in SSF processes.

This study was aimed to study the effect of commercial cellulases (Celluclast 1.5 LFG) on Kluyveromyces marxianus CECT 10875 growth and ethanol production in SSF processes. Preliminary tests carried out in glucose (50 g/L) fermentation medium showed that high enzyme amounts (2.5-3.5 FPU/mL) could cause a negative effect on K. marxianus growth rate and viable cells number. However, the maximum ethanol production was not affected and about 86% of the theoretical (22 g/L) was reached in all cases independently of the enzyme dosage. In SSF experiments, cell viability was always affected by enzyme loading. Nevertheless, slight differences observed on cell viability during glucose fermentation processes with the detected concentrations of the additives did not justify the negative effect observed in SSF experiments.

Application of Fenton's reaction to steam explosion prehydrolysates from poplar biomass.

The application of Fenton's reaction to enhance the fermentability of prehydrolysates obtained from steam explosion pretreatment of poplar biomass was studied. Reaction conditions of temperature and H2O2 and Fe(II) concentrations were studied. The fermentability of prehydrolysate treated by Fenton's reaction was tested by using different inoculum sizes of thermotolerant strain Kluyveromyces marxianus CECT 10875. The highest percentages of toxic compound degradation (ranging from 71 to 93% removal) were obtained at the highest H2O2 concentration tested (50 mM). However, a negative effect on fermentability was observed at this H2O2 concentration at the lower inoculum loading. An increase in inoculum size to 0.6 g/L resulted in an enhanced ethanol fermentation yield of 95% relative to control.

Selection of thermotolerant yeasts for simultaneous saccharification and fermentation (SSF) of cellulose to ethanol.

A total of 27 yeast strains belonging to the groups Candida, Saccharomyces, and Kluyveromyces were screened for their ability to grow and ferment glucose at temperatures ranging 32-45 degrees C. K. marxianus and K. fragilis were found to be the best ethanol producing organisms at the higher temperature tested and, so, were selected for subsequent simultaneous saccharification and fermentation (SSF) studies. SSF experiments were performed at 42 and 45 degrees C, utilizing Solkafloc (10%) as cellulose substrate and a cellulase loading of 15 FPU/g substrate. Best results were achieved at 42 degrees C with K. marxianus L. G. and K. fragilis L. G., both of which produced close to 38 g/L ethanol and 0.5 ethanol yield, in 78 h.

Enzymatic hydrolysis of lignocellulosic biomass from Onopordum nervosum.

Some properties of the cellulolytic complex obtained from Trichoderma reesei QM 9414 grown on Solka floc as carbon source and its ability to hydrolyze the lignocellulosic biomass of Onopordum nervosum Boiss were studied. The optimum enzyme activity was found at temperatures between 50 and 55 degrees C and pH ranging from 4.3 to 4.8. Hydrolysis of 4-nitropnenyl-beta-D-glucopyranoside (4-NPG) and cellobiose by the beta-glucosidase of the complex, showed competitive inhibition by glucose with a K(i) value of 0.8 mM for 4-NPG and 2. 56 mM for cellobiose. Enzymatic hydrolysis yield of Onopordum nervosum, evaluated as glucose production after 48 h, showed a threefold increase by pretreating the lignocellulosic substrate with alkali. When the loss of glucose incurred by de pretreatment was taken into account, a 160% increase in the final cellulose to glucose conversion was found to be due to the pretreatment.