The vegetal oil processing industry involves the extraction and processing of oils and fats from vegetable sources. The oils and fats are extracted from a variety of fruits, seeds and nuts. The preparation of raw materials includes husking, cleaning, crushing and conditioning. The extraction processes are generally mechanical (boiling for fruits, pressing for seeds and nuts) or involve the use of a solvent such as hexane. Residues are conditioned (for example, dried) and are reprocessed to yield by-products. Crude oil refining includes degumming, neutralization, bleaching, deodorization and further refining (BMZ, 1995). Continuous sampling and measuring of key production parameters allow production losses to be identified and reduced, thus reducing the waste load. Since the pollutants generated by the industry are very largely losses in production, improvements in production efficiency, as described above, are recommended to reduce pollutant loads. Under this view, pollution prevention practices in the industry focus on the following main areas:
• prevent the formation of moulds on edible materials by controlling and monitoring air humidity;
• use citric acid instead of phosphoric acid, where feasible, in degumming operations;
• where appropriate, give preference to physical refining rather than chemical refining of crude oil, as active clay has a lower environmental impact than the chemicals generally used;
• reduce product losses through better production control;
• maintain volatile organic compounds (VOCs) well below explosive limits. Hexane should be below 150 mg m-3 of air;
• provide dust extractors to maintain a clean workplace, recover product and control air emissions;
• recover solvent vapours to minimize losses;
• optimize the use of water and cleaning chemicals;
• recirculate cooling waters;
• collect waste product for use in by-products such as animal feed, where feasible without exceeding cattle-feed quality limits.
Wastewater loads are typically 3-5 m3 per tonne of feedstock; plant operators should aim to achieve lower rates at the intake of the effluent treatment system. Hexane, if used, should be below 50 mg L-1 in wastewater. The BOD level should be less than 2.5 kg per tonne of product, with a target of 1-1.5 kg per tonne. 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. Pre-treated effluents can be discharged to a municipal sewerage system, if capacity exists, with the approval of the relevant authority. Proper circulation of air, using an extracting and cleaning system, is normally required to maintain dust at acceptable levels. Odour control is by ventilation, but scrubbing may also be required and implemented where necessary to achieve acceptable odour quality for nearby residents. Fabric filters should be used to control dust from production units to below 50 milligrams per normal cubic metre (mg Nm-3). The liquid effluents should meet some important requirements. Table 20.4 indicates the effluent levels to be achieved. Monitoring of the final effluent for the parameters listed above should be carried out weekly, or more frequently if the flows vary significantly. Monitoring data should be analyzed and reviewed at regular intervals and compared with the operating standards so that any necessary corrective actions can be taken. The key production and control practices that will lead to compliance with emissions requirements can be summarized as follows:
• monitor key production parameters to reduce product losses;
• prefer citric acid to phosphoric acid in degumming operations;
• give preference to physical refining over chemical refining of crude oil, where appropriate;
• hold levels of hexane, if used, below 150 mg m-3;
• design and operate the production system to achieve recommended wastewater loads;
• collect wastes for use in by-products or as fuel.
Table 20.4 Effluents from vegetable oil processing (adapted from Bressan et al., 2004)
Parameter Maximum value pH 6-9
Oil and grease 10
Total nitrogen 10
Temperature increase <3 °C*
* The effluent should result in a temperature increase of no more than 3 °C at the edge of the zone where initial mixing and dilution take place. TSS, total suspended solids.
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