Digestate and compost quality - case studies at anaerobic disgestion plants with the BTA Process
The key for the successful anaerobic digestion of biowaste and/or municipal solid waste is an efficient removal of the impurities before the digestion step. This is not only indispensable for the reliability of the process and therefore the plant availability, but furthermore for the quality of the produced digestate or final compost. With the existing
developments within the EU, the quality of the digestate / compost has become a focus of attention.
Further Authors:
P. Bolzano - Biotec Sistemi s.r.l.
J. Del’haye - IVVO
The most efficient way to remove these impurities is in the wet medium. This leads to a better quality of the 'washed†rejects and to a higher recovery of the digestible organics to the anaerobic digestion. The BTA Process offers a corresponding technical solution which has proven its flexibility, robustness and reliability in over 40 plants installed worldwide since 1990 for different types of waste (e.g. food waste, commercial waste, biowaste or municipal solid waste). The BTA® Process consists of two steps: an hydromechanical pre-treatment in which non-biodegradable contaminants bigger than 10 mm are separated as a 'heavy†and a 'light†fraction in the BTA Waste Pulper and as a 'grit†fraction smaller than 10 mm in the BTA Grit Removal System. The clean organic suspension is pumped to the methanization step for its energetic valorization. Generally the digestate is dewatered in a solid-liquid separation step. The liquid phase is widely recirculated to the hydromechanical pre-treatment while the solid phase is aerobically stabilized by composting. In the present paper, the further processing and destination of the digestate for the following reference projects is presented:
• In Castelleone (Italy), where biowaste and commercial waste are digested as co-substrates together with agricultural residues, the produced digestate is spread on the fields of nearby farms
• In the Biowaste Methanization Plant Kirchstockach (Germany) the dewatered digestate is mixed with green waste to produce quality compost
• In the Biowaste Methanization Plant Ieper (Belgium) the dewatered digestate is further processed to produce quality compost
• In the MBT Plant Valorlis (Portugal) the dewatered digestate is further processed to produce final (stabilized) compost
Finally, analysis carried out during the start-up and the operation tests for the MBT Plant Valorlis (Portugal), which treats Municipal Solid Waste, showed that the parameters for the content of physical impurities (content of plastics and of stones, glass splitters) are met. These are examples which prove that it is possible to obtain a quality product out of the digestate without an extensive refining step but a simple screening for the partial recovery of structure material, demonstrating the advantage of an efficient removal of impurities in the upfront of the anaerobic digestion.
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