Assessment of heavy metal removal technologies for biowaste by physico-chemical fractionation

Home and community composts in Nantes city (France): quality and safety regarding trace metals and metalloids

Home and community composting are key strategies for local organic waste management. The quality and safety of industrial composts are controlled, but those of home and community composts are not, and this could make them unsafe for use in kitchen gardens. Home (n = 20) and community (n = 41) composts, from urban and suburban areas including mildly Pb-contaminated allotment gardens, were analyzed for quality and safety regarding trace metals and metalloids (TMM) using mid-infrared Fourier transform spectrometry (FT-MIR) and portable X-ray fluorescence spectrometry, respectively. Home composts had a significantly higher Pb content (98 mg.kg^-1 ± 10 mg.kg^-1) than community composts (21 mg.kg^-1 ± 2 mg.kg^-1). Numerous home composts (85%) and a few community composts (17%) exceeded the organic farming thresholds for Pb (45 mg.kg^-1) and Zn (100 mg.kg^-1). The high mineral matter content and the relative abundance of chemical functions attributable to silicates (up to 35%) highly paralleled with TMM contents, mostly concentrated in the fine fraction. Co-inertia analysis highlighted strong and significant links between TMM contents and the whole chemical signature delivered by FT-MIR spectrometry. Pb-contaminated soil could be carried into home compost by green waste or by voluntary addition. Covariance analyses indicated that mineral matter and chemical functions only partly explained the variability in Pb content, suggesting a more complex combination of drivers. Community composting appears as a suitable local solution resulting in high-quality compost that complies with European organic farming regulations, while home composting from allotment gardens should be seriously evaluated to comply with such safety requirements.

Organic contaminants of emerging concern in Norwegian digestates from biogas production

The aim of this study was to analyze a variety of environmental organic contaminants of emerging concern (CEC) and their metabolites in representative digestate samples from Norwegian biogas production plants. Biogas digestates can be a valuable source for soil amendments and/or fertilizers in commercial agriculture. It is important to assess whether the digestates contain harmful contaminants in order to avoid unintended exposure of human consumers. In total 19 biogas digestates from 12 biogas production plants in Norway were collected and analyzed. Furthermore, process related parameters such as pretreatment of substrates, additives, flocculation and temperature conditions were considered for interpretation of the results. The CEC levels found in the digestates were shown to be dependent on the original composition of the substrate, dry-matter content, and conditioning of the substrate. The sunscreen octocrylene (147 μg L-1) and acetaminophen (paracetamol; 58.6 μg L-1) were found at the highest concentrations in liquid digestates, whereas octocrylene (>600 ng g-1, on a wet weight basis = ww) and the flame retardant TCPP (tris(1-chloro-2-propyl) phosphate, >500 ng g-1 ww) were found at the highest levels in solid digestates, exceeding even the upper limit of quantification (uLOQ) threshold. The highest levels of total CECs were measured in solid digestates (1411 ng g-1 ww) compared to liquid digestates (354 μg L-1 equals 354 ng g-1). The occurrence of CECs in digestate samples, even after extensive and optimized anaerobic digestion, indicates that the operational conditions of the treatment process should be adjusted in order to minimize CEC contamination.

Assessment of a particle size fractionation as a technology for reducing heavy metal, salinity and impurities from compost produced by municipal solid waste

A physical fractionation of a compost obtained by municipal solid wastes (MSW) was conducted by dry-sieving process, to quantify coarse impurities and assess the distribution of nutrients, heavy metals and salinity values in particle size fractions of 2, 1.2-2, 0.8-1.2, 0.4-0.8, 0.2-0.4, 0.1-0.2 and <0.1mm diameter. The whole unfractionated compost and all physical fractions were analyzed for the same chemical parameters. The results showed that the studied compost was of a low grade due to high salinity and heavy metal concentrations, and the presence of coarse impurities, mainly glass. The physical fractionation analysis showed that heavy metal and base cations concentrations, and salinity values significantly increased with decreasing of particle size, whereas macro nutrients such as C, N and P were more evenly distributed among the different particle size fractions. Overall, our results showed that the removal of selected particle size fractions <0.8mm and coarse impurities (e.g. glass impurity >2mm) could significantly improve the compost quality without reduce its fertilization potential. We concluded that particle size fractionation is a feasible and sustainable approach to improve composted MSW materials for their safe recycle in agriculture.

Nondestructive characterization of the contaminated biodegradable fraction of municipal solid waste using synchrotron radiation-induced micro-X-ray fluorescence

Synchrotron radiation-induced micro-X-ray fluorescence (SR-μXRF) was employed to elucidate the elemental characteristics of contaminated biodegradable fraction of municipal solid waste (BFMSW). Six sectioned BFMSW samples were selected for SR-μXRF mapping and 50 individual fine particles sorbed onto BFMSW were analyzed using SR-μXRF point scanning. The results showed that heavy metals tended to be concentrated on the surface of BFMSW and highly localized to some "hot-spots". Marked differences in heavy metal sequestering potentials among various kinds of BFMSW, and the significant role of heavy metal "hot-spots" in BFMSW contamination were identified. The lower heavy metal levels in the simulated samples compared to field samples (with longer and more intense mixing) indicated that inter-contamination during waste handling contributed significantly to heavy metal accumulation in BFMSW. Furthermore, additional information regarding elemental characteristics of individual particles could be acquired using SR-μXRF.

Use of organic acids in the composting of municipal solid waste: a pilot-scale study

Compost made from municipal solid waste (MSW) contains heavy metals that can interfere with the use of organic amendment in soil. In order to find effective ways to reduce the potential risk of heavy metals, we have investigated a novel approach by use of organic acid during MSW composting. Citric and oxalic acid dissolutions (0.25 mol x (-1)) were used at determined ratios (kg dried MSW: cm(3) acid). Cr and Ni concentrations were similar in compost, independent of acid contribution. By contrast, Cu concentrations decreased by 63% (at citric acid ratio 1:15), 65% (at citric acid ratios 1:20 and 1:40) and 83% (at oxalic acid ratio 1:40); furthermore, Pb concentrations reduced by 71% (at citric acid ratios 1:20 and 1:40 and at oxalic acid ratio 1:40) and Zn concentrations reduced by 67% (at citric acid ratios 1:10 and 1:20) and 70% (at oxalic acid ratio 1:40). The total metal mass decreased by an average of 12% in the compost fraction, whereas the total percentage of the residual fraction increased by an average of 20%. The acid addition in the studied ratios improved compost quality without negatively influencing biostabilization.

Source analysis of heavy metals and arsenic in organic fractions of municipal solid waste in a mega-city (Shanghai)

Heavy metals and arsenic contamination in municipal solid waste (MSW) and its treatment products has garnered increasing attention. This study investigated the heavy metals and arsenic flows in organic fractions of MSW (OFMSW) in Shanghai, China, through a one-year monitoring program. The OFMSW separated directly from the source (source-separated, pure organic waste), obtained from the treatment facilities were sampled and compared with pure foodstuffs. The heavy metals and arsenic contents in the source-separated OFMSW resembled those in foodstuffs, whereas the OFMSW from the treatment facilities was significantly contaminated with heavy metals and arsenic and failed to meet the government standards for land use. Using flow analysis, > 80% of heavy metals and arsenic were from extrinsic inorganic waste with high ash content that was combined with OFMSW during MSW collection, transfer, transportation, and storage stages. Based on source analysis of heavy metals and arsenic, suggestions for reducing heavy metals and arsenic contents in the current MSW management system in Shanghai are presented.

Advanced bioconversion of biowaste for production of a peat substitute and renewable energy

Traditional composting systems for biowaste generally produce low quality composts that may endanger recycling. A pilot-scale bioconversion process yielding quality compost and renewable energy was designed and tested. The process consisted of a set of wet physical separation units, composting and anaerobic digestion. Biowaste was divided in four streams by physical separation: (1) organic fraction >2 mm, (2) inorganic fraction 0.05-2 mm, (3) residual fraction composed of organics 0.05-2 mm and the fraction <0.05 mm and (4) a fraction solubilised in the washing water. The organic fraction >2 mm was composted and the compost, high in organic matter and low in EC and heavy metals, aimed at replacing peat in horticulture. The inorganic fraction 0.05-2 mm was completely made up of sand and can be used as a construction material. Solubilised organic matter in the washing water was converted to CH(4) by anaerobic digestion. The residual fraction can be used as landfill cover material.