Post-Removal of Phosphorus from Biologically Treated Wastewater and Recovering It as Fertilizer: Pilot-Scale Attempt—Project PhoReSe

: The major issue of raw materials’ depletion, and more specifically, of phosphorous (an important fertilizer) has currently become an emergent aspect due to expected depletion problems needing immediate handling. This was the reason for the implementation of the PhoReSe project that aimed to remove and recover phosphorus from the secondary (biologically treated) effluent of a municipal wastewater (biological) treatment plant (WWTP “AINEIA”, located near Thessaloniki, N. Greece), treating the wastewaters of the nearby touristic area. Regarding the phosphorous supplementary removal and recovery treatment options, two methods were examined, initially at the laboratory scale (batch experiments), i.e., (1) the adsorption of phosphorous, and (2) the chemical precipitation of phosphorus. Both methods were further applied at the pilot scale by initially performing the adsorption of phosphorous onto the AquAsZero commercial sorbent, which is a mixed manganese iron oxy-hydroxide, followed by the chemical precipitation of phosphorous implemented after the desorption process of the previously saturated adsorbent. The final precipitate of this procedure was examined as an alternative/supplementary fertilizer, this way returning phosphorus into the natural cycle. These experiments, as applied successfully in at the pilot scale, set the basis for larger-scale relevant applications for similar WWTP facilities.


Introduction
Even though the Life Cycle Assessment (LCA) method and the concept of the Circular Bio/Economy have dynamically entered industrialized optimization processes, nevertheless, the linear use of materials is still quite widely applied, resulting in the loss or unnecessary disposal of useful raw materials [1].Apart from sustainability aspects, the increased processing costs for the achievement of even stricter disposal limits, as imposed by respective regulation norms to avoid potential environmental problems, usually require further potential exploitation of used end-of-life materials, such as phosphorus (P), to reduce the respective costs [2].Phosphorus is among the important nutrients, pointing out the recycled/recovered options during each step of the wastewater treatment process due to its significance for the growth of plants and living organisms, as well as not being considered as a convenient renewable material in nature [3,4].The crystallization of struvite (i.e., magnesium ammonium phosphate (MAP), MgNH 4 PO 4 •6H 2 O) during certain wastewater treatment processes, in regard to P-rich streams (usually after the anaerobic digestion of sludge), was once considered as a serious operational problem, but nowadays, it is mainly used to retrieve phosphorus from these streams when the existing conditions (i.e., mainly Mg and nutrients' content, pH, and temperature) allow for its formation, since it can be subsequently effectively applied as a potential fertilizer, therefore enabling the recovery of phosphorus from wastewaters and its environmental reuse [5,6].
Phosphates' gradual depletion due to the overuse of fertilizers (expected to be from 7.5 to 12.5 Tg/year during 2005-2050) results in a high phosphorous footprint, which can also be related to methane production (being an important part (around 13-20%) of the total greenhouse gases effects) due to the potential creation of algae blooms in water bodies, caused by the resulting eutrophication problem from their respective mismanagement/disposal, and following their death, which can produce methane under anaerobic conditions, hence seriously affecting climate change overall [7,8].Moreover, the eutrophication problem of water resources is accelerated by anthropogenic activities that include the disposal of municipal (not-sufficiently treated) wastewaters.An increase in the release of nutrients into water bodies, and more specifically, of nitrogen and phosphorus, can result in severe algal blooms, as well as the presence of potentially harmful cyanobacterial species that could, in turn, produce dangerous cyanotoxins [9,10].As a global issue, eutrophication must be mitigated by implementing efficient treatment strategies that will allow the removal, recovery, and re-entry of phosphorus into its natural bio-geo-environmental cycle.Since there is a tendency to increase the total phosphorus content in raw wastewaters, the target should be to obtain low residual concentration levels (<1 mg/L) in the treated effluents disposed in water bodies; hence, eliminating the undesirable phenomena of eutrophication [11,12].In the future, if not recycled, phosphorus scarcity will result in the "phosphogeddon" phenomenon, affecting not only climate change, but food production and quality as well [13].Phosphorus is an essential element for agriculture and cannot be substituted, making the identification of potential strategies to secure sufficient phosphorous supply and avoid the expected deficiency by 2035 very important [14,15].These strategies must be implemented worldwide towards the exploitation and recovery of phosphorous from several waste streams [16].
Initially, the respective treatment processes regarding phosphorous are aimed at removing it from wastewater to avoid the adverse environmental effects of its disposal.However, nowadays, the problems of phosphorous depletion have led to practices that promote the closing of the P-cycle by implementing processes to recover and recycle it instead [17].It should also be noted that most current studies focus on the recovery/recycling of P from P-rich streams (e.g., from food production wastes, from municipal wastewater treatment plants (anaerobically treated sewage sludge), and from aqueous sewage liquors and animal wastes etc.) [15][16][17][18][19][20][21][22].The European Sustainable Phosphorus Platform (ESPP) annually updates the nutrient recovery technologies catalogue, reporting the applied technologies for phosphorus mostly in the forms of struvite, hydroxyapatite, calcium phosphates, and phosphoric acid.Among the 53 respective technologies in the 2023 report, only 1 was reported for the recovery of phosphorus from lower-concentrated steams [23,24].Bearing in mind the serious economic consequences that will be negatively affected by the "phosphogeddon" depletion effect, but also the respective environmental benefits, the scientific community is oriented to achieve P-recovery not only from P-rich streams (which are mainly in the form of inorganic phosphates/PO 4 3− ), but also from lower-concentrated P-streams, such as secondary (treated) wastewater treatment effluents, as well as the meat industry and steelmaking slag, etc. [17,25].
The relevant mitigation techniques may include chemical, physical, and biological processes.P-rich streams in the wastewater treatment process, when the phosphorus recovery is required, usually include urine influent (when separated), secondary wastewater treated effluents, the anaerobic digester supernatant of sewage sludge treatment, and specifically incinerated sewage sludge ash [26].The wet chemical (hydrometallurgical) method is a simple technique that allows for the recovery of phosphorus mainly from sewage sludge ash or from the anaerobic digester supernatant in the form of high-purity products, such as struvite.However, it presents a relatively high operating cost due to the added chemical reagents and its operational procedures [27].Sewage sludge ash can be also treated with more complex and higher-energy-consuming thermal methods to recover phosphorus, producing high-purity products and the minimum pollutant content.Diverse technologies, such as pyrolysis and electrochemistry, can be also applied to sewage sludge to recover phosphorus.Even though pyrolysis shows a high recovery rate, its main problems are the potential dioxin and furans gases produced during this process.Electrochemical methods are still under investigation, but mostly at the laboratory scale [28,29].
On the other hand, adsorption is an easy handling process, showing a high selectivity for the removal of phosphorus and being applicable in aqueous phase streams; usually, a proper pre-treatment step for the enhanced separation of suspended solids is required.This method is also highly dependent on the applied sorbent material's capacity for capturing phosphorus [30,31].
To achieve phosphorus removal and recovery from treated secondary wastewater effluents, in this case, the P-stream that PhoReSe examined, several chemical agents can be added, such as lime, aluminum, or iron salts.However, due to the cost and required quantities of these agents, the process can become viable economically, but only for P-rich streams, while, at the same time, a secondary treatment to eliminate the remaining potential microbial pollutant loads, etc., may be commonly required [32,33].On the other hand, the application of biological processes cannot handle the phosphorus content constantly and with a high removal efficiency, with around a 70% treatment efficiency for most of relevant cases, also being a highly temperature-dependent process [34,35].Moreover, the alternative application of micro-or ultra-filtration membrane-specific processes, apart from their rather high implementation costs, can also result in a supernatant that still contains quite high quantities of phosphorus, hence contributing to secondary pollution problems when disposed of, or requiring additional post-treatment [9,36].Currently, pilot and fullscale applications are also oriented to the combination of adsorption and precipitation processes, such as those examined in the PhoReSe project [31,37].
Since phosphorus scarcity is one of major issues that must be dealt with in the 21st century, the realization of the PhoReSe project was held within the framework of the circular economy and a sustainable development framework, targeting the environmental achievements of closing the phosphorus loop and deficiency and preventing its forecasted elimination as a raw material source.This review paper aims to highlight the successful completion of the PhoReSe project, recovering phosphates from treated wastewaters and presenting selected results to exhibit the potential of its implementation, as supported by previous relevant publications [31,[38][39][40][41][42][43].The importance of the PhoReSe project lies beyond the state of the art of other relevant studies, as it has produced an applicable solution for the recovery of phosphorus from streams of treated municipal wastewater before its disposal in the receiving water bodies, primarily producing an environmentally valuable product that enables the re-entry/reuse of phosphates as a nutrient source, while, at the same time, avoiding the eutrophication phenomena caused by its disposal in rivers, lakes, and even the sea [44].

Statistical Variations Determination
For the realization of this research, it is important to examine the stability of wastewater treatment processes and determine the phosphorus concentration that can be potentially recovered.For this reason, the statistical variations of secondary (biologically treated) effluents from a WWTP, handling about 8 × 10 3 m 3 /d of influent and consisting of 7 × 10 3 m 3 /d of municipal wastewater and 1 × 10 3 m 3 /d of domestic septage waste, were examined for the period of 12 months.The mean higher phosphorus concentrations in this effluent were the determinant values for the design of experimental tests [39], because it is crucial for the recovery process to be able to effectively treat the whole range (min.-max.) of respective phosphorus concentrations [45].The measurement of the statistical variations in the main parameters was conducted using the IBM SPSS statistic software 23 package.

WWTP Effluent
To achieve the required residual concentrations, lower than the commonly applied legislation limit for PO 4 3− (i.e., 1 mg total P/L, or 3 mg PO 4 3− /L), the continuous recovery of phosphorus disposed with the treated secondary effluents was examined [11].The municipal WWTP of "AINEIA" treats wastewaters produced from the nearby touristic areas of Thessaloniki, which usually contain (mean) phosphorus concentrations of 4-6 mg P-PO 4 3− /L [31,[38][39][40].The phosphorus adsorption experiments were conducted by using certain hydrous iron oxyhydroxides, either commercially available (i.e., GEH, Bayoxide, and AquAsZero), or produced in the laboratory (schwertmannite and lepidocrocite), as well as the organic resin PuroliteA200 EMBCL supplied by Purolite, Milan, Italy, aiming to examine whether the disposal of phosphorus effluent was achievable below the regulation limit and could be further recovered.Regarding the recovery of phosphorus, the preliminary desorption, in an alkaline environment, of the P-loaded sorbent was investigated, followed by subsequent chemical precipitation, by using certain salts as coagulant agents [31].The produced precipitated phosphate salts, respectively, were examined further as potential fertilizers by checking their phosphate content and soil bioavailability [38].

Adsorption Capacity Evaluation
The adsorption experiments evaluated the adsorption capacity by the batch isotherms, thermodynamics, and pH effect fitted in the Langmuir and Freundlich models, in which a quantity of adsorbent was placed in the phosphate solution and examined for its adsorption capacity; also, by using the Rapid Small-Scale Column Tests (RSSCTs), the adsorption capacity was examined in continuous flow.The examined commercially available adsorbents were the following: • GEH (supplied by GEH Wasserchemie GmbH & Co. KG, Osnabrück, Germany), consisting mainly form akaganeite.
The main physico-chemical characteristics of the above-mentioned adsorbents, as previously reported by Kalaitzidou et al., 2016, are presented in Table 1 [31].The adsorption capacity Q was calculated by the following Equation ( 1): where: Q: adsorption capacity C Initial : the initial phosphate concentration of the solution used for the adsorption experiment C Residual : the residual concentration of phosphates after the adsorption experiment C Adsorbent : the adsorbent concentration in the solution In addition, two laboratory-synthesized iron oxyhydroxides were examined, prepared under intensive oxidative conditions after the oxidation and precipitation reaction of FeSO 4 •H 2 O (supplied by Loufakis Chemicals S.A., Thessaloniki, Greece, with >99.0%purity), producing, as final products, schwertmannite and lepidocrocite, but only in the experiments examining the temperature effect [46].

Chemical Precipitation Experiments
The PhoReSe experimental investigation included an examination of the chemical precipitation of phosphorus, with the addition of commercial forms of inorganic salts, i.e., magnesia (MgO), calcium hydroxide (Ca(OH) 2 ), huntite (Ca.Mg 3 (CO 3 ) 4 ), ferric chlorosulfate (FeClSO 4 ), and sodium aluminate (NaAlO 2 ), in various water matrixes, i.e., aquatic solutions of potable or distilled water and of biologically treated wastewater (secondary effluent), containing an initial concentration of about 5 mg P-PO 4 3− /L.These salts were examined as Ca 2+ , Mg 2+ , Fe 3+ , and Al 3+ sources to form the respective insoluble precipitates of phosphorus [41].According to the relevant speciation diagram that corresponds to a phosphate concentration of 10 mg/L in the National Sanitation Foundation (NSF) water matrix, which is used as a typical element concentration in natural waters, the respective calcium and magnesium salts can be formed at pH values of >6 at 293 K with the negatively charged phosphate anions [40,47].The chemical precipitation experiments were conducted in three different matrixes, i.e., distilled, potable, and treated wastewater after the secondary (biological) wastewater treatment, and the main characteristics are shown in Table 2.The phosphate stock solution was prepared by using anhydrous KH 2 PO 4 (supplied by Panreac, Applichem, Castellar del Vallés, Barcelona, Spain, >98.0%purity).The coagulation/precipitation procedure was investigated initially by batch experiments, adding the respective coagulant doses to the phosphate solutions and examining the optimum phosphate removal, as well as the required dosages and treatment time.The used chemical agents investigated during this project as "coagulants" for the eventual precipitation of PO 4 3− were the following inorganics:

Scaling up to Pilot Scale
The sustainability aspect was the main target of the PhoReSe project, aiming to engage phosphorus from the secondary treated wastewater effluent back into the phosphorus environmental cycle [48].The promising results obtained from the laboratory-scale experiments led to the design, construction, and operation of a relevant pilot plant, put in operation in the WWTP of AINEIA (see Supplementary Materials) [49].The pilot plant set-up also included a membrane pre-filtration unit for the efficient removal of residual suspended solids, an adsorption-desorption column, and a final chemical precipitation tank, alongside a full automation panel for remotely controlling the whole recovery process [31,34].The respective materials and methods are explained in more detail in the previous relevant publications [31,[38][39][40].

Precipitate Evaluation
The resulting chemical precipitates of the phosphorus in the alkaline effluent after the desorption procedure were investigated in terms of phosphorus bioavailability by the extraction of phosphorus with the Mehlich-3 solution (consisting from 0.001 M EDTA, supplied by Merck, with >99.0%purity; 0.015 M NH 4 F, supplied by Merck, with >98.0%purity; 0.2 M CH 3 COOH, supplied by Merck, with >99.8% purity; 0.25 M NH 4 NO 3 ; 0.013 M HNO 3 , supplied by Merck, as 65.0% a.r.) or the Olsen solution (NaHCO 3 0.5 M) for the acidic and alkaline soil samples, respectively.Moreover, the assessment of the soil samples' phytotoxicity was conducted by using the commercially available toxicity bioassay "Phytotox kit micro-biotest" [38].

Statistical Analysis
During the PhoReSe project, as reported by Raptopoulou et al., 2016, the statistical results showed that notable variations took place regarding the phosphorus content, espe-Water 2024, 16, 1527 7 of 15 cially during the summer period in the secondary treated wastewater effluent, indicating the need for additional post-treatment [39].

Adsorption Capacity
The results evaluation of the adsorbents' efficiencies, as reported by Kalaitzidou et al., 2016, showed the superiority of iron-based adsorbents, as compared to the lower adsorption/ion exchange capacity of the Purolite A200 EMBCL material [31].Figure 1 presents the data fittings, applying the Freundlich and Langmuir isotherm models by plotting the adsorption capacity Q of the material versus the residual concentration (C Residual ) after the adsorption experiment, and shows the major findings regarding the high iron-based adsorbents' removal efficiencies.Data for Purolite A200 EMBCL are not included in this plot due to the near-zero adsorption capacity found (0.5 mg PO 4 3− /g ads ) [31].The obtained data fitted the Freundlich model better, due to the presence of heterogeneous adsorption sites, while it is obvious that the AquAsZero adsorbent material presented the optimum adsorption capacity (Q), with Q max 60 mg PO 4 3− /g ads , whereas Q 3 , which was calculated to comply with the regulation limit of 3 mg PO 4 3− /L, was found to be 57.4 mg PO 4 3− /gads.It is remarkable that the adsorption was favored by the co-existing ions, showing a better adsorption capacity when the adsorbents dispersed in the NSF water matrix.The amorphous FeOOH and AquAsZero presented a lower PZC and higher C total (mmol [OH − ]/g), as presented in Table 1, and showed a superior exchange capacity compared to the crystalline adsorbents GEH and Bayoxide.The adsorption was favored by the charge on the adsorbent surface and the SO 4 2− content (crystalline or adsorbed), as reported by Kalaitzidou et al., 2020 [31,40].The higher presence of crystalline SO 4 2− existing in FeOOH, which was 5.4 wt% as compared to the respective value of AquAsZero 1.5 wt%, was the main factor that hindered adsorption when there were co-existing ions.For this reason, AquAsZero presented the optimum sorption results [40].

Statistical Analysis
During the PhoReSe project, as reported by Raptopoulou et al., 2016, the statistical results showed that notable variations took place regarding the phosphorus content, especially during the summer period in the secondary treated wastewater effluent, indicating the need for additional post-treatment [39].

Adsorption Capacity
The results evaluation of the adsorbents' efficiencies, as reported by Kalaitzidou et al., 2016, showed the superiority of iron-based adsorbents, as compared to the lower adsorption/ion exchange capacity of the Purolite A200 EMBCL material [31].Figure 1 presents the data fittings, applying the Freundlich and Langmuir isotherm models by plotting the adsorption capacity Q of the material versus the residual concentration (CResidual) after the adsorption experiment, and shows the major findings regarding the high iron-based adsorbents' removal efficiencies.Data for Purolite A200 EMBCL are not included in this plot due to the near-zero adsorption capacity found (0.5 mg PO4 3− /gads) [31].The obtained data fitted the Freundlich model better, due to the presence of heterogeneous adsorption sites, while it is obvious that the AquAsZero adsorbent material presented the optimum adsorption capacity (Q), with Qmax 60 mg PO4 3− /gads, whereas Q3, which was calculated to comply with the regulation limit of 3 mg PO4 3− /L, was found to be 57.4 mg PO4 3− /gads.It is remarkable that the adsorption was favored by the co-existing ions, showing a better adsorption capacity when the adsorbents dispersed in the NSF water matrix.The amorphous FeOOH and AquAsZero presented a lower PZC and higher Ctotal (mmol [OH − ]/g), as presented in Table 1, and showed a superior exchange capacity compared to the crystalline adsorbents GEH and Bayoxide.The adsorption was favored by the charge on the adsorbent surface and the SO4 2− content (crystalline or adsorbed), as reported by Kalaitzidou et al., 2020 [31,40].The higher presence of crystalline SO4 2− existing in FeOOH, which was 5.4 wt% as compared to the respective value of AquAsZero 1.5 wt%, was the main factor that hindered adsorption when there were co-existing ions.For this reason, Aq-uAsZero presented the optimum sorption results [40].For a better evaluation of AquAsZero, the effects of different pH values were also examined, i.e., pH 6, 7, and 8, with the optimum adsorption results shown for the lower pH value.Moreover, the effect of temperature on the adsorption of phosphate onto the iron-based adsorbents was thoroughly examined, and an exothermic process was reported when the adsorption took place in the distilled water matrix, while an endothermic process was found for almost all examined iron-based adsorbents, except of Bayoxide, when the matrix was the NSF water.The difference in thermodynamics can be attributed to the co-existing ions in the NSF water matrix that favored the adsorption of phosphates onto the iron-based adsorbents.Higher temperatures seemed to favor phosphate removal by adsorption onto iron oxyhydroxides, with 90% removal occurring after 1 h of contact time.The physisorption mechanism, as previously reported by Kalaitzidou et al., 2016 and2022, resulted in the efficient/easy desorption of the adsorbed phosphates onto AquAsZero, due to the weak forces that were affected by the alkaline environment that shifted the equilibrium to finally desorb the adsorbed phosphates [31,40].
The removal/separation of phosphates by adsorption onto the iron-based adsorbents was also investigated with the continuous running of Rapid Small-Scale Column Tests (RSSCTs), which can more efficiently simulate full-scale applications than the previous batch tests, by using as a matrix the secondary treated effluent from the WWTP "AINEIA", which, according to the statistical variation results of Table 1, can contain about 12-17 mg of PO 4 3− /L.The breakthrough curves for the first cycle of adsorption resulted in 34, 31, 19, and 15 mg PO 4 3− /g ads for the materials AquaAsZero, FeOOH, Bayoxide, and GEH, respectively, as calculated for the regulation discharge limit of 3 mg PO 4 3− /L.As expected, the different chemical contents of this aqueous matrix negatively affected the adsorption procedure [31].

Chemical Precipitation
The examined phosphate concentration for the preliminary chemical precipitation experiments was approximately 10 mg PO 4 3− /L.The chemical precipitation of phosphorus, due to the addition of magnesia (MgO), calcium hydroxide (Ca(OH) 2 ), or huntite (Ca.Mg 3 (CO 3 ) 4 ), requires an alkaline environment; hence, a pH greater than 10 was applied to achieve the required efficient capture/removal of phosphorus, and the residual concentration had to be lower than the set regulation limit of 3 mg/L, by using all examined water matrixes, i.e., potable or distilled water and wastewater (secondary effluent).For the same matrix, the addition of 5 mg Al 3+ /L in the pH range of 6-8 could achieve removals lower than the required regulation limit of 3 mg PO 4 3− /L, which, however, increased significantly for even higher pH values due to the corresponding increase in aluminum solubility.Moreover, this also occurred for the addition of 7.5 mg Fe 3+ /L and the reaction ratios of 0.5:1 for Al 3+ :PO 4 3− and 0.75:1 for Fe 3+ :PO 4 3− , as estimated for the optimum removal at pH 7 (Figure 2).Additionally, the temperature also affected the PO 4 3− removal, as shown in Figure 3, with the temperature increase slightly favoring removal for the cases of Ca(OH) 2 or Ca.Mg 3 (CO 3 ) 4 additions, while slightly decreasing removal for the rest of theexamined chemical agents.In addition, the formed precipitates contained almost 50.1% PO 4 3− for the case of the Ca(OH) 2 addition, whereas only 1.2% PO 4 3− for the case of Ca.Mg 3 (CO 3 ) 4 and 19.0%PO 4 3− for MgO, 8.7% PO 4 3− for NaAlO 2 , and 27.3% PO 4 3− for FeClSO 4 , when the experimental matrix was distilled water [41].

Pilot-Plant Operation
Similar to the RSSCTs results, an adsorption capacity of 32 mg PO 4 3− /g ads , as calculated for the regulation discharge limit of 3 mg PO 4 3− /L, was reported for the following pilot-scale implementation (Figure 4) by using AquAsZero as the adsorbent medium.The pilot-plant successfully processed the secondary treated effluent from the WWTP "AINEIA" both in the adsorption process with an Empty Bed Velocity (EBV) of 10 m/h, as well as in the desorption mode with an EBV of 5-20 m/h (seven adsorption and six desorption cycles for the pilot plant were performed), where continuous adsorption-desorption cycles with an approximately 20% loss of adsorption sites (and efficiency) between the first and second cycle and 10% more loss between the subsequent adsorption cycles were noticed.This capacity loss can be attributed to the phosphates strongly bound onto the iron oxy hydroxides' adsorption sites that could not be easily desorbed during the subsequent treatment cycle [31,38,42,48].More specifically, even though during the RSSCTs experiments, the desorption succeeded by applying the NaOH solution at the pH of 12.5 ± 0.2 achieved in approximately 3 h to complete the adsorbents' regeneration, permitting the reuse of the adsorbent for the pilot-plant operation, the requirement of a regeneration optimization process was reflected in the non-expected reduction in the adsorption capacity during the subsequent operation cycles, as shown in Figure 5.An even higher pH value (12.8 ± 0.2) was applied in the pilot plant to optimize the regeneration process at the fourth regeneration cycle, favoring the adsorption capacity after the fifth operation cycle.Besides the loss of adsorption capacity, this regeneration process is vital for the economic viability of the overall recovery process [43].

Fertilizer
The alkaline solution from the desorption process was collected in the sedimentation tank, as shown in Figure 4, where, according to the preliminary chemical precipitation experiments, the contained phosphates can be best precipitated and separated with the presence of Ca 2+ or Mg 2+ addition of salts.Since the precipitate was intended to be finally used as a fertilizer, the maximum PO4 3-content in the precipitate was desired; therefore, Ca 2+ was selected for this process.The non-toxic precipitate, as collected from the pilotplant application (Figure 6) for the six adsorption-desorption cycles, was approximately

Fertilizer
The alkaline solution from the desorption process was collected in the sedimentation tank, as shown in Figure 4, where, according to the preliminary chemical precipitation experiments, the contained phosphates can be best precipitated and separated with the presence of Ca 2+ or Mg 2+ addition of salts.Since the precipitate was intended to be finally used as a fertilizer, the maximum PO4 3-content in the precipitate was desired; therefore,

Fertilizer
The alkaline solution from the desorption process was collected in the sedimentation tank, as shown in Figure 4, where, according to the preliminary chemical precipitation experiments, the contained phosphates can be best precipitated and separated with the presence of Ca 2+ or Mg 2+ addition of salts.Since the precipitate was intended to be finally used as a fertilizer, the maximum PO 4 3− content in the precipitate was desired; therefore, Ca 2+ was selected for this process.The non-toxic precipitate, as collected from the pilotplant application (Figure 6) for the six adsorption-desorption cycles, was approximately 5 kg and presented a high phosphate content (35.4 wt.%) that could be easily subsequently applied as a phosphate-based fertilizer, mostly in acidic soils, where the phosphate release is expected to be better.At the same time, no adverse effects regarding the plants' growth may be caused, as reported during the phytotoxicity investigation [31,38].

PhoReSe Significance
Comparing PhoReSe with the relevant REM NUT ® process, which also uses adsorption onto the Cu-form resin Dowex M-4195, a rather expensive adsorbent, and uses the secondary effluent from Massafra (S.E.Italy) Wastewater Treatment Plant as a matrix in the respective pilot plant, it becomes obvious that the AquAsZero adsorbent can treat greater volumes of the secondary wastewater, noting that both comparatively examined effluents contain similar concentration of phosphates; hence, PhoReSe presents, in this way, a lower cost for the whole treatment process [50].Like the PhoReSe project, the secondary wastewater effluent from another municipal sewage treatment plant was treated at the pilot scale by applying phosphate adsorption onto the ZnFeZ material, which also subsequently subjected to desorption and finally, phosphorus precipitated as struvite.Even though the obtained results in this case study seem quite promising, nevertheless, the operational cost is elevated due to the additional magnetic separator required, whereas that process is operated in batch mode (another disadvantage) [51].Additionally, a similar adsorption process was reported using the HAIX-Layne RT (HAIX) resin for the phosphate anion exchange from the secondary treated wastewater, with a removal capacity of 4.1 mg PO4 3− /gresin, which is significantly lower than the capacity found for Aq-uAsZero (32 mg PO4 3− /gads) [52].
The phosphorus content in the municipal waste streams corresponds to the largest percentage (approximately 23.6%) of the whole lost phosphorus from the respective cycle of use, being, at the same time, the most promising recovery option from secondary sources [53].Effective circulation from the wastewater streams to finally reach (and fertilize) the soil is of high importance, since phosphorus deposits are being quickly depleted worldwide [54,55].Therefore, sustainable strategies based on the best available technique should be applied to recover the disposed phosphorus from secondary wastewater treated effluents as an added-value material (fertilizer) that will release phosphorus again in the agricultural soil, partially closing, in this way, the respective material balance and reducing the actual phosphorus needs from virgin (mineral) sources [56].
Future work should be oriented to the optimization of fertilizer production by reduc-

PhoReSe Significance
Comparing PhoReSe with the relevant REM NUT ® process, which also uses adsorption onto the Cu-form resin Dowex M-4195, a rather expensive adsorbent, and uses the secondary effluent from Massafra (S.E.Italy) Wastewater Treatment Plant as a matrix in the respective pilot plant, it becomes obvious that the AquAsZero adsorbent can treat greater volumes of the secondary wastewater, noting that both comparatively examined effluents contain similar concentration of phosphates; hence, PhoReSe presents, in this way, a lower cost for the whole treatment process [50].Like the PhoReSe project, the secondary wastewater effluent from another municipal sewage treatment plant was treated at the pilot scale by applying phosphate adsorption onto the ZnFeZ material, which also subsequently subjected to desorption and finally, phosphorus precipitated as struvite.Even though the obtained results in this case study seem quite promising, nevertheless, the operational cost is elevated due to the additional magnetic separator required, whereas that process is operated in batch mode (another disadvantage) [51].Additionally, a similar adsorption process was reported using the HAIX-Layne RT (HAIX) resin for the phosphate anion exchange from the secondary treated wastewater, with a removal capacity of 4.1 mg PO 4 3− /g resin , which is significantly lower than the capacity found for AquAsZero (32 mg PO 4 3− /g ads ) [52].The phosphorus content in the municipal waste streams corresponds to the largest percentage (approximately 23.6%) of the whole lost phosphorus from the respective cycle of use, being, at the same time, the most promising recovery option from secondary sources [53].Effective circulation from the wastewater streams to finally reach (and fertilize) the soil is of high importance, since phosphorus deposits are being quickly depleted worldwide [54,55].Therefore, sustainable strategies based on the best available technique should be applied to recover the disposed phosphorus from secondary wastewater treated effluents as an added-value material (fertilizer) that will release phosphorus again in the agricultural soil, partially closing, in this way, the respective material balance and reducing the actual phosphorus needs from virgin (mineral) sources [56].
Future work should be oriented to the optimization of fertilizer production by reducing the CaCO 3 content in the precipitate.Moreover, a combination of proper materials, such as the serpentinized solid wastes from Grecian Magnesite SA mining operations, could be also used as Mg 2+ sources, recycling two different types of wastes and belonging in the general context of sustainability [57].A supplementary addition that can be also made in the pilot set-up is that of a second adsorption column operating in parallel to the first one, treating the effluent continuously, even when one of these columns is under the regeneration mode.

Conclusions
Phosphorus, due to depletion problems, is considered to be a finite resource, alongside the ecological vulnerability of receiving water bodies towards the presence of excess phosphorus, causing eutrophication problems.This has led to the investigation of post-treatment for the secondary effluents from municipal WWTPs, considering both the phosphorus removal and recovery options.Initially, the statistical variations in phosphorus content in an operation year were determined as 12-17 mg PO 4 3− /L.During the performed (preliminary) chemical precipitation experiments, it was noticeable that the removal of phosphorus could be enhanced by the co-presence in the water to be treated with wastewater common ions; the precipitation process can also be positively influenced (after the desorption/regeneration process) by an increase in alkalinity, rather than by the excess addition of chemical reagents.Calcium and magnesium compounds can effectively remove phosphates when the solution pH value is greater than 9, whereas iron and aluminum salts can effectively remove phosphorus in the pH range of 6-9.
Summarizing, the PhoReSe project achieved effective phosphorus recovery from low-concentrated treated wastewater effluent (in the range of few mg/L) in the form of a fertilizer that can re-introduce phosphorus into agricultural cropland.It is of high importance that the circular economy loop of phosphorus can be (partially) closed, a fact that presents important benefits, such as an increased availability of phosphorusbased fertilizers that, nowadays, face severe scarcity issues, also reducing respected high fertilizers' cost, as well as the potential to prevent eutrophication problems, since the load of phosphorus in the disposal of post-treated effluents can be significantly lower.
The adsorption evaluation during laboratory-scale experiments pointed out that the AquAsZero material presents the optimum adsorption capacity of 34 mg PO 4 3− /L, an efficiency also verified during the subsequent pilot-scale experiments (32 mg PO 4 3− /L), as calculated for the current disposal regulation limit of 3 mg PO 4 3− L; hence, this indicated the proper design of the examined process and implied its suitability for full-scale application at the operating pH of around 7. Both the (small) columns (at the laboratory scale) and the larger ones at the pilot plant could be effectively regenerated by using the NaOH solution at pH > 12.5, resulting in an enriched PO 4 3− solution that could be further treated by adding Ca 2+ as the chemical agent to precipitate phosphorus, producing the respective fertilizers that could contribute to the sustainable/proper management of phosphorous.As presented, the PhoReSe project showed a high removal capacity (32 mg PO 4 3− /g ads ) and recovery efficiency (>95%), resulting in the superiority of the examined treatment process in comparison with other (relevant) pilot-scale plants previously presented in the literature.The recovered phosphorus can effectively re-enter the phosphorus cycle as a fertilizer, which is produced by using the desorption effluent, adding Ca(OH) 2 , which leads to the phosphate fertilizer.The significance of this project is the potential of its implementation for treated secondary wastewater effluents from municipal WWTPs, a fact that is of great environmental importance, as it complies with the current regulation limits and is considered as economically feasible, unlike most other implemented technologies.The realization of this project is in accordance also with green chemistry and engineering

Figure 1 .
Figure 1.Comparison of phosphates' adsorption efficiency using several adsorbents in NSF water matrix at pH 7 and 20 • C.

Figure 4 .
Figure 4.The pilot-plant plc operation; (a) pre-treatment tank with hollow-fiber commercial membranes for the preliminary TSS removal, (b) the adsorption-desorption column, (c) the solution tanks (containing NaOH and CaCl2), and (d) the precipitation tank.

Figure 5 .
Figure 5. Adsorption capacity of phosphorus for the pilot-plant experiments after the subsequent operation cycles.

Figure 4 .
Figure 4.The pilot-plant plc operation; (a) pre-treatment tank with hollow-fiber commercial membranes for the preliminary TSS removal, (b) the adsorption-desorption column, (c) the solution tanks (containing NaOH and CaCl 2 ), and (d) the precipitation tank.

Figure 4 .
Figure 4.The pilot-plant plc operation; (a) pre-treatment tank with hollow-fiber commercial membranes for the preliminary TSS removal, (b) the adsorption-desorption column, (c) the solution tanks (containing NaOH and CaCl2), and (d) the precipitation tank.

Figure 5 .
Figure 5. Adsorption capacity of phosphorus for the pilot-plant experiments after the subsequent operation cycles.

Figure 5 .
Figure 5. Adsorption capacity of phosphorus for the pilot-plant experiments after the subsequent operation cycles.

Table 2 .
Main chemical characteristics (parameters) of Thessaloniki's tap (potable) water, NSF water, and of the WWTP AINEIA secondary treated effluent used in the experiments.

Table 2 .
Cont.According to the requirements of the respective experiment the pH values of NSF water adjusted respectively.