Key reasons to improve waste management in vegetable oil processing

There are several environmental and economic benefits arising from the application of new waste management processes.

540 Handbook of waste management and co-product recovery 20.2.1 Economic and environmental reasons

In the case of olive oil, the application of the two-phase extraction process produces only a solid by-product that can be dried and extracted by solvent. The new DOR can be used for cogeneration of electric power, used in combination with saprobic fungi for removal of monomeric phenols. The by-product of the two-step olive mill process can represent an alternative to synthetic fertilizers and amendments. The development of environmentally acceptable methods for disposal of vegetable oil wastewaters still remains a problem, whereas the degradation of the toxic compounds contained in this wastewater will certainly enhance the quality of the remediated waters and of the sedimentation muds, in view of their safe utilization as fertilizers. From this view point, bioremediation of vegetable oil mill wastewater by chemical alteration induced for example by Azotobacter vinelandii and use of the end product as a conditioner and liquid organic fertilizer appears an economically convenient approach. A different approach regards the use of superabsorbent polymers in an innovative process that allows vegetable mill wastewaters to be used as fertilizers through the removal of molecular weight fractions of COD and phenolic compounds in an integrated treatment process of oil mill effluent. Vegetable oil effluents can be effectively pre-treated and discharged into a municipal sewerage system. Pre-treatment of effluents comprises screening and air flotation to remove fats and solids; it is normally followed by biological treatment. If space is available, land treatment or pond systems are potential treatment methods. Other possible biological treatment systems include trickling filters, rotating biological contactors and activated sludge treatment. The pre-treating process also includes proper circulation of air, using an extracting and cleaning system, to maintain dust at acceptable levels.

Vegetable oil wastewaters are rich in organic and inorganic compounds and may also be regarded as an inexpensive source of products to be recovered because of their potential economic interest and/or ability to be transformed into products for use in agriculture and industry. OMWW are rich in antioxidant compounds that could be recovered from the matrix and employed both in preservative chemistry and, following appropriate trials to evaluate their safety and efficacy, as prophylactic agents in the prevention of certain radical-induced human diseases. The possibility of isolating natural antioxidants like hydroxytyrosol from wastewater extracts is high. Hydroxytyrosol, the most active component of OMWW extracts, is of particular interest because it is amphiphilic and thus it acts at the oil-water interface and in systems where both oil and water phases are present, such as emulsions (Auroma et al., 1998). Hydroxytyrosol is characterized by a high antioxidant activity, which is comparable with that of the usual synthetic antioxidants such as 2,6-di-tert-butyl-p-hydroxytoluene (BHT) and 3-tert-butyl-6-hydroxyanisole (BHA). In vitro it also inhibits the oxidation of low-density lipoproteins (LDLs) and confers both cell protection and dietetic properties to virgin olive oil. Analysis of OMWW shows that their phenolic compounds content fluctuates from 0.5 to 1.8%. Results in vitro demonstrate that hydroxytyrosol inhibits human LDL oxidation (a process included in the pathogenesis of atherosclerosis), scavenges free radicals, inhibits platelet aggregation and the production of leucotriene for human neutrophyls (which is indicative of anti-inflammatory properties) and confers cell protection. It has also been demonstrated that hydroxytyrosol acts in vitro against both Gram-positive and Gram-negative bacteria, which are causes of infections in the respiratory and intestinal tracts. Nevertheless, and despite recent bioavailability studies, more studies are required to demonstrate the antioxidant and antimicrobial effectiveness of hydroxy-tyrosol in vivo. Larger amounts of this compound are required at competitive prices, so that it can be used, for instance, as a preservative in foods. Fernández-Bolaños et al. (2002) showed that large quantities of phenolic compounds, especially hydroxytyrosol, can be obtained from olive cake (three-phase process) and olive stones, in both cases by means of steam treatment. Hydroxytyrosol can also be recovered from OMWW (three-phase process) and from the washing water using the Spanish-style green table olive process. Diverse synthesis procedures for the production of hydroxytyrosol have also been developed. However, the production methods so far proposed are expensive and produce low yields. Other components such as flavonoids, anthocyanins and tannins are of potential biological interest due to their antioxidant activities (Vinson et al., 1995). From a commercial point of view, the most widely employed antioxidants are those indigenous to foods, the water-soluble ascorbate and the lipid-soluble butylated hydroxytoluene, butylated hydroxyanisole, the esters of 3,4,5-trihydroxybenzoic acids and vitamin E. Plant extracts are also in use, and their share of the antioxidant market is expected to grow by 15% by the year 2006 (Krishnakumar and Gordon, 1996).

Nowadays, there is growing interest in novel sources of natural antioxi-dants due to the recognized involvement of reactive oxygen species in the onset of several human diseases (Auroma et al., 1998) and in the oxidative degradation of food, animal feed and other goods such as cosmetics. Other added-value compounds such as xylose, arabinose, glucose, oligosaccha-rides, mannitol, vitamin E, sterols and protein can also be isolated; waste from frying oils can be a valid economic substrate for biosurfactant production. From an economic point of view, vegetable wastewaters can also be exploited as a growth medium for the production of extracellular enzymes such as laccase and manganese peroxidase and also for the extraction of polymerins as bioamendments and metal biointegrators. Waste cooking oils can become an important alternative to conventional fossil fuels in the production of biodiesel. Fat- and oil-containing wastes from the fat or oil separators of hotels, canteens, kitchens and bakeries are considered to be an excellent supplementary substrate for biogas plants due to the high specific methane yield. It is also feasible to synthesize vegetable oil-derived esters as a diesel fuel substitute or additive using methanol and KOH as catalyst. Biodiesel is created via transesterification, a chemical reaction between a fatty substance such as vegetable oil, an alcohol such as ethanol or methanol, and a catalyst such as lye. The two by-products of this reaction are biodiesel and glycerin. Since Rudolf Diesel's invention of the compression (diesel) engine over 100 years ago, it has been known that the engine can operate on vegetable oils. Biodiesel, the transesterified product of vegetable oil, is considered to be the most promising diesel fuel substitute. Biodiesel development can now be found in 28 countries, with Germany and France so far being the world's largest producers of biodiesel fuel. Recently, Japan started a project in Kyoto to use biodiesel at a commercial-level with municipal city-owned trucks running on 100% biodiesel fuel; this trial has been extended to 81 municipal buses with a blend of 20% biodiesel and 80% petroleum diesel fuel. Most biodiesel is produced from soybean oil, though biodiesel is also commonly produced from rapeseed (canola) and mustard seed, and any oil from crops or animal fat can be used. It is also possible to make biodiesel using waste vegetable oil from restaurant fryers. The benefits of biodiesel are as follows: it reduces all emissions except NOx; it comes from renewable sources such as soybeans; it is carbon neutral and thus does not contribute to the greenhouse effect; it biode-grades faster than sugar and is less toxic than table salt; it has excellent lubricating properties, is an engine detergent and is sulphur-free. Biodiesel can 'gel' at temperatures below about 25 °F; this problem can be avoided by adding petroleum diesel during cold snaps.

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