Mechanisms and Model Process Parameters in Bioelectrochemical

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Mechanisms and model process parameters in bioelectric wet phosphate recovery from iron phosphate sewage sludge Maxime Blatter, Marion Vermeille, Clément Furrer, Géraldine Pouget, and Fabian Fischer ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05781 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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ACS Sustainable Chemistry & Engineering

Mechanisms and model process parameters in bioelectric wet phosphate recovery from iron phosphate sewage sludge

Maxime Blatter1, Marion Vermeille1, Clément Furrer1, Géraldine Pouget1, Fabian Fischer1,2*

1) Institute

of Life Technologies, HES-SO Valais, University of Applied Sciences Western

Switzerland, Route du Rawyl 64, CH-1950 Sion 2) Institut

de Recherche Energie et Environnement, University of Applied Sciences Western

Switzerland, Route du Rawyl 47, CH-1950 Sion Switzerland. Email: [email protected]. Tel.: +41 27 606 86 58, Fax.: +41 27 606 86 15.

Abstract: Phosphate recovery from sewage sludge is possible with a bioelectrochemical system (BES) also referred to as microbial fuel/electrolysis cell (MFC, MEC). The investigated process is based on phosphate removal with iron salts, which is extensively used in wastewater treatment. The mechanisms and reaction parameters of the bioelectric phosphate recovery process was examined by modelling and model reactions for future scale up works. The mechanistic analyses concerned the electron reduction process, the role of the pH as well as the observed metal removal capacity. Iron oxidation state analyses showed that the iron reduction mechanism was of negligible importance under microbial electrolysis cell conditions. The cathodic iron reduction was outperformed by fast iron precipitation and phosphate remobilization process depended largely on chemical base (OH-). 1 ACS Paragon Plus Environment

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Fluid particle kinetics and shrinking core modelling determined the relevancy of the reaction parameters in order to accelerate phosphate remobilisation. Rate enhancements were possible at higher pH, increased temperature and faster stirring. With the elucidated mechanisms and reaction kinetics parameters the scale-up of bioelectrochemical system based phosphate recovery was given a foundation for scale-up works. Keywords Urban mining, fertilizer, phosphorous, caustic soda, kinetics, simulation, microbial fuel cell, microbial electrolysis cell Introduction It was shown some years ago that pure phosphate is recoverable using a microbial fuel cell (MFC). It worked by adding digested sewage sludge into the basic cathode.1 By changing from the MFC to a microbial electrolysis cell (MEC) the recovery increased from 80% well beyond 95%. It remained uncertain, what mechanisms were behind the good to quantitative yields. In this paper the mechanistic questions were investigated in a series of model reactions and the modelling of possible rate limitations. The initial idea to use a Bioelectrochemical System (e.g. MFC or MEC) for phosphate recovery was based on the fact that phosphate rock is a depleting resource, while being essential to all life forms.2,3 It is contained in food and beverages in sufficient quantities for human nutrition.4 Phosphorus is also used in non-food products such as batteries,5,6 catalysts7,8 and detergents.9 The most important use is as fertilizer in agriculture where its application becomes more and more important as the world population grows while the arable land surface decreases. This is a considerable challenge, particularly in large and highly populated settlement areas.10 Moreover, there are phosphate fertilizer supply chain rupture risks due to the depletion itself and

2 ACS Paragon Plus Environment

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diverse stakeholder needs influencing market availability and prices.11 Mined phosphate rock has therefore become a commodity with volatile and constant price increases.12 There is no alternative to phosphate fertilizers because phosphorus cannot be replaced by any other compound.13 Phosphate rock reserves decline14 rapidly and a production peak became predictable for 203015 or as we approach this moment it appears to be somewhat later,16 but the looming fertilizer shortage remains a great challenge for future food security. In order to mitigate the looming phosphate shortage, more efficient use and recycling are widely researched.17,18 With an annual worldwide potential of 3 million tons of renewable phosphate, wastewater treatment plants (WWTP) are an important unexploited phosphate source and therefore envisioned to substitute phosphate rock mines19,20,21. In most modern WWTPs, iron salts (FeyXz) are used since decades to remove phosphate from wastewater due to their optimal activity in the wastewater matrix.22 The efficiency of phosphate removal depends on accurate iron salt dosage and other factors allowing a routine phosphate removal of 93%.23 Iron chloride (FeCl3) is preferred as it generates readily insoluble strengite (FePO4 .2H2O) (Eq.1) with a solubility product of Ksp = 9.91.10-16 .24 The same applies for FeCl2 or FeSO4 use, which crystalizes with phosphate as vivianite (Fe3(PO4)2 . 8H2O (Eq.2), which is even less soluble than strengite with a Ksp = 1.07 . 10-29.25 Stengite transforms during wastewater treatment (WWT) into vivianite due to anoxic conditions where electrogenic microbes reduce ferric iron (Fe3+) to ferrous iron (Fe2+) (Eq.3,4). This respiratory metabolism is performed by dissimilatory iron-reducing bacteria (DIRB) contained in wastewater using Fe3+ as external electron acceptor.26 Beside strengite and vivianite more complex iron phosphates (FePs) were proposed to be formed during WWT. These are lipscombite, beraunite or rockbridgeite as well as other compounds.27



𝐹𝑒𝐶𝑙3 (𝑎𝑞) + 𝑃𝑂34 ―(𝑎𝑞)→𝐹𝑒𝑃𝑂4 (𝑠)↓ + 3𝐶𝑙

―

𝐹𝑒3 + (𝑎𝑞)

𝐷𝐼𝑅𝐵

𝐹𝑒2 + (𝑎𝑞)

(1)

(𝑎𝑞)

3𝐹𝑒𝐶𝑙2 (𝑎𝑞) +2𝑃𝑂34 ―(𝑎𝑞)→𝐹𝑒3(𝑃𝑂4)2 (𝑠)↓ + 6𝐶𝑙

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―

(𝑎𝑞)

(2)

(3)

3 𝐹𝑒𝑃𝑂4 (𝑠)↓ +3𝑒 ― →𝐹𝑒3(𝑃𝑂4)2 (𝑠)↓ + 𝑃𝑂34 ― (𝑎𝑞)

(4)

Iron-phosphate sewage sludge generated in WWTPs is a voluminous waste and incineration reduces its mass to fit limited landfill space. The alternative use in agriculture as fertilizer is forbidden in many countries28 due to the presence of toxic metals and micro-pollutants.29 Consequently, phosphorus has to be extracted from wet sewage sludge or ashes and then purified before being used as fertilizer.30 The refining of sewage sludge is described as not economic.18 Three major recovery methods (i-iii) are distinguishable: (i) extraction or precipitation directly from wastewater as biomass, struvite or similar; (ii) extraction from wet digested sewage sludge; or (iii) release from sewage sludge ashes. Most often, phosphorus is extracted from ashes using acids31 or bases32. Acid leaching from ashes (Eq.5) is effective but heavy metals are equally released. The use of chemical base for ash leaching was less investigated (Eq.6) as it is slow and low yielding.32 Nevertheless, the advantage of chemical base induced ash phosphate extraction is the low level of metal contaminations present in recovered phosphate. Beside acid or base driven methods, less proven and effective technologies are phosphate fertilizer generation methods used during wastewater treatment. They extract approximately only 40% of phosphorus contained in wastewater.30 Such low 4 ACS Paragon Plus Environment

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efficiencies do not resolve the phosphate pollution problem for rivers and lakes, which receive partly purified effluents.33 Additionally carbon dioxide injection under high pressure34 or the use of caustic soda (OH-)35 can leach phosphate from wet digested sewage sludge containing FePs. These two methods provide with CO2 stirring > temperature > sludge concentration > and particle size. In the following the phosphate remobilisation is discussed. First purely on chemical base



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use and then additional effects that need to be considered, when using a bioelectrochemical system to enable sustainable base generation.

Phosphate remobilization modelling Phosphate remobilisation modelling was studied to understand what the limiting reaction step is. The kinetic analysis was based on fluid particle kinetic shrinking core reaction modelling theory (Figure 3). This enabled to simulate three types of possible remobilisation reactions:47 (i) the diffusion of chemical base through the surrounding liquid toward the solid particle (Eq. 8), (ii) the diffusion of the base in the particle matrix (Eq. 9) and (iii) a none diffusional chemical reaction (Eq. 10). To the modelled kinetic curves based on experimental data, they were added to see the match them with one of the three model reaction curves (reference experiment in Figure 5a). The experimental curves did in many cases not ideally fit with the theoretical models for part or the full length of the phosphate remobilisation experiments (Figure 5). This showed that the kinetic model had limitations but all such data indicated that the diffusion inside the particle was the rate limiting step to overcome to enhance phosphate remobilisation rates.

Here Figure 5

Highly concentrated base enhanced phosphate remobilization rates A very high pH (~ 13) accelerated phosphate remobilisation most clearly. This was in line with the hypothesis that the base driven reaction mechanism was of primary importance. The chemical base was effective when the (OH-) concentration remained high throughout the phosphate remobilization process. Such high and stable base concentration allowed also to apply pseudo first order reaction kinetics (Eq. 12) and


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calculate reaction constants, k’. Higher phosphate yields were possible above pH = 12.4 and with pH ≥ 13 best yields were achieved. A drop of the pH below 12.4 was not advisable as phosphate extractions became clearly sluggish (Figure 6c) and pseudo first order reaction kinetics insufficiently accurate. The pH interval used here looks small from a first glance, but there is 10 times more chemical base at pH 13 than with pH 12 what explains the huge influence of the basicity on phosphate remobilisation from FePs. The high phosphate recovery rates with high pH values confirmed results obtained with the bioelectrochemical system under MEC conditions (Figure 6). Therefore, the controlled examination outside of the MEC in a well defined experimental setup was possible to monitor the remobilisation process (Figure 2).36 Further explanations for the importance of a high pH were the solubility products of the principal FePs and Fick’s first law (Eq.17). The excess of OH- anions led to a high concentration gradient between the reaction surface of the sludge particle and the bulk solution (∂ci(x) ∂x), involving a greater diffusive flux of OH- species (𝐽𝑑,𝑖(𝑥)). Therefore, with a higher pH, hydroxide anions moved faster through the particles sludge layer to react with solid iron phosphates (Figure 2). Hence, greater remobilisation yields up to 94% of remobilized ortho-phosphate were possible (Figure 6).36

∂𝑐𝑖 (𝑥)

𝐽𝑑,𝑖(𝑥) = ―𝐷𝑖

∂𝑥

(17)

To support the assumptions made based on the Fick’s law, the reactions kinetic resistance was simulated by the shrinking core model. Above a certain base concentration, the reaction followed the diffusion model inside the particle (Figure 5a). This was considered as the limiting process to overcome for a faster reaction. However, with an increasing basicity of pH 17 ACS Paragon Plus Environment


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= 13 or above there was no longer a direct match with the modelled reaction curves as the experimental reactions became presumably too fast for at line analytics. The used assay was a comparatively slow and no faster method was available at the time (Figure 5b). The experimental data from this fast base induced process indicated that the limiting step was the inside particle diffusion resistance. In the next sections, the other reaction parameters were analysed and compared to shrinking core modelling kinetics.

Faster stirring enhanced phosphate remobilization Higher stirring rates allowed remobilizing phosphate in a fast manner. By doubling the stirring velocity from 200 to 400 rpm phosphate release from FePs increased from 25 to 92% (Figure 6b). The enhanced stirring induced higher base mobility (𝑣(𝑥)) in sewage sludge particles resulting in higher mass transfer (𝐽𝑐,𝑖(𝑥)) (Eq.18). This was confirmed by experimental data comparison with the closest modelling curve (Figure 5c, blue line). Moreover, the stirring remained the whole reaction time influential, which was not observed with other reaction parameters according to the model reaction for the diffusion in the particle. This was deduced from the experimental data, which matched well with the inside particle diffusion model up to the end of the reaction (Figure 5c). Such a close correlation of experimental data and model reaction was not observed with the other kinetic parameters such as pH and temperature. This finding about the stirring effect was important given that it could lower the energy needs to remobilize phosphate because mechanical stirring consumes less energy than for example heating.

𝐽𝑐,𝑖(𝑥) = 𝑐𝑖𝑣(𝑥)

(18)


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Temperature effect and activation energy in phosphate remobilisation Increasing reaction temperatures improved phosphate remobilisation rates. The lowest remobilisation temperature was 15 °C. This was a well controllable starting point to monitor temperature effects on reaction rates (at pH = 12.4) (Figure 6a) obtaining a rate constant of k’ = 7.3 x 10-6 s-1. From this value, it was clear that the process was reasonably fast even at low 15 °C, which represents close to upper operation temperatures in many WWTPs. The reaction at standard conditions (25 °C) was somewhat faster with a k’ = 1.7 x 10-5 s-1. When the remobilization was performed at 40 °C the rate constant increased again to k’ = 4.5 x 10-5 s-1. This clear temperature dependence (Figure 6d) enabled to determine an accurate activation energy Ea by the Arrhenius’ law (Eq. 13) and a Ea = 55 kJ/mol was calculated.36 Although there was a clear temperature effect it became only well detected at a pH value below 12.4. Above this pH the temperature effect was less or not visible as high chemical base concentration became more influential on the reaction rates than the temperature. The temperature reaction was compared to modelled reactions and it was in line with the diffusion limitation in the sludge particles. With a higher temperature of 70 °C experimental data were not a close fit to any of the modelling curves (Figure 5d) but remained on the side of the in-particle diffusion mechanism. The use of heat is of interest to know if the remobilisation process works rather at lower than higher temperature. It was shown that the process works at standard conditions. It works also at lower temperature as shown and stirring and pH regulation can be used to accelerate the process at lower temperatures. Heating to mesophilic or even thermophilc conditions is costly and appears not beneficial with the BES design in a WWTP (Figure 1) but when performed as a non-biotic process in a separate



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reactor an option but due to the application of aforementioned kinetic parameters it will be rarely needed as far the results indicated.

Sludge solubility and concentration in phosphate recovery reactions From a practical point of view it was important to examine the phosphate remobilisation with as much as possible of digested sewage sludge per volume of concentrated base. However, the miscibility of sludge particles was limited in order to keep the resulting suspensions steerable. Otherwise, sludge particles accumulated on top of the suspension and impeded kinetic assays. Concentrations of up to 20 g/L of digested sewage sludge particles were well treatable in the used setup and accurate kinetic data were obtained. With low 6.7 g/L the remobilisation efficiency was best with 68% after 20 h against 26% with 20 g/L sludge particles (Figure 6d). The higher sludge concentration correlated with a higher iron phosphate content requiring more chemical base and consequently the pH and extraction efficiency dropped. This effect was also visible in experimental data comparisons with the modelling curves, where in the beginning the in-particle diffusion dominated and then the process became slow and changed to a liquid diffusion controlled process (Figure 5e).

Here Figure 6

Sludge particle diameter and phosphate remobilisation rates To assess the importance of the particle diameter on phosphate remobilisation rates three types of powder were generated by milling dry digested sewage sludge. The average particle diameters (d50) obtained and used were 26, 56 and 84 µm. Reducing 20 ACS Paragon Plus Environment

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particle diameters by half or three times did not result in significant rate changes (Figure 6e) and phosphate recoveries stagnated between 48 and 51% after 24 h. The initial solution pH = 12.4 was set relatively low due to the need to reduce reaction rates in order to observe particle size effects on reaction rates. The reason for the low impact of the particle size under these conditions was explained by the heterogeneity of the generated sludge particles. They were different in terms of composition, shape and probably lack of stability. Moreover, the mill generated a broad range of particles diameters with a size distribution from nano to micrometers according to optical microscopy and Malvern analysis (Figure 7). The irregular particle shapes resulted from the heterogeneity of sewage sludge and shear forces during the milling. The comparison of experimental data with the modelling curves showed, as in other cases, a vicinity match with the in-particle diffusion controlled reaction model (Figure 5f).

Here Figure 7

Iron oxidation states in digested sewage sludge FeCl3 was used to remove ortho-phosphate from wastewater in all five involved WWTPs in this work (Eq. 1). The Fe3+(aq) addition instantly formed strengite (FePO4.2H2O), a crystalline precipitate and the Fe3+ therein was not reduced at this point. The iron oxidation state was expected to change from Fe3+ to Fe2+during waste water treatment (WWT).44 The examined digested sludge samples contained indeed minor (18.1%) to good (65.2%) quantities of Fe2+ (Figure 8), although an even higher percentage of Fe2+ was expected to be contained due to microbial reduction under anaerobic conditions26 in the concerned WWTPs. Therefore, the presence of important quantities of Fe3+ (Figure 8) in all samples was to some degree surprising. The reason



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for the high Fe3+ content was explained by the fact that WWTPs need to fulfil primarily legal requirements. These lead to (i) FeCl3 overdosing, (ii) fast processing (iii) and therefore probably insufficient time for the expected iron reduction to happen and neither the formation of minerals that are composed of Fe2+ at given pH values. Moreover, FeCl3 is equally added to suppress of H2S degassing to avoid bad odours45 and because it supports flocculation during WWT. On the other side the digested sludge was for some time exposed to air-oxygen after reaching the sludge container at the end of the WWT process. At that point, a spontaneous oxidation of Fe2+ to Fe3+ occurred to some degree, but when analysing digested sewage sludge samples, which were not in contact with air-oxygen, rather similar Fe3+ quantities were detected and oxidation after WWT was not the primary cause for the important quantities of ferric ions.

The oxidation state analysis was validated and it confirmed the previously observed oxidation states.42 The accuracy was in line with other reports using the same methodology.43,40 The oxidation state assay provided results on total iron and ferrous iron (Fe2+) contents and enabled to calculate the Fe3+ fraction. ICP-OES analytics confirmed the total iron content. It was estimated that no elemental iron (Fe°) was in the sewage sludge samples due to the aqueous nature of WWT. In addition, any Fe° presence would increase the Fe2+ content and not the Fe3+ fraction whose relatively high concentration was here the debatable observation. The high Fe3+ quantity showed that an excess of hydroxides was used to remobilize phosphate from FePs in digested sewage sludges (Figure 8). The excess to iron was about 71% in relation to the formation of vivianite (Fe3(PO4)3), the ideal precipitation product. But, the situation was certainly not ideal, moreover, one has to keep in mind that the iron excess served


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equally H2S removal in WWT. This later function of the FeCl3 use was not investigated in detail.

Here Figure 8 Conclusions Quantitative phosphate remobilization from iron phosphates in digested sewage sludge was possible. High pH, thermal (hot aqueous solution) processing and fast stirring were critical to rate enhancements in the remobilisation. Performed particle fluid kinetics and related shrinking core modelling showed that phosphate remobilisation was limited by hydroxide diffusion in the sewage sludge particles. The iron reduction mechanism in the microbial electrolysis cell was not observed because iron cations precipitated faster than any iron reduction occurred. Chemical base formation in the MEC was fast and the resulting pH readily formed iron hydroxides. Base induced phosphate recovery was highly effective with up to 95%. The quantity of chemical base needed depended on phosphate content and the overall quantity of iron cations as well as other charges that formed solids. Iron phosphates in digested sewage sludge from five WWTP contained clearly more Fe3+ than expected due to FeCl3 overdosing. Finally, wet phosphate remobilisation in the MEC setup became predictable with the elaborated parameters. In particular, the possible origin for conflicting data in the literature35,38,37 was shown to be related most likely to a too low pH < 12 of the reaction mixtures at the end of the phosphate remobilisation process leading to lower yields. The stirring and temperature need also to be taken in consideration, when the yields run to low. With this the scale-up of phosphate remobilisation beyond the pilot scale should be examined and to demonstrate the 23 ACS Paragon Plus Environment


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feasibility of the base propelled phosphate recovery process from wet digested sewage sludge of any WWTP that works with metal salt phosphate removal.

Acknowledgements The work was supported by the Swiss Federal Office for the Environment (Project: UTF 393.27.11), and others such as: City of Sion (WWTP Châteauneuf, Valais), City of Martigny, Valais (WWTP), Lonza Ltd. Visp, WWTP Worblental (Berne), SATOM, Altis, and Landor (fenaco). We thank Marion Jaussi for help with editing.

List or Abbreviations Abbreviations: A BES CFeP

Pre-Exponential factor Bioelectrochemical System Iron phosphate concentration

CFeP, t CFeP, t = 0 Ci COHCP, t CPO43-, t CSludge d50 Di DIRB Ea FeP FeyXx

Iron phosphate concentration at a specific time Initial iron phosphate concentration Concentration of species i Hydroxide ion concentration Phosphorus concentration at a specific time Phosphate concentration at a specific time Digested sewage sludge concentration Average particle diameter Diffusion coefficient of species i Dissimilatory Iron-Reducing Bacteria Activation energy Iron Phosphate Iron Salt

ICP-OES Jc, i Jd, i K k' Ksp MEC MFC Mn(OH)m PVC

Inductively Coupled Plasma Optical Emission Spectrometry Convection flux of species i Diffusive flux of species i Rate constant Observed rate constant Solubility product constant Microbial Electrolysis Cell Microbial Fuel Cell Metal Hydroxide Polyvinyl Chloride


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R rc RVC ʋ wFeP wP WWT WWTP X XFeP YPO43-, FeP

𝜏

Gas constant or particle radius Unreacted core radius Reticulated Vitreous Carbon Linear velocity of the solution Proportion of iron phosphate in sludge Proportion of phosphorus in sludge Wastewater Treatment Wastewater Treatment Plant Distance Iron phosphate fraction converted Remobilized phosphate from iron phosphate in sludge Time for total conversion

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(15) Cordell, D.; Drangert, J. O.; White, S. The story of phosphorus: Global food security and food for thought. Glob. Environ. Chang. 2009, 19, 292-305. (16) Walan, P.; Davidsson, S.; Johansson, S.; Höök, M. Phosphate rock production and depletion: Regional disaggregated modeling and global implications. Resour. Conserv. Recycl. 2014, 93, 178-187. (17) Baker, A.; Ceasar, S. A.; Palmer, A. J.; Paterson, J. B.; Qi, W.; Muench, S. P.; Baldwin, S. A. Replace, reuse, recycle: improving the sustainable use of phosphorus by plants. J. Exp. Bot. 2015, 66, 3523-3540. (18) Ludwig, H. Rückgewinnung von Phosphor aus der Abwassereinigung. Eine Bestandesaufnahme. Umwelt-Wissen Nr. 0929. Bundesamt für Umwelt, 196 S, Bern, 2009. (19) Karunanithi, R.; Szogi, A.A.; Bolan, N.; Naidu, R.; Loganathan, P.; Hunt, P.G.; Vanotti, M.B.; Saint, C.P.; Ok, Y.S.; Krishnamoorthy, S. Phosphorus recovery and reuse from waste streams. In Advances in agronomy, Academic Press 2015, 131, 173-250. (20) Marchi, A.; Geerts, S.; Weemaes, M.; Wim, S.; Christine, V. Full-scale phosphorus recovery from digested waste water sludge in Belgium–part I: technical achievements and challenges. Water Sci. Technol. 2015, 71, 487-494. (21) Geerts, S.; Marchi, A.; Weemaes, M. Full-scale phosphorus recovery from digested wastewater sludge in Belgium–part II: economic opportunities and risks. Water Sci. Technol. 2015, 71, 495-502. (22) Zhang, T.; Ding, L.; Ren, H.; Guo, Z.; Tan, J. Thermodynamic modeling of ferric phosphate precipitation for phosphorus removal and recovery from wastewater. J. Hazard. Mater. 2010, 176, 444-450.



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(23) Happe, M.; Sugnaux, M.; Cachelin, C. P.; Stauffer, M.; Zufferey, G.; Kahoun, T.; Salamin, P-A. Egli, T.; Comninellis, C.; Grogg, A-F. Fischer, F. Scale-up of phosphate remobilization from sewage sludge in a microbial fuel cell. Bioresour. Technol. 2016, 200, 435-443. (24) Lide, D. R. Handbook of Chemistry and Physics, CRC Press, USA, 2005. (25) Chen, P.; Faust, S. The Solubility Product of Ferrous Phosphate. Environ. Lett. 1974, 6, 287-296. (26) Lovley, D. R. Dissimilatory Fe(III) and Mn(IV) reduction. Microbiol. Rev. 1991, 55, 259-287. (27) Wilfert, P.; Kumar, P. S.; Korving, L.; Witkamp, G. J.; Van Loosdrecht, M. C. The Relevance of Phosphorus and Iron Chemistry to the Recovery of Phosphorus from Wastewater: A Review. Environ. Sci. Technol. 2015, 49, 9400-9414. (28) The Swiss Confederation, Ordonance on the Reduction of Risks relating to the Use of Certain Particularly Dangerous Substances, Preparations and Articles, Art.3, Annex 2.6, 2005. (29) Fijalkowski, K.; Rorat, A.; Grobelak, A.; Kacprzak, M. J. The presence of contaminations in sewage sludge–The current situation. J. Environ. Manag. 2017, 203, 1126-1136. (30) Sartorius, C.; Horn, J.; Tettenborn, F.; Phosphorus Recovery from WastewaterExpert Survey on Present Use and Future Potential. Water Environ. Res. 2012, 84, 313-322. (31) Biswas, B. K.; Inoue, K.; Harada, H.; Ohto, K.; Kawakita, H. Leaching of phosphorus from incinerated sewage sludge ash by means of acid extraction followed by adsorption on orange waste gel. J. Environ. Sci. 2009, 21, 1753-1760.


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(32) Levlin, E.; Hultman, B. Phosphorus recovery from sewage sludge - ideas for further studies to improve leaching. Joint Polish - Swedish Reports, Stockholm, Sweden, 2005. (33) Smith, V. H.; Tilman, G. D.; Nekola, J. C. Eutrophication: impacts of excess nutrient inputs on freshwater, marine, and terrestrial ecosystems. Environ. Pollut. 1999, 100, 179-196. (34) Stössel, E. Budenheim Carbonic Acid Process–An environment friendly process for recovering phosphates from sewage sludge. In Braunschweig (4th International Symposium „Re-Water Braunschweig “) 2013. (35) Sano, A.; Kanomata, M.; Inoue, H.; Sugiura, N.; Xu, K.; Inamori, Y. Extraction of raw sewage sludge containing iron phosphate for phosphorus recovery. Chemosphere 2012, 89, 1243-1247. (36) Fischer, F.; Zufferey, G.; Sugnaux, M.; Happe, M. Microbial electrolysis cell accelerates phosphate remobilisation from iron phosphate contained in sewage sludge. Environ. Sci.: Processes Impacts 2015, 17, 90-97. (37) Maier, W. ; Weidelener, A. ; Krampe, J. ; Rott, I. Entwicklung eines Verfahrens zur Phosphat-Rückgewinnung aus ausgefaultem Nassschlam oder entwässertem Faulschlamm als gut pflanzenverfügbares Magnesium-Ammonium-Phosphat (MAP): Schlussbericht: Teil 1: Zusammenfassung und Wertung der Ergebnisse, 2005, 160 pp. (38) Suresh Kumar, P. Phosphate recovery from wastewater via reversible adsorption. Diss. 2018, DOI:10.4233/uuid:f75d3713-8ef2-4f92-884f-06664b040f47 (39) Havukainen, J.; Nguyen, M. T.; Hermann, L.; Horttanainen, M.; Mikkilä, M.; Deviatkin, I.; Linnanen, L. Potential of phosphorus recovery from sewage sludge and manure ash by thermochemical treatment. Waste Manag. 2016, 49, 221-229.



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(40) Wilfert, P.; Mandalidis, A.; Dugulan, A. I.; Goubitz, K.; Korving, L.; Temmink, H.; Witkamp G. J.; Van Loosdrecht, M. C. M. Vivianite as an important iron phosphate precipitate in sewage treatment plants. Water Res. 2016, 104, 449-460. (41) Rasmussen, H.; Nielsen, H. Iron reduction in activated sludge measured with different extraction techniques. Water Res. 1996, 30, 551-558. (42) Lovley, D. R.; Phillips, E. J. P. Organic Matter Mineralization with Reduction of Ferric Iron in Anaerobic Sediments. Appl. Environ. Microbiol. 1986, 51, 683-689. (43) Dziedzic‐Kocurek, K.; Banaś, A.; Kwiatek, W. M.; Stanek, J. Iron valence states in organic samples and tissues investigated by XANES and Mössbauer spectroscopy. X‐Ray Spectrom. 2008, 37, 219-225. (44) Nielsen, P. H.; Frølund, B.; Spring, S.; Caccavo Jr, F. (1997). Microbial Fe (III) reduction in activated sludge. Syst. Appl. Microbial. 1997, 20, 645-651. (45) Nielsen, A. H.; Lens, P.; Vollertsen, J.; Hvitved-Jacobsen, T. Sulfide-iron interactions in domestic wastewater from a gravity sewer. Water Res. 2005, 39, 27472755. (46) Blatter, M.; Sugnaux, M.; Comninellis, C.; Nealson, K.; Fischer, F. (2016). Modeling of Sustainable Base Production by Microbial Electrolysis Cell. ChemSusChem 2016, 9, 15701574. (47) Levenspiel, O. Chemical Reaction Engineering. ed. John Wiley & Sons, NY, 1999, 25, 566-582. (48) Viollier, E.; Inglett, P.; Hunter, K.; Roychoudhury, A.; Cappellen, P. The ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters. Appl. Geochem. 2000, 15, 785-790. (49) Nielsen, P. H. The significance of microbial Fe (III) reduction in the activated sludge process. Water Sci. Technol. 1996, 34, 129-136. 30 ACS Paragon Plus Environment

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Figure 1: The possible location of a bioelectrochemical system for phosphate recovery in a state-of-the-art wastewater treatment plant. It replaces by design the biological pond and remobilizes phosphate while purifying wastewater and generating P-free sewage sludge as biofuel. P-fluxes based on real data for FeCl3 dosage and Premoval.23


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Figure 2: The kinetics of phosphate remobilisation in the cathode of a bioelectrochemical system was examined at the outside in clearly defined environment with chemical base. Left: a BES with small cathode for in-situ phosphate remobilisation, chemical base generation, iron reduction and iron precipitation experiments.



Experiment

Kinetic parameters examined in paricular

Initial pH

Temperature

Stirring

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Sludge

Average particle size [-] [°C] [rpm] [g/L] [µm] 1 Set as reference* 12.7 25 300 10 26 2 pH 12.4 25 300 10 26 3 pH 12.7 25 300 10 26 4 pH* 13 25 300 10 26 5 Temperature 12.7 25 300 10 26 6 Temperature 12.7 40 300 10 26 7 Temperature* 12.7 70 300 10 26 8 Stirring 12.7 25 200 10 26 9 Stirring 12.7 25 300 10 26 10 Stirring 12.7 25 350 10 26 11 Stirring* 12.7 25 400 10 26 12 Sludge 12.7 25 300 6.67 26 13 Sludge 12.7 25 300 10 26 14 Sludge 12.7 25 300 15 26 15 Sludge* 12.7 25 300 20 26 16 Diameter 12.7 25 300 10 26 17 Diameter 12.7 25 300 10 56 18 Diameter* 12.7 25 300 10 84 Table 1: Screening parameter in kinetic assay of the semi-model phosphate remobilisation process from digested sewage sludge using NaOH (Figure 6). *) Comparison of best performing conditions in experiments 1,4,7,11,15 and 18 with shrinking core modelling reactions (Figure 5). All experiments were performed in triplicate.


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Figure 3: Shrinking core model for particle fluid reactions between solid iron phosphates (FePs) and hydroxide anions (OH-) in inert sewage sludge particles liberating ortho-phosphate (𝑃𝑂34 ― ). Top: Three shrinking core model reaction types (Eq. 8-10). Bottom: According to all analyses, diffusion in the particle sludge matrix was the rate limiting step. The reaction frontier zone moved progressively toward the centre of the particle also described as shrinking core process.47



A)

C)

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B)

D)

Figure 4: A) Cathodic phosphate remobilisation with a bioelectrochemical system from FeP contained in sewage sludge. B) Quantitative recovery of phosphate with a basic BES cathode. C) Current generated with the BES using an anodic Shewanella oneidensis MR-1 cultivation. D) pH increase (●) in the cathode followed closely a corresponding model curve46 (full line) for chemical base production in the BES cathode.


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A)

B)

C)

D)

E) F)

Figure 5: Experimental data (red dotted lines) compared with three shrinking core modelling curves (solid lines). A) Remobilisation reaction under reference conditions (Table 1, Entry 1, see also colour codes (green, violet and blue) for model curves in Figure A). Best working parameter values (dotted lines) in contrast to reference conditions in Figure A: B) high pH, C) fast stirring, D) elevated temperature, E) particle quantity, and F) particle size. The remobilisations performed under experimental conditions A, E and F did not permit to obtain high phosphate recoveries in the given time and the end of the reactions were estimated, for A > 10 days, E > 25 days and F > 40 days. This explains the absence of experimental data between 0.1 t/ and 1 t/. The experimental conditions are found in Table 1, Entries 1,4,7,11,15 and 18.



A)

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B)

)

C)

D)

E)

Figure 6: Phosphate remobilisation kinetics for five reaction parameters (Table 1, Entries 1-18). A) Initial pH. B) stirring velocity, C) temperature influence, D) sludge quantities and E) particle size. General remobilisation conditions: 10 g/L of digested sewage sludge particles (all form the same lot, WWTP Châteauneuf), 26 µm particle diameter, 25 °C, pH 12.4 and 300 rpm.


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Figure 7: Particle size distribution (d50) of dry milled digested sewage sludge by Malvern analysis and optical microscopy (top left).



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Figure 8: Phosphorus, iron mass as well as iron oxidation states assayed in dry digested sewage sludge from five Swiss wastewater treatment plants (WWTPs). Average iron excess in view of phosphorus contents was about ~71%, which originated from over dosage and influents. Châteauneuf, Worblental and Martigny are urban municipal WWTPs, Lonza in Visp is an industrial/municipal WWTP, and Bagnes an alpine resort/municipal WWTP.


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"For Table of Contents Use Only"

Bioelectrochemical system modelling of phosphate recovery from sewage for renewable fertilizer and P-free solid biofuel generation



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Mechanisms and Model Process Parameters in Bioelectrochemical

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