Trace metal mass transfer during thermophilic composting
A concern regarding MSW compost quality is the trace metal content. A study was initiated to assess the impact of various metal contaminants on compost quality.
One critical step in the study was to estimate metal transfer during composting, which was achieved by measuring the corrosion rate of different contaminants during the high rate composting stage for 3 weeks. The experiment was carried out using alfalfa hay and straw as composting substrate. Substrate moisture content was kept at 70% (w.b.) and temperature around 55°C during the experiment period, with pH ranging from 8.3 to 9.5.
A total of 7 different contaminants were tested. Specimens were sealed with glue or paint to keep an open surface area at least 1 cm in one dimension. All specimens were cleaned to remove dirt and grease before corrosion exposure. Each specimen was weighed and the exposed surface area determined before putting into compost. Species were placed in a 1 m3 pilot-scale composter for three weeks. After composting, all specimens were screened out and submitted to corrosion analysis. Specimens were cleaned to remove dirt and materials that are not part of corrosion products. Weight loss/gain measurements were performed twice: first after initial cleaning and second after corrosion products were removed. Among the 7 types of contaminants, stainless steel screws, brass screws, and light bulb Aluminium alloy thread contacts showed little weight changes.
This suggests minimum concerns for these materials in terms of metal release during composting. The highest metal release per unit area was from light bulb foot contacts, which was 3.66×10-3 g/cm2, while galvanized steel nails and zinc plated screw had a metal release rate in the range of 1.5~1.9×10-3 g/cm2. The low metal release rates might be partly due to the high pH values (8.3 to 8.8) of the composting substrate. As pH is a significant corrosion parameter, the synthetic composting feedstock recipe should be adjusted to obtain a lower starting value. Scanning Electron Microscope (SEM) was employed to determine the chemistry of corrosion products and change of surface morphology by corrosion. X-ray diffraction/spectrum may be formed to determine the type of corrosion products formed on the surface of some selected samples. In addition to the species originating from the specimens, other elements such as Al, Si, S, Cl, Ca, K, O, C were consistently detected on the surface of specimens in EDS spectrum, which might be either from tap water or soil. These elements might be deposited on the surface of metal contaminants affecting the release/dissolution of metals from metal contaminants. However, corrosive species such as S, O, C were not detected to be associated with metal ions from metal contaminants. This could be due to the fact that the corrosion products on the surface were insignificant and beyond the detection limit of XRD.
As there is little literature on this subject, methodology and data from this work can be served as a scientifically sound
reference to not only academic, but also industry and legislators.
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