Deliverable 3.2 - Contaminant fate
Among “hot spots” of using waste as a raw material, the possible content of priority pollutants which can migrate from the waste through the technology chain and into the end-products is a matter of concern. As an example, the agronomic reuse of compost, wastewater sludge and/or anaerobic digestate as soil improvers is presently the main way to recover nutrients from waste streams but also subjected to severe limitations not only by environmental regulation but also someway by the social perception. The “End of Waste Criteria for Biodegradable Waste Subject to Biological Treatment (Digestate and Compost)” (as defined by the Joint Recent Centre) are the present reference for what can be considered or not a waste when transforming a biodegradable waste to compost or digestate. In RES URBIS, polyhydroxyalkanoate (PHA) is presently being considered to be produced from a wide range of organic waste such as municipal solid waste, sludge from wastewater treatment, food-processing waste, and park/garden waste, also in combination thereof. However, when new materials are recovered or synthesized from biodegradable waste, the above cited criteria are not anymore applicable; it is therefore necessary to define the status of new materials by opening the way for a future adaptation of End of Waste criteria. To a greater reason, this issue holds for the specific case of waste biorefinery because different urban organic waste streams can be combined (especially the organic fraction of municipal solid waste and the excess sludge from urban wastewater treatment) and conversion processes are also novel with respect to state of the art (i.e. biological synthesys of PHA from ad hoc derived precursors instead of simply composting).
Hence, it is evident that the problem of migration of contaminants possibly contained in the organic waste to the end-products (namely PHA) is highly specific, depending on
a) contaminant families,
b) type of feedstock,
c) production technologies (especially PHA extraction from biomass), and
d) end products.
With reference to point a), three families of potential contaminants have been selected for this study, i.e.
- Inorganic elements, including toxic metals
- Polyciclic aromatic hydrocarbons (PAHs)
- Polychlorinated biphenyls (PCBs)
With reference to point b), two types of feedstock have been considered, namely
- The mixture of organic fraction of municipal solid waste and excess sludge from urban wastewater treatment (which has been investigated by UNIRM/UNIVE, in the pilot plant of Treviso)
- From fruit waste (by NOVAID, in the pilot plant in Lisbona)
Moreover, for comparison purposes, three types of commercial PHA that are produced from crops have been considered as blanks of minimal presence of contaminants.
With reference to point c), the attention has been focsed on the extraction step. Thus, both PHA-rich biomass before extraction and purified PHA after extraction from biomass were considered. As for the latter, three different types of extraction were considered.
With reference to point d), the existing regulatory framework has been considered which depends on the type of products under consideration.
Overall, 24 different samples from 6 different conditions were analysed; each sample for more than 100 different compounds.
- The PHA content of contaminants is generally low, i.e in the range between ppb and a few ppm, but for alkaline and alkaline earth metals (which are of little environmental concern) RES URBIS (GA 730349)
- The type of feedstock affects the contaminant contents, with the PHA from fruit waste generally showing a lower content than PHA from the mixture of organic fraction of municipal waste and sludge from wastewater treatment. The commercial PHA which is derived from crops has lower concentrations of heavy metals than waste-based PHA, but similar concentrations of PAH and PCB.
- The type of PHA stabilization and extraction also affects the contaminant contents, where acid stabilization and extraction with aqueous inorganic extractants brings to generally lower contants than thermal stabilization and extraction with either hypochlorite or chloroform.
- Although a specific regulation does not exist yet, a comparison has been made with the regulation and guidelines for similar materials and/or applications. It resulted that all tested PHA types meet present regulatory standards and guidelines (e.g. limits for Cd and PHA in plastic materials based on REACH regulation, including toys; limitis PCB in Recycling Plastics from Shredder Residue, based on EPA guidelines).
- On the other hand, waste-based PHA slightly exceed the limits for heavy metals which are set by the EU Directive n. 10/2011 on PHA-based plastic materials and articles to come into contact with food. Thus, based on available results so far, the use of waste-based PHA in direct contact with food would require a slight improvement.
- Future work is planned to also measure the release of contaminants through migration tests under standardized conditions. As an example, the Directive 2009/48/EC on the safety of toys specifies limits for the migration of 19 elements (aluminium, antimony, arsenic, barium, boron, cadmium, chromium (III), chromium (VI), cobalt, copper, lead, manganese, mercury, nickel, selenium, strontium, tin, organic tin and zinc) from toy materials and from parts of toys, to be measured under standardised test procedure (EN 71-3:2013+A3:2018).