A novel approach using ureolytic induced MAP formation, for the recovery of phosphate, has been economically evaluated. The ureolytic MAP crystallizationon has been tested on anaerobic effluent of a potato processing company in a pilot plant, with MgCl2.6H2O as magnesium source. The pilot plant showed a high phosphate removal efficiency of 82 ± 9 %, resulting in a final effluent concentration of 13 ± 7 mg/L PO4-P. XRD analyses confirmed the presence of struvite in the precipitate. During operation pH and the molar magnesium : ammonium : phosphate ratio are the most important operational parameters influencing MAP crystallization. Results show that for high phosphate concentrations in wastewater (e.g. 100 mg/L PO4-P) the ureolytic phosphate precipitation is a cost effective method (6.1 € kg-1 Premoved). Moreover, the technique is competitive with the chemical phosphate precipitation of struvite (6.2 € kg-1 Premoved)

Desmidt, Evelyn

Ghyselbrecht, Karel

Maes, Betty

Meesschaert, Boudewijn

Monballiu, Annick

Verstraete, Willy

Ghent University Academic Bibliography

Economic Evaluation of the Precipitation of Phosphate as Struvite at Pilotscale    E. Desmidt*, K. Ghyselbrecht*, A. Monballiu*, B. Maes*, W. Verstraete*** and B.D. Meesschaert*/**  * Department of Industrial Science and Technology, Katholieke Hogeschool Brugge-Oostende, Associated to the Katholieke Universiteit Leuven, Zeedijk 101, Ostend, Belgium  ** Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems (M2S), Faculty of Bio-engineering Sciences, Katholieke Universiteit Leuven, – Heverlee, Belgium *** Laboratory of Microbial Ecology and Technology (LabMET), Faculty of Bio-engineering Sciences, University of Ghent, Belgium    Abstract A novel approach using ureolytic induced MAP formation, for the recovery of phosphate, has been economically evaluated. The ureolytic MAP crystallizationon has been tested on anaerobic effluent of a potato processing company in a pilot plant, with MgCl2.6H2O as magnesium source. The pilot plant showed a high phosphate removal efficiency of 82 ± 9 %, resulting in a final effluent concentration of 13 ± 7 mg/L PO4-P. XRD analyses confirmed the presence of struvite in the precipitate.  During operation pH and the molar magnesium : ammonium : phosphate ratio are the most important operational parameters influencing MAP crystallization. Results show that for high phosphate concentrations in wastewater (e.g. 100 mg/L PO4-P) the ureolytic phosphate precipitation is a cost effective method (6.1 € kg-1 Premoved). Moreover, the technique is competitive with the chemical phosphate precipitation of struvite (6.2 € kg-1 Premoved). Keywords: Phosphate removal; nutrient recovery; ureolytic activity   INTRODUCTION  Phosphorus is an important element, making a major contribution to agricultural and industrial development, but its release to natural water bodies is one of the main causes of eutrophication. It is desirable that water treatment facilities remove phosphorus from the wastewater before it is returned to the environment. In most countries, total removal or at least a significant reduction of phosphorus is obligatory, but is not always fulfilled. In recent times two new processes have been developed for P precipitation from wastewater, the calcium phosphate crystallization process and the magnesium ammonium phosphate (MAP) crystallization process. (Doyle and Parsons, 2002). The recovered products can be used as the alternative of agricultural fertilizer or as the phosphate rock for phosphate industry (Li and Zhao, 2003; de-Bashan and Bashan, 2004).  Anaerobically treated wastewater mostly contains large amounts of phosphate and ammonium.  For the recovery of phosphate as struvite an increase of pH and the addition of a magnesium source is necessary. When dealing with anaerobically treated wastewaters, pH increase can be achieved by simple air stripping. If air stripping does not suffice, additional alkaline reagent must be added, e.g. sodium hydroxide, although this alkaline addition can be combined with the supplementation of magnesium using either MgO or Mg(OH).    Corresponding authors, Tel: +32 59 56 90 53; fax: +32 59 56 90 01 E-mail address: boudewijn.meesschaert@khbo.be; evelyn.desmidt@khbo.be   An alternative process is the ureolytic MAP crystallization. In this process pH is increased by means of bacterial urease activity. Ureolytic activity decomposing urea in to ammonium leads to MAP formation when phosphate and Mg2+-ions are present. (Meesschaert et al., 2007; Carballa et al., 2008; Desmidt et al., 2009).    MATERIALS AND METHODS  Description of the pilot plant Ureolytic MAP crystallization was tested on anaerobically treated wastewaters in a pilot plant (figure 1). Influent was fed into a compressor-connected stripper by a centrifugal pump P1. MgCl2.6H2O (14.2 %) and urea (4%) were dosed with volumetric pumps P2 and P3 to the crystallization reactor. Influent flow rates to the stripper varied between 0.3 and 0.35 m³h-1, which resulted in a hydraulic retention time of around 0,7 hours. The effluent of the stripper flowed over to the crystallization reactor, provided with ureolytic sludge. The HRT varied between 2.9 and 3.3 hours. The effluent of the crystallisation reactor flowed to a settler, where separation between effluent and solids occurred.  The heavy fraction of the struvite precipitate remained in the settler while the sludge and the smaller struvite crystals flowed over to the decantor and were recirculated to the crystallization reactor after precipitation.    Figure 1. Scheme of the pilot plant used for ureolytic MAP crystallization  Wastewater characteristics The pilot plant was fed with the effluent from an upflow anaerobic sludge bed (UASB) reactor from a potato processing industry. Table 1 shows the main characteristics of this wastewater, which corresponds with the influent of the MAP crystallizator.      Analytical methods pH was measured with a pH meter (Mettler Toledo seven multi). Dissolved oxygen was measured with an oxygen meter Oxi 315 (WTW). Ammonium and magnesium were determined with a Dionex DX-100 chromatograph equipped with a conductivity detector. Phosphate was measured with a Dionex series 4500i ion chromatography equipped with a conductivity detector. Dry matter content and its mineral fraction were determined according to standard methods (Greenberg et al, 1992). Urea was determined colorimetically with diacetyl monoxime. XRD analyses were done by using a Stoe Stadi P unit with Cu Kα radiation at 0.1541 nm with an image plate as detector.   Determination of the ureolytic activity For the determination of the ureolytic activity 20 ml of sludge was taken from the crystallization reactor and washed several times with phosphate buffer by centrifugation during 15 minutes at 5000 g. After washing the sludge was suspended in 95 ml of salt solution (0.5 g KHCO3 and 0.5 NaCl suspended in 330 ml tap water and 660 ml demineralised water) and 5 ml of a 2000 mg.L-1 urea-nitrogen solution. The solution was incubated and shaked at 30°C. During 2 hours samples were taken and analyzed for ammonium by ion chromatography.   RESULTS AND DISCUSSION The ureolytic MAP crystallization has been tested on anaerobic effluent of a potato processing company in a pilot plant, with a reactor volume of 1 m³, during several months. MgCl2.6H2O was used as magnesium salt during this process, because of the higher solubility compared to MgO and MgOH (Münch and Barr, 2001). During operation pH and the molar magnesium : ammonium : phosphate ratio are the most important operational parameters influencing MAP crystallization (Song et al., 2007).  The characteristics of the influent, stripper and effluent, during ureolytic MAP crystallization, of a potato processing wastewater are given in table 1.   Table 1. Characteristics of the influent, stripper and effluent obtained during MAP crystallization of a potato processing wastewater  Parameter Influent Stripper Effluent pH 7.41 ± 0.31 7.75 ± 0.27 8.04 ± 0.26 NH4+(mg/L) 432 ± 76 415 ± 64 462 ± 66 Mg2+ (mg/L) 48 ± 16 41 ± 16 130 ± 14 PO4-P (mg/L) 99 ± 40 78 ± 30 13 ± 7 Inorganische Carbon IC (mg/L) 785 ± 165 765 ± 160 800 ± 145  From the results given in table 1 it is clear that some MAP-formation occurred in the stripper, thus by simple aeration of the influent. The pilot plant showed a high phosphate removal efficiency of 82 ± 9 %, resulting in a final effluent concentration of 13 ± 7 mg/L PO4-P. During this period the added molar ratios of magnesium and urea-N were 2.0 ± 0,8 Mg.P-1 and 2.4 ± 0.9 urea-N.P-1 (phosphate concentrations in the influent). In the past the pilot plant has been running at higher pH 8.4 ± 0.2  (data not shown), which resulted in an even higher phosphate removal efficiency of 92 ± 7 % and a final effluent concentration of 5.9 ± 5.5 mg/L PO4-P. During this period the added molar ratios of magnesium and urea-N were 2.1 ± 0.8 Mg.P-1 and 3.3 ± 0.7 urea-N.P-1. So, a higher ammonium : phosphate ratio also results in a higher phosphate removal efficiency.    Figure 2. Phosphate concentration in influent (■), effluent (□) and phosphate removal (line) during ureolytic MAP crystallization     To evaluate the activity of the micro-organisms, urea concentration and ureolytic activity were tested during the operation. The mean value of the volatile suspended solids and specific ureolytic activity were respectively 7.8 g.L-1 and 0.30 g NH4-N.g VSS-1d-1 (figure 3). This corresponds with an ureolytic activity of 0.056 mM urea.min-1, which is comparable with the ureolytic activity determinated by Hammes et. al during the microbial CaCO3 precipitation (Hammes et al., 2003). In addition no urea was found in the effluent of the treated wastewater. XRD analyses confirmed the presence of struvite in the precipitate.      Figure 3. Specific ureolytic activity (line) and volatile suspended solids (VSS) (□) during ureolytic MAP crystallization  01020304050607080901000204060801001201 15 18 21 25 28 30 39 42 46 49 51 56 60 72 74 78 80% Phosphate r emovalConcentration PO4-P (mg L-1 )Time (days)00,20,40,60,811,21,41,61,82024681012143 17 23 37 44 51 58 72 86Specific ureolytic activity (g NH4-N gVSS-1.d-1)VSS (g L-1)Time (days) The cost for the ureolytic MAP crystallization according to an influent phosphate-phosphorus concentration of 100 mg/L is given in table 2 and is estimated at 6.1 € kg-1 Premoved. The cost for struvite precipitation by air stripping and addition of MgCl2 and NaOH is estimated at 6.2 € kg-1 Premoved (Carballa et. al., 2008; personal communication). So, the ureolytic MAP crystallization is competitive with the chemical phosphate precipitation as struvite.    Table 2. Cost estimation of ureolytic and air stipping/MgCl2/NaOH MAP crystallization (Carballa et. al., 2008; personal communication) according to influent phosphate-phosphorus concentrations of 100 mg/L.  Ureolytic MAP crystallization Air stripping/ MgCl2/NaOH MgCl2 (€ kg-1 Premoved) 2.6 2.6 Urea (€ kg-1 Premoved) 2.6 - NaOH (€ kg-1 Premoved) - 3 Energy and sludge (€ kg-1 Premoved) 0.9 0.6 Total (€ kg-1 Premoved) 6.1 6.2   CONCLUSIONS Phosphate recovery by MAP crystallization is a alternative way of removing phosphate from wastewaters, especially for anaerobically treated wastewater. During operation pH and molar magnesium : ammonium : phosphate ratio are important. The optimal pH is around 8.5 although results show also a good removal efficiency at a pH of 8. Increasing the ammonium : phosphate ratio resulted in a better removal efficiency.  The ureolytic MAP crystallization requires, besides a magnesium source, the addition of urea. During operation no urea was found in the effluent, meaning complete hydrolysis of urea by the sludge, present in de crystallization reactor.   Results show that for high phosphate concentrations in wastewater (e.g. 100 mg/L PO4-P) the ureolytic phosphate precipitation is a cost effective method. Moreover, the technique is competitive with the chemical phosphate precipitation as struvite (table 2).    ACKNOWLEDGMENTS We wish to thank the IWT–Vlaanderen (Instituut voor de Aanmoeding van Innovatie door Wetenschap en Techniek; = IWT-Flanders, The Institute for the promotion of Innovation by Science and Technology in Flanders; HOBU-grand 30089), VITO (Vlaamse Instelling voor Technologisch Onderzoek; MIP-project EDROR-RENUD), VeGeBe (Vereniging Groenteverwerkende Bedrijven en industriegroenten = Belgian association for vegetable processing and trade in industrial vegetables), BELGAPOM (Belgische aardappelhandel en -verwerking = Belgian federation for the potato trade and processing) and Agristo NV for financial support.   REFERENCES Carballa, M., Moerman, W., De Windt, W., Grootaerd, H. and Verstraete, W. (2008) Strategies to optimize phosphate removal from industrial anaerobic effluents by magnesium ammonium phosphate (MAP) production. J Chem. Technol. Biotechnol., 84, 63-68. De-Bashan, L.E., Bashan, Y. (2004) Recent advances in removing phosphorus from wastewater and its future use as fertilizer (1997-2003). Water Res., 38, 4222-4246. Desmidt, E., Verstraete, W., Dick. J, Meesschaert, B.D., and Carballa, M. (2009) Ureolytic phosphate precipitation from anaerobic effluents. Wat. Sci. Tech., 59 (10), 1983-1988. Doyle, J.D., Parsons, S.A. (2002) Struvite formation, control and recovery. Water Res., 38, (3925-3940).   Greenberg, A.E., Clesceri, L.S. and Eaton, A.S. (1992) Standard methods for the examination of water and wastewater. American Public Health Association, Washington DC. Hammes, F., Seka, A., van Hege, K., van de Wiele, T., Vanderdeelden, J., Sicilliano, S. and Verstraete, W. (2003) Calcium removal from industrial wastewater by bio-catalytic CaCO3 precipitation.  J Chem. Technol. Biotechnol., 78, 670-677. Li, X.Z., Zhao, Q.L. (2003) Recovery of ammonium-nitrogen from landfill leachate as a multi-nutrient fertilizer. Ecol. Eng., 20, 171-181. Meesschaert, B.D., Desmidt, E., Dick, J. and Verstraete, W. (2007) Ureolytic phosphate precipitation from wastewater. In: Programme and abstract book 6th IWA Specialist conference on Wastewater Reclamation and Reuse for Sustainability (6th WRRS): Guiding the growth of water reuse, Antwerp, pp. 49. Münch E.V. and Barr K. (2001) Controlled struvite crystallization for removing phosphorus from anaerobic digester sidestreams. Wat. Res. 35(1), 151-159. Song Y., Yuan P., Zheng B., Peng J.,Yuan F. and Goa Y.   (2007) Nutrients removal and recovery by crystallization of magnesium ammonium phosphate from synthetic swine wastewater. Chemosphere, 69, 319-324.      

Economic evaluation of the precipitation of phosphate as struvite at pilotscale

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Economic evaluation of the precipitation of phosphate as struvite at pilotscale

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