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.

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Nereda® aerobic granular sludge plants will be urban biorefineries in near future. The development of Kaumera Nereda® Gum extractions from granular sludge was the first step. Kaumera is a biopolymer that can substitute oil derived polymers. Here we share our experiences in the EU funded Water Mining project. A transportable Kaumera extraction installation is build, additional technologies will be integrated: (I) nitrogen recovery, (II) phosphorus recovery and (III) alkaline fermentation of organic sludge residuals.
Together with industrial and public partners we combine fundamental research (alkaline fermentation and phosphorus recovery) with classical engineering (Kaumera Nereda® Gum extraction and nitrogen stripping) and process design. This technological process is supplemented with modern approaches from social, political, economic and environmental sciences to stimulate the commercialisation of these technologies and their concomitant recovery products. @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, FeS_x formation. This was confirmed in low dose sulfide experiments without significant FeS_x formation. Phosphate release from vivianite was very efficient because sulfide reacts directly (1:1) with Fe(II) to form FeS_x, 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.

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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.

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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.

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Biogenic iron oxides (BioFeO) formed by Leptothrix sp. and Gallionella sp. were compared with chemically formed iron oxides (ChFeO) for their suitability to remove and recover phosphate from solutions. The ChFeO used for comparison included a commercial iron-based adsorbent (GEH) and chemically oxidized iron precipitates from groundwater. Despite contrary observations in earlier studies, the batch experiments showed that BioFeO do not have superior phosphate adsorption capacities compared to ChFeO. However, it seems multiple mechanisms are involved in phosphate removal by BioFeO which make their overall phosphate removal capacity higher than that of ChFeO. The overall phosphate removal capacity of Leptothrix sp. deposits was 26.3 mg P/g d.s., which could be attributed to multiple mechanisms. This included adsorption on the solid phase (6.4 mg P/g d.s.) as well as removal via precipitation and/or adsorption onto suspended complexes released from the BioFeO of Leptothrix sp. (19.6 mg P/g d.s.). Only a very small part of phosphorus (0.3 mg P/g d.s.) was retained in the Leptothrix sp. sheats during bacterial growth. Deposits of Gallionella sp. had an overall phosphate removal capacity of 39.6 mg P/g d.s. Significant amounts of phosphate were apparently incorporated into the Gallionella sp. stalks during their growth (31.0 mg P/g d.s.) and only one-fifth of the total phosphate removal can be related to adsorption (8.6 mg P/g d.s.). Their overall ability to immobilize large quantities of phosphate from solutions indicates that BioFeO could play an important role in environmental and engineered systems for removal of contaminants.

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Phosphate recovery from sewage sludge is essential in a circular economy. Currently, the main focus in centralized municipal wastewater treatment plants (MWTPs) lies on struvite recovery routes, land application of sludge or on technologies that rely on sludge incineration. These routes have several disadvantages. Our study shows that the mineral vivianite, Fe₂(PO₄)₃ × 8H₂O, is present in digested sludge and can be the major form of phosphate in the sludge. Thus, we suggest vivianite can be the nucleus for alternative phosphate recovery options. Excess and digested sewage sludge was sampled from full-scale MWTPs and analysed using x-ray diffraction (XRD), conventional scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), environmental SEM-EDX (eSEM-EDX) and Mössbauer spectroscopy. Vivianite was observed in all plants where iron was used for phosphate removal. In excess sludge before the anaerobic digestion, ferrous iron dominated the iron pool (≥50%) as shown by Mössbauer spectroscopy. XRD and Mössbauer spectroscopy showed no clear correlation between vivianite bound phosphate versus the iron content in excess sludge. In digested sludge, ferrous iron was the dominant iron form (>85%). Phosphate bound in vivianite increased with the iron content of the digested sludge but levelled off at high iron levels. 70–90% of all phosphate was bound in vivianite in the sludge with the highest iron content (molar Fe:P = 2.5). The quantification of vivianite was difficult and bears some uncertainty probably because of the presence of impure vivianite as indicated by SEM-EDX. eSEM-EDX indicates that the vivianite occurs as relatively small (20–100 μm) but free particles. We envisage very efficient phosphate recovery technologies that separate these particles based on their magnetic properties from the complex sludge matrix.

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The scope of this thesis was to lay the basis for a phosphate recovery technology that can be applied on sewage sludge containing iron phosphate. Such a technology should come with minimal changes to the existing sludge treatment configuration while keeping the use of chemicals or energy as small as possible. The research focused on understanding the exact mechanism for phosphate release from iron in sewage sludge in order to find a method to release phosphate in an elegant way. Phosphate is an essential nutrient for plant growth, but at the same time the resources of phosphate are limited and concentrated in a few countries outside Europe. Recovery of phosphate can secure the access to phosphate for food production and is therefore an important topic. Iron based phosphate removal is still used by a majority of sewage treatment plants (STPs) but no viable technology is available to recover phosphate from sludge without sludge incineration. The addition of iron is a convenient way for removing phosphate from wastewater, but this is often considered to limit phosphate recovery. Struvite precipitation is currently used to recover phosphate, and this approach has attracted much interest. However, it requires the use of enhanced biological phosphate removal (EBPR). Phosphate removal relying solely on EBPR is not yet widely applied and the recovery potential is low (<50%). Other phosphate recovery methods, including sludge application to agricultural land or recovering phosphate from sludge ash, also have limitations. Energy-producing STPs increasingly rely on phosphate removal using iron, but the problem (as in current processes) is the subsequent recovery of phosphate from the iron. In contrast, phosphate is efficiently mobilized from iron by natural processes in sediments and soils. Iron–phosphate chemistry is diverse, and many parameters influence the binding and release of phosphate, including redox conditions, pH, presence of organic substances, and particle morphology. The current poor understanding of iron and phosphate chemistry in sewage systems is preventing processes being developed to recover phosphate from iron–phosphate rich wastes like municipal wastewater sludge. In the first chapter parameters that affect phosphate recovery were reviewed, and methods are suggested for manipulating iron–phosphate chemistry in wastewater treatment processes to allow phosphate to be recovered. Iron is omnipresent in STPs. It can be present unintentionally, for e.g. due to groundwater seepage into sewers, or it is intentionally added for odour and corrosion control, phosphate removal or prevention of hydrogen sulphide emissions into the biogas. The strong affinity of iron to phosphate has advantages for efficient removal of phosphate from sewage but it may also reduce recovery efficiencies in struvite precipitation technologies or for some phosphate recovery methods from ash. On the other hand iron may also have positive effects on phosphate recovery. Acid consumption was reported to be lower when leaching phosphate from sewage sludge ash with higher iron content. Also, phosphate recovery efficiencies may be higher if an iron phosphate compound, like vivianite, Fe(II)3(PO4)2x8H2O, could be harvested from sewage sludge. Developers of phosphate recovery technologies should be aware of the potential and obstacles the iron and phosphate chemistry bears. The mineral vivianite, is already present in digested sewage sludge and can be an alternative phosphate recovery option to current technologies. To evaluate this, surplus and digested sewage sludge was sampled from full-scale STPs and analysed using XRD, (e)SEM-EDX and Mössbauer spectroscopy. Vivianite was observed in all plants where iron was used for phosphate removal. In surplus sludge before the anaerobic digestion ferrous iron dominated the iron pool (≥50%). XRD and Mössbauer spectroscopy showed no clear correlation between vivianite bound phosphate versus the iron content in surplus sludge. In digested sludge, ferrous iron was the dominant iron form (>85%). Phosphate bound in vivianite increased with the iron content of the digested sludge but levelled off at high iron levels. 70-90% of all phosphate was bound in vivianite in the sludge with the highest iron content (molar Fe:P = 2.5). The quantification of vivianite was difficult and bears some uncertainty probably because of the presence of impure vivianite as indicated by SEM-EDX. eSEM-EDX indicates that the vivianite occurs as relatively small (20 -100 µm) but free particles that could potentially be separated from the sludge. We hypothesize that chemical/microbial Fe(III) reduction is relatively quick and triggers vivianite formation in the treatment lines. Once formed, vivianite may endure oxygenated treatment zones due to slow oxidation kinetics and due to oxygen diffusion limitations into sludge flocs. It was shown that vivianite can indeed form relatively quickly in activated sludge systems. Kinetics of iron reduction, the microbial community and the mechanism of vivianite formation in activated sludge from two STPs were studied; one STP with a low iron dosing (STP Leeuwarden, EBPR) and the other STP with a high iron dosing (STP Cologne, applying chemical phosphorous removal, CPR) were studied. The sludges were incubated under anaerobic conditions in batch experiments. The iron reduction rate in the CPR sludge (2.99 mg-Fe g VS-1 h-1) was 3 times higher than the rate observed in the EBPR sludge (1.02 mg-Fe g VS-1 h-1). The higher iron reduction rate in the CPR sludge is probably caused by its 3 times higher iron content. The rate constants (k) in both sludges are comparable (0.06 h-1 in EBPR sludge vs 0.05 h-1 in CPR sludge), thus the potential rates in both sludges are similar. For calculating the time it takes to turn over all Fe(III) to Fe(II) in the sludge, the Fe(III) reduction rates at the total ferric iron content of the experiments were used and assumed to be constant over time. Calculations then suggest that all iron in STP Leeuwarden and STP Cologne can be turned over within 15 h and 44 h respectively. Sequencing showed that both of the sludges were dominated by proteobacteria (65 – 89% of all operational taxonomic units, OTUs) and that the dominant class of bacteria were β-proteobacteria (38-63% of all OTUs). The microbial communities in both sludges contained genera that comprise iron oxidizing and iron reducing bacteria. These genera were more abundant in the CPR sludge with a higher iron content. XRD and Mössbauer spectroscopy showed that significant quantities of vivianite were formed in the sludges within 24 h. Our study suggests that iron metabolizing bacteria are more abundant in sludge which is rich in iron and that significant vivianite formation can already take place before the anaerobic digestion process. Based on the findings, vivianite is the most important phosphate phase provided enough iron is present, vivianite separation from sewage sludge was studied using a tailor made magnetic separator. Vivianite particles are paramagnetic and present as free particles. Magnetism is an elegant technology as it exclusively separates the liberated and paramagnetic vivianite (and perhaps some pyrite or iron carbonates that are present in the sludge). For this purpose a magnetic separator with Jones magnetic plates was designed and tested on two digested sewage sludges with different iron content. Varying feeding rates were used for the separation. A higher phosphate separation efficiency was achieved with sludge that contained more iron (up to 60% of all input phosphate was recovered) compared to the sludge with lower iron contents (up to 40% of all phosphate could be recovered). The iron and phosphate content was double sometimes even three times higher in the separated (magnetic) fraction when compared to the initial sludge solids. The crystalline fraction of the separated material consisted mainly of vivianite (68%) but also quartz was found (32%) as shown by XRD. The separated material had still a relatively high volatile solid content ranging between 30 – 40% of the dry matter. This fraction is related to organic compounds and other compounds that lose weight during heating (such as carbonates or vivianite). Based on these observations a new phosphate recovery technology for vivianite containing sludge was proposed that makes use of relatively cheap magnetic separation equipment from the mining industry. In this process iron is dosed in high quantities during the treatment process. This would result not only in low effluent phosphate concentrations but, additionally, vivianite formation is not limited by iron during the anaerobic digestion and this would probably result in the transformation of all available phosphate to vivianite. Then vivianite can be separated using a magnetic separator. This separation could be combined with a liberation or pre-separation step by using e.g. a hydrocyclone. Once vivianite is separated from sludge it could be directly used, preferably to produce high valuable products, or it could be dissolved to produce fertilizer. Pure vivianite can easily be dissolved at alkaline pH of about 12. At this pH, phosphate goes in solution and iron and most other metals remain in the precipitate. The phosphate solution obtained from the separated vivianite can directly be used for fertilizer production. Iron could be re-used for phosphate elimination in the STP. In another study it was tested whether sulphide can help to release and recover phosphate from sewage sludge. A series of batch experiments were conducted on different synthetic iron phosphates: Fe(III)P purchased from Sigma, Fe(III)P synthesized in the lab and vivianite. Sulphide was added to these different iron phosphates in a molar Fe:S ratio of 1 to evaluate the total phosphate release and the kinetics of phosphate release into solution. Phosphate release was usually completed within 1 hour. The maximum phosphate release was 92%, 60% and 76% from vivianite, Sigma Fe(III)P and Fe(III)P synthesized in the lab, respectively. However, rebinding of the released phosphate by Fe(II), only in the experiment with Fe(III)P that was synthesized in the lab, reduced the net phosphate release to about 56%. Sulphide induced phosphate release from vivianite is more efficient because sulphide reacts directly with Fe(II) to form FeSx and releases phosphate. No additional sulphide is needed for reducing Fe(III) to Fe(II). At the same time Fe(II) in vivianite is probably more efficient, or as efficient, as Fe(III) in retaining phosphate. Phosphate release from Fe(III)P was, at its maximum (before re-sorption/re-precipitation of the phosphate to other compounds in the sludge) higher than stoichiometry would suggest. Probably because sulphide was acting as a reducing agent, without significant formation of FeSx. FeSx formation requires a larger sulphide input. The high efficiency (moles P released / moles S input) of sulphide acting as a reducing agent to release phosphate was confirmed in additional experiments where sulphide was slowly added to Fe(III)P. Moreover, sulphide addition experiments showed that up to 30% of all phosphate could be released from digested sewage sludge. The highest phosphate release was achieved in experiments with the highest iron content. The total phosphate release from digested sludge was not as high as expected, earlier measurements using XRD and Mössbauer spectroscopy, that were used to quantify iron bound phosphate in the digested sludges, suggested that more phosphate should be iron bound and hence sulphide extractable. The dewaterability (determined using capillary suction test) in digested sludge (0.13 ±0.015 g2(s2 m4)-1) dropped significantly after sulphide was added (0.06 ±0.004 g2(s2 m4)-1). This strongly suggests that sulphide addition to sewage sludge will result in higher sludge disposal costs. Only insignificant phosphate release (1.5%) was observed from sewage sludge ash in response to sulphide addition. Overall, sulphide showed to be a useful tool to release phosphate bound to iron from sewage sludge for its subsequent recovery. Drawbacks are the deterioration of the sludge dewaterability and a net phosphate release that is lower than expected. In a side project of this thesis biogenic iron oxides (BioFeO) formed by Leptothrix sp. and Gallionella sp. were compared with chemically formed iron oxides (ChFeO) for their suitability to remove and recover phosphate from solutions. The ChFeO used for comparison included a commercial iron based adsorbent (GEH®) and chemical precipitates. Despite contrary observations in earlier studies, our batch experiments showed that BioFeO do not have superior phosphate adsorption capacities compared to ChFeO. However, it seems multiple mechanisms are involved in phosphate removal by BioFeO which make their overall phosphate removal capacity higher than that of ChFeO. The overall phosphate removal capacity of Leptothrix sp. was 26.3 mg P/g dry matter (d.m.), of which less than 6.4 mg P/g d.m. was attributed to adsorption. The main removal is likely due to formation of organic iron phosphate complexes (19.6 mg P/g d.m.). Gallionella sp. had an overall phosphate removal capacity of 39.6 mg P/g d.m. Significant amounts of phosphate were apparently incorporated into the Gallionella sp. stalks during their growth (31.0 mg P/g d.m.) and only one fourth of the total phosphate removal can be related to adsorption (8.6 mg P/g d.m.). Their overall ability to immobilize large quantities of phosphate from solutions indicates that BioFeO could play an important role in environmental and engineered systems for removal of contaminants such as phosphate or arsenic. This thesis showed that the iron phosphate chemistry in STPs has been neglected in the past and that more research is necessary to understand the complex interactions between iron and phosphate. This knowledge would help to improve the use of iron in STPs for phosphate removal further and pave the way for new phosphate recovery technologies from iron rich sewage sludge. Within the framework of this research the mineral vivianite was identified as a main iron phosphate phase in sewage sludge. Phosphate recovery technologies via vivianite might lead to a significantly higher recovery efficiency compared to routes relying on struvite. Magnetic separation of vivianite from sewage sludge was achieved using equipment from the mining industry. This process will be tested on pilot scale next. Future research related to vivianite based phosphate recovery has to focus on (I) understanding the formation of vivianite in STPs, (II) improving the separation efficiency of vivianite from sewage sludge using equipment that is tailor made for the type of vivianite which is contained in the sludge (density, magnetic susceptibility etc.) or by manipulating the formation of vivianite (by e.g. increasing its particle size) and (III) evaluating the purity of vivianite in sewage sludge to determine its economic value. @en

Iron is omnipresent in sewage treatment systems. It can unintentionally be present because of, e.g., groundwater seepage into sewers, or it is intentionally added for odor and corrosion control, phosphate removal, or prevention of hydrogen sulfide emissions. The strong affinity of iron for phosphate has advantages for efficient removal of phosphate from sewage, but it is also often considered a disadvantage for phosphate recovery. For instance, the strong affinity between iron and phosphate may reduce recovery efficiencies via struvite precipitation or for some phosphate recovery methods from ash. On the other hand, iron may also have positive effects on phosphate recovery. Acid consumption was reported to be lower when leaching phosphate from sewage sludge ash with higher iron content. Also, phosphate recovery efficiencies may be higher if a Fe-P compound like vivianite (Fe₃(PO₄)₂ 8H₂O) could be harvested from sewage sludge. Developers of phosphate recovery technologies should be aware of the potential and obstacles the iron and phosphate chemistry bears.

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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.

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