OMWW has a sugar content of about 1.6-5% (w/v) that can serve as a source for alcohol production (Fiestas Ros de Ursinos J.A., 1961a,b, 1967; Fernandez-Bolanos J. et al., 1983). A possibility for the utilization of sugars is their transformation to ethanol and recovery of the alcohol by distillation (Martinengri G.B., 1963; Oliveira de J.S., 1974). Oliveira de (1974) studied the effect of the yeasts Saccharomyces wine 31 B2, S. mollasses, bread yeast, Candida utilis and the natural fauna on ethanol production from OMWW and, with the exception of some reports (Martinengri G.B., 1963), no essential differences in the amounts of alcohol produced 0.5-0.57% (w/v) were found. Some of early efforts extracted alcohol from OMWW by evaporation. The extracted alcohol was used in foods, fuels, cosmetics, etc. (PT69240, 1979; PT69785, 1979).
OMWW has, though, a toxic effect on yeasts and to counteract this effect some investigators have recommended a dilution to 2% sugar (Fiestas Ros de Ursinos J.A., 1967). Bambalov G. et al. (1989) confirmed that fresh OMWW was unfavorable to yeast growth. Eight culture-collection yeast strains of various species and five yeast strains isolated and identified by the authors were tested for both growth in OMWW and fermentation of the sugars in the same media. The culture-collection yeast strains did not grow in an effluent containing 2.86% sugar (w/v), 8 g/ l phenolic substances, 4.58 g/l titratable acidity and pH 4.96, whereas the isolated strains of Torulopsis sp44. MK-1, Saccharomyces norbensis MC-1, S. oleaceus MC-2, and S. oleaginous grew well and fermented the sugars and produced alcohol in amounts of 1.63 to 1.38%, respectively. None of the yeasts grew in OMWW vacuum-concentrated to over 13-14% of dry matter. The strain of Torulopsis sp. MK-1 showed a higher stability.
One of the objectives of the EC project: AIR3-CT94-1987 ''BIOWARE'' was the recovery of ethanol by selecting yeast strains able to degrade the sugars present in OMWW, with a conversion efficiency near the theoretical values. It was concluded that the quantity of ethanol that might be produced using yeast strains to treat the OMWW is too low, due to the low levels of sugars and hence the production of ethanol was of no economical interest. All these studies confirmed that alcohol fermentation by yeasts is not an economical way of OMWW utilization, mainly due to the toxicity of the substrate and to a low alcohol concentration in the fermentation broth.
In another study solventogenic Clostridium spp. were used for butanol production from OMWW considering also the removal of COD (Wahner R.S. et al., 1988). As the OMWW did not support growth of the strains even if it was supplemented with nutrients, all the experiments were performed with diluted OMWW. A 50% dilution (COD value 100-120 g/l) was the lowest one, which allowed normal growth and solvent production. Butanol yields, ranging from 0.09 to 0.29 g per gram of sugar content and COD removals as high as 85% were achieved in small-scale experiments. The yield of butanol based on total sugar content was of the same order as the values reported for other agricultural wastes (Maddox I.S. and Murray A.E., 1983). The concentration of butanol achieved (2.8-8 g/l) and the COD reduction of 85% for OMWW with a high COD value (100-120 g/l) suggest that OMWW is an interesting raw material for butanol production. In addition, the acetone-butanol fermentation produces a significant reduction of COD value.
The recent development of a new two-phase centrifugation process for extracting olive oil in Spain has substantially reduced water consumption, thereby minimizing wastewater. However, a new high sugar content residue is still generated (2POMW). In a study the two fractions present in this residue (olive pulp and fragmented stones) were assayed as substrate for ethanol production by the simultaneous saccharification and fermentation (SSF) process (Ballesteros Perdices I. et al., 2001). Pretreatment of fragmented olive stones by sulfuric acid-catalyzed steam explosion was the most effective treatment for increasing enzymatic digestability. However, a pretreatment step was not necessary to bioconvert the olive pulp into ethanol.
44Torulopsis is considered by some authorities to be a synonym of Candida.
The olive pulp and fragmented olive stones were tested by the SSF process using a fed-batch procedure. By adding the pulp three times at 24 h intervals, 76% of the theoretical SSF yield was obtained. Experiments with fed-batch pretreated olive stones provided SSF yields significantly lower than those obtained at standard SSF procedure. The preferred SSF conditions to obtain ethanol from olive stones (61% of theoretical yield) were 10% substrate and addition of cellulases at 15 filter paper units/g of substrate.
ES2056745 (1994) discloses a process for obtaining mannitol45 and derived products from OMWW, olive twigs, leaves, and stalks. The process comprises the steps of (i) partial or complete drying; (ii) extraction of mannitol by means of alcohols or hydroalcohols; (iii) the extracts obtained are defecated by the addition of basic lead acetate so as to eliminate the materials which accompany them; (iv) the extracts so purified, are concentrated and the mannitol is isolated by crystallization with alcohol (96°). The process is slightly different depending on the raw material which is processed, needed prior concentration, and even drying where the raw material is OMWW. A modified process for obtaining mannitol and its derivatives from 2POMW is illustrated in Fig. 10.6 (ES2060549, 1994).
ES2143939 (2000) discloses the use of a steam explosion process to extract mannitol from olive cake coming from a three-phase centrifugation system. With this process the olive cake is treated in a 2 l steam explosion unit at temperatures around 200°C for time periods of 2-4 minutes, there then occurring an abrupt decompression and the subsequent unloading of the reactor. All the mannitol present in olive cake is separated out and recovered and by means of various simple purification stages (ultrafiltration, ion exchange, and fractionated crystallization) permissible in food industry, can achieve a yield with a high degree of purity.
As part of the effort to contribute to the recycling of OMWW, it was investigated whether OMWW could be used as raw material for biosurfactant production (Mercade M.E. et al., 1993). Different biosurfactant-producing strains were assayed and several strains of Pseudomonas sp. were able to grow on OMWW as the sole carbon source and accumulate rhamnolipids. Samples of OMWW were diluted depending on their composition and it was only necessary to add NaNO3 (0.25 g/l). Conversion yields of 0.058 g of rhamnolipid per gram of substrate (OMWW) were achieved and COD of OMWW was reduced approximately 50% in 72 h. An improved process was developed for rhamnolipid production from OMWW as feedstock in a stirred tank fermentor with Pseudomonas aeruginosa (Mercade; M.E. and Manresa M.A., 1994). P. aeruginosa JAMM (NCIB 400440) was selected for its capacity to decrease surface tension when grown on OMWW (Mercade M.E., 1990 and Mercade M.E. et al., 1993). The surfactant concentration increased during
45White crystalline, sweetish, water-soluble, carbohydrate alcohol, HOCH2(CHOH)4CH2OH, occurring in three optically different forms.
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