Introduction

Vegetable oils and fats are principally used for human consumption but are also used in animal feed, for medicinal purposes and for certain technical applications. The vegetable oil market is strongly correlated to the protein meal market as both are largely co-products resulting from the processing of oilseeds. Supply and demand conditions in one market affect the other. For 1999-2000, world production of edible oil was 86.4 million tonnes, consisting of 85.2 million tonnes of vegetable oil and 1.2 million tonnes of marine oil (AAFC, 2000). Among vegetable oils, palm oil is the oil produced in greatest amounts. In 1998, 18.2 million tonnes of palm oil were produced worldwide (AAFC, 2000). The production in south-east Asia was 14.95 million tonnes per year, with Malaysia contributing 8.6 million, Indonesia 5.9 million, and Thailand 0.45 million tonnes per year (FAO, 1998). Due to the rising demand for plant oil for food, feed and technical applications, oil palm plantations were rapidly extended in Indonesia, which is the leading palm oil producer together with Malaysia, and led to an increased production of 7.5 million tonnes in 2000 in Indonesia (Pamin, 1998).

In the past few decades huge amounts of vegetable oil wastes have been produced worldwide. For the production of 1 tonne of crude palm oil, far more than 1 tonne of empty fruit bunches is left behind as a waste product, consisting of 16% lignin, 50% cellulose, 23% hemicellulose, 3.6% oil and 8% ash. The solid wastes are either burnt to generate steam for steaming the nuts in the fresh bunches, deposited in land depressions or composted. The latter is only executed to a minor extent, mainly due to phytohygienic concerns (for instance, the distribution of Ganoderma, which causes stem fouling) and a lack of experience. An alternative to burning of empty fruit bunches is pyrolysis to generate charcoal (Lua and Guo, 1999).

20.1.1 Waste characteristics

In the Mediterranean countries, olivoculture and production of olive oil represents one of the most important, and oldest agricultural activities. The olive oil extraction industry produces liquid effluents termed olive oil mill wastewaters (OMWW). The disposal of OMWW is one of the main environmental problems in the Mediterranean area, where the greatest quantities of olive oil are produced, with a large volume of wastewaters within only a few months (from November to February). Nevertheless, a series of environmental issues, such as the recovery and detoxification of the effluents from olive oil mill plants, appear to be far from being resolved. A large amount of waste is produced during oil processing. Dust is generated in materials handling and in the processing of raw materials, including in the cleaning, screening and crushing operations. For palm fruit, about 2-3 m3 of wastewater are generated per metric tonne of crude oil. The wastewater is high in organic content, resulting in a biochemical oxygen demand (BOD) of 20 000-35 000 mg L-1 and a chemical oxygen demand (COD) of 30 00060 000 mg L-1. In addition, the wastewaters are high in dissolved solids (10 000 mg L-1), oil and fat residues (5000-10 000 mg L-1), organic nitrogen (500-800 mg L-1) and ash residues (4000-5000 mg L-1). Seed dressing and edible fat and oil processing generate approximately 10-25 m3 of waste-water per metric tonne of product. Most of the solid wastes (0.7-0.8 tonnes per tonne of raw material), which are mainly of vegetable origin, can be processed into by-products or used as fuel. Moulds may be found on peanut kernels, and aflatoxins may be present. The high polluting activity of OMWW is linked to their high content of organic molecules, especially polyphenolic mixtures (1-10 g L-1), as well as their acidity and high concentrations of potassium, magnesium and phosphate salts. Besides aromatic compounds, OMWW contains other organic molecules - including nitrogen compounds, sugars, organic acids and pecans - that increase their organic load (COD = 80-200 g L-1; BOD = 50-100 g L-1). Furthermore, the physico-chemical characteristics of OMWW are rather variable, depending on climatic conditions, olive cultivars, degree of fruit maturation, storage time and extraction procedure (Ielmini et al., 1976).

20.1.2 Industry description and practices

Presently there are three methods for extracting the juice from the olive. They are classified according to the technology used in the phased extraction process, from which oil, solids and aqueous fractions may arise. This technology consists basically of either the press or the centrifugation system.

The press system, the traditional method, is a batch process, whereas cen-trifugation is usually continuous. The latter can also be further divided into two different systems depending on the number of phases produced: the two-phase and three-phase systems. The three-phase system is applied in Italy, Greece and other Mediterranean countries, whereas the two-phase is widely used in Spain, which is the main olive oil producing country. Industries working under the three-phase system generate two main residues: a solid waste (olive pomace) and an aqueous liquid (OMWW), which is a highly pollutant matrix. To reduce this pollution, the new two-phase system was developed during the past decade, and it produces only a solid by-product. It contains a higher proportion of water than the olive pomace and a great amount of lignin, cellulose, hemicellulose and phenolic compounds. Thus, in the three-phase system, oil, olive cake and OMWW are obtained, while in the two-phase system, oil and a semisolid waste made up of olive cake and concentrated OMWW are generated. Each method produces different volumes of by-products and as a consequence the effluents differ in their characteristics (Fig. 20.1 and Table 20.1). The use of the modern two-phase processing technique, in which no water is added, generates a new by-product called 'alperujo' (AL) that is a combination of liquid and solid waste. This new two-phase centrifugation process is used for the separation of the oil from the vegetable material, which includes all mineral and organic fractions (fats, proteins, sugars, organic acids, cellulose, hemicellulose, pectins, gums, tannins and polyphenols), but it produces a new contaminated wastewater as well as a solid residue. Alternatively, AL can be dried and subjected to chemical extraction with hexane, which has been a common practice for the olive pomace of the old two-phase system. This new dry olive mill residue (DOR) can be used for cogeneration of electric power. However, some problems have been raised lately such as the low residual level of oil in the unextracted solid cake, changes in cogen-eration subsidies and the discovery of polycyclic aromatic hydrocarbons in olive oil pomace, which make the study of alternative uses for this solid by-product necessary. In many European countries, such as Spain, a massive change from the traditional three-phase to the new two-phase process has taken place, and large volumes of waste (3-5 million tonnes per year) are generated. An integrated approach to this waste as fertilizer or animal feed or through recovery of residual oil and/or extraction of high added value products contributes to diminish its environmental impact and will provide a way to make the wastes from the olive mill plant profitable.

20.1.3 Pollution issues

Effluents from olive production are currently one of the most serious environmental concerns in the Mediterranean basin, largely as a result of both the sheer volume of waste generated per year (around 30 million m3 of OMMW) and their recalcitrant characteristics and toxic effects for the

Press system

Olive (100 kg)

Three-phase system (3Ph)

Milling

Oily must

Natural decantation

Virgin olive oil (20 kg)

Two-phase system (2Ph)

Washings-

Olive (100 kg)

Milling

Malaxation

Centrifugation 3-phases decanter i Oil OMWW

(with some water)

Washings

Olive (100 kg)

Milling

Malaxation

Centrifugation OMWW

Virgin olive oil (20 kg)

► Centrifugation—► Olive cake+OMWW 2-phases decanter (80 kg, 70% moisture)

(with some water) Centrifugation^— Hot water

Virgin olive oil (20 kg)

Oil washings

Fig. 20.1 Processes for olive oil extraction (adapted from Lopez et al., 2001).

OQ a

g ui

Table 20.1 Characteristics of OMWW from different oil extraction systems (adapted from BMZ, 1995)

System

Volume generated (L t-1 olive)

Total solids

(g L-1)

(g L-1)

(g L-1)

Press

800

120

7-5

80

Three phases

1300

60

19

50

Two phases

300

5

-

20

environment (Perez et al., 1990). Direct discharge of untreated waste is not allowed by law in most countries and many efforts have been made in order to either ensure waste purification or recycling (Ramos-Cormenzana et al., 1995). Indeed, the discharge of large quantities of this pollutant in the sewage system is not possible without any treatment. Because of the expense of new technologies for pre-treatment and the difficulty of conventional treatment methods, some regulations, such as the Italian law, allow the spreading on agricultural soil of up to 50 m3 ha-1 for OMMW obtained by press and 80 m3 ha-1 for OMMW obtained by centrifugation. The untreated release of OMMW on land produces potential danger for the surrounding environment, and several studies have shown that simple OMMW phenolic compounds of low molecular mass are toxic to seeds, aquatic organisms and bacteria. However, there is a controversy over what the phytotoxic components of the olive residues are. Most researchers have reported a high phytotoxicity against plant and microbial growth by low molecular mass phenols, but high molecular mass polyphenols or lignin-like polymers have also shown toxic activity and must also be considered. Furthermore, some researchers have not found a relationship between detoxification and removal of monomeric phenols from olive residues.

20.1.4 Disposal and recycling of oil wastes

Recently, Aliotta et al. (2002) reported the phytotoxicity of polyphenols from OMWW on seed germination, and Yesilada et al. (1999) reported their toxic effects on the soil bacterium Pseudomonas aeruginosa, but only a few studies have reported the toxic potential of this matrix on the typical organisms of the freshwater food chain. For these reasons, different biological and chemical/physical methods have been proposed to reduce the organic matter, polyphenols and tannins present in OMMW in order to avoid the toxic effects on the environment. Several pre-treatment techniques have been worked out to reduce the impact of OMWW on municipal plants and on the receiving water bodies by using microorganisms and chemical or physicochemical methods. Thus, a wide range of bioremedi-ation processes, such as aerobic (Benitez et al., 1997) and anaerobic digestion (Borja et al., 1997) or decolorization by lignolytic microorganisms

(Peréz et al., 1998), have been proposed. Oxidation systems have often been used as a pre-treatment to decrease OMWW toxicity and allow biological degradation (Lopéz et al., 1996). Composting (Monteoliva-Sánchez et al., 1996) and the production of industrially useful microbial products (González-López et al., 1996) have been shown to be worthwhile alternatives. However, the most suitable procedures seem to be treatments involving recycling rather than detoxification. Due to these characteristics, which increase the organic load of COD (80-200 g L-1) and BOD (50-100 g L-1) to values 200-400 times higher than those of a typical municipal sewage, the annual disposal of several million cubic metres of OMMW is a major environmental problem for agriculture in the Mediterranean area. Another important disposal approach deals with the recovery of the organic components for use in agriculture and in industry. The olive fruits contain a wide variety of bioactive components. Among these, hydroxytyrosol stands out as a compound of high added value, due to its high antioxidant properties and beneficial properties (with regard to both nutrition and oil stability), that could be recovered from the solid by-product. Hydroxytyrosol plays a role in enhancing the oxidative stability of olive oil and also has a positive effect on human health.

Recycling of oil waste material is a major task for sustainable development as for example the use of rape for biodiesel production from rapeseed oil (Ma and Hanna, 1999). The residues of biological treatment methods may serve as fertilizers or soil conditioners, while the slugs from waste incineration could be separated into a granulated fraction replacing sand in concrete and a fine-particulate/dust fraction, which needs to be deposited in hazardous waste sites due to its content of heavy metals and other toxic components. Whenever this waste material can be recycled it must be re-introduced into production processes and the non-recyclable fractions should be used as a fuel for energy recovery. Nowadays there is an urgent need to upgrade the processes for the treatment of the organic fractions from vegetable oil production to produce more valuable re-usable products or at least to recover their energy content. The main upgrading processes of oil organic wastes include composting, biogas fermentation, production of organic acids, polyphenols, biopolymers or biosurfactant production (Bolaüos et al., 2004). Moreover, a variety of oil plants store a large amount of carbohydrates which can be lost in the production wastes and may serve as a raw material for a biotechnological conversion by microorganisms to new 'value-added' products.

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