Holger Ecke

Sequestration of metals in carbonated municipal solid waste incineration (MSWI) fly ash

Waste management is in need of a reliable and economical treatment method for metals in fly ashes from municipal solid waste incineration (MSWI). However, no state-of-the-art technique has gained wide acceptance yet. This paper is a synthesis of five elsewhere published investigations covering a project which aimed to assess the possibilities and limitations of adding carbon dioxide (CO2) to fly ash as a stabilization method. Carbonation factors that were studied are the partial pressure of carbon dioxide (CO2), the addition of water, the temperature, and the reaction time. Laboratory experiments were performed applying methods such as factorial experimental design, thermal analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and leaching assays including pHstat titration and sequential extraction. Leaching data were verified and complemented using chemical equilibrium calculations. Data evaluation was performed by means of multivariate statistics such as multiple linear regression, principal component analysis (PCA), and partial least squares (PLS) modeling. It was found that carbonation is a good prospect for a stabilization technique especially with respect to the major pollutants lead (Pb) and zinc (Zn). Their mobility decreased with increasing factor levels. Dominating factors were the partial pressure of CO2 and the reaction time, while temperature and the addition of water were of minor influence. However, the treatment caused a mobilization of cadmium (Cd), requiring further research on possible countermeasures such as metal demobilization through enhanced silicate formation.

Solidification with water as a treatment method for air pollution control residues.

The process of solidification with water was studied on air pollution control (APC) residues from incineration of refuse-derived fuel (RDF) regarding mechanical strength and leaching behaviour of solidified material. Factorial design in two levels was applied to investigate the impact of water addition, time, and temperature to mechanical strength of solidified material. Factors time and temperature, as well as the interaction between the addition of water and time significantly (alpha=0.05) influenced the mechanical strength of solidified material. The diffusion-leaching test NEN 7345 was performed to investigate if the leaching behaviour of elements from solidified material was determined by diffusion. Since it was found that leaching is not diffusion controlled, the long-term leaching behaviour was not assessed. However, the investigation showed that some of the studied components (Al, Hg, Mn, Pb, Si, and Zn) could be considerably demobilised by solidification with water. Concentrations of As, Cd, Co, Cu, Fe, and Ni were either below or not quite above the detection limits to be included in the analysis of leaching behaviour. The elements least demobilised by solidification were Cl, Cr, K, and Na.

Accuracy of COD Test for Landfill Leachates

COD (chemical oxygen demand) has historically been considered to be an estimate of organic matter, and though this is no longer the case, for most kinds of water it is still a fair approximation. Landfill leachates may, however, be one of the exceptions. Landfill leachate contains many inorganic substances and, in certain circumstances, high concentrations of volatile organic compounds like acetic acid; the COD value may be affected by these conditions. Designed experiments were performed to determine how COD could be affected by the composition of landfill leachates. The factors studied include the content of iron(II), manganese(II), sulphide, ethanol, acetic acid, ammonia, and chloride as well as different aspects of the COD analysis design. The results show that up to about one-third of COD may be due to the inorganic components of leachates. The main conclusion from the experiments is that COD cannot be used solely as a measure of the organic matter of landfill leachate since inorganic substances as well as interactions between substances may interfere with the COD results.

Hydraulic conductivity of fly ash : sewage sludge mixes for use in landfill cover liners

Secondary materials could help meeting the increasing demand of landfill cover liner materials. In this study, the effect of compaction energy, water content, ash ratio, freezing, drying and biological activity on the hydraulic conductivity of two fly ash-sewage sludge mixes was investigated using a 2(7-1) fractional factorial design. The aim was to identify the factors that influence hydraulic conductivity, to quantify their effects and to assess how a sufficiently low hydraulic conductivity can be achieved. The factors compaction energy and drying, as well as the factor interactions material x ash ratio and ash ratio x compaction energy affected hydraulic conductivity significantly (alpha=0.05). Freezing on five freeze-thaw cycles did not affect hydraulic conductivity. Water content affected hydraulic conductivity only initially. The hydraulic conductivity data were modelled using multiple linear regression. The derived models were reliable as indicated by R(adjusted)(2) values between 0.75 and 0.86. Independent on the ash ratio and the material, hydraulic conductivity was predicted to be between 1.7 x 10(-11)m s(-1) and 8.9 x 10(-10)m s(-1) if the compaction energy was 2.4 J cm(-3), the ash ratio between 20% and 75% and drying did not occur. Thus, the investigated materials met the limit value for non-hazardous waste landfills of 10(-9)m s(-1).

Anaerobic Reduction of Hexavalent Chromium in Filter Sludge of an Electrochemical Process

The potential of an anaerobic system to remediate hexavalent chromium containing waste was investigated. Experiments were performed at laboratory scale in the batch mode. The variables studied were the concentrations of waste, of inoculum, of yeast as an additional carbon source, of sulfate and the initial pH. The gas development, the final pH and the final concentrations of hexavalent and total chromium were measured. The evaluation was based on Multivariate data analysis. It could be shown that a remediation is possible. The influence of pH and sulfate were negligible in the given range. High yeast concentrations and low waste concentrations were favorable for a strong gas development and a complete elimination of chromate. Possible mechanisms are the active reduction of chromate by the microorganisms, its use as electron acceptor in bacterial respiration, and its sorption on organic material. Especially the last explanation is strongly supported by the experiments.

Impact of Grassed Swales on the Fate of Metals Leached from Roads Built with Municipal Solid Waste Incineration Bottom Ashes

Roads built with municipal solid waste incineration (MSWI) bottom ashes may possess a risk to the environment if contaminants (e.g. heavy metals) are leached out. In this study, roadside swales were constructed and studied as a trap for pollutants leached from an experimental road. Soil from the slopes and bottoms of the swales was sampled twice yearly (in spring and autumn) and analysed for metal content. Common plant species in Northern Sweden were planted and a grass mixture was sowed in the swales. The aerial plant parts were sampled in the autumn and analysed for Cu, Cr, Pb, and Zn. One season of observations did not reveal any significant effect of the road on the soil and plants. The metal concentrations in the soil of the swales varied insignificantly between the samples collected in the spring and autumn. Metal concentration in the soil did not exceed the generic guideline values for Swedish contaminated soils. The dry summer of 2002 could have hindered the formation of ash leachate from the road. As the soil properties (pH, metal content) of all studied swale sections remained unaltered, a lack of humidity might be the reason for the observed poor plant establishment in the experimental and reference swales. The impact of the road material on the soil quality started to show up during the second season (2003). Slopes of the experimental swale became contaminated with Cu. Yet the metal translocation from grassed swales through the trophic chain is unlikely; in soil containing higher concentrations of metals grew vegetation with lower metal uptake.

Hydrometallurgical treatment of metal‐containing solid waste using anaerobic processes

The potential to damage the environment is the major motive to treat solid wastes. One main group of pollutants comprises metals such as cadmium, chromium, copper, mercury, nickel, lead, and zinc. This paper is a synthesis of five elsewhere published investigations focusing on the possibilities and limits to control the flux of metals from solid wastes using anaerobic processes. In particular, the treatment of sulfide forming elements and hexavalent chromium (Cr(VI)) were studied at both bench‐, pilot‐, and full‐scale. It was found that two‐step processes have the potential to significantly improve the quality of solid wastes. In a first step, metals were leached and transferred to the process water. In a second step, the metal‐enriched process water was treated under methanogenic conditions facilitating metal trapping through precipitation as sulfides or hydroxides. Anaerobic hydrometallurgy is judged to be a promising technique that has the potential to gain wide acceptance in the treatment of metal‐con...

Vitrified forts as anthropogenic analogues for assessment of long-term stability of vitrified waste in natural environments

The area’s natural analogues, vitrifi ed forts, combustion technology, and vitrifi ed waste have been reviewed.The purpose was to identify if investigations of vitrifi ed rock in hill forts might b ...