ORBIT e.V.

Rest or mixed waste sorting-digestion-separation for the recovery of recyclables and energy

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Courtesy of ORBIT e.V.

Source separate collection of biowaste has been implemented in many countries in Europe, especially in the German- speaking countries and the Benelux. By separating the biowaste fraction, a waste fraction called rest waste or grey waste is left over. This fraction is in most instances landfilled or incinerated. However, rest waste still contains a considerable amount of organics (up to 65%) which could be stabilised biologically. Several plants have been constructed, demonstrating the feasibility for biological treatment of the rest waste by means of aerobic composting. This fraction can also be treated by means of digestion which gives the added advantage that energy can be recovered in the form of biogas as is the case with biowaste digestion plants.

On the other hand, many European countries have so far chosen not to implement source separate collection and continue to collect mixed waste. This has been due mainly to the additional costs and logistics required for source separate collection. After demonstration of the feasibility of digesting rest waste, it is certainly possible to digest the organic fraction coming from mixed waste, which in many instances is much easier to handle than rest waste.

Both the digestion of mixed and rest waste have been successfully implemented on a large scale. The question remains however which kind of quality of end products, such as compost fractions and recyclable components could be produced after the digestion of rest or mixed solid waste. This is difficult to do once the digested residue is dewatered and dried as in conventional treatment. In order to achieve the desired quality, the SORDISEP process utilises advanced dry sorting in a first phase, followed by dry digestion and wet separation, including several screening and washing steps, in order to produce high quality materials that could otherwise not be produced under dry conditions.

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