Pernille Erland Jensen

Extracting phosphorous from incinerated sewage sludge ash rich in iron or aluminum

Ashes from mono-incineration of sewage sludge (ISSA) generally contain high concentrations of phosphorous (P) and can be regarded as secondary P resources. ISSA has no direct value as fertilizer as P is not plant available. The present paper experimentally compares P extraction in acid from two different ISSAs; one rich in Al (67g/kg) and the other in Fe (58g/kg). The difference related to P precipitation at the waste water treatment facilities. Another major difference between the ashes was that flue gas purification products were mixed into the first ash and it contained about 5% activated carbon. The Al rich ash had a significantly higher buffering capacity and required more acid for extraction of P. When acid extraction of P from ISSA is the method for recovery, it is thus beneficial to go back to the waste water treatment facility and e.g. choose Fe for P precipitation rather than Al. Formation of a high amount of gypsum crystals in both ashes after extraction in H2SO4 was seen by SEM-EDX. H2SO4 is the cheapest mineral ash, but the gypsum formation must be taken into account when either finding possibility for using the remaining ash in e.g. construction materials or if the choice is deposition, as the gypsum increases the volume significantly.

Comparison of two different electrodialytic cells for separation of phosphorus and heavy metals from sewage sludge ash

With decreasing availability of phosphorus from primary resources its recovery from waste streams becomes increasingly more important. Sewage sludge ash is rich in phosphorus, but the direct use as fertilizer is limited because of inorganic contaminants such as heavy metals and strong bonding of phosphorous in the ash. Electrodialysis (ED) can be used to recover phosphorus and simultaneously remove heavy metals. The present work is an experimental screening of different options for ED in relation to experimental setup and combination with acid addition. Experiments for stirred ash suspensions utilizing a three compartment cell setup where the anode, cathode and stirred suspension are separated by ion exchange membranes are reported. Simplifying this experimental setup by removing the anion exchange membrane brings the anode in direct contact with the stirred ash suspension. Through this adjustment, half-reactions at the anode contribute to the acidity of the stirred suspension resulting in increased dissolution of both phosphorus and heavy metals (Cd, Cu, Cr, Pb, Zn, Ni) and better separation of most heavy metals from the stirred ash suspension. When the ash is suspended in an acidic solution, these effects increase significantly in early stages of the experiments. The combination of ED in a two compartment setup and initial acidification of the stirred suspension is most effective in dissolving of phosphorus and separation of heavy metals. In this setup, up to 96% of the phosphorus in the ash was dissolved after 7 d. Using the three compartment setup and initially suspending the ash in distilled water, resulted in 53% dissolution of the total recovered phosphorus after 7 d.

Utilization of electromigration in civil and environmental engineering—Processes, transport rates and matrix changes

Electromigration (movement of ions in an applied electric field) is utilized for supply or extraction of ions from various porous materials within both civil and environmental engineering. In civil engineering, most research has been conducted on the removal of chlorides from concrete to hinder reinforcement corrosion while in environmental engineering remediation of heavy metal polluted soil is the issue most studied. Never the less, experiments have been conducted with utilization for several other materials and purposes within both engineering fields. Even though there are many topics of common interest in the use of electromigration for the two fields, there is no tradition for collaboration. The present paper is a review with the aim of pointing out areas of shared interest. Focus is laid on the purposes of the different processes, transport rates of various ions in different materials and on changes in the matrix itself. Desorption and dissolution of the target elements into ionic form is a key issue to most of the processes, and can be the limiting step. The removal rate is generally below 1 cm day− 1, but it can be much less than 1 mm day− 1 when desorption is slow and insufficient. Matrix changes occurs under the action of the applied electric field and it includes both physico-chemical and hydrological changes. Some of the solid phases is weathered and new can be formed. Increased fundamental understanding of the effects and side effects, when applying the electric field to a porous material, can lead to improvement of the known technologies and possibly to new applications.

Electrodialytic Separation of Phosphorus and Heavy Metals from Two Types of Sewage Sludge Ash

During sewage sludge incineration phosphorus (P) is retained in the ash in a form not directly available to plants. As P is a sparse resource, it is important to develop techniques for recovery of P from incinerated sewage sludge ashes (ISSA). Heavy metals are concentrated in ISSA and separation of P and heavy metals is required. The present work is an experimental screening of a new combination of acid extraction and electrodialysis–electrodialytic separation (EDS) for simultaneous P recovery and removal of heavy metals. Experiments were conducted with two different ashes; rich in Fe or Al. The separation method was best suited for the Fe-rich ash, where it was possible to separate P into one processing solution, heavy metals (Cu, Zn, Ni, Pb) into another, keeping the ash suspended in a third solution (which though still contained P after 1 week of EDS). For the Al rich ash, the separation was not similarly encouraging. The high release of Al during the extraction influenced the speciation of P and negatively charged P complexes were not prevailing. On the contrary to Al, the Fe containing ash particles were insoluble so Fe did not interfere with P speciation and separation after extraction.

Electrodialytic remediation of suspended soil – Comparison of two different soil fractions

Electrodialytic remediation (EDR) can be used for removal of heavy metals from suspended soil, which allows for the soil remediation to be a continuous process. The present paper focused on the processing parameters for remediation of a soil polluted with Cu and As from wood preservation. Six electrodialytic treatments lasting from 5 to 22 days with different liquid to solid ratio (L/S) and current intensity were conducted. Among treatments, the highest removal was obtained from the soil fines with 5 mA current at L/S 3.5 after 22 days where 96% of Cu and 64% of As were removed. Comparing the removal from the original soil and the soil fines in experiments with identical charge transportation, higher removal efficiency was observed from the soil fines. Constant current with 5 mA could be maintained at L/S 3.5 for the soil fines while not for the original soil. Doubling current to 10 mA could not be maintained for the soil fines either, and doubling L/S to 7 at 5 mA entailed a very fast acidification which impeded the removal. The results showed that a very delicate balancing of current density and L/S must be maintained to obtain the most efficient removal.

Preliminary treatment of MSW fly ash as a way of improving electrodialytic remediation

In the current work electrodialytic remediation (EDR) was applied to remove heavy metals from municipal solid waste (MSW) fly ash, a hazardous waste collected during flue gas treatment. Tests were conducted to evaluate if EDR could be improved by introducing a preliminary treatment in which very soluble salts were removed. Three different preliminary treatments were conducted with different L:S ratios and pH. Treatment in which metal release and L/S ratio were lower was selected for EDR. Electrodialytic remediation was performed at a constant current of 38 mA, for 14 days, using gluconate as a solubilisation enhancement agent. Conductivity and pH were monitored and electrolyte samples were collected every 4 days to evaluate metal release over time. It was found that the preliminary treatment reduces fouling of the ion-exchange membranes used in EDR and drastically increases the removal of metals. Remediation time was also considerably reduced. Additionally, preliminary washing reduces energy consumption during EDR, since electric current is not wasted in the transport of soluble salts. Sequential extraction was performed in the untreated and treated samples to help identify how metals are bond to the fly ash. It was seen that at the end metals are mainly found in the strongly bonded and residual phases. This indicates that the combined treatment (washing + EDR) is successful in reducing the environmental risk posed by fly ash.

Multivariate methods for evaluating the efficiency of electrodialytic removal of heavy metals from polluted harbour sediments

Chemometrics was used to develop a multivariate model based on 46 previously reported electrodialytic remediation experiments (EDR) of five different harbour sediments. The model predicted final concentrations of Cd, Cu, Pb and Zn as a function of current density, remediation time, stirring rate, dry/wet sediment, cell set-up as well as sediment properties. Evaluation of the model showed that remediation time and current density had the highest comparative influence on the clean-up levels. Individual models for each heavy metal showed variance in the variable importance, indicating that the targeted heavy metals were bound to different sediment fractions. Based on the results, a PLS model was used to design five new EDR experiments of a sixth sediment to achieve specified clean-up levels of Cu and Pb. The removal efficiencies were up to 82% for Cu and 87% for Pb and the targeted clean-up levels were met in four out of five experiments. The clean-up levels were better than predicted by the model, which could hence be used for predicting an approximate remediation strategy; the modelling power will however improve with more data included.

Electrodialytic remediation of soil slurry-removal of Cu, Cr, and As

Abstract Severe soil contamination is often found at old wood preservation sites and a common combination of pollutants is Cu, Cr, and As. In the present work it is tested if simultaneous removal of Cu, Cr, and As can be obtained in an electrodialytic cell where the polluted soil is remediated as a stirred suspension (placed as the desalination compartment in accordance to the position of the ion exchange membranes). The soil for the experiments was sampled at an abandoned wood preservation site and contained 2170 mg Cu/kg, 710 mg Cr/kg and 3200 mg As/kg. SEM-EDX analysis showed that Cu, Cr, As and oxygen formed particles that were cementing soil minerals together. The soil was suspended in distilled water, distilled water with I2 crystals to have an oxidizing environment, or in an acidified environment at pH about 1.0. The experiments lasted from 1 to 3 weeks. Good results were obtained in two experiments; an experiment where the soil was suspended in distilled water and the remediation lasted 3 weeks with 2.5 mA and an experiment with acidification of the soil suspension with HNO3 to pH about 1.0 (2 weeks and 5 mA). The best separation of pollutants and soil was obtained in the experiment with suspension in distilled water. Based on soil concentrations, good Cu removal (95%) was obtained in both experiments. Removal of Cr was most efficient from the acidified soil suspension (74%). Both Cu and Cr concentrations were below the limiting values after the remediation. The As concentration, however, was not even although 61% was removed. In the soil remained about 1070 mg As/kg soil and since the limiting value is 40 mg As/kg, the removal was not efficient enough. So simultaneous removal was possible, but the target values were only met in the case of Cu and Cr, and more research is needed to remove As to a sufficiently low concentration, as well.

Chemometric analysis for pollution source assessment of harbour sediments in Arctic locations.

Pollution levels, pollutant distribution and potential source assessments based on multivariate analysis (chemometrics) were made for harbour sediments from two Arctic locations; Hammerfest in Norway and Sisimiut in Greenland. High levels of heavy metals were detected in addition to organic pollutants. Preliminary assessments based on principal component analysis (PCA) revealed different sources and pollutant distribution in the sediments of the two harbours. Tributyltin (TBT) was, however, found to originate from point source(s), and the highest concentrations of TBT in both harbours were found adjacent to the former shipyards. Polyaromatic hydrocarbons (PAH) ratios and PCA plots revealed that the predominant source in both harbours was pyrogenic related to coal/biomass combustion. Comparison of commercial polychlorinated biphenyls (PCB) mixtures with PCB compositions in the sediments indicated relation primarily to German, Russian and American mixtures in Hammerfest; and American, Russian and Japanese mixtures in Sisimiut. PCA was shown to be an important tool for identifying pollutant sources and differences in pollutant composition in relation to sediment characteristics.

Pulse current enhanced electrodialytic soil remediation--comparison of different pulse frequencies.

Energy consumption is an important factor influencing the cost of electrodialytic soil remediation (EDR). It has been indicated that the pulse current (in low frequency range) could decrease the energy consumption during EDR. This work is focused on the comparison of energy saving effect at different pulse frequencies. Based on the restoration of equilibrium, the relaxation process of the soil-water system was investigated by chronopotentiometric analysis to find the optimal relaxation time for energy saving. Results showed that the pulse current decreased the energy consumption with different extent depending on the pulse frequency. The experiment with the frequency of 16 cycles per day showed the best restoration of equilibrium and lowest energy consumption. The energy consumption per removed heavy metals was lower in pulse current experiments than constant current and increased with the pulse frequency. It was found that the transportation of cations through the cation exchange membrane was the rate controlling step both in constant and pulse current experiments, thus responsible for the major energy consumption. Substitution of the cation exchange membrane with filter paper resulted in a dramatic decrease in energy consumption, but this change impeded the acidification process and thus the removal of heavy metals decreased significantly.