• Energy Research

Abstract In the transition from a fossil to a bio-based economy, it has become an important challenge to maximally recuperate valuable nutrients coming from waste streams. Nutrient resources are rapidly depleting, significant amounts of fossil energy are used for the production of chemical fertilizers, whereas costs for energy and fertilizers are increasing. In the meantime, biogas production through anaerobic digestion produces nutrient-rich digestates. In high-nutrient regions, these products cannot or only sparingly be returned to agricultural land in its crude unprocessed form. The consequent processing of this digestate requires a variety of technologies producing lots of different derivatives, which could potentially be re-used as green fertilizers in agriculture. As such, a sustainable alternative for fossil-based mineral fertilizers could be provided. This study aims to characterize the physico-chemical properties of digestates and derivatives, in order to identify the fertilizer value and potential bottlenecks for agricultural re-use of these products, in line with European legislative constraints. In addition, the economic and ecological benefits of substituting conventional fertilizers by bio-based alternatives are quantified and evaluated. Waste water from acidic air scrubbers for ammonia removal shows potential for application as N-S fertilizer. Analogously, concentrates resulting from membrane filtrated liquid fraction of digestate show promise as N-K fertilizer. Substituting conventional fertilizers by digestate derivatives in different cultivation scenarios can result in significant economic and ecological benefits for the agriculturist. Starting from theoretical scenarios outlined in the current study, field test validation will be required to confirm the potential substitution of fossil-based mineral fertilizers by bio-based alternatives.

Controlled flooding of lowlands is considered as a potential water management strategy to minimize the risk of flooding of inhabited areas during high water periods. However, due to industrial activities, river water, sediments and soils are often contaminated with metals which may have adverse effects on the ecosystem's structure and functioning. Additionally, salinity may greatly affect the bioavailability and toxicity of metals present or imported into these systems. The effect of contaminated soils under different flooding and salinity exposure scenarios on the growth, reproduction and metal accumulation in the oligochaete Tubifex tubifex (Müller, 1774) were examined. In these bioassays metal contaminated soils were flooded with water of different salinities (0 and 3 psu), and tested after 0, 6 and 12 months of permanent inundation. We indeed found that inundation time had significant decreasing effects on Cu and Zn accumulation; although initial accumulation of Cu and Zn was higher in the previously unflooded soil at the start of the flooding treatment, these differences seem to disappear after 6 months of permanent inundation. Moreover, the complex interaction between substrate type and salinity suggests that redox potential is probably of major importance.

Extractability of Cd, Cr, Cu, Ni, Pb, and Zn in a dredged sediment disposal site was assessed using single extraction procedures (H2O; 0.01 M CaCl2; 1 M NH4OAc; NH4OAc-EDTA; CaCl2-TEA-DTPA). Only Cd and Zn were found to exceed statutory threshold values for total content. The field was planted with Salix viminalis "Orm" and accumulation of heavy metals in bark, leaves, roots, and wood was evaluated at seven sampling locations along an observed gradient in texture and pollution. Biomass production was high, ranging from 13.2 to 17.8 t ha(-1) y(-1) dry weight. Metal accumulation in aboveground plant parts was low, amounting to the following annually extracted mass of metals per ha: 5034 g Zn, 83 g Cd, 145 g Cu, 83 g Pb, 12 g Ni and 6 g Cr. The use of accumulating clones and the use of soil amendments might enhance extraction efficiency in future research.

The combination of energy production from biomass with phytoremediation opens perspectives for bringing land with moderate metal contamination into safe and beneficial use. Small scale fixed bed downdraft gasifier installations (150 kWe-1 MWe) can be employed for on-site conversion of biomass to electricity and heat. To assess the fate of heavy metals, batch gasification operations were run in a small scale fixed bed downdraft gasifier (100 kW thermal power). The gasification of I kg of wood resulted in the production of 40 g of ashes, 1.2 kWh of electricity and 9 MJ of heat. Per kg dry mass of willow wood, 31 g bottom ashes, 7 g cyclone ashes,. 2 g filter ashes, and 18 g DM of gasifier bed ashes were produced. Cd, Zn and Pb were enriched by a factor 7-100 in the fly ashes compared to the bottom ashes. Low mass recoveries (30-40%) were found for Cd, Cr, Ni and Pb. This was mainly attributed to measurement uncertainties. 60% and more of recovered Cd, Pb and Zn were present in the finer filter and cyclone ashes, while Cr, Cu and Ni were mainly found in the bottom ashes and to a lesser extent the bed ashes. The scrubber placed after the hot cyclone has all essential role in intercepting the more volatile heavy metals before combustion of the gas. The bottom ashes constituted the largest fraction and ideally should be recycled if limits for metal contents are met. Only Cd and Zn exceeded Flemish threshold values for use of this fraction as a fertilizer, albeit by a small margin. This leaves perspectives for future research to optimise process parameters towards the production of bottom ashes that can be recycled. (C) 2005 Elsevier Ltd. All rights reserved.

Metal concentration of plants growing on contaminated soils among other factors may depend on changes in the hydrological regime of the soil. Foliar and stem metal concentrations in Salix cinerea (grey sallow) were measured in 2 consecutive growing seasons on a submerged sediment-derived soil that underwent gradual terrestrialisation. Foliar and stem cutting concentrations for Cd, Zn and Mn increased on plots that were submerged during the first year, but emerged in the second year of monitoring. The litter layer was sampled under the shrubs of a plot with a recent abrupt change in hydrological regime and on the reference plot. It was separated in three size fractions through sieving. Analysis of the litter fractions suggested that Cd and Zn concentrations remained constant during fragmentation. However, Cr, Cu, Ni and Pb concentrations increased, which was attributed to adhesion of mineral soil particles on the fine fraction. After correction for the metal content in the mineral fraction, an increase in Cd, Mn and Cu concentration during fragmentation of the organic part of the litter layer was observed for the polluted plot. Net litter layer decomposition rate was low, which may indicate low colonisation by the decomposing community. Terrestrialisation resulted in higher Cd, Mn and Zn uptake by willows. The deviant litter layer metal concentrations for Cd, Zn and Mn and low decomposition rate must be further monitored. Feasibility of measures aiming at re-establishing wetland conditions for the dredged sediment landfill must be considered.

Marginal abatement cost (MAC) CO2; Metal contamination; Subsidy; Energy crops; Adapted gross income (AGI); Phytoremediation

Fast growing biomass plants such as Salix species are promising for use in phytoremediation of contaminated land. This study assessed the seasonal variations and changes with stand age in metal concentrations of S. fragilis L. and S. triandra L. grown in field conditions on contaminated dredged sediment substrates with comparable properties. A lesser proportion of total soil Cd was extractable by ammonium-acetate in the 6-year-old stand (6%) compared to the 1-year-old stand (17%). This suggests that the potential to remove metals from the site declines with tree age. Metal concentrations in willow biomass compartments decreased with stand age. Concentrations of Cd and Zn in leaves, wood, and bark increased toward the end of the growing season, irrespective of the species. Only Cd behavior offered limited prospects for targeting effective removal of the metal from the sediment through repeated harvest. The most efficient removal of Cd would require the combined harvest of stems and leaves; at the same time the risk of spreading Cd and Zn to the surroundings with leaf fall would be avoided.

Worldwide there are numerous regions where conventional agriculture is affected by the presence of elevated amounts of plant-available trace elements, causing economic losses and food and feed quality and safety. The Belgian and Dutch Campine regions are a first-class example, with approximately 700 km(2) diffusely contaminated by historic atmospheric deposition of Cd, Zn and Pb. Primary land use in this region is agriculture, which is frequently confronted with crops exceeding the European standards for heavy metal contents in food and feed-stuffs. Phytoremediation as a soil remediation technology only appears feasible if the produced biomass might be valorised in some manner. In the current case, we propose the use of energy maize aiming at risk-reduction and generation of an alternative income for agriculture, yet in the long run also a gradual reduction of the pollution levels. Since the remediation aspect is demoted to a secondary objective with sustainable risk-based land use as first objective, we introduce the term 'phytoattenuation': this is in analogy with 'natural attenuation' of organic pollutants in soils where also no direct intended remediation measures but a risk-based management approach is implemented. In the current field experiment, cultivation of energy maize could result in 33,000-46,000 kW h of renewable energy (electrical and thermal) per hectare per year which by substitution of fossil energy would imply a reduction of up to 21 x 10(3)kg ha(-1) y(-1) CO(2) if used to substitute a coal fed power plant. Metal removal is very low for Cd and Pb but more significant for Zn with an annual reduction of 0.4-0.7 mgkg(-1) in the top soil layer.

A pot study was used to examine the effects of amendments such as EDTA and elemental sulfur on the growth potential, gas exchange features, uptake and mobilization of Pb by wheat (Triticum aestivum L.) in two texturally different contaminated soils at three levels of EDTA (2, 4, 8 mmol kg(-1) dry soil) and two levels of elemental sulfur (100, 200 mmol kg(-1) dry soil). EDTA resulted in more solubilization of Pb than elemental sulfur in both soils. Application of EDTA and elemental sulfur increased shoot dry matter in the loamy sand soil, whereas in the sandy clay loam soil EDTA treated plants produced lower shoot dry matter compared to that observed with elemental sulfur. Application of EDTA 10d prior to harvest increased the amount of Pb accumulated into wheat shoots with more Pb accumulated by plants from the loamy sand than from the sandy clay loam soil. However, evaluation of the relative extraction efficiency expressed as the percentage of solubilized Pb that is subsequently also effectively accumulated by the plant shoots reveals that the relatively low efficiency does not warrant the massive mobilization induced by the environmentally persistent EDTA chelator. More modest mobilization of Pb induced by elemental sulfur and the higher relative extraction of mobilized Pb therefore deserves further attention in future research. In particular, attention needs to be paid to determining soil types in which elemental sulfur can induce significant impact on soil pH and metal mobility after application of a practically realistic dosage.