Phosphorous extraction and heavy metal separation from sewage sludge ash by two-compartment electrodialysis in an upscaled tube reactor

Two-compartment electrodialytic extraction (2C-ED) is a one-step process for the simultaneous phosphorous extraction and separation of heavy metals from sewage sludge ash (SSA). The process is driven by an applied electric DC field, which can be supplied from renewable sources. The proof-of-concept of the method was conducted in small laboratory cells; however, upscaling to a continuous 2C-ED process, which additionally can treat SSA suspensions at a low liquid-to-solid (L:S) ratio, requires a new design. This paper presents such a new design. In principle, ED consists of two compartments separated by a cation exchange membrane. One compartment contains a suspension of SSA in water and the anode. A cathode is placed in the other compartment. Electrolysis at the anode acidifies the suspension causing the dissolution of phosphorous and heavy metals. The heavy metals are separated from the suspension by electromigration into the catholyte, whereas the dissolved phosphorous remains in the dispersion solution. In the new design, the SSA was suspended in a tube-shaped reactor with the cation exchange membrane covering the outside. The reactor was placed in a container with the catholyte. Periodically turning off the reactor kept SSA in suspension even at a low L:S ratio without corners and pockets where the SSA otherwise tends to settle. Five 2C-ED experiments were conducted with 1.5 to 3 kg SSA at varying currents and durations. Up to 89% P was extracted. The extracted P was concentrated in the dispersion solution of the SSA suspension, where the obtained P-related concentrations of heavy metals were far below the limiting values for spreading on agricultural land. The experiments underlined that treating the SSA in a suspension with a low L:S ratio is advantageous. A comparison to previous laboratory experiments in small cells treating 50 g SSA shows a significantly more efficient use of the applied current in the new reactor setup. Thus, the new reactor design for 2C-ED fulfilled the set criteria for the operation and did additionally result in a higher efficiency than the laboratory setups, i.e., the design can be the first step towards an upscaling.

Phosphorus recovery from sewage sludge ash (SSA): An integrated technical, environmental and economic assessment of wet-chemical and thermochemical methods

Incineration is a promising disposal method for sewage sludge (SS), enriching more than 90% of phosphorus (P) in the influent into the powdered product, sewage sludge ash (SSA), which is convenient for further P recovery. Due to insufficient bioavailable P and enriched heavy metals (HMs) in SSA, it is limited to be used directly as fertilizer. Hence, this paper provides an overview of P transformation in SS incineration, characterization of SSA components, and wet-chemical and thermochemical processes for P recovery with a comprehensive technical, economic, and environmental assessment. P extraction and purification is an important technical step to achieve P recovery from SSA, where the key to all technologies is how to achieve efficient separation of P and HMs at a low economic and environmental cost. It can be clear seen from the review that the economics of P recovery from SSA are often weak due to many factors. For example, the cost of wet-chemical methods is approximately 5∼6 €/kg P, while the cost of recovering P by thermochemical methods is about 2∼3 €/kg P, which is slightly higher than the current P fertilizer (1 €/kg P). So, for now, legislation is significant for promoting P recovery from SSA. In this regard, the relevant experience in Europe is worth learning from countries that have not yet carried out P recovery from SSA, and to develop appropriate policies and legislation according to their own national conditions.

Phosphorus recovery from incinerated sewage sludge ash using electrodialysis coupled with plant extractant enhancement technology

Phosphorus (P) is an integral mineral nutrient for the growth of plants and animals. As the increasing population worldwide, the demand for P resources keeps increasing. Therefore, it is necessary to recover P from secondary resources. Unlike conventional P recovery processes, this work focused on the recovery of P from incinerated sewage sludge ash (ISSA) using electrodialysis as the main technology coupled with plant extractants. In this study, Amaranthus and hydrolyzed polymaleic anhydride (HPMA) were used as P extractants, investigating the effects of HPMA concentration and pH of the compound agent on the migration of P and heavy metals from ISSA. The results showed that the concentration of HPMA and pH of the compound agent had a significant influence on the mobility of P and heavy metals. Meanwhile, the impacts of eggshell additions and voltage on the recovery efficiency of P was also studied by using waste eggshells as calcium sources. We found that when eggshells were added at 10 g/L and the voltage was 10 V, the recovery efficiency of P reached 96.05%. Moreover, XRD patterns revealed that the mineral phase of recovered P-containing products was predominantly hydroxyapatite, which had good environmental benefits. Generally, the favorable results have been achieved in the recovery efficiency of P and has practical implications for P recovery from ISSA.

Acidification and alkalinization pretreatments of biowastes and their effect on P solubility and dynamics when placed in soil

BACKGROUND

Sustainability concerns as well as recent increases in fertilizer prices exacerbates the need to optimise the use of biowastes as fertilizers. For this reason, we investigated how different pretreatments affect the P dynamics when biofertilizers are placed in the soil.

METHODS

Sewage sludge (SS), sewage sludge ash (SS-ash), meat and bone meal (MBM), and the solid fraction of biogas digestate (BGF) were pretreated with H₂SO₄, NaOH, and Ca(OH)₂ and incubated for 2 and 12 days, respectively, in a one-dimensional reaction system for detailed studies of the interactions in the biomaterial-soil interface and the soil adjacent to the placement zone.

RESULTS

Our results showed that acidification and treatment with NaOH increased the P solubility of the biomaterials. The P loss from the biomaterial layer to the soil was correlated with water-extractable P in the biomaterials (0.659) and water-extractable P in the soil (0.809). Acidification significantly increased the total amount of P depleted from the biomaterial to the soil whereas NaOH pre-treatment did not. However, for NaOH-treated SS and SS-ash, the apparent recoveries were significantly higher compared to the acidification due to a decrease in soil P sorption capacity as the soil pH increased due to residual alkalinity in the biomaterials.

CONCLUSIONS

Acidification showed promising results by increasing the P solubility of all the biomaterials, and the alkalinization of SS and SS-ash with NaOH by increasing the apparent recovery in the soil. However, further studies are needed to assess the effects of these treatments on plant growth and P uptake.

Hydration mechanism and environmental impacts of blended cements containing co-combustion ash of sewage sludge and rice husk: Compared with blended cements containing sewage sludge ash

In this study, the hydration mechanism and environmental impacts of blended cements with the co-combustion ash of rice husk and sewage sludge (CCA) were investigated and compared to those of blended cements with sewage sludge ash (SSA). CCA possesses lower phosphate contents than SSA, leading to lower inhibition effects on early hydration of cement clinker. Moreover, the pozzolanic activity of CCA is higher than that of SSA. Thus, more hydration products from the pozzolanic reaction of CCA are generated in CCA-based blended cements. Compared to the matrix of SSA-based blended cements, that of their CCA-based counterpart is filled with more hydration products, which promotes porosity refinement and strength development of CCA-based blended cements at later ages. CCA-based blended cements exhibit greater environmental benefits than SSA-based blended cements because fossil consumption and toxic substance emissions during the co-combustion of rice husk and sewage sludge is lower than that during the mono-combustion of sewage sludge.

Improvement of fungal extraction of phosphorus from sewage sludge ash by Aspergillus niger using sludge filtrate as nutrient substrate

Fungal extraction is a promising approach for reclaiming phosphorus (P) from sewage sludge ash (SSA). However, this approach faces notable technical and economic challenges, including an unknown P speciation evolution and the addition of expensive chemical organic carbon. In this study, the use of an organic-rich effluent produced in sludge dewatering as nutrient source is proposed to initiate the fungal extraction of SSA-borne P with Aspergillus niger. The changes in P speciation in the ash during fungal treatment was analyzed by combined sequential extraction, solid-state ³¹P nuclear magnetic resonance, and P X-ray absorption near edge spectroscopy. Results showed that after 5 days of fungal treatment using sludge-derived organics, 85 % of P was leached from SSA. Dominantly, this considerable release of P resulted from the dissolution of Ca₃(PO₄)₂, AlPO₄, FePO₄, and Mg₃(PO₄)₂ in the ash, and their individual contribution rates to P released accounted for 28.0 %, 24.3 %, 20.6 %, and 18.8 %, respectively. After removal of metal cations (e.g., Mg²⁺, Al³⁺, Fe³⁺, and heavy metals) by cation exchange resin (CER), a hydroxyapatite (HAP) product with a purity of > 85 % was harvested from the extract by precipitation with CaCl₂. By contrast, without CER purification, a crude product of Ca/Mg-carbonates and phosphates mixture were obtained from this extract. A total of 73.2 wt% of P was ultimately recovered from SSA through integrated fungal extraction, CER purification, and HAP crystallization. These findings provide a mechanistic basis for the development of waste management strategies for improved P reclamation with minimal chemical organics consumption.

Sewage sludge ash-based mortar as construction material: Mechanical studies, macrofouling, and marine toxicity

Incinerated sewage sludge ash is tested here as a cement and aggregate substitute in mortar blocks. It can be used at various percentages to reduce the overall cost of production and promote ash recycling. The compressive strength of the cast blocks was tested at 28 days to determine the optimal combination of ball milled ash (replacing cement) and sewage sludge ash (replacing sand). This was compared with a control block made of cement and sand only. The cast blocks with the optimal ash formulation were tested for their flexural strength and other properties such as surface functional groups, constituent phases and porosity. The control and ash mortars exhibited similar properties. A potential application of these blocks is to use them as part of seawalls. These blocks were thus suspended in the sea for 6 months. Marine organism attachment was observed over time in both control and ash mortar blocks. There was no significant difference between the mortars after 6 months. The mortar blocks were also subjected to leaching tests (NEN-7345). The leachates did not exhibit toxicity to microalgae. In contrast, mild toxicity was observed in the sea urchin embryo development assay. Overall, the study suggests that sewage sludge ash is a potential material to be used for seawall construction as it has the desirable mechanical properties. However, there remain some residual marine toxicity concerns that need to be further addressed.

Phosphorus solubility and dynamics in a tropical soil under sources derived from wastewater and sewage sludge

Conventional phosphate fertilizers are usually highly water-soluble and rapidly solubilize when moistened by the soil solution. However, if this solubilization is not in alignment with plants demand, P can react with the soil colloidal phase, becoming less available over time. This is more pronounced in acidic, oxidic tropical soils, with high P adsorption capacity, reducing the efficiency of P fertilization. Furthermore, these fertilizers are derived from phosphate rock, a non-renewable resource, generating an environmental impact. To assess these concerns, waste-recycled P sources (struvite, hazenite and AshDec®) were studied for their potential of reducing P fixation by the soil and improving the agronomic efficiency of the P fertilization. In our work, we compared the solubilization dynamics of struvite, hazenite, AshDec® to triple superphosphate (TSP) in a sandy clay loam Ferralsol, as well as their effect on solution pH and on soil P pools (labile, moderately-labile and non-labile) via an incubation experiment. Leaching columns containing 50 g of soil with surface application of 100 mg per column (mg col^-1) of P from each selected fertilizer and one control (nil-P) were evaluated for 60 days. Daily leachate samples from the column were analyzed for P content and pH. Soil was stratified in the end and submitted to P fractionation. All results were analyzed considering p < 0.05. Our findings showed that TSP and struvite promoted an acid P release reaction (reaching pHs of 4.3 and 5.5 respectively), while AshDec® and hazenite reaction was alkaline (reaching pHs of 8.4 and 8.5 respectively). Furthermore, TSP promoted the highest P release among all sources in 60 days (52.8 mg col^-1) and showed rapid release dynamic in the beginning, while struvite and hazenite showed late release dynamics and lower total leached P (29.7 and 15.5 mg col^-1 P respectively). In contrast, no P-release was detected in the leachate of the AshDec® over the whole trial period. Struvite promoted the highest soil labile P concentration (7938 mg kg^-1), followed by hazenite (5877 mg kg^-1) and AshDec® (4468 mg kg^-1), all higher than TSP (3821 mg kg^-1), while AshDec® showed high moderately-labile P (9214 mg kg^-1), reaffirming its delayed release potential.

Sewage sludge ash-incorporated stabilisation/solidification for recycling and remediation of marine sediments

Finding suitable disposal sites for dredged marine sediments and incinerated sewage sludge ash (ISSA) is a challenge. Stabilisation/solidification (S/S) has become an increasingly popular remediation technology. This study sheds light on the possible beneficial use of ISSA together with traditional binders to stabilise/solidify marine sediments. The performance of the binders on S/S of sediment 1 (clean) and sediment 2 (contaminated) was also compared. The results showed that the use of ISSA as part of the binder was effective in promoting the strength of the sediment with a high initial moisture content due to ISSA porous and high water absorption characteristics. The sediments treated with 10% cement and 20% ISSA attained the highest strength. Also, cement hydration as well as pozzolanic reactions between ISSA and Ca(OH)₂ made contributions to the strength development. This was supported by the microstructural analysis, in particular the porosity results. In terms of environmental impacts, two leaching tests (toxicity characteristic leaching procedure and synthetic precipitation leaching procedure) found that all the S/S treated sediment by 10% lime and 20% ISSA resulted in the lowest leachate concentrations under the on-site reuse scenario or under simulative acidic rainfall conditions. Therefore, recycling waste ISSA with lime can be used as an appealing binder to replace cement to stabilise/solidify dredged marine sediments for producing fill materials.

Sewage sludge ash-derived materials for H2S removal from a landfill biogas

H₂S removal is a key step for biogas cleaning because this component can lead to premature corrosion of the equipment and its cleaning has a significant cost. The aim of the present work was to assess the use of sewage sludge derived ash (SSA)-materials for H₂S removal from a landfill biogas. SSA and mixtures made with SSA, activated carbon (AC) and sand were tested for H₂S removal. The best removal efficiency was obtained with the mixture 80%m SSA and 20%m AC, while SSA alone was not a good adsorbent under tested experimental conditions. The materials characterization helped the adsorption mechanism understanding. Indeed, results highlighted that SSA presence stabilizes the pH on a basic range, favorable for H₂S dissociation into HS^- then its chemisorption. On the other hand, with the microporosity of AC, the contact surface between H₂S and oxygen was sufficiently large for chemisorption kinetics. It also appeared that the mixture with sand and AC adorbs non selectively H₂S but also other volatile organic pollutants present in biogas. Contrariwise, with SSA/AC mixtures, H₂S seems to be selectively chemisorbed.

Impact of time and phosphorus application rate on phosphorus bioavailability and efficiency of secondary fertilizers recovered from municipal wastewater

Demand for phosphorus (P) resources other than non-renewable P rock has driven the development of several P recovery technologies from municipal wastewater treatment and directed recovery of P into valuable fertilizers (struvite, ash, iron phosphate, etc.). Although the bioavailability of novel secondary P fertilizers has been examined in previous studies, insufficient attention has been paid to defining optimal plant growth duration and monitoring conditions to assess the dynamic changes in P. Accordingly, five fertilizers recovered from municipal wastewater: two struvites (STRSL and STRLQ), two ashes (ASH1 and ASH2), and iron-phosphate pelletized sludge (FeP) using triple superphosphate (TSP) as a positive control and blank (zero P) as a negative control, were applied to P poor-sand at three P doses (equivalent to 30, 60, and 90 kg P₂O₅ ha^-1). Fertilizer impact on perennial ryegrass (Lolium perenne) dry matter (DM) and P concentration were evaluated on a monthly basis for seven months. DM and relative agronomical efficiency (RAE) have shown the same trend between the fertilizers, but only at the lowest P dose (corresponding to 30 kg P₂O₅ ha^-1). At higher P doses (60, and 90 kg P₂O₅ ha^-1) the differences in DM and RAE among the fertilizers diminished. STRLQ, STRSL, ASH1 and FeP expressed a rather steady P release pattern, while ASH2 had a delay of four cuts and increase afterward. Monitoring the P uptake during four months of perennial ryegrass growth turned out to be the minimum, and seven months the optimum period for reaching the full capacity of the slow-release P fertilizers.

Immobilization of high-Pb contaminated soil by oxalic acid activated incinerated sewage sludge ash

Identifying effective and low-cost agents for the remediation of Pb-contaminated soil is of great importance for field-scale applications. In this study, the feasibility of reusing incinerated sewage sludge ash (ISSA), a waste rich in phosphorus, under activation by oxalic acid (OA) for the remediation of high-Pb contaminated soil was investigated. ISSA and OA were mixed at different proportions for the treatment of the high-Pb contaminated soil (5000 mg/kg). The Pb immobilization efficacy was further examined by both the standard deionized water leaching test and the toxicity characteristic leaching procedure (TCLP). The overall results showed that the use of the ISSA alone and an appropriate mixture of the ISSA and OA could effectively reduce the leachability of Pb from soil. 20% ISSA together with 30% OA (0.2 mol/L) reduced leached Pb concentration by 99%. The main stabilization mechanisms were then explored by different microstructural and spectroscopic analytical techniques including SEM, XRD and FTIR. Apparently, OA released phosphate from the ISSA and Pb from soil via acid attack, which combined and precipitated as stable lead phosphate minerals. However, excessive OA could cause high leaching of phosphate and zinc from the ISSA. Overall, this study indicates that ISSA could be used together with OA to remediate high-Pb contaminated soil, but careful design of mix proportions is necessary before practical application to avoid excessive leaching of phosphate and zinc from the ISSA.

Sewage sludge ash contaminated with radiocesium: Solidification with alkaline-reacted metakaolinite (geopolymer) and Portland cement

This study contributes toward developing measures for the disposal of radiocesium-contaminated sewage sludge ash (SSA). Here, we prepared two types of solidified bodies containing 30 wt% radiocesium-bearing SSA. The material used for the two solidified bodies were alkaline-reacted metakaolinite (geopolymer) and ordinary Portland cement (OPC). Cement has been used for solidification of low-level radioactive wastes, and geopolymer is a candidate of cement alternative materials. The characteristics of these solidified bodies were investigated by various aspects including mechanical strength, transformation of SSA components during solidification, and radiocesium confinement ability by leaching test. The compressive strength of geopolymer- and OPC-solidified bodies at 30 wt% SSA content was more than 40 MPa. After static leaching test at 60 °C, ¹³⁷Cs was hardly leached out from the geopolymer-solidified bodies containing SSA at 30 wt% to ultrapure water (<0.1%), whereas more than 30% ¹³⁷Cs was leached from the OPC-solidified bodies containing SSA at 30 wt% even though only ~9% of ¹³⁷Cs in the SSA is soluble. These results strongly indicate that geopolymer is far superior to OPC for solidifying radiocesium-bearing SSA.

Transformation of TiO2 (nano)particles during sewage sludge incineration

Titanium dioxide (TiO₂) (nano)particles are produced in large quantities and their potential impacts on ecosystems warrants investigations into their fate after disposal. TiO₂ particles released into wastewater are retained by wastewater treatment plants and accumulate in digested sludge, which is increasingly incinerated in industrialized countries. Therefore, we investigated the changes of the Ti-speciation during incineration of as-received sludge and of sludge spiked with anatase (d=20-50 nm) or rutile (d=200-400 nm) using X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM). In the as-received sludge, rutile and anatase were the dominant Ti bearing minerals and both remained unaffected by the anaerobic treatment. During incineration, Ti reacts with hematite to members of the hematite-ilmenite solid solution series (Hem-Ilm). Up to 80% of the Ti spiked as anatase transformed into Hem-Ilm, a distorted 6-fold coordinated Ti (Ti(IV)sulfate) and rutile during incineration. Up to 30% and 60% of rutile transformed into Hem-Ilm and Ti(IV)sulfate represented phases in fly and bottom ash, respectively. Fe and Ti were spatially correlated in ash derived from as-received and anatase spiked sludge, whereas only a thin layer of the spiked rutile reacted with Fe, in line with XAS data. This study highlights the transient nature of nano-Ti species during sewage sludge incineration.

Flow analysis of major and trace elements in residues from large-scale sewage sludge incineration

Increase of sewage sludge (SS) has led to the construction of more incineration plants, exacerbating to the production of SS incineration residues. However, few studies have considered the mass balance of elements in large-scale SS incineration plants, affecting the residues treatment and utilization. In this study, flow analysis was conducted for major and trace elements in the SS, the fly ash (sewage sludge ash, SSA) and bottom ash from two large-scale SS incineration plants. The elemental characteristics were compared with those of coal fly ash (CFA), and air pollution control residues from municipal solid waste incineration (MSWIA), as well as related criteria. The results showed that the most abundant major element in SSA was Si, ranging from 120 to 240 g/kg, followed by Al (76-348 g/kg), Ca (26-113 g/kg), Fe (35-80 g/kg), and P (26-104 g/kg), and the trace elements were mainly Zn, Ba, Cu, and Mn. Not all the major elements were derived from SS. Most trace elements in the SS incineration residues accounted for 82.4%-127% of those from SS, indicating that SS was the main source of trace elements. The partitioning of heavy metals in the SS incineration residues showed that electrostatic precipitator ash or cyclone ash with high production rates were the major pollutant sinks. The differences in some major and trace elements could be indicators to differentiate SSA from CFA and MSWIA. Compared with related land criteria, the pollutants in SSA should not be ignored during disposal and utilization.

Incineration of sewage sludge and recovery of residue ash as building material: A valuable option as a consequence of the COVID-19 pandemic

Circular economy principles were adopted by European Commission, to support a sustainable growth. They contain general rules that should be considered in all situations. At present, during pandemic, some waste disposal practices are under evaluation to guarantee safety conditions. For example, in view of the recent results reporting the presence of SARS-CoV-2 virus in sewage sludge, the possibility that it diffuses in the environment is alarming. The situation may result critical in densely populated cities, which are the largest sources of sewage sludge. In this frame the diffused practice of reuse of this waste in agriculture is under revision. In this context, incineration may represent a valuable alternative strategy to manage sewage sludge during pandemic. Indeed, due to thermal treatment, the destruction of organic micropollutants and pathogens, eventually present in the waste, is guarantee. Moreover, it is fundamental to highlight that also if the management of sewage sludge changes, the ash resulting from its combustion may have suitable reuse opportunities, and their landfilling should be avoided. This work presents the available possibilities of sewage sludge ash recovery in building applications and shows the results obtained by the analysis of their sustainability. The approach is based on the use of embodied energy and carbon footprint values, to make a simple and fast new method able to be a suitable tool to support and promote sustainability also in critical situations (such as pandemic) and when all the information about a technology are not available, making not possible to perform a full-LCA approach. This work aims to be not only a reference paper for promotion of strategies able to increase waste management safety, but also an example showing that circular economy principles should be pursued also if boundary conditions can change.

Simultaneous recovery of phosphorus and nitrogen from sewage sludge ash and food wastewater as struvite by Mg-biochar

The drawback of biochar as a soil ameliorant is its low-nutrient content while the bottleneck of struvite production is its high chemical cost. This drew the idea of using designed biochar for nutrient recovery from nutrient-rich wastewater as struvite. Mg-biochar was used for simultaneous P and N recovery from sewage sludge ash (SSA) and food wastewater (FW) by using ground coffee bean (GCB) and palm tree trunk (PTT) waste. PTT Mg-biochar could recover 92.2% of PO₄^3--P and 54.8% of NH₄⁺-N while GCB Mg-biochar could recover 79.5% of PO₄^3--P and 38.6% of NH₄⁺-N. Adsorption, precipitation and cation-exchange mechanisms are involved in the Mg-biochar for the simultaneous recovery of PO₄^3--P and NH₄⁺-N as struvite. Mg-biochars also showed higher struvite selectivity than the control samples. This method not only supports waste recycling and pollution mitigation but also highlights economical struvite production and the benefits of CO₂ sequestration.

Development of a novel phosphorus recovery system using incinerated sewage sludge ash (ISSA) and phosphorus-selective adsorbent

Phosphorus is an essential nutrient but faces foreseeable resource depletion. The incinerated sewage sludge ash (ISSA) is a promising source for recovering phosphorus. In this study, we proposed a new system for recovering phosphorus from ISSA. This innovative system uses phosphorus-selective adsorbent to purify phosphorus from the ISSA acid leachate. Laboratory scale batch and column tests were performed to demonstrate the feasibility of the system. Note that >70% of phosphorus in ISSA can be recovered as a high-purity recovery product. The product showed a structure similar to hydroxyapatite (Ca₅(PO₄)₃OH). The total amount of Ca, P, and O in the product was above 90 wt%. The content of trace elements (As, Cd, Cr, and Pb) in the product was below the fertilizer limits, suggesting that the health and environmental risks of using fertilizer in agriculture are negligible. The expected costs of the system were estimated. The reusability of the adsorbent can reduce the operational costs to a satisfactory level. This study provides a practical alternative for recovering phosphorus from ISSA.

Characterization of sewage sludge incineration ashes from multi-cyclones and baghouse dust filters as possible cement substitutes

Incineration is not a final disposal method for sludge management since a significant amount of ash is generated. Although some published literature reported on the use of sewage sludge ashes (SSAs) as a cement replacement, none of them referred to the air pollution control unit of the incinerator where the SSA is collected. The properties of SSAs from different air pollution control units were investigated with the purpose of usage as a cement replacement in the present study. The SSA samples obtained from multi-cyclone units (MC-SSA), where SSA is collected at the bottom of the unit with the help of inertia and centrifugal forces, and baghouse dust filters (BHD-SSA), where SSA is collected at the fabrics of the unit, of the sludge incinerator were analyzed for chemical and physical characteristics, including heavy metal leaching, mineralogy (XRD), and particle morphology (scanning electron microscopy). Mortar samples were prepared with 15 and 30% of MC-SSA and BHD-SSA additions and analyzed for workability, strength activity, strength development, and freeze-thaw resistance. Heavy metal leaching results of both of the MC-SSA and BHD-SSA and their mortars were below the legal requirements of landfills, inferring that the toxic elements in SSA would not be of environmental concern in case SSA is used in cement matrices. The present study identified the variability of the physical and chemical properties of both SSAs over time and their differences. Incompliances of the SSAs to the pozzolanic material standards were observed, although moderate pozzolanic activity was concluded. In order to obtain the required flow value, different amounts of polycarboxylate ether-based superplasticizer were used in all of the mixtures. The levels of amorphous phases were found to be 23.3 and 39.3% for MC-SSA and BHD-SSA, respectively. MC-SSA exhibited less porosity with larger agglomerates than BHD-SSA. Ninety-day compressive strength levels of MC-SSA mortars with 15% replacement, which was found as 50.53 MPa, was higher than that of BHD-SSA mortars by 6% (47.65 MPa). The freeze-thaw resistance and water adsorption capacity of SSA mortars were comparable with that of the cement mortars. It was inferred that the contribution of SSA substitution to the strength development was influenced by hydraulic activity more than pozzolanic activity.Graphical abstract.

To incinerate or not? - Effects of incineration on the concentrations of heavy metals and leaching efficiency of post-precipitated sewage sludge (RAVITA™)

The major element and heavy metal concentrations of post-precipitated sewage sludge (PPS) and its ash residue (PPA) were determined using microwave digestion followed by inductively coupled plasma optical emission spectrometry (ICP-OES) and mass spectrometry (ICP-MS). To the best of our knowledge, this has not been previously done. In both PPS and PPA the heavy metal concentrations were clearly below the average concentrations than those encountered in sewage sludge in Europe. The leaching efficiency of the metal (Al/ Fe) used as a precipitation agent from post-precipitated sludge and its ash residue with phosphoric acid was also investigated with previously optimized leaching conditions. Tests resulted in leaching efficiencies for Al of 85 ± 1% and 99.5 ± 0.7% for PPS and PPA, respectively which were produced with aluminum as precipitation agent for phosphorus. Sludge, which was produced using iron as a precipitation agent, had a leaching efficiency of Fe 36.6 ± 0.9% and 68.0 ± 1.1%, for PPS and PPA, respectively. The leaching efficiency for P was 94 ± 3% and 96 ± 5% for Al-PPA and Fe-PPA, respectively.

Chromium (VI) in phosphorus fertilizers determined with the diffusive gradients in thin-films (DGT) technique

Phosphorus (P) fertilizers from secondary resources became increasingly important in the last years. However, these novel P-fertilizers can also contain toxic pollutants such as chromium in its hexavalent state (Cr(VI)). This hazardous form of chromium is therefore regulated with low limit values for agricultural products even though the correct determination of Cr(VI) in these fertilizers may be hampered by redox processes, leading to false results. Thus, we applied the novel diffusive gradients in thin-films (DGT) technique for Cr(VI) in fertilizers and compared the results with the standard wet chemical extraction method (German norm DIN EN 15192) and Cr K-edge X-ray absorption near-edge structure (XANES) spectroscopy. We determined an overall good correlation between the wet chemical extraction and the DGT method. DGT was very sensitive and for most tested materials selective for the analysis of Cr(VI) in P-fertilizers. However, hardly soluble Cr(VI) compounds cannot be detected with the DGT method since only mobile Cr(VI) is analyzed. Furthermore, Cr K-edge XANES spectroscopy showed that the DGT binding layer also adsorbs small amounts of mobile Cr(III) so that Cr(VI) values are overestimated. Since certain types of the P-fertilizers contain mobile Cr(III) or partly immobile Cr(VI), it is necessary to optimize the DGT binding layers to avoid aforementioned over- or underestimation. Furthermore, our investigations showed that the Cr K-edge XANES spectroscopy technique is unsuitable to determine small amounts of Cr(VI) in fertilizers (below approx. 1% of Cr(VI) in relation to total Cr).

Transformation of phosphorus and stabilization of heavy metals during sewage sludge incineration: the effect of suitable additives and temperatures

Phosphorus (P), an irreplaceable nutrient for all living organisms, is facing scarcity via phosphate resources. In this research, the effect of suitable additives and temperature on P and heavy metals speciation during sewage sludge (SS) thermochemical treatment was investigated. The results demonstrated that additives (CaO and MgO) could promote the conversion of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP). X-ray diffraction measurements indicated that the phosphorus mineral phase in sewage sludge ash (SSA) mainly was AP, with addition of MgO and CaO. Moreover, orthogonal testing revealed that the optimal molar ratio of Mg:Ca:P for P recovery as AP was 1:3.5:1 at 750 °C. Risk index results implied that the heavy metals in the phosphorus-enriched SSA have low potential ecological risk. Thermodynamic equilibrium calculations revealed that P reacted with the other metal ions was in the following order: Ca²⁺ > Mg²⁺ > Al³⁺ > Fe³⁺ > Zn²⁺ > K⁺. Graphical Abstract.

Evaluation of sewage sludge incineration ash as a potential land reclamation material

This study evaluated the potential of utilising sewage sludge incineration ash as a land reclamation material. Toxicity assessment of the leachate of the ash was carried out for both terrestrial and marine organisms. Both the fruit fly Drosophila melanogaster and barnacle Amphibalanus amphitrite showed that both bottom and fly ash leached at liquid-to-solid (L/S) ratio 5 did not substantially affect viabilities. The leachate carried out at L/S 10 was compared to the European Waste Acceptance Criteria and the sewage sludge ashes could be classified as non-hazardous waste. The geotechnical properties of the sewage sludge ash were studied and compared to sand, a conventional land reclamation material, for further evaluation of its potential as a land reclamation material. It was found from direct shear test that both bottom and fly ashes displayed similar and comparable shear strength to that of typical compacted sandy soil based on the range of internal friction angle obtained. However, the consolidation profile of bottom ash was significantly different from sand, while that of fly ash was more similar to sand. Our study showed that the sewage sludge ash has the potential to be used as a land reclamation material.

Survival and phosphate solubilisation activity of desiccated formulations of Penicillium bilaiae and Aspergillus niger influenced by water activity

The impact of formulation and desiccation on the shelf life of phosphate (P)-solubilising microorganisms is often under-studied, particularly relating to their ability to recover P-solubilisation activity. Here, Penicilllium bilaiae and Aspergillus niger were formulated on vermiculite (V) alone, or with the addition of protectants (skimmed milk (V + SM) and trehalose (V + T)), and on sewage sludge ash with (A + N) and without nutrients (A), and dried in a convective air dryer. After drying, the spore viability of P. bilaiae was greater than that of A. niger. V formulations achieved the highest survival rates without being improved by the addition of protectants. P. bilaiae formulated on V was selected for desiccation in a fluidised bed dryer, in which several temperatures and final water activities (aw) were tested. The highest spore viability was achieved when the formulation was dried at 25 °C to a final aw >0.3. During three months' storage, convective air dried formulations were stable for both strains, except in the presence of skimmed milk for P. bilaiae which saw a decrease in spore viability. In the fluidised bed-dried formulations, when aw >0.3, the loss in viability was higher, especially when stored at 20 °C, than at aw <0.1. P-solubilisation activity performed on ash was preserved in most of the formulations after desiccation and storage. Overall, a low drying temperature and high final aw positively affected P. bilaiae viability, however a trade-off between higher viability after desiccation and shelf life should be considered. Further research is needed to optimise viability over time and on more sustainable carriers.

Hydraulic and mechanical behavior of landfill clay liner containing SSA in contact with leachate

Sewage sludge ash (SSA) produced by municipal sludge can be used as a modified additive for clay liner, and improves the working performance of landfill clay liner in contact with leachate. Under the action of landfill leachate, the permeability, shear strength, phase composition, and pore structure of the modified clay are investigated through the flexible wall permeability test, triaxial shear test, thermal gravimetric and differential thermal analysis, and low-temperature nitrogen adsorption test, respectively. The hydraulic conductivity of the modified clay containing 0-5% SSA is in the range of 3.94 × 10^-8-1.16 × 10^-7 cm/s, and the pollutant concentration of the sample without SSA was higher than others. The shear strength of the modified clay is more than that of the traditional clay liner, the cohesion rate of modified clay increases from 32.5 to 199.91 kPa, and the internal friction angle decreases from 32.5° to 15.6°. Furthermore, the weight loss rates of the samples are 15.69%, 17.92%, 18.06%, and 20.68%, respectively, when the SSA content increases from 0% to 5%. The total pore volume and average pore diameter of the modified clay decrease with the increase in the SSA content, respectively. However, the specific area of the modified clay increases with the increase in the SSA content.

Performance of secondary P-fertilizers in pot experiments analyzed by phosphorus X-ray absorption near-edge structure (XANES) spectroscopy

A pot experiment was carried out with maize to determine the phosphorus (P) plant-availability of different secondary P-fertilizers derived from wastewater. We analyzed the respective soils by P K-edge X-ray absorption near-edge structure (XANES) spectroscopy to determine the P chemical forms that were present and determine the transformation processes. Macro- and micro-XANES spectroscopy were used to determine the chemical state of the overall soil P and identify P compounds in P-rich spots. Mainly organic P and/or P adsorbed on organic matter or other substrates were detected in unfertilized and fertilized soils. In addition, there were indications for the formation of ammonium phosphates in some fertilized soils. However, this effect was not seen in the maize yield of all P-fertilizers. The observed reactions between phosphate from secondary P-fertilizers and co-fertilized nitrogen compounds should be further investigated. Formation of highly plant-available compounds such as ammonium phosphates could make secondary P-fertilizers more competitive to commercial phosphate rock-based fertilizers with positive effects on resources conservation.

Comparing the use of sewage sludge ash and glass powder in cement mortars

This study explored the suitability of using sewage sludge ash (SSA) and mixed-colored glass powder (MGP) as construction materials in cement mortars. Positive findings from this study may help promote the recycling of waste SSA and MGP in construction works. The results indicated that the SSA decreased while MGP improved the mortar workability. The SSA exhibited very low pozzolanic activity, but the cement mortar prepared with 20% SSA yielded strength values slightly superior to those of the glass mortars due to its water absorption ability. MGP can serve as a pozzolan and when 20% of cement was replaced by MGP, apparent compressive strength gains were found at later curing ages. The SSA could be used to mitigate ASR expansion while the MGP was superior in resisting drying shrinkage.

Fertilisers production from ashes after sewage sludge combustion - A strategy towards sustainable development

Sustainable development and circular economy rules force the global fertilizer industry to develop new phosphorous recovery methods from alternative sources. In this paper a phosphorus recovery technology from Polish industrial Sewage Sludge Ashes was investigated (PolFerAsh - Polish Fertilizers form Ash). A wet method with the use of mineral acid and neutralization was proposed. Detailed characteristic of SSA from largest mono-combustion plans were given and compared to raw materials used on the market. The technological factors associated with such materials were discussed. The composition of the extracts was compared to typical industrial phosphoric acid and standard values characterizing suspension fertilizers. The most favorable conditions for selective precipitation of phosphorus compounds were revealed. The fertilizers obtained also meet EU regulations in the case of the newly discussed Cd content. The process was scaled up and a flow mass diagram was defined.

Sequential electrodialytic recovery of phosphorus from low-temperature gasification ashes of chemically precipitated sewage sludge

Phosphorus recycling from secondary materials like sewage sludge ashes offers an alternative to mining of phosphates from primary resources and a mean to counteract the current phosphorous rock depletion concern. A separation of P from the bulk ash is normally required, due to its low plant availability and the presence of heavy metals. Previously, more than 80% of P was recovered from incineration sewage sludge ashes using a two-compartment electrodialytic cell. In contrast, the recovery was below 30% for ashes from low-temperature gasification using the same setup. The low recovery was due to a high presence of Al- and Fe(III)-P bindings. In the present study, an electrodialytic process combining sequentially a pair of two-compartment cells allowed a recovery of up to 70% of phosphorus from these ashes. The use of a second cell, where the ash was suspended in an alkaline solution, allowed the P solubilisation from aluminium and ferric phosphates. In addition, P was separated from most metals as they became insoluble under the prevailing chemical environment. The obtained ratio of Al, Fe, Mg and most heavy metals to P was comparable to wet process phosphoric acid. Therefore, this sequential process was found to be suitable to recycle P and potentially use it in the production of common fertilizers like diammonium phosphate.

Chemical state of mercury and selenium in sewage sludge ash based P-fertilizers

Phosphorus-fertilizers from secondary resources such as sewage sludge ash (SSA) will become more important in the future as they could substitute conventional fertilizers based on the nonrenewable resource phosphate rock. Thermochemical approaches were developed which remove heavy metals from SSA prior to its fertilizer application on farmlands. We analyzed the chemical state of mercury and selenium in SSA before and after thermochemical treatment under different conditions for P-fertilizer production by X-ray absorption near edge structure (XANES) spectroscopy. In some incineration plants the mercury loaded carbon adsorber from off-gas cleaning was collected together with the SSA for waste disposal. SSAs from those plants contained mercury mainly bound to carbon/organic material. The other SSAs contained inorganic mercury compounds which are most probably stabilized in the SSA matrix and were thus not evaporated during incineration. During thermochemical treatment, carbon-bound mercury was removed quantitatively. In contrast, a certain immobile fraction of inorganic mercury compounds remained in thermochemically treated SSA, which were not clearly identified. HgSe might be one of the inorganic compounds, which is supported by results of Se K-edge XANES spectroscopy. Furthermore, the chemical state of selenium in the SSAs was very sensitive to the conditions of the thermochemical treatment.

Phosphorous recovery from sewage sludge ash suspended in water in a two-compartment electrodialytic cell

Phosphorus (P) is indispensable for all forms of life on Earth and as P is a finite resource, it is highly important to increase recovery of P from secondary resources. This investigation is focused on P recovery from sewage sludge ash (SSA) by a two-compartment electrodialytic separation (EDS) technique. Two SSAs are included in the investigation and they contained slightly less P than phosphate rock used in commercial fertilizer production and more heavy metals. The two-compartment electrodialytic technique enabled simultaneous recovery of P and separation of heavy metals. During EDS the SSA was suspended in water in the anolyte, which was separated from the catholyte by a cation exchange membrane. Electrolysis at the anode acidified the SSA suspension, and hereby P, Cu, Pb, Cd and Zn were extracted. The heavy metal ions electromigrated into the catholyte and were thus separated from the filtrate with P. More than 95% P was extracted from both SSAs. The charge transfer to obtain this varied when treating the two SSAs, and for one ash it was about 30% higher than for the other as a result of a higher buffering capacity against acidification. The repeatability of EDS results between experiments with the same SSA and the same experimental conditions was good, which shows that the process is easy to control at the studied laboratory conditions. About 80% P and 10% of the heavy metals remained in the filtrate from the anolyte after treatment of both SSAs. The heavy metal content relative to P in the filtrate by far meet the limiting values for use of industrial wastes as fertilizers, thus the filtrate is ready for direct processing into P-fertilizer.

Potential of phosphorus recovery from sewage sludge and manure ash by thermochemical treatment

All life forms require phosphorus (P), which has no substitute in food production. The risk of phosphorus loss from soil and limited P rock reserves has led to the development of recycling P from industrial residues. This study investigates the potential of phosphorus recovery from sewage sludge and manure ash by thermochemical treatment (ASH DEC) in Finland. An ASH DEC plant could receive 46-76 kt/a of sewage sludge ash to produce 51-85 kt/a of a P-rich product with a P2O5 content of 13-18%, while 320-750 kt/a of manure ash could be supplied to produce 350-830 kt/a of a P-rich product with a P content of 4-5%. The P2O5 potential in the total P-rich product from the ASH DEC process using sewage sludge and manure ash is estimated to be 25-47 kt/a, which is significantly more than the P fertilizer demand in Finland's agricultural industries. The energy efficiency of integrated incineration and the ASH DEC process is more dependent on the total solid content and the subsequent need for mechanical dewatering and thermal drying than on the energy required by the ASH DEC process. According to the results of this study, the treated sewage sludge and manure ash using the ASH DEC process represent significant potential phosphorus sources for P fertilizer production.

Sewage sludge ash--A promising secondary phosphorus source for fertilizer production

Sewage sludge incineration is extensively practiced in some European countries such as the Netherlands, Switzerland, Austria and Germany. A survey of German sewage sludge ash showed that the recovery potential is high, approx. 19,000 t of phosphorus per year. However, the survey also discovered that the bioavailability of phosphorus in the sewage sludge ash is poor and that more than half of the ashes cannot be used as fertilizers due to high heavy metal content. A new thermochemical process for sewage sludge ash treatment was developed that transforms the ash into marketable fertilizer products. Sewage sludge ash was thermochemically treated with sodium and potassium additives under reducing conditions, whereby the phosphate-bearing mineral phases were transformed into plant available phosphates. High P-bioavailability was achieved with a molar Na/P ratio >1.75 in the starting materials. Sodium sulfate, carbonate and hydroxide performed comparably as additives for this calcination process. Potassium carbonate and -hydroxide have to be added in a molar K/P ratio >2.5 to achieve comparable P-solubility. The findings of the laboratory scale investigations were confirmed by an industrial demonstration trial for an ash treatment with sodium sulfate. Simultaneously, the volatile transition metal arsenic (61% removal) as well as volatile heavy metals such as cadmium (80%), mercury (68%), lead (39%) and zinc (9%) were removed via the off-gas treatment system. The product of the demonstration trial is characterized by high bioavailability and a toxic trace element mass fraction below the limit values of the German fertilizer ordinance, thus fulfilling the quality parameters for a P-fertilizer.

Transformation of apatite phosphorus and non-apatite inorganic phosphorus during incineration of sewage sludge

The recovery of phosphorus from incinerated sewage sludge ash (SSA) is assumed to be economical. Transformation from non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP), which has a higher bioavailability and more extensive industrial applications, was studied at 750-950°C by sewage sludge incineration and model compound incineration with a calcium oxide (CaO) additive. Thermogravimetric differential scanning calorimetry analysis and X-ray diffraction measurements were used to analyze the reactions between NAIP with CaO and crystallized phases in SSA. High temperatures stimulated the volatilization of NAIP instead of AP. Sewage sludge incineration with CaO transformed NAIP into AP, and the percentage of AP from the total phosphorus reached 99% at 950°C. Aluminum phosphate reacted with CaO, forming Ca2P2O7 and Ca3(PO4)2 at 750-950°C. Reactions between iron phosphate and CaO occurred at lower temperatures, forming Ca(PO3)2 before reaching 850°C.

Recovery potential of German sewage sludge ash

Incineration of sewage sludge is expected to increase in the future due to growing concerns about the direct use of sludge in agriculture. Sewage sludge is the pollutant sink of wastewater treatment and thus loaded with contaminants that might pose environmental hazards. Incineration degrades organic pollutants efficiently, but since the ash is currently mostly disposed of, all valuable component like phosphorus (P) and technologically relevant metals present in the sewage sludge ash (SSA) are removed from the economic cycle entirely. We conducted a complete survey of SSA from German mono-incineration facilities and determined the theoretical recovery potential of 57 elements. German SSA contains up to 19,000 t/a P which equals approximately 13% of phosphorus applied in the German agriculture in form of phosphate rock based mineral fertilizers. Thus, SSA is an important secondary resource of P. However, its P-solubility in ammonium citrate solution, an indicator for the bioavailability, is only about 26%. Treatment of SSA is recommended to enhance P bioavailability and remove heavy metals before it is applied as fertilizer. The recovery potential for technologically relevant metals is generally low, but some of these elements might be recovered efficiently in the course of P recovery exploiting synergies.

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.

Heavy metal removal and speciation transformation through the calcination treatment of phosphorus-enriched sewage sludge ash

On the basis of the heavy metal (Cd, As, Pb, Zn, Cu, Cr, and Ni) control problem during the thermochemical recovery of phosphorus (P) from sewage sludge (SS), P-enriched sewage sludge ash (PSSA) was calcined at 1100°C. The effect of organic chlorinating agent (PVC) and inorganic chlorinating agent (MgCl2) on the fixed rate of P removal and the speciation transformation of heavy metal was studied. The removal of heavy metals Cd, Pb, As, Zn, and Cr exhibited an increasing tendency with the addition of chlorinating agent (PVC). However, an obvious peak under 100gCl/kg of PSSA appeared for Cu, owing to the presence of carbon and hydrogen in PVC. MgCl2 was found to be more effective than PVC in the removal of most heavy metals, such that up to 98.9% of Cu and 97.3% of Zn was effectively removed. Analyses of heavy metal forms showed that Pb and Zn occurred in the residue fraction after calcination. Meanwhile, the residue fraction of Cr, Ni, Cd, and Cu exhibited a decreasing tendency with the increase in the added chlorinating agent (MgCl2). Losses of P from PSSA were around 16.6% without the addition of chlorinating agent, which were greatly reduced to around 7.7% (PVC) and to only 1.7% (MgCl2).

Chemical state of chromium in sewage sludge ash based phosphorus-fertilisers

Sewage sludge ash (SSA) based P-fertilisers were produced by thermochemical treatment of SSA with Cl-donors at approximately 1000°C. During this thermochemical process heavy metals are separated as heavy metal chlorides via the gas phase. Chromium cannot be separated under normal conditions. The risk of the development of toxic Cr(VI) during the thermochemical process was investigated. X-ray Absorption Spectroscopy measurements showed that SSA and thermochemically treated SSA with CaCl2, MgCl2 and NaCl contain Cr(III) compounds only. In contrast, treating SSA with elevated quantities of Na2CO3, to enhance the plant-availability of the phosphate phases of the fertiliser, developed approximately 10-15% Cr(VI). Furthermore, Raman microspectroscopy showed that using Mg-carbonate reduces the risk of a Cr(VI) development during thermochemical treatment. Additionally, leaching tests showed that only a Cr-water solubility>10% is an indicator for Cr(VI) in SSA based P-fertilisers.

Phosphorus recovery from sewage sludge ash through an electrodialytic process

The electrodialytic separation process (ED) was applied to sewage sludge ash (SSA) aiming at phosphorus (P) recovery. As the SSA may have high heavy metals contents, their removal was also assessed. Two SSA were sampled, one immediately after incineration (SA) and the other from an open deposit (SB). Both samples were ED treated as stirred suspensions in sulphuric acid for 3, 7 and 14 days. After 14 days, phosphorus was mainly mobilized towards the anode end (approx. 60% in the SA and 70% in the SB), whereas heavy metals mainly electromigrated towards the cathode end. The anolyte presented a composition of 98% of P, mainly as orthophosphate, and 2% of heavy metals. The highest heavy metal removal was achieved for Cu (ca. 80%) and the lowest for Pb and Fe (between 4% and 6%). The ED showed to be a viable method for phosphorus recovery from SSA, as it promotes the separation of P from the heavy metals.