Comparative environmental assessment of end-of-life carbonaceous water treatment adsorbents.

This study evaluates and compares the environmental impacts arising from the disposal of different carbonaceous sorbents used for wastewater treatment. Three different adsorption materials were considered, i.e. activated carbon, biochar and hydrochar, and three end-of-life management approaches, i.e. incineration, regeneration and landfilling. The highest overall environmental impact was of Carcinogenic effects and Freshwater Ecotoxicity due to emissions of heavy metals during production of all types of sorbents. The use of materials with higher adsorption capacities and regeneration of carbonaceous materials were considered and shown to be an efficient way for reducing the overall environmental impacts of the different adsorbents. The compensation of fossil fuel incineration by using recovered heat led to negative impacts in all categories. Recirculation of HTC process water reduced the impact on Freshwater Ecotoxicity and Eutrophication.

Closing the phosphorus cycle: Multi-criteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash.

The presence of heavy metals in concentrations above legal limit values is one of the main obstacles preventing closure of the phosphorus (P) cycle through directly applying wastewater treatment sludge ash as a fertilizer. Therefore, an alternative procedure is proposed to recover the valuable P from the sludge ash via wet chemical extraction. This comprehensive study uses several inorganic and organic acids, chelating agents and an alkaline solution to establish optimal and cost-effective conditions for wet P extraction from sludge ash. The optimization takes into account co-extraction of the following heavy metals: Cd, Cr, Cu, Ni, Pb and Zn. Design of experiments results show extraction liquid concentration, liquid/solid ratio and contact time all affect P and heavy metal extraction efficiency, both individually and through interaction. In addition, type of extraction liquid and pH at the end of the extraction procedure also affect P and heavy metal extraction efficiency. Combining results of XRD and SEM-EDX analysis with extraction data shows that at a pH <2, both Ca- and Al-phosphates in the ash dissolve easily. However, at slightly higher pH only Ca-phosphates dissolve well and at alkaline pH only Al-phosphates. The best trade-off between high P extraction, low heavy metal co-extraction and low operational costs is obtained with H2SO4 (0.5 N, 10 ml/g, 120 min) and oxalic acid (0.5 N, 12.8 ml/g, 120 min). H2SO4 outperforms the other extraction liquids in terms of extraction liquid costs per kg P extracted, whereas extraction with oxalic acid results in the lowest heavy metal co-extraction, thus reducing the downstream processing costs. None of the extraction liquids considered is appropriate for heavy metal removal prior to P extraction due to loss of P and insufficient heavy metal removal.

Legal situation and current practice of waste incineration bottom ash utilisation in Europe.

Almost 500 municipal solid waste incineration plants in the EU, Norway and Switzerland generate about 17.6 Mt/a of incinerator bottom ash (IBA). IBA contains minerals and metals. Metals are mostly separated and sold to the scrap market and minerals are either disposed of in landfills or utilised in the construction sector. Since there is no uniform regulation for IBA utilisation at EU level, countries developed own rules with varying requirements for utilisation. As a result from a cooperation network between European experts an up-to-date overview of documents regulating IBA utilisation is presented. Furthermore, this work highlights the different requirements that have to be considered. Overall, 51 different parameters for the total content and 36 different parameters for the emission by leaching are defined. An analysis of the defined parameter reveals that leaching parameters are significantly more to be considered compared to total content parameters. In order to assess the leaching behaviour nine different leaching tests, including batch tests, up-flow percolation tests and one diffusion test (monolithic materials) are in place. A further discussion of leaching parameters showed that certain countries took over limit values initially defined for landfills for inert waste and adopted them for IBA utilisation. The overall utilisation rate of IBA in construction works is approximately 54 wt%. It is revealed that the rate of utilisation does not necessarily depend on how well regulated IBA utilisation is, but rather seems to be a result of political commitment for IBA recycling and economically interesting circumstances.

Theory and practice of corrosion related to ashes and deposits in a WtE boiler.

Corrosion of heat-exchanging components is one of the main operational problems in Waste-to-Energy plants, limiting the electrical efficiency that can be reached. Corrosion is mainly related to the devolatilization and/or formation of chlorides, sulphates and mixtures thereof on the heat-exchanging surfaces. Theoretical considerations on this corrosion were already put forward in literature, but this paper now for the first time combines theory with a large scale sampling campaign of several Waste-to-Energy plants. Based on the outcome of elemental and mineralogical analysis, the distribution of Cl and S in ashes sampled throughout the plant during normal operation is explained. Cl concentrations are high (15-20%) in the first empty pass, decrease in the second and third empty pass, but increase again in the convective part, whereas the S concentrations show an inverse behavior, with the highest concentrations (30%) observed in the second and third empty pass. Sampling of deposits on specific places where corrosion possibly occurred, gives a better insight in the mechanisms related to corrosion phenomena in real-scale WtE plants and provides practical evidence for some phenomena that were only assumed on the basis of theory or lab scale experiments before. More specific, it confirms the role of oxygen content, temperatures in the different stages of the boiler, the presence of polysulphates, Pb and Zb, and the concentrations of HCl and SO2 in the flue gas for different types of boiler corrosion.

Immobilization of antimony in waste-to-energy bottom ash by addition of calcium and iron containing additives.

The leaching of Sb from waste-to-energy (WtE) bottom ash (BA) often exceeds the Dutch limit value of 0.32mgkg(-1) for recycling of BA in open construction applications. From the immobilization mechanisms described in the literature, it could be concluded that both Ca and Fe play an important role in the immobilization of Sb in WtE BA. Therefore, Ca and Fe containing compounds were added to the samples of the sand fraction of WtE BA, which in contrast to the granulate fraction is not recyclable to date, and the effect on the Sb leaching was studied by means of batch leaching tests. Results showed that addition of 0.5 and 2.5% CaO, 5% CaCl2, 2.5% Fe2(SO4)3 and 1% FeCl3 decreased the Sb leaching from 0.62±0.02mgkgDM(-1) to 0.20±0.02, 0.083±0.044, 0.25±0.01, 0.27±0.002 and 0.29±0.02mgkgDM(-1), respectively. Due to the increase in pH from 11.41 to 12.53 when 2.5% CaO was added, Pb and Zn leaching increased and exceeded the respective leaching limits. Addition of 5% CaCO3 had almost no effect on the Sb leaching, as evidenced by the resulting 0.53mgkgDM(-1) leaching concentration. This paper shows a complementary enhancement of the effect of Ca and Fe, by comparing the aforementioned Sb leaching results with those of WtE BA with combined addition of 2.5% CaO or 5% CaCl2 with 2.5% Fe2(SO4)3 or 1% FeCl3. These lab scale results suggest that formation of romeites with a high Ca content and formation of iron antimonate (tripuhyite) with a very low solubility are the main immobilization mechanisms of Sb in WtE BA. Besides the pure compounds and their mixtures, also addition of 10% of two Ca and Fe containing residues of the steel industry, hereafter referred to as R1 and R2, was effective in decreasing the Sb leaching from WtE BA below the Dutch limit value for reuse in open construction applications. To evaluate the long term effect of the additives, pilot plots of WtE BA with 10% of R1 and 5% and 10% of R2 were built and samples were submitted to leaching tests at regular intervals over time. The Sb leaching from untreated WtE BA was just below or above the Dutch limit value. The Sb leaching from the pilot plots of BA with additives first remained stable around 0.13mgkg(-1) but had a tendency to slightly increase after 6months, indicating the need for further research on the effect of weathering, and more specifically of carbonation, on Sb leaching from WtE BA.

Recycling of spent adsorbents for oxyanions and heavy metal ions in the production of ceramics.

Spent adsorbents for oxyanion forming elements and heavy metals are classified as hazardous materials and they are typically treated by stabilization/solidification before landfilling. The use of lime or cement for stabilization/solidification entails a high environmental impact and landfilling costs are high. This paper shows that mixing spent adsorbents in the raw material for the production of ceramic materials is a valuable alternative to stabilize oxyanion forming elements and heavy metals. The produced ceramics can be used as construction material, avoiding the high economic and environmental impact of stabilization/solidification followed by landfilling. To study the stabilization of oxyanion forming elements and heavy metals during the production process, two series of experiments were performed. In the first series of experiments, the main pollutant, Mo was adsorbed onto iron-based adsorbents, which were then mixed with industrial sludge (3 w/w%) and heated at 1100°C for 30 min. Mo was chosen, as this element is easily adsorbed onto iron-based adsorbents and it is the element that is the most difficult to stabilize (i.e. the highest temperatures need to be reached before the concentrations in the leachate are reduced). Leaching concentration from the 97/3 sludge/adsorbent mixture before heating ranged between 85 and 154 mg/kg; after the heating process they were reduced to 0.42-1.48 mg/kg. Mo was actually stabilized, as the total Mo concentration after addition was not affected by the heat treatment. In the second series of experiments, the sludge was spiked with other heavy metals and oxyanion forming elements (Cr, Ni, Cu, Zn, As, Cd and Pb) in concentrations 5 times higher than the initial concentrations; after heat treatment the leachate concentrations were below the regulatory limit values. The incorporation of spent adsorbents in ceramic materials is a valuable and sustainable alternative to the existing treatment methods, saving raw materials in the ceramics production process and avoiding the use of stabilizing agents. Besides, spent adsorbents added to the raw material for ceramic products, may improve their aesthetic and structural properties.

Destruction and formation of dioxin-like PCBs in dedicated full scale waste incinerators.

Destruction and formation of dioxin-like PCBs in full scale waste incinerators is studied by analysing input waste streams and boiler and fly ash of a grate furnace incinerator (GFI) incinerating MSW, of a Fluidised Bed Combustor (FBC) incinerating a mix of 50% sludge, 25% refuse derived fuel (RDF) and 25% automotive shredder residue (ASR) and of a rotary kiln incinerator (RKI) incinerating hazardous waste. The dioxin-like PCB fingerprints of the waste inputs show that PCB oils Aroclor 1242 and Aroclor 1254 late are the major dioxin-like PCB contamination source of sludge, RDF and ASR. The dioxin-like PCB fingerprints of the waste inputs are clearly different from the fingerprints of the outputs, i.e. boiler and fly ash, indicating that in full scale waste incinerators dioxin-like PCBs in the input waste are destroyed and other dioxin-like PCBs are newly formed in the post combustion zone. The dioxin-like PCB fingerprint of boiler and fly ash of all three incinerators corresponds well to the fly ash fingerprint obtained in lab scale de novo synthesis experiments, indicating that dioxin-like PCBs are mainly formed through this mechanism. The high PCB concentration in the input waste mix of the RKI does not promote the formation of dioxin-like PCBs through precursor condensation.

Environmental impact of incineration of calorific industrial waste: rotary kiln vs. cement kiln.

Rotary kiln incinerators and cement kilns are two energy intensive processes, requiring high temperatures that can be obtained by the combustion of fossil fuel. In both processes, fossil fuel is often substituted by high or medium calorific waste to avoid resource depletion and to save costs. Two types of industrial calorific waste streams are considered: automotive shredder residue (ASR) and meat and bone meal (MBM). These waste streams are of current high interest: ASR must be diverted from landfill, while MBM can no longer be used for cattle feeding. The environmental impact of the incineration of these waste streams is assessed and compared for both a rotary kiln and a cement kiln. For this purpose, data from an extensive emission inventory is applied for assessing the environmental impact using two different modeling approaches: one focusing on the impact of the relevant flows to and from the process and its subsystems, the other describing the change of environmental impact in response to these physical flows. Both ways of assessing emphasize different aspects of the considered processes. Attention is paid to assumptions in the methodology that can influence the outcome and conclusions of the assessment. It is concluded that for the incineration of calorific wastes, rotary kilns are generally preferred. Nevertheless, cement kilns show opportunities in improving their environmental impact when substituting their currently used fuels by more clean calorific waste streams, if this improvement is not at the expense of the actual environmental impact.