3.1 Organic so lid wastes and anaerobic s ludge inocu I aSeveral organic solid wastes were analyzed in this study in order to examine the possibility of their use as a substrate in anaerobic digestion for energy recovery. These substrates were: source-sorted OFMSW (later called biowaste) as the main substrate, pressing leachate from OFMSW composting plant (press water) as the main and co¬substrate, source-sorted foodwaste (foodwaste) as co-substrate, and sludge from a potato industry wastewater treatment plant (potato sludge) as co-substrate.3.1.1 BiowasteThe biowaste suspension used in this study was the same as that which was prepared from source-sorted domestic biowaste and that was treated in the biowaste treatment plant of Karlsruhe/Durlach. This full-scale biowaste treatment plant applies the BTA/MAT process for the preparation of the biowaste suspension. The digester has a total volume of 1,300 m3and a working volume of 1,000 m3. More than 11,000 tons source-sorted OFMSW per year are processed and digested (the plant was actually sized for 8,000 tons per year). The operation of this full-scale methane reactor is the basic reference of this study. The separately collected biowaste fraction is squeezed in a mill to tear apart plastic bags and then defibered in the BTA/MAT hydropulper after addition of two parts of process water (supernatant of centrifuged digester effluent + rain water). The addition of ~12 m3 process water to 6 tons of biowaste for hydropulping results in a moisture content of more than 90% in order to perform a wet anaerobic digestion. Heavy materials (cans, stones, ceramics, knifes, forks and spoons, etc.) sediment at the bottom and are withdrawn from the bottom while light materials (mostly plastics) form a scum layer at the top of the hydropulper during and after hydropulping and is scimmed of. Fine sand separation is achieved by two hydrocyclones during interim storage. The different steps involved in the biowaste treatment plant are depicted in Figure 3.1. The suspension samples for the laboratory experiments were collected after the hydro-pulper and light and heavy material removal, before entering the full-scale digester. The samples were collected monthly from the interim storage tank and stored in a refrigerator until it was used.
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