Biogas potential of soapstock and bleaching earth

An effective method of increasing the productivity of small scale agricultural biogas plants is the addition of wastes from industry, particularly the fish processing industry. This Norwegian study investigates the biogas potential of two currently non-utilised by-products of the Omega 3 fish oil refining process, soapstock and bleaching earth. Other by-products such as stearin are also suitable for biogas production but may be better utilised for direct combustion at the processing plant or for conversion to biodiesel in the future if economically viable.

However, fish oil soapstock and spent bleaching earth both contain large amounts of oils and fats which have a high biogas potential. The annual quantity of these by-products from the Norwegian Omega 3 processing industry has been estimated at 20,000 tons of soapstock and 30,000 tons of bleaching earth. Currently, much of this material (soapstock) is shipped to Denmark for utilization in biogas plants, whereas the bleaching earth is sometimes deposited as landfill.

The refining and concentration of Omega 3 oils (a valuable product for human consumption) from oily fish is a multi-step process. The first step is neutralization where an alkaline solution removes unstable fatty acids into a liquid by-product called soapstock. The next step is a bleaching process using montmorillonite clay, commonly referred to as bleaching earth.  The spent bleaching earth is an oil-rich by-product produced at this stage. Winterization is a filtering step to remove any remaining unstable fatty acids. The by-product here is stearin, a waxy substance. The deodorization step is a distillation procedure to remove most of the Omega 3 oils. Following deodorization is an esterification of the remaining material to produce ethyl esters which are again distilled to concentrate any remaining Omega 3 oils. The non-Omega 3 oil at this stage is mostly used as a fuel for the processing plant.

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