Hybrid anion exchange adsorbents (HAIX) seem promising to prevent eutrophication and recover phosphate (P). HAIX consist of an anion exchange resin (AIX) backbone, promoting anion physisorption (outer-sphere complex), impregnated with iron (hydr)oxide nanoparticles (NPs), for selective P chemisorption (inner-sphere complex). In this work, for the first time, as far as we know, Zn-doped iron (hydr)oxide NPs were embedded in AIX, and the performances compared with conventional HAIX, both commercial and synthesized. Zn-doped HAIX displayed improved P adsorption performances. Mössbauer spectroscopy (MS) revealed the goethite nature of the NPs, against the “amorphous hydrous ferric oxide” claimed in literature. The P adsorption comparisons, made in synthetic solution and real wastewater, underlined the crucial role of the NPs for selective P adsorption, while improving the understanding on the competition between physisorption and chemisorption. In pure P synthetic solutions, especially at high P concentrations, physisorption can “hide” chemisorption. This depends also on the anion form of the AIX, due to their higher affinity for multivalent anions, which affects HAIX adsorption selectivity and P desorption. In fact, a mild alkaline regeneration over three adsorption–desorption cycles revealed a complex interaction between the regenerant OH− and the adsorbed P. OH− molecules are consumed to transform phosphate speciation, causing (stronger) P re-adsorption and preventing desorption. Finally, Mössbauer spectroscopy revealed NPs agglomeration/growth after the three cycles plus final regeneration at pH 14. This study provides further understanding on the P adsorption–desorption mechanism in HAIX, drawing attention on the choice of experimental conditions for reliable performance assessment, and questioning HAIX consistent P removal and efficient P recovery in the long-term.@en

The presence of soluble iron and phosphorus in wastewater sludge can lead to vivianite scaling. This problem is not often reported in literature, most likely due to the difficult identification and quantification of this mineral. It is usually present as a hard and blue deposit that can also be brown or black depending on its composition and location. From samples and information gathered in 14 wastewater treatment plants worldwide, it became clear that vivianite scaling is common and can cause operational issues. Vivianite scaling mainly occurred in 3 zones, for which formation hypotheses were discussed. Firstly, iron reduction seems to be the trigger for scaling in anaerobic zones like sludge pipes, mainly after sludge thickening. Secondly, pH increase was evaluated to be the major cause for the formation of a mixed scaling (a majority of oxidized vivianite with some iron hydroxides) around dewatering centrifuges of undigested sludge. Thirdly, the temperature dependence of vivianite solubility appears to be the driver for vivianite deposition in heat exchanger around mesophilic digesters (37 °C), while higher temperatures potentially aggravate the phenomenon, for instance in thermophilic digesters. Mitigation solutions like the use of buffer tanks or steam injections are discussed. Finally, best practices for safe mixing of sludges with each other are proposed, since poor admixing can contribute to scaling aggravation. The relevance of this study lays in the occurrence of ironphosphate scaling, while the use of iron coagulants will probably increase in the future to meet more stringent phosphorus discharge limits.@en

It was recently discovered that vivianite (Fe3(PO4)2.8H2O) could be magnetically extracted from digested activated sludge which opened a new route for phosphorus recovery (Wijdeveld et al. 2022). While its formation in digested sludge is regularly reported, it is not yet studied for fresh, undigested activated sludge. In particular, the extent to which vivianite could form during sludge storage is missing. The current research showed that iron reduction was completed after 2–4 days of anaerobic storage, and the vivianite appeared to form quickly from the pool of reduced iron made available. After sludge thickening at the wastewater treatment plant (30 h retention time), around 11% of the iron was vivianite. With subsequent 1–3 days of anaerobic storage, this fraction increased to 50–55%. After this storage, almost all the vivianite that could potentially form did form. This research concluded that efficient vivianite formation can be achieved without a sludge digester, showing phosphorus recovery potential from undigested sludge via vivianite recovery. Besides, the recovery of vivianite from undigested sludge presents advantages like the reduction of the sludge to dispose of and mitigation of the vivianite scaling formation.@en

Eutrophication and the resulting formation of harmful algal blooms (HAB) causes huge economic and environmental damages. Phosphorus (P) from sewage effluent and agricultural run-off has been identified as a major cause for eutrophication. Phosphorous concentrations greater than 100 μg P/L are usually considered high enough to cause eutrophication. The strictest regulations however aim to restrict the concentration below 10 μg P/L. Orthophosphate (or phosphate) is the bioavailable form of phosphorus. Adsorption is often suggested as technology to reduce phosphate to concentrations less than 100 and even 10 μg P/L with the advantages of a low-footprint, minimal waste generation and the option to recover the phosphate. Although many studies report on phosphate adsorption, there is insufficient information regarding parameters that are necessary to evaluate its application on a large scale. This review discusses the main parameters that affect the economics of phosphate adsorption and highlights the research gaps. A scenario and sensitivity analysis shows the importance of adsorbent regeneration and reuse. The cost of phosphate adsorption using reusable porous metal oxide is in the range of $ 100 to 200/Kg P for reducing the phosphate to ultra-low concentrations. Future research needs to focus on adsorption capacity at low phosphate concentrations, regeneration and reuse of both the adsorbent and the regeneration liquid.@en

Adsorption is often suggested for to reach very low phosphate levels in municipal wastewater effluent and even to recover phosphate. Adsorbent performance is usually associated with surface area but the exact role of the pore size distribution (PSD) is unclear. Here, we show the effect of the PSD on phosphate adsorption. Granular activated carbons (GACs) with varying PSDs were treated with potassium permanganate followed by reaction with ferric chloride to form iron oxide coated GACs (Fe-GACs). Energy dispersive X-ray and kinetics experiments confirmed that manganese anchored on the GAC is important for subsequent iron attachment. Mössbauer spectroscopy showed presence of ferrihydrite in Fe-GAC. Transmission electron microscopy showed that the iron oxide particles are not present in the micropores of the GACs. Phosphate adsorption isotherms were performed with the Fe-GACs and adsorption at lower phosphate concentrations correlated with the porous area of >3 nm of the adsorbents, a high fraction of which is contributed by mesopores. These results show that high surface areas of GACs resulting from micropores do not contribute to adsorption at low phosphate concentrations. This can be explained by the micropores being difficult to coat with iron oxide nanoparticles, but in addition the diffusion of phosphate into these pores could also be hindered. It is therefore recommended to use backbones having high mesoporous areas. This information is useful for developing adsorbents particularly for applications treating low phosphate concentrations, for e.g. in municipal wastewater effluent polishing.@en

Sulfide is frequently suggested as a tool to release and recover phosphate from iron phosphate rich waste streams, such as sewage sludge, although systematic studies on mechanisms and efficiencies are missing. Batch experiments were conducted with different synthetic iron phosphates (purchased Fe(III)P, Fe(III)P synthesized in the lab and vivianite, Fe(II)3(PO4)2*8H2O), various sewage sludges (with different molar Fe:P ratios) and sewage sludge ash. When sulfide was added to synthetic iron phosphates (molar Fe:S = 1), phosphate release was completed within 1 h with a maximum release of 92% (vivianite), 60% (purchased Fe(III)P) and 76% (synthesized Fe(III)P). In the latter experiment, rebinding of phosphate to Fe(II) decreased net phosphate release to 56%. Prior to the re-precipitation, phosphate release was very efficient (P released/S input) because it was driven by Fe(III) reduction and not by, more sulfide demanding, FeSx formation. This was confirmed in low dose sulfide experiments without significant FeSx formation. Phosphate release from vivianite was very efficient because sulfide reacts directly (1:1) with Fe(II) to form FeSx, without Fe(III) reduction. At the same time vivianite-Fe(II) is as efficient as Fe(III) in binding phosphate. From digested sewage sludge, sulfide dissolved maximally 30% of all phosphate, from the sludge with the highest iron content which was not as high as suggested in earlier studies. Sludge dewaterability (capillary suction test, 0.13 ± 0.015 g2(s2m4)−1) dropped significantly after sulfide addition (0.06 ± 0.004 g2(s2m4)−1). Insignificant net phosphate release (1.5%) was observed from sewage sludge ash. Overall, sulfide can be a useful tool to release and recover phosphate bound to iron from sewage sludge. Drawbacks -deterioration of the dewaterability and a net phosphate release that is lower than expected-need to be investigated.@en

Iron is an important element for modern sewage treatment, inter alia to remove phosphorus from sewage. However, phosphorus recovery from iron phosphorus containing sewage sludge, without incineration, is not yet economical. We believe, increasing the knowledge about iron-phosphorus speciation in sewage sludge can help to identify new routes for phosphorus recovery. Surplus and digested sludge of two sewage treatment plants was investigated. The plants relied either solely on iron based phosphorus removal or on biological phosphorus removal supported by iron dosing. Mössbauer spectroscopy showed that vivianite and pyrite were the dominating iron compounds in the surplus and anaerobically digested sludge solids in both plants. Mössbauer spectroscopy and XRD suggested that vivianite bound phosphorus made up between 10 and 30% (in the plant relying mainly on biological removal) and between 40 and 50% of total phosphorus (in the plant that relies on iron based phosphorus removal). Furthermore, Mössbauer spectroscopy indicated that none of the samples contained a significant amount of Fe(III), even though aerated treatment stages existed and although besides Fe(II) also Fe(III) was dosed. We hypothesize that chemical/microbial Fe(III) reduction in the treatment lines is relatively quick and triggers vivianite formation. Once formed, vivianite may endure oxygenated treatment zones due to slow oxidation kinetics and due to oxygen diffusion limitations into sludge flocs. These results indicate that vivianite is the major iron phosphorus compound in sewage treatment plants with moderate iron dosing. We hypothesize that vivianite is dominating in most plants where iron is dosed for phosphorus removal which could offer new routes for phosphorus recovery.@en

In a wastewater treatment plant (WWTP), several sludge streams exist and the composition of their liquid phase varies with time and place. For evaluating the potential for formation of precipitates and equilibria for weak acids/bases, the ionic strength and chemical composition need to be known. This information is often not available in literature, and even neglected in chemical model-based research. Based on a literature review, we proposed three ranges of concentration (low, typical and high) for the major constituents of the liquid phase of the different streams in a WWTP. The study also discusses the reasons for the concentration evolution, and the exceptional cases, to allow readers to consider the right range depending on their situation. The ionic strength of the different streams and the contribution of its constituents were calculated based on the ionic composition. The major contributors to the ionic strength for the wastewater-based streams (influent, effluent and mixed sludge) were Na+, Cl-, Mg2+ and Ca2+, representing 50-70% of the ionic strength. For digestate, NH4+ and HCO3- accounted for 65-75% of the ionic strength. Even though the ionic strength is recognized to impact several important wastewater treatment processes, its utilization in literature is not always adequate, which is discussed in this study.@en

Phosphorus (P) is an essential resource for food production and chemical industry. Phosphorus use has to become more sustainable and should include phosphorus recycling from secondary sources. About 20% of P ends up in sewage sludge, making this a substantial secondary P source. There is currently a technological gap to recover P from sludge locally at wastewater treatment plants (WWTP) that remove P by dosing iron. Vivianite (Fe3(PO4)2•8(H2O)) is the main iron phosphate mineral that forms during anaerobic digestion of sewage sludge, provided that enough iron is present. Vivianite is paramagnetic and can be recovered using a magnetic separator. In this study, we have scaled up vivianite separation from lab-scale to bench- and pilot-scale. Bench-scale tests showed good separation of vivianite from digested sewage sludge and that a pulsation force is crucial for obtaining a concentrate with a high P grade. A pilot-scale magnetic separator (capacity 1.0 m3/h) was used to recover vivianite from digested sewage sludge at a WWTP. Recirculating and reprocessing sludge allows over 80% vivianite recovery within three passes. A concentrated P-product was produced with a vivianite content of up to 800 mg/g and a P content of 98 mg/g. P recovery is limited by the amount of P bound in vivianite and can be increased by increased iron dosing. With sufficient iron dosing, the vivianite content can be increased, and subsequently more P can be recovered. This would allow compliance with existing German legislation, which requires a P recovery larger than 50%.@en

The study evaluated the combined phosphorus, nitrogen, methane, and extracellular polymeric substances (EPS) recovery from aerobic granular sludge (AGS) wastewater treatment plants. About 30% of sludge organics are recovered as EPS and 25–30% as methane (≈260 ml methane/g VS) by integrating alkaline anaerobic digestion (AD). It was shown that 20% of excess sludge total phosphorus (TP) ends in the EPS. Further, 20–30% ends in an acidic liquid waste stream (≈600 mg PO4-P/L), and 15% in the AD centrate (≈800 mg PO4-P/L) as ortho-phosphates in both streams and is recoverable via chemical precipitation. 30% of sludge total nitrogen (TN) is recovered as organic nitrogen in the EPS. Ammonium recovery from the alkaline high-temperature liquid stream is attractive, but it is not feasible for existing large-scale technologies because of low ammonium concentration. However, ammonium concentration in the AD centrate was calculated to be 2600 mg NH4-N/L – and ≈20% of TN, making it feasible for recovery. The methodology used in this study consisted of three main steps. The first step was to develop a laboratory protocol mimicking demonstration-scale EPS extraction conditions. The second step was to establish mass balances over the EPS extraction process on laboratory and demonstration scales within a full-scale AGS WWTP. Finally, the feasibility of resource recovery was evaluated based on concentrations, loads, and integration of existing technologies for resource recovery.@en

Phosphorus (P) removal from freshwater bodies to ultra-low concentrations is fundamental to prevent eutrophication, while its recovery is necessary to close the P usage cycle. Iron oxide-based adsorbents seem promising candidates, being abundant, cheap, and easy to synthesize compounds, with good affinity for P. Affinity is the key parameter when targeting ultra-low concentrations. Also, adsorbent regeneration and re-use is fundamental for the economic viability, hence the adsorbent stability is important. Goethite, (α-FeOOH), is one of the most stable iron (Fe3+) (hydr)oxide species, with higher affinity, but lower adsorption capacity (per kg) compared to other species. Doping could change goethite surface properties, to boost the adsorption capacity, while preserving the high stability and affinity for P. In this work, pure goethite was compared to goethite doped (5%at.) with different elements of different preferential oxidation states: Zn2+, Mn3+, and Zr4+. Doping was successfully achieved for all elements, albeit Zr showed a lower Fe substitution than targeted. Zn doping increased the goethite point of zero charge and adsorption capacity (per mass and per surface area), preserving the high affinity, while Mn- and Zr- doping displayed a decrease in all the parameters. These could be explained with surface protonation as a charge compensation mechanism in Zn2+-for-Fe3+ substitution. The regeneration test showed improved P recovery for Zr- and Zn-doped goethite. All samples remained stable throughout the whole process. This work provides promising insights on doping as a strategy to manipulate iron oxides surface properties and for developing a highly performing and long-lasting goethite-based adsorbent.@en

The eutrophic Bouvigne pond (Breda, The Netherlands) regularly suffers from cyanobacterial blooms. To improve the water quality, the external nutrient loading and the nutrient release from the pond sediment have to be reduced. An enclosure experiment was performed in the pond between March 9 and July 29, 2020 to compare the efficiency of dredging, addition of the lanthanum-modified bentonite clay Phoslock® (LMB), the aluminum-modified zeolite Aqual-P™ (AMZ) and FeCl2 to mitigate nutrient release from the sediment. The treatments improved water quality. Mean total phosphorus (TP) concentrations in water were 0.091, 0.058, 0.032, 0.031, and 0.030 mg P L-1 in controls, dredged, FeCl2, LMB and AMZ treated enclosures, respectively. Mean filterable P (FP) concentrations were 0.056, 0.010, 0.009, 0.005, and 0.005 mg P L-1 in controls, dredged, FeCl2, LMB and AMZ treatments, respectively. Total nitrogen (TN) and dissolved inorganic nitrogen (DIN) were similar among treatments; lanthanum was elevated in LMB treatments, Fe and Cl in FeCl2 treatments, and Al and Cl in AMZ treatments. After 112 days, sediment was collected from each enclosure, and subsequent sequential P extraction revealed that the mobile P pool in the sediments had reduced by 71.4%, 60.2%, 38%, and 5.2% in dredged, AMZ, LMB, and FeCl2 treatments compared to the controls. A sediment core incubation laboratory experiment done simultaneously with the enclosure experiment revealed that FP fluxes were positive in controls and cores from the dredged area, while negative in LMB, AMZ and FeCl2 treated cores. Dissolved inorganic nitrogen (DIN) release rate in LMB treated cores was 3.6 times higher than in controls. Overall, the applied in-lake treatments improved water quality in the enclosures. Based on this study, from effectiveness, application, stakeholders engagement, costs and environmental safety, LMB treatment would be the preferred option to reduce the internal nutrient loading of the Bouvigne pond, but additional arguments also have to be considered when preparing a restoration.@en

The recovery of phosphorus from secondary sources like sewage sludge is essential in a world suffering from resources depletion. Recent studies have demonstrated that phosphorus can be magnetically recovered as vivianite (Fe(II)3(PO4)2∗8H2O) from the digested sludge (DS) of Waste Water Treatment Plants (WWTP) dosing iron. To study the production of vivianite in digested sludge, the quantity of Fe dosed at the WWTP of Nieuwveer (The Netherlands) was increased (from 0.83 to 1.53 kg Fe/kg P in the influent), and the possible benefits for the functioning of the WWTP were evaluated. Higher Fe dosing is not only relevant for P-recovery, but also for maximal recovery of organics from influent for e.g. biogas production. The share of phosphorus present as vivianite in the DS increased from 20% to 50% after the increase in Fe dosing, making more phosphorus available for future magnetic recovery. This increase was directly proportional to the increase of Fe in DS, suggesting that vivianite could be favored not only thermodynamically, but also kinetically. Interestingly, analyses suggest that several types of vivianite are formed in the WWTP, and could differ in their purity, oxidation state or crystallinity. These differences could have an impact on the subsequent magnetic separation. Following the Fe dosing increase, P in the effluent and H2S in the biogas both decreased: 1.28 to 0.42 ppm for P and 26 to 8 ppm for H2S. No negative impact on the nitrogen removal, biogas production, COD removal or dewaterability was observed. Since quantification of vivianite in DS is complicated, previous studies were reviewed and we proposed a more accurate Mössbauer spectroscopy analysis and fitting for sludge samples. This study is important from a P recovery point of view, but also because iron addition can play a crucial role in future resource recovery wastewater facilities.@en

Phosphate is a vital nutrient for life but its discharge from wastewater effluents can lead to eutrophication. Adsorption can be used as effluent polishing step to reduce phosphate to very low concentrations. Adsorbent reusability is an important parameter to make the adsorption process economically feasible. This implies that the adsorbent can be regenerated and used over several cycles without appreciable performance decline. In the current study, we have studied the phosphate adsorption and reusability of commercial iron oxide based adsorbents for wastewater effluent. Effects of adsorbent properties like particle size, surface area, type of iron oxide, and effects of some competing ions were determined. Moreover the effects of regeneration methods, which include an alkaline desorption step and an acid wash step, were studied. It was found that reducing the adsorbent particle size increased the phosphate adsorption of porous adsorbents significantly. Amongst all the other parameters, calcium had the greatest influence on phosphate adsorption and adsorbent reusability. Phosphate adsorption was enhanced by co-adsorption of calcium, but calcium formed surface precipitates such as calcium carbonate. These surface precipitates affected the adsorbent reusability and needed to be removed by implementing an acid wash step. The insights from this study are useful in designing optimal regeneration procedures and improving the lifetime of phosphate adsorbents used for wastewater effluent polishing.@en

To prevent eutrophication of surface water, phosphate needs to be removed from sewage. Iron (Fe) dosing is commonly used to achieve this goal either as the main strategy or in support of biological removal. Vivianite (Fe(II) 3 (PO 4 ) 2 * 8H 2 O) plays a crucial role in capturing the phosphate, and if enough iron is present in the sludge after anaerobic digestion, 70–90% of total phosphorus (P) can be bound in vivianite. Based on its paramagnetism and inspired by technologies used in the mining industry, a magnetic separation procedure has been developed. Two digested sludges from sewage treatment plants using Chemical Phosphorus Removal were processed with a lab-scale Jones magnetic separator with an emphasis on the characterization of the recovered vivianite and the P-rich caustic solution. The recovered fractions were analyzed with various analytical techniques (e.g., ICP-OES, TG-DSC-MS, XRD and Mössbauer spectroscopy). The magnetic separation showed a concentration factor for phosphorus and iron of 2–3. The separated fractions consist of 52–62% of vivianite, 20% of organic matter, less than 10% of quartz and a small quantity of siderite. More than 80% of the P in the recovered vivianite mixture can be released and thus recovered via an alkaline treatment while the resulting iron oxide has the potential to be reused. Moreover, the trace elements in the P-rich caustic solution meet the future legislation for recovered phosphorus salts and are comparable to the usual content in Phosphate rock. The efficiency of the magnetic separation and the advantages of its implementation in WWTP are also discussed in this paper. @en

Iron oxide-based adsorbents showed potential to reach ultra-low phosphorus (P) concentrations to prevent eutrophication and recover P. High affinity, high capacity at low P concentrations (<1 mg L−1), good stability, and reusability of the adsorbent are key factors for economic viability. In this study, nanoparticles of goethite (α-FeOOH), a highly stable phase, have been synthesized with increasing Zn2+-doping, 0–20 %at. Zn/Fe, to manipulate the surface properties, following the results of a previous work. Mössbauer spectroscopy showed preserved goethite phase and increased point of zero charge (pzc) at low Zn-doping percentages, while at higher percentages (>5%at.) co-existing phases with increased specific surface area formed. Low concentrations (0.1–10 mg L−1) batch adsorption tests showed increased P removal per unit mass with increasing doping. However, the highest pzc, affinity and P removal per unit area were observed for the 5%at. doped sample, suggesting this dopant concentration to provide the most effective surface. A regeneration test, performed at a lower pH than usual, showed preserved, even improved P desorption with increasing doping. Mössbauer spectroscopy showed that the nanoparticle phase and composition, up to 5%at., doping was preserved throughout the process. These results are promising to develop a stable effective Zn-doped goethite-based adsorbent for P recovery at ultra-low concentrations.@en

Phosphate is a vital nutrient but its presence in surface waters even at very low concentrations can lead to eutrophication. Adsorption is often suggested as a step for reducing phosphate down to very low concentrations. Porous metal oxides can be used as granular adsorbents that have a high surface area and hence a high adsorption capacity. But from a practical point of view, these adsorbents also need to have good adsorption kinetics. The surface area of such adsorbents comes from pores of varying pore size and the pore size distribution (PSD) of the adsorbents can affect the phosphate adsorption kinetics. In this study, the PSD of 4 different adsorbents was correlated with their phosphate adsorption kinetics. The adsorbents based on iron and aluminium (hydr)oxide were grinded and the adsorption performance was studied as a function of their particle size. This was done to identify diffusion limitations due to the PSD of the adsorbents. The phosphate adsorption kinetics were similar for small particles of all the adsorbents. For larger particles, the adsorbents having pores larger than 10 nm (FSP and DD6) showed faster adsorption than adsorbents with smaller pores (GEH and CFH). Even though micropores (pores < 2 nm) contributed to a higher portion of the adsorbent surface area, pores bigger than 10 nm were needed to increase the rate of adsorption.@en

Kinetics of iron reduction, formation of vivianite and the microbial community in activated sludge from two sewage treatment plants (STPs) with low (STP Leeuwarden, applying enhanced biological phosphate removal, EBPR) and high (STP Cologne, applying chemical phosphate removal, CPR) iron dosing were studied in anaerobic batch experiments. The iron reduction rate in CPR sludge (2.99 mg-Fe g VS −1 h −1 ) was 3-times higher compared to EBPR sludge (1.02 mg-Fe g VS −1 h −1 ) which is probably caused by its 3-times higher iron content. Accordingly, first order rate constants in both sludges are comparable (0.06 ± 0.001 h −1 in EBPR vs 0.05 ± 0.007 h −1 in CPR sludge), thus potential rates in both sludges are comparable. The measured Fe(III) reduction rates suggest that all iron in STP Leeuwarden and STP Cologne can be turned over within 15 h and 44 h respectively. Mössbauer spectroscopy and X-ray diffraction (XRD) indicated vivianite formation within 24 h in both sludges. After 24 h, 53% and 34% of all iron were bound in vivianite in the EBPR and CPR sludge respectively. Next generation sequencing (NGS) showed that the microbial community in the CPR sludge comprised more genera with iron-oxidizing and iron-reducing bacteria. Iron reduction and vivianite formation commence once activated sludge is exposed to oxygen free conditions. Our study reveals that the biogeochemistry of iron in STPs is very dynamic. By understanding the interactions between iron and phosphate crucial processes in modern sewage treatment, such as chemical phosphate removal or phosphate recovery from sewage sludge, can be optimized. @en