As stated above, the conversion to biodiesel of waste oils and fats from the meat and fish processing industries represents an opportunity to valorize a residue and obtain a higher value product (biodiesel). In many situations, the adequate disposal of residual oils and fats represents an operational cost, as they cannot be burnt directly in a boiler without special equipment. Thus, from this point of view, there is an economic incentive to valorize those residues.

However, depending on the total quantity of residual oils and fats generated, dif­ferent approaches have to be considered. If the total amount is small, as it is the case of the waste frying oils generated in restaurants, it will be easier to make the selective collection of those materials to be processed in a centralized production facility. If a good logistic system is developed and properly implemented, and incentives are available for the residue producers, this is proved to be a good option [8], applicable even for the small — and medium-sized companies of the meat and fish processing industries. This situation may change if small and compact units for the production of small quantities of biodiesel from a wide variety of feedstocks become available, although the costs of energy and raw materials, and the hazards involved in the manipulation of dangerous chemicals may render this possibility impracticable.

However, if the quantity of residues generated is large, the option of having an in-house facility for the production of biodiesel may be viable from an economic point of view. In any case, the reduction in the consumption of fuel by the company, either by the utilization of biodiesel or by burning of the glycerol produced in the process, must be compared with the investment in equipment and operational costs due to the consumption of materials and energy necessary to produce biodiesel. With the increase in price of fossil fuels, this option is expected to become more and more attractive. It is also relevant for companies operating close by or even interconnected, that generate large quantities of residual fats, and that may be interested on a com­mon processing plant to take care of all the fat residues generated in their activities.

From an environmental point of view, the valorization of residual oils and fats to biodiesel production makes sense. This corresponds to the reutilization of a waste material originated from a renewable source, thus reducing the consumption of non­renewable fossil fuels. Although at a first glance there is a reduction in greenhouse gas emissions, in particular of carbon dioxide [71], the actual reduction depends on how the residues are collected and transported to the production site. To minimize those emissions, the logistical network should be properly optimized, for example, by giving the residue generators special containers for storing the waste fats and defining the more adequate collection routes. Although for in-house biodiesel pro­duction facilities this problem does not occur, additional savings may be accom­plished through an adequate process optimization and integration.

Moreover, with the advent of more stringent limits for greenhouse gas emissions and the development of trading schemes for carbon emissions, the production of biodiesel from residual oils and fats can be a good form to combine the environmen­tal and the economical aspects to one’s advantage. However, the current legislation and regulations still need to be improved or even created to be able to have a clear vision of the trade-offs involved on these decisions.

3 Conclusions

This article presents and discusses the main questions regarding the utilization of residual oils and fats for biodiesel production. Some key aspects are identified and strategies to deal with them are presented. Among them, the high content of FFA and moisture in waste frying oils and animal fats, as compared to fresh edible oils, makes the alkaline-catalyzed transesterification reaction to be less efficient for biodiesel production. A pre-treatment method suitable to handle this type of feedstocks is presented. Also, more efficient and robust production processes are presented that are able to use feedstocks with the characteristics normally encoun­tered in waste fats.

As the global demand for biodiesel increases and the pressure to be more envi­ronmental friendly, yet maintaining market competitiveness, increases, more and more waste residues will be seen as valuable raw materials. Besides helping compa­nies to fulfil their goals, policy targets defined at governmental and regional levels may be easier to reach.