Energy and organic fertilizer from the organic fraction of MSW Solid State AD: real case study

The Organic Fraction of Municipal Solid Waste (OFMSW) represents a relevant source for renewable energy production exploitable by Anaerobic Digestion (AD). The EU27 OFWSM production ranges from about 75 to 100 Mtonnes per year corresponding to an energetic potential of about 11-29 TWh. Currently more than 40 Mtonnes per year of OFMSW arising from Source Segregated (SS) collection are treated in composting plants for mass recovery aims and only 6 Mtonnes per year are treated in AD facilities. Even if the biogas produced by AD is considered a renewable source and for this reason significantly supported by economic incentives (Green Certificate, Emission Trading, ...), AD diffusion in MSW management remains quite low. The reason of this phenomenon arises from different factors as the reliability of the AD process and its viability. Infact the most diffused technologies for MSW AD are base on wet and semi-dry process. These are continuous feed AD reactors working with a Total Solids concentration ranging from 10 to 18% (w/w). Further the different facilities components as pumps, valves and similar, need a high fluidity level along with the absence of impurities of large size. Considering the nature and the features of the OFMSW, arising from SS collection or not, important pretreatments are required involving the adoption of costly and complicate pre treatment devices as shredder, grinder, pulper, grit chamber, and similar. Another relevant question that represents a serious obstacle in AD adoption in many European areas is represented by the management of the liquid fraction of the digestate (about 40-70% w/w). Disposal by spreading on field is not permitted by several local rules binding the treatment in Waste Water Treatment Plants with a relevant increase in the whole operating costs. A technology that seems to avoid the aforesaid problems is represented by the Solid State Anaerobic Digestion (SSAD) due to the slight pretreatment required and the narrowed liquid fraction discharged, even if some problems concerning process stability and biogas yield needs to be carefully investigate and managed. By the aid of a SSAD experimental apparatus several runs have been performed for evaluating both the process performances and the solid digestate quality. The amount of biogas producible ranges between 100 to 220 Nl per kg VS treated in the reactor, being strongly influenced by the Organic Load (OL) (gVS/kg). In the runs it ranged from 270 to 330 gVS/kg. Higher was OL lower was the biogas yield. The digestate shows characteristics interesting for an agricultural use as a fertiliser. In fact Total Organic Carbon (TOC) content ranges between 35 and 42% expressed on a dry basis, whereas Total N concentration is about 1.5%, 90% of that represented by Organic N. The quality of organic matter is testified by a concentration of humic and fulvic acids ranging between 6.4 and 8.8% (on a dry basis) with a mean value of 8.0%. Interesting also K and P concentration, both elements important for plant nutrition. Even if the urban origin of row material, heavy metal concentration results negligible from an environmental point of view. The germination index (GI) is relatively low, with a maximum peak of 53.10 % for the digestate obtained by the 3rd run. The waste liquid fraction produced by the process was always not higher than 10%w/w of the whole masse processed in the anaerobic reactor.