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In the transition from a fossil to a bio-based economy, it has become an important challenge to maximally recuperate and recycle valuable nutrients coming from manure and digestate processing. Membrane filtration is a suitable technology to separate valuable nutrients in easily transportable concentrates which could potentially be re-used as green fertilizers, in the meantime producing high quality water. However, traditional membrane filtration systems often suffer technical problems in waste stream treatment. The aim of this study was to evaluate the performance of vibratory shear enhanced processing (VSEP) in the removal of macronutrients (N, P, K, Na, Ca, Mg) from the liquid fraction of digestates, reducing their concentrations down to dischargeable/re-usable water. In addition, the re-use potential of VSEP-concentrates as sustainable substitutes for fossil-based mineral fertilizers was evaluated. Removal efficiencies for N and P by two VSEP filtration steps were high, though not sufficient to continuously reach the Flemish legislation criteria for discharge into surface waters (15 mg N l −1 and 2 mg P l −1 ). Additional purification can occur in a subsequent lagoon, yet further optimization of the VSEP filtration system is advised. Furthermore, concentrates produced by one membrane filtration step showed potential as N–K fertilizer with an economic value of €6.3 ± 1.1 t −1 fresh weight (FW). Further research is, however, required to evaluate the impact on crop production and soil quality by application of these new potential green fertilizers.

Enhanced phytoextraction proposes the use of soil amendments to increase the heavy-metal content of above-ground harvestable plant tissues. This study compares the effect of synthetic aminopolycarboxylic acids [ethylenediamine tetraacetatic acid (EDTA), nitriloacetic acid (NTA), and diethylenetriamine pentaacetic acid (DTPA)] with a number of biodegradable, low-molecular weight, organic acids (citric acid, ascorbic acid, oxalic acid, salicylic acid, and NH 4 acetate) as potential soil amendments for enhancing phytoextraction of heavy metals (Cu, Zn, Cd, Pb, and Ni) by Zea mays. The treatments in this study were applied at a dose of 2 mmol/kg −1 1 d before sowing. To compare possible effects between presow and postgermination treatments, a second smaller experiment was conducted in which EDTA, citric acid, and NH 4 acetate were added 10 d after germination as opposed to 1 d before sowing. The soil used in this screening was a moderately contaminated topsoil derived from a dredged sediment disposal site. This site has been in an oxidized state for more than 8 years before being used in this research. The high carbonate, high organic matter, and high clay content characteristic to this type of sediment are thought to suppress heavy-metal phytoavailability. Both EDTA and DTPA resulted in increased levels of heavy metals in the above-ground biomass. However, the observed increases in uptake were not as large as reported in the literature. Neither the NTA nor organic acid treatments had any significant effect on uptake when applied prior to sowing. This was attributed to the rapid mineralization of these substances and the relatively low doses applied. The generally low extraction observed in this experiment restricts the use of phytoextraction as an effective remediation alternative under the current conditions, with regard to amendments used, applied dose (2 mmol/kg −1 soil), application time (presow), plant species (Zea mays), and sediment (calcareous clayey soil) under study.

Water soluble and/or dissolved metals represent the most ecotoxicologically relevant fraction of metals in the environment. However, water extractions may be prone to errors. This study aims to evaluate the performance of 5 filters as well as Rhizon soil moisture samplers, with respect to metal adsorption and/or release by the filter. In addition, the effect of equilibration time on water extractions of different types of soils was evaluated (silty loam, silty clay loam, loamy sand). Filtrations of synthetic solutions containing 40 μg 1^sup -1^ Zn, 20 μg 1^sup -1^ Cu, Ni and Pb, 10 μg 1^sup -1^ Cr and 2 μg 1^sup -1^ Cd were conducted using the different filters. The synthetic solutions either contained (i) no other competitive cations (Ca, Mg, Na, K, Fe, Mn, Al) (A ^sub 1^), (ii) competitive cations at concentrations similar to those observed in soil solutions (A ^sub 2^), (iii) competitive cations at 10 times lower concentrations than those in the synthetic soil solution (A ^sub 3^). Whiteband filters were observed to retain considerable amounts of trace metals (except Cr), both in the presence and absence of other competitive cations. Millipore filters did not exhibit metal retention. Rhizon soil moisture samplers did not retain trace metals from the synthetic soil solution (A ^sub 2^), whereas at lower concentrations of competitive cations (A ^sub 1^, A ^sub 3^) retention of Cu and Pb was observed. Whiteband filters without a predefined pore diameter allowed colloidal material of unknown particle size to pass into the filtrate, making interpretation of results very difficult and comparison between studies using different filters impossible. Millipore filters with a predefined pore diameter are to be recommended for this purpose. However, particular attention must be paid to potential constitutive Zn release by the sintered glass filter holders, the effect of which can be reduced by rigorous acid washing prior to and following every use. Rhizon samplers were also considered to be useful tools with well-defined and sufficiently small pore diameters to withhold colloidal material. However, in the absence or at reduced concentrations of competitive cations (A ^sub 1^, A ^sub 3^) retention of trace metals by the Rhizons, particularly Cu and Pb, was observed. Finally, short equilibration times may be insufficient for full assessment of the water extractable pool of trace metals in the soil.[PUBLICATION ABSTRACT]

Willow (Salix spp.) stands are often proposed as vegetation covers for the restoration and stabilization of contaminated and derelict land. Planting willows on dredged sediment disposal sites for biomass production can be an alternative to traditional capping techniques. However, with the introduction of willow stands on dredged sediment disposal sites, the possibility of increased contaminant availability in the root zone must be acknowledged as it can increase the risk of leaching. Two trials investigated the availability of Cd, Zn, Cu, and Pb in the root zones of willows grown on contaminated sediment. To assess the effects of willow root growth on metal extractability and mobility, bulk and rhizosphere sediment samples were extracted with deionized water, ammonium acetate at pH 7, and ammonium acetate-EDTA at pH 4.65. A rhizobox experiment was used to investigate the short-term effect of willow roots on metal availability in oxic and anoxic sediment. Longer-term effects were assessed in a field trial. The rhizobox trial showed that Cd, Zn, and Cu extractability in the rhizosphere increased while the opposite was observed for Pb. This was attributed to the increased willow-induced oxidation rate in the root zone as a result of aeration and evapotranspiration, which masked the direct chemical and biological influences of the willow roots. The field trial showed that Cu and Pb, but not Cd, were more available in the root zone after water and ammonium acetate (pH 7) extraction compared with the bulk sediment. Sediment in the root zone was better structured and aggregated and thus more permeable for downward water flows, causing leaching of a fraction of the metals and significantly lower total contents of Cd, Cu, and Pb. These findings indicate that a vegetation cover strategy to stabilize sediments can increase metal availability in the root zone and that potential metal losses to the environment should be considered.

Intensive animal farming has resulted in overproduction of manure in parts of the industrialized world. The localized excessive nutrient emission has detrimental effects on both ecology and economy. In response to European guidelines, Flanders (Belgium) has imposed obligations on the agricultural sector to process a portion of the produced animal manure. To achieve the stringent criteria for discharge of processed waste water, overly expensive engineering techniques are currently required (mostly membrane technology). This paper examines the potential of more economic physico-chemical techniques for P-removal from pre-treated liquid animal manure. Two techniques for dephosphatation were performed sequentially: (i) the use of flocculants (FeCl sub(3), FeCl sub(2), FeClSO sub(4), Poly Aluminium Chloride (PAC), Na Aluminate) and subsequently, (ii) the use of substrates to further reduce P-levels by adsorption or precipitation (expanded clay (argex), Fe-oxide powder, clay, calcinated clay, alganite, sand). The P-removal efficiencies obtained in this research are promising to consider implementation of physico-chemical P-removal as an intermediary step in animal manure processing after secondary pre-treatment in an activated sludge reactor and before tertiary treatment.

The concept of an integrated multifunctional river management is gaining importance. For major rivers, restoring the contact between rivers and alluvial plains is an important goal, as riparian areas have a specific role for several riverine processes. However, former and current human activities are an obstruction or a limitation for river restoration. We studied the influence of former dredging activities along the river Leie on the alluvial plain quality. A soil survey and an archive query for reconstructing the history of dredging operations were conducted simultaneously. The geographical impact expressed as topographical changes and covering of the original soil profile and related processes and biota was large. The pollution status of dredged sediment-derived soils was found to be far from negligible: concentrations of Cd, Cr, and Zn were, in 10% of the cases, higher than 20, 480, and 2800 mg kg(-1) DM, respectively. Both agriculture and nature rehabilitation on dredged sediment-derived soils can only be accepted after profound risk assessment, and management should focus on ecological risk reduction. Results indicate the importance of soil quality assessment in alluvial plains for an integrated river management, rather than a priori assuming pristine soil conditions. The collected \"off-line\" sediment data can be used as a reconstruction of past sediment pollution, especially when long-term sediment monitoring programs are not available.

The removal of Co, Ni, Cu and Zn from synthetic industrial wastewater was studied in subsurface flow constructed wetland microcosms filled with gravel or a gravel/straw mixture. Half of the microcosms were planted with Phragmites australis and half were left unplanted. All microcosms received low-strength wastewater (1 mg L−1 of Co, Ni, and Zn, 0.5 mg L−1 Cu, 2,000 mg L−1 SO4) during seven 14-day incubation batches. The pore water was regularly monitored at two depths for heavy metals, sulphate, organic carbon and redox potential. Sorption properties of gravel and straw were assessed in a separate experiment. A second series of seven incubation batches with high-strength wastewater (10 mg L−1 of each metal, 2,000 mg L−1 SO4) was then applied to saturate the substrate. Glucose was added to the gravel microcosms together with the high-strength wastewater. Sorption processes were responsible for metal removal during start-up, with the highest removal efficiencies in the gravel microcosms. The lower initial efficiencies in the gravel/straw microcosms were presumably caused by the decomposition of straw. However, after establishment of anaerobic conditions (Eh∼−200 mV), precipitation as metal sulphides provided an additional removal pathway in the gravel/straw microcosms. The addition of glucose to gravel microcosms enhanced sulphate reduction and metal removal, although Phragmites australis negatively affected these processes in the top-layer of all microcosms.

Since 2003, the pig industry in Flanders (Belgium) is obliged to process a portion of the nutrient overproduction. In general, pig manure processing occurs as follows: i) separation into liquid and solid fractions, ii) conversion of the solid fraction to an exportable product (e.g. composting) and iii) reduction of nutrient contents in the liquid fraction before discharge into surface water or spreading on arable land. The aim of this study was to evaluate the potential of constructed wetlands (CWs) planted with Phragmites australis to reduce nitrogen (N), phosphorus (P) and chemical oxygen demand (COD) in the liquid fraction to levels below discharge criteria. In addition, the removal efficiency of heavy metals (Cu, Zn) present at elevated levels in the liquid fraction was evaluated. A greenhouse experiment was conducted with subsurface flow (SSF) reed beds (2 × 0.125 × 0.11 m) filled with sand, loam, clayey sand or expanded clay (argex). The liquid manure load was set at 1 mm per day. Removal efficiencies varied between 64-75% for COD, 73-83% for N and 71-92% for P, depending on the matrix material used. However, effluent levels still remained significantly above the Flemish legal discharge criteria of 2 mgl^sup -1^, 15 mgl^sup -1^ and 125 mgl^sup -1^ for P, N and COD respectively.[PUBLICATION ABSTRACT]

Heavy metals in dredged sediment disposal sites may be transported through runoff and percolation. In the present study metal fluxes by runoff and percolation were determined under simulated rainfall at a slope of 19% and a rainfall intensity of approximately 40 mm h super(-1). These circumstances correspond to half the mean annual erosivity of rain under Belgian weather conditions. Surface runoff and percolating water samples were analysed for suspended solids, total dissolved carbon and Cd, Cr, Cu, Ni, Pb and Zn.Runoff rate and sediment yields were highest for asilt loam sediment, characterised by a low clay andorganic matter content. Metal concentrations in runoff and percolating water varied widely between these diments studied and were related to the total metal content in the sediment. In runoff and percolating water from the contaminated sediments, metal concentrations strongly exceeded the Netherlands Areference values for ground water quality. Very high metal fluxes were observed for the recently oxidised dredged sediment. Metal transport per unit surface area through percolating water was from two to more than twenty times greater than that in surface runoff.

The potential risk of surface and ground water contamination by trace metals leached from contaminated land‐disposed dredged sediment is a major environmental concern. The objective of this study was to evaluate trace metal leachability, leachate quality, and environmental effects of land‐disposed dredged sediments (LDDS). In addition to chemical characterization and determination of potential and actual leachability, dredged sediments were also subjected to solid phase fractionation using a sequential extraction procedure. Potential leachability, defined as the maximum metal pool that may become available for leaching at a constant pH 4, decreased in the following order: Zn ∼ Cd > Mn > Ni > Co > Cu ∼ As > Pb > Cr. Potential metal leachability was controlled mainly by solid phase distribution of metals in sediments. The acid‐extractable fraction of most metals correlated well with the potentially leachable metals extracted with a solution of pH 4 (r2 > 0.67), except for Pb. The kinetics of metal release were determined using the cascade leaching test (CLT) with water at pH 4. Except for Ni and Zn in the first fraction, metal concentrations in all consecutive leaching fractions remained below the maximum permissible level in water for human consumption. Actual metal leachability expressed as a percentage of potential leachability varied widely among trace metals and decreased in the order: As > Cu > Cr > Ni > Zn > Cd > Co > Pb. Our results indicate low risk of surface and ground water contamination resulting from land disposal of dredged sediments.