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Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Struvite formation for enhanced dewaterability of digested wastewater sludge a
b
c
a
a
B.J.C. Bergmans , A.M. Veltman , M.C.M. van Loosdrecht , J.B. van Lier & L.C. Rietveld a
Faculty of Civil Engineering and Geosciences, Department of Water Management, Section Sanitary Engineering, Delft University of Technology, PO Box 5048, Delft 2600 GA, The Netherlands b
Waternet, PO Box 94370, Amsterdam 1090 GJ, The Netherlands
c
Faculty of Applied Sciences, Department of Biochemical Engineering, Section Environmental Biotechnology, Delft University of Technology, PO Box 5046, Delft 2600 GA, The Netherlands Published online: 01 Oct 2013.
To cite this article: B.J.C. Bergmans, A.M. Veltman, M.C.M. van Loosdrecht, J.B. van Lier & L.C. Rietveld (2014) Struvite formation for enhanced dewaterability of digested wastewater sludge, Environmental Technology, 35:5, 549-555, DOI: 10.1080/09593330.2013.837081 To link to this article: http://dx.doi.org/10.1080/09593330.2013.837081
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2014 Vol. 35, No. 5, 549–555, http://dx.doi.org/10.1080/09593330.2013.837081
Struvite formation for enhanced dewaterability of digested wastewater sludge B.J.C. Bergmansa∗ , A.M. Veltmanb , M.C.M. van Loosdrechtc , J.B. van Liera and L.C. Rietvelda a Faculty
of Civil Engineering and Geosciences, Department of Water Management, Section Sanitary Engineering, Delft University of Technology, PO Box 5048, Delft 2600 GA, The Netherlands; b Waternet, PO Box 94370, Amsterdam 1090 GJ, The Netherlands; c Faculty of Applied Sciences, Department of Biochemical Engineering, Section Environmental Biotechnology, Delft University of Technology, PO Box 5046, Delft 2600 GA, The Netherlands
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(Received 24 March 2013; final version received 14 August 2013 ) One of the main advantages of controlled struvite formation in digested sludge is an improvement in dewaterability of the digested sludge, which eventually leads to lower volumes of dewatered sludge that need to be transported. The effects of the control parameters for struvite formation, magnesium concentration and pH, on digested sludge dewaterability were investigated and are discussed in relation to the efficiency of struvite formation. Laboratory experiments with digested activated sludge were performed in a 20 L batch reactor. CO2 was stripped from the digested sludge using a bubble aerator and magnesium chloride was added to induce struvite formation. The dewaterability of the sludge was determined by gravity filtration tests. In the experiments, either the pH or the molar magnesium to phosphate ratio (Mg:PO4 ) was varied. The results confirm improved sludge dewaterability after struvite formation. Magnesium to phosphate ratios above 1.0 mol/mol did not further improve dewaterability. The addition of magnesium did not prevent the need for polymer addition for sludge dewatering. An increase in pH led to a deterioration in dewaterability. The best dewaterability results were found at the lowest pH value (pH = 7.0), while stirring the sludge instead of using the bubble aerator. At these settings, an orthophosphate removal of around 80% was achieved. Keywords: struvite; digested sludge; dewaterability; pH; magnesium dosing
Introduction Struvite (MgNH4 PO4 · 6H2 O) is a crystalline phosphate mineral that is formed under specific chemical conditions in solutions in which all its components (magnesium, ammonium and phosphate) are present. Stimulated struvite formation is applied to the liquid phase of anaerobically digested activated sludge for the following reasons: to prevent scaling in pipes and installations downstream, to reduce the phosphate load in the reject water that is returned to the wastewater treatment plant (WWTP) inlet and to explore whether the formed struvite can be used as a fertilizer of commercial interest.[1–4] Currently, the controlled formation of struvite is mostly applied to reject water resulting from sludge dewatering.[4,5] Struvite formation before dewatering, directly after anaerobic digestion,[6,7] has the additional advantage that scaling problems in the remainder of the sludge line are prevented. On top of that, it has been observed that struvite formation in digested sludge leads to a better sludge dewaterability.[6] An improvement in dewaterability reduces the volume of the dewatered sludge and hence the costs for transport and disposal of the sludge. The control parameters of practical interest for struvite formation in the liquid phase of digested sludge are magnesium concentration and pH. Generally, magnesium is added because it is the limiting component for crystallization in the ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
liquid phase of the digested sludge.[8] The pH value is controlled because it is directly related to the availability of the ionic struvite components magnesium (Mg2+ ), ammonium 3− (NH+ 4 ) and phosphate (PO4 ), and therefore to the solubility of struvite.[9] As struvite solubility is minimal around pH = 10 [10] and the pH of digested sludge is typically around 7,[11] the pH is generally increased, for example, by CO2 stripping. Although it is known that dewaterability of the digested sludge is influenced by magnesium addition [12] and pH,[13] the precise effects of these parameters on sludge dewaterability have not been determined. In this paper, the influence of magnesium concentration and pH on sludge dewaterability was investigated and is discussed in relation to the efficiency of struvite formation. Laboratory experiments were conducted using anaerobically digested sludge from WWTP Amsterdam West in the Netherlands.[14] The optimal process settings for both sludge dewaterability and orthophosphate removal were investigated. Materials and methods Digested sludge The digested sludge was collected from the outlet of the anaerobic digesters at WWTP West. The influent of WWTP
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West with a maximum flow of 30,000 m3 /h is biologically treated in an activated sludge system based on the Modified University of Cape Town (MUCT) principle.[15] In addition to the sludge produced at the site, WWTP West processes sludge from several external sources. All primary and secondary sludge is thickened, anaerobically digested and dewatered in decanter centrifuges. The chemical composition and physical characteristics of the digested sludge were determined during the experiments. Experimental set-up Reactor design and operation The digested sludge was treated in a batch reactor with a diameter of 20 cm and a height of 100 cm. The reactor was filled with 20 L of digested sludge and CO2 was stripped up to a pH set point value of pH = 7.4, 7.6, 7.8 and 8.0, using a fine bubble aerator operated with an airflow of 500 L/h. Magnesium chloride (33% (weight) solution) was added instantaneously at the sludge surface. The applied magnesium to phosphate ratios of Mg:PO4 = 0.5, 1.0, 1.5 and 2.0 are expressed as the molar magnesium concentration directly after dosing, i.e. the sum of the dosed magnesium and the background magnesium concentration, divided by the molar orthophosphate concentration as measured before dosing. As protons are released during the formation of struvite, the addition of magnesium led to a sudden pH decrease. After magnesium addition, CO2 was stripped from the digested sludge until the pH set point was again reached, to mimic a full-scale reactor that is operated at continuous pH. At that point, a sample was taken in the middle of the well-mixed reactor from a pre-installed sampling tap. The first flow of digested sludge was drained off to gain a representative sample. Sample analysis The samples were analysed on dry solids content (DSC) using a Sartorius MA35 DSC measuring device, and on concentrations of the components orthophosphate (PO4 –P), ammonium (NH4 –N) and magnesium. To measure the component concentrations, part of the sample was centrifuged, filtered over a 45 μm nylon Syringe filter, diluted if necessary and analysed using a Dr. Lange LASA 50 cuvette test apparatus. To determine the dewaterability before treatment, a 200 g test sample was prepared using the digested sludge sample and cationic polymer solution (0.21%) in such a way that 10 g of polymer was available per kg of dry solids. Digested sludge and polymer were intensively mixed for 3 s in a blender with a timer connected to its power supply. The mixture was instantaneously brought on a woven synthetic filter with minimal filter resistance. The amount of percolated water was measured with a balance (accuracy = 0.01 g) after 5, 10, 15, 20, 30, 40, 50, 60, 90 and 120 s. The dewaterability after treatment, i.e. CO2 stripping and/or MgCl2 dosage, was determined in the
same way and with the same mass of sludge and polymer as before treatment. This was done to assure that a change in dewaterability was not caused by a change in polymer addition. Calculation of free ionic concentrations and supersaturation Under supersaturated conditions, struvite is formed by a 3− chemical reaction between free Mg2+ , NH+ 4 and PO4 ions, with the incorporation of six H2 O molecules. In the solution, magnesium, ammonium and phosphate are present in different forms. For the calculation of struvite solubility, at least the following dissolved ionic species are 2− 3− to be considered [16]: H3 PO4 , H2 PO− 4 , HPO4 , PO4 , + − + + 2+ MgH2 PO4 , MgHPO4 , MgPO4 , MgOH , Mg , NH4 and NH3 . Considering additional soluble and solid species such as carbonate, bicarbonate and calcium phosphates increases accuracy as well as complexity of the calculation.[1] In this paper, other soluble and solid species were neglected. For each combination of pH and component concentrations that were measured during the experiments, the free ionic concentrations of magnesium [Mg2+ ], ammonium 3− [NH+ 4 ] and phosphate [PO4 ] were calculated in MATLAB by solving the system of thermodynamic equations as described by Ali and Schneider.[10] The value used for the ionic strength (I = 0.02 mol/L) was adapted from example calculations in digested sludge.[11] During the experiments the temperature of the sludge decreased from ±30◦ C to ±24◦ C. In the MATLAB model, a constant temperature of 3− 25◦ C was assumed. The product of [NH+ 4 ] and [PO4 ] was 2+ plotted against [Mg ] to obtain a graphical presentation of the equilibria reached in the experiments. Using the system of thermodynamic equations as described by Ali and Schneider,[10] the observed minimum struvite solubility product pKS0 was calculated. For the formation of pure struvite, equimolar amounts of magnesium, ammonium and orthophosphate are used. As ammonium was abundant and magnesium was added during the experiments, the efficiency of struvite formation was represented as the observed magnesium/orthophosphate removal ratio CT ,Mg : CT ,PO4 . Using the observed struvite solubility product, the average observed magnesium/orthophosphate removal ratio and the system of thermodynamic equations as described by Ali and Schneider,[10] curves were constructed in MATLAB that predict the final orthophosphate concentration as a function of pH and magnesium to phosphate ratio. For the construction of these curves, the initial orthophosphate concentration was considered equal to the average orthophosphate concentration as measured in the untreated digested sludge in the different experiments. The NH4 –N concentration was assumed constant and was calculated as the average concentration of all NH4 –N measurements before and after treatment. It was assumed that this simplification was permitted because the relative removal of NH4 –N was
Environmental Technology Table 1.
Overview of the experiments.
Experiment no. 1 2 3 4 5 6 7 8 9
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Description
Volume (L)
pH set point
Mg:PO4
Aeration (no dosing) Reference Mg:PO4 variation Mg:PO4 variation Mg:PO4 variation pH variation pH variation pH variation Stirring
20 20 20 20 20 20 20 20 0.4
7.5, 7.75, 8.0 7.6 7.6 7.6 7.6 7.4 7.8 8.0 7.0
– 1.5 0.5 1.0 2.0 1.5 1.5 1.5 1.5
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expected to be small as compared with the relative removal of orthophosphate and magnesium. This was verified after completion of the experiments. Overview of the experiments Table 1 presents an overview of the experiments. In experiment 1, the influence of pH increase without magnesium addition was investigated by aerating the sludge and analysing the samples taken at different pH values. Experiment 2 was used as a reference. In experiments 3–8, the pH and the magnesium to phosphate ratio were varied according to Table 1. In experiment 9, it was investigated whether stirring the sludge leads to better dewaterability results than using the bubble aerator. A small amount of digested sludge (0.4 L) was stirred in a beaker using a magnetic stirrer (800 rotations per minute), while magnesium was added to reach a magnesium to phosphate ratio of 1.5. The sludge was stirred for 30 min, after which it was analysed in the same way as the samples from other experiments. To investigate whether costs savings on polymer could be achieved, dewaterability tests were performed on treated sludge from the reference experiment using different polymer dosing ratios. Results Sludge dewaterability We evaluated the effect of pH change only (Figure 1), the effect of varying magnesium additions (Figure 2), the effect of magnesium addition and varying pH adjustments (Figure 3) and the effect of polymer dosage (Figure 4) on digested sludge dewaterability. The dewaterability before treatment is the average dewaterability of the raw digested sludge as found in the nine experiments. Error bars represent the standard deviation. An increase in pH by CO2 stripping without magnesium addition led to a deterioration of the dewaterability (Figure 1). This was also observed visually, as the sludge/polymer mixture, directly after intensive mixing, showed a strong floc formation at pH = 7.5, while at pH = 8.0 no floc formation was observed. At a constant pH of 7.6, achieved by CO2 stripping, the addition of magnesium led to an improvement in dewaterability (Figure 2). At a
Figure 1. Dewaterability of digested sludge after CO2 stripping to varying pH values, without magnesium dosing.
Figure 2. Dewaterability of digested sludge after magnesium addition at varying magnesium to phosphate ratios and CO2 stripping to pH = 7.6.
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Figure 3. Dewaterability of digested sludge after magnesium addition (Mg:PO4 = 1.5 mol/mol) and CO2 stripping to varying pH values, or after magnesium addition (Mg:PO4 = 1.5 mol/mol) and 30 min of stirring.
magnesium to phosphate ratio of 0.5, only a slight improvement was found. A magnesium to phosphate ratio of 1.0 led to a further improvement. Higher magnesium to phosphate ratios (1.5 and 2.0) led to the same dewaterability results as a magnesium to phosphate ratio of 1.0. At a constant magnesium to phosphate ratio of 1.5, lower pH values led to a better dewaterability (Figure 3). For all pH values, the dewaterability after treatment improved as compared with the initial dewaterability. Stirring the sludge instead of using the bubble aerator led to the best dewaterability result. A decrease in polymer dosing led to a deterioration in dewaterability (Figure 4). Adding 80% of the standardized amount of polymer (that was added before treatment) to the treated sludge of the reference experiment resulted in a dewaterability slightly below the average dewaterability before treatment.
Struvite formation The formation of struvite was investigated by evaluating the chemical equilibria that were reached during the experiments. Table 2 presents the pH values and concentrations of magnesium, ammonium and orthophosphate that were measured in the experiments. Using these data, the theoret3− ically available concentrations [Mg2+ ], [NH+ 4 ] and [PO4 ] were calculated. Figure 5 presents the product of [NH+ 4] 2+ and [PO3− ] as a function of [Mg ]. The trend line through 4 the plotted values represents the observed struvite solubility curve and corresponds with an observed struvite solubility 0 product (pKS;observed ) of 12.41. The average observed magnesium/orthophosphate removal ratio (CT ,Mg : CT ,PO4 ) was 0.84 mol/mol. No
Figure 4. Dewaterability of the treated digested sludge from the reference experiment at varying polymer dosing ratios.
clear correlations were found between CT ,Mg : CT ,PO4 and pH, and between CT ,Mg : CT ,PO4 and Mg:PO4 . The relative removal of ammonium was only 3% on average while the relative removal of orthophosphate and magnesium was 85% and 49% on average, respectively. Using the 0 observed struvite solubility product (pKS;observed = 12.41), the observed average magnesium/orthophosphate removal ratio (CT ,Mg : CT ,PO4 = 0.84), an initial orthophosphate concentration of 318 mgP/L and a constant ammonium concentration of 871 mgN/L, curves were constructed in MATLAB that predict the final orthophosphate concentration as a function of pH and Mg:PO4 (Figure 6). Discussion Sludge dewaterability In our experiments an increase in pH without magnesium addition led to a deterioration in sludge dewaterability, which is in agreement with earlier research by Shao et al.,[17] who suggested that the deterioration in dewaterability at high pH values is caused by the shifting of proteins from the pellet and tightly bound extracellular polymeric substances (EPS) fraction to the slime fraction of the sludge, retaining a higher degree of bound water. Previous studies [12,18] emphasized the important role of divalent cations, such as magnesium, in sludge dewaterability. Therefore, it was expected that the improvement in sludge dewaterability resulting from struvite formation was predominantly caused by the increase in magnesium concentration. In a comparative study by Sobeck and Higgins [18] it was concluded that the divalent cation bridging (DCB) theory explains the role of cations best. According to the DCB theory, divalent cations bridge the negatively charged groups present on the EPS that are
Environmental Technology Table 2.
pH and component concentrations measured during the experiments.
Before treatment (average) After treatment (experiment 1–9)
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Experiment no.
pH (−)
PO4 –P (mg/L)
NH4 –N (mg/L)
Mg (mg/L)
– 1 2 3 4 5 6 7 8 9
7.1 – 7.6 7.6 7.6 7.6 7.4 7.8 8.0 7.0
318 – 13 137 62 26 25 10 8 58
900 – 853 880 787 885 855 794 819 –
25 – 142 28 79 332 179 155 151 182
Figure 5. Equilibria reached during the experiments. Dosing magnesium (a) leads to a higher magnesium concentration and hence to a lower negative logarithm of the magnesium concentration. Reaction of magnesium, ammonium and phosphate to struvite (b) decreases all component concentrations and hence increases the negative logarithm of these concentrations. Based on these equilibria, the observed 0 struvite solubility product was calculated (pKS;observed = 12.41).
Figure 6. Prediction of the final orthophosphate concentration as a function of pH and Mg:PO4 for pKS0 = 12.41, CT ,Mg : CT ,PO4 = 0.84, PO4 –Pinitial = 318 mgP/L, NH4 –N = 871 mgN/L and T = 25◦ C.
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produced by microorganisms. This bridging helps to aggregate and stabilize the matrix of EPS and microorganisms, thereby promoting floc formation, hence improving sludge dewaterability.[12] In our experiments, adding magnesium chloride to the digested sludge at a constant pH (pH = 7.6) led to an improvement in sludge dewaterability. However, at magnesium to phosphate ratios above 1.0 mol/mol no significant further improvement was observed. This implies that the improvement in sludge dewaterability might have been caused by a decrease in orthophosphate concentration or by precipitant formation rather than by an increase in magnesium concentration. Moreover, it was observed that the addition of magnesium chloride could not compensate for the need of polymer addition. As the working principle of (cationic) polymer addition [11] is similar to the working principle of divalent cations according to the DCB theory, the persistent need for polymer addition was considered an additional proof that the working principle of struvite formation for better sludge dewaterability differed from the working principle of polymer addition and that the mechanisms of these working principles can work parallel to each other. In line with the above, the best dewaterability result was found using a magnesium to phosphate ratio above 1.0 mol/mol (in this case Mg:PO4 = 1.5 mol/mol) at the lowest pH (pH = 7). This relatively low pH was achieved by stirring the sludge instead of using the bubble aerator.
dewaterability result (Mg:PO4 = 1.5 mol/mol, pH = 7), around 80% of orthophosphate was removed.
Struvite formation Many researchers report on the value of the standard struvite solubility product, with pKS0 values ranging from 12.6 to 13.36.[19] These values, which are determined exclusively in chemically well-defined matrices and at relatively low ionic strengths, are higher than the negative logarithm of the observed struvite solubility product as derived from 0 our experiments (pKS;observed = 12.41). This means that in our experiments less struvite was formed than expected based on known pKS0 values. Similar results are described in research on calcium carbonate precipitation during anaerobic digestion by Langerak et al.,[20] who concluded that the difference between the actually prevailing solubility product and the thermodynamic value can be attributed to kinetic inhibition of crystal formation by wastewater constituents. As the magnesium/orthophosphate removal ratio CT ,Mg : CT ,PO4 was below 1 mol/mol (0.84 mol/mol on average), the experimental results suggest that the formed precipitate did not solely consist of struvite. Phosphate could have also been removed by other reactions, forming different precipitates such as hydroxyapatite (Ca5 (PO4 )3 OH),[21] which has a relatively high pKS0 value of 57.5.[22] At all process settings, a significant orthophosphate removal was achieved, varying from 57% (Mg:PO4 = 0.5 mol/mol, pH = 7.6) to 97% (Mg:PO4 = 1.5 mol/mol, pH = 8.0). At the process settings that led to the best
References
Conclusions In this paper, the influence of magnesium concentration and pH on sludge dewaterability was investigated and was discussed in relation to the efficiency of struvite formation. The main conclusions are shown below: An increase in pH without magnesium addition led to a deterioration in digested sludge dewaterability. At a constant magnesium to phosphate ratio of 1.5 mol/mol, lower pH set points led to better sludge dewaterability results. The addition of magnesium to digested sludge improved the sludge dewaterability. However, magnesium to phosphate ratios above 1.0 mol/mol did not further improve dewaterability and the addition of magnesium could not compensate for the need of polymer addition. Therefore, it seemed that the working principle of struvite formation for better sludge dewaterability differed from the working principle of polymer addition and that the mechanisms of these working principles can work parallel to each other. The best sludge dewaterability result was found at pH = 7.0 and a magnesium to phosphate ratio above 1.0 mol/mol (in this case Mg:PO4 = 1.5 mol/mol), while stirring the sludge instead of using a bubble aerator. At these settings, an orthophosphate removal of about 80% was achieved.
[1] Hanhoun M, Montastruc L, Azzaro-Pantel C, Biscans B, Frèche M, Pibouleau L. Temperature impact assessment on struvite solubility product: a thermodynamic modelling approach. Chem Eng J. 2011;167:50–58. [2] Berg U, Knoll G, Kaschka E, Weidler PG, Nüesch R. Is phosphorus recovery from waste water feasible? Environ Technol. 2007;28:165–172. [3] Latifian M, Liu J, Mattiasson B. Struvite-based fertilizer and its physical and chemical properties. Environ Technol. 2012;33:2691–2697. [4] Martí N, Pastor L, Bouzas A, Ferrer J, Seco A. Phosphorus recovery by struvite crystallization in WWTP’s: influence of the sludge treatment line operation. Water Res. 2010;44:2371–2379. [5] Xu H, He P, Gu W, Wang G, Shao L. Recovery of phosphorus as struvite from sewage sludge ash. J Environ Sci. 2012;24:1533–1538. [6] Heinzmann B, Engel G. Induced magnesium ammonium phosphate precipitation to prevent incrustations and measures for phosphate recovery. Water Pract Technol. 2006;1:511–518. [7] Shimamura K, Ishikawa H, Mizuoka A, Hirasawa I. Development of a process for the recovery of phosphorus resource from digested sludge by crystallization technology. Water Sci Technol. 2008;57:451–456. [8] Stratful I, Scrimshaw MD, Lester JN. Conditions influencing the precipitation of magnesium ammonium phosphate. Water Res. 2001;35:4191–4199. [9] Doyle JD, Oldring K, Churchley J, Price C, Parsons SA. Chemical control of struvite precipitation. J Environ Eng. 2003;129:419–426.
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[10] Ali MI, Schneider PA. An approach of estimating struvite growth kinetic incorporating thermodynamic and solution chemistry, kinetic and process description. Chem Eng Sci. 2008;63:3514–3525. [11] Tchobanoglous G, Burton FL, Stensel HD, editor. Wastewater engineering: treatment and reuse. 4th ed. (International Edition). New York: McGraw-Hill; 2004. [12] Peeters B, Herman S. Monitor cations in CPI wastewater for better performance. Chem Eng. 2007;114:56–62. [13] Chen Y, Yang H, Gu G. Effect of acid and surfactant treatment on activated sludge dewatering and settling. Water Res. 2001;35:2615–2620. [14] van Nieuwenhuijzen AF, Havekes M, Reitsma BA, de Jong P. Wastewater treatment plant Amsterdam West: new, large, high-tech and sustainable. Water Pract Technol. 2009;4: 1–8. [15] van Haandel A, van der Lubbe J. Handbook biological waste water treatment: design and optimisation of activated sludge systems. Leidschendam: Quist Publishing; 2007. [16] Cervantes FJ. Environmental technologies to treat nitrogen pollution. London: IWA Publishing; 2009.
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[17] Shao L, He P, Yu G, He P. Effect of proteins, polysaccharides, and particle sizes on sludge dewaterability. J Environ Sci. 2009;21:83–88. [18] Sobeck DC, Higgins MJ. Examination of three theories for mechanisms of cation-induced bioflocculation. Water Res. 2002;36:527–538. [19] Ronteltap M, Maurer M, Gujer W. Struvite precipitation thermodynamics in source-separated urine. Water Res. 2007;41:977–984. [20] Langerak EPA, Beekmans MMH, Beun JJ, Hamelers HVM, Lettinga G. Influence of phosphate and iron on the extent of calcium carbonate precipitation during anaerobic digestion. J Chem Technol Biotechnol. 1999;74:1030–1036. [21] Gadekar S, Pullammanappallil P, Varshovi A. Validation of a comprehensive chemical equilibrium model for predicting struvite precipitation. International conference on nutrient recovery from wastewater streams. London: IWA Publishing; 2009. p. 111–120. [22] Udert KM, Larsen TA, Gujer W. Estimating the precipitation potential in urine-collecting systems. Water Res. 2003;37:2667–2677.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Struvite formation for enhanced dewaterability of digested wastewater sludge a
b
c
a
a
B.J.C. Bergmans , A.M. Veltman , M.C.M. van Loosdrecht , J.B. van Lier & L.C. Rietveld a
Faculty of Civil Engineering and Geosciences, Department of Water Management, Section Sanitary Engineering, Delft University of Technology, PO Box 5048, Delft 2600 GA, The Netherlands b
Waternet, PO Box 94370, Amsterdam 1090 GJ, The Netherlands
c
Faculty of Applied Sciences, Department of Biochemical Engineering, Section Environmental Biotechnology, Delft University of Technology, PO Box 5046, Delft 2600 GA, The Netherlands Published online: 01 Oct 2013.
To cite this article: B.J.C. Bergmans, A.M. Veltman, M.C.M. van Loosdrecht, J.B. van Lier & L.C. Rietveld (2014) Struvite formation for enhanced dewaterability of digested wastewater sludge, Environmental Technology, 35:5, 549-555, DOI: 10.1080/09593330.2013.837081 To link to this article: http://dx.doi.org/10.1080/09593330.2013.837081
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2014 Vol. 35, No. 5, 549–555, http://dx.doi.org/10.1080/09593330.2013.837081
Struvite formation for enhanced dewaterability of digested wastewater sludge B.J.C. Bergmansa∗ , A.M. Veltmanb , M.C.M. van Loosdrechtc , J.B. van Liera and L.C. Rietvelda a Faculty
of Civil Engineering and Geosciences, Department of Water Management, Section Sanitary Engineering, Delft University of Technology, PO Box 5048, Delft 2600 GA, The Netherlands; b Waternet, PO Box 94370, Amsterdam 1090 GJ, The Netherlands; c Faculty of Applied Sciences, Department of Biochemical Engineering, Section Environmental Biotechnology, Delft University of Technology, PO Box 5046, Delft 2600 GA, The Netherlands
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(Received 24 March 2013; final version received 14 August 2013 ) One of the main advantages of controlled struvite formation in digested sludge is an improvement in dewaterability of the digested sludge, which eventually leads to lower volumes of dewatered sludge that need to be transported. The effects of the control parameters for struvite formation, magnesium concentration and pH, on digested sludge dewaterability were investigated and are discussed in relation to the efficiency of struvite formation. Laboratory experiments with digested activated sludge were performed in a 20 L batch reactor. CO2 was stripped from the digested sludge using a bubble aerator and magnesium chloride was added to induce struvite formation. The dewaterability of the sludge was determined by gravity filtration tests. In the experiments, either the pH or the molar magnesium to phosphate ratio (Mg:PO4 ) was varied. The results confirm improved sludge dewaterability after struvite formation. Magnesium to phosphate ratios above 1.0 mol/mol did not further improve dewaterability. The addition of magnesium did not prevent the need for polymer addition for sludge dewatering. An increase in pH led to a deterioration in dewaterability. The best dewaterability results were found at the lowest pH value (pH = 7.0), while stirring the sludge instead of using the bubble aerator. At these settings, an orthophosphate removal of around 80% was achieved. Keywords: struvite; digested sludge; dewaterability; pH; magnesium dosing
Introduction Struvite (MgNH4 PO4 · 6H2 O) is a crystalline phosphate mineral that is formed under specific chemical conditions in solutions in which all its components (magnesium, ammonium and phosphate) are present. Stimulated struvite formation is applied to the liquid phase of anaerobically digested activated sludge for the following reasons: to prevent scaling in pipes and installations downstream, to reduce the phosphate load in the reject water that is returned to the wastewater treatment plant (WWTP) inlet and to explore whether the formed struvite can be used as a fertilizer of commercial interest.[1–4] Currently, the controlled formation of struvite is mostly applied to reject water resulting from sludge dewatering.[4,5] Struvite formation before dewatering, directly after anaerobic digestion,[6,7] has the additional advantage that scaling problems in the remainder of the sludge line are prevented. On top of that, it has been observed that struvite formation in digested sludge leads to a better sludge dewaterability.[6] An improvement in dewaterability reduces the volume of the dewatered sludge and hence the costs for transport and disposal of the sludge. The control parameters of practical interest for struvite formation in the liquid phase of digested sludge are magnesium concentration and pH. Generally, magnesium is added because it is the limiting component for crystallization in the ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
liquid phase of the digested sludge.[8] The pH value is controlled because it is directly related to the availability of the ionic struvite components magnesium (Mg2+ ), ammonium 3− (NH+ 4 ) and phosphate (PO4 ), and therefore to the solubility of struvite.[9] As struvite solubility is minimal around pH = 10 [10] and the pH of digested sludge is typically around 7,[11] the pH is generally increased, for example, by CO2 stripping. Although it is known that dewaterability of the digested sludge is influenced by magnesium addition [12] and pH,[13] the precise effects of these parameters on sludge dewaterability have not been determined. In this paper, the influence of magnesium concentration and pH on sludge dewaterability was investigated and is discussed in relation to the efficiency of struvite formation. Laboratory experiments were conducted using anaerobically digested sludge from WWTP Amsterdam West in the Netherlands.[14] The optimal process settings for both sludge dewaterability and orthophosphate removal were investigated. Materials and methods Digested sludge The digested sludge was collected from the outlet of the anaerobic digesters at WWTP West. The influent of WWTP
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West with a maximum flow of 30,000 m3 /h is biologically treated in an activated sludge system based on the Modified University of Cape Town (MUCT) principle.[15] In addition to the sludge produced at the site, WWTP West processes sludge from several external sources. All primary and secondary sludge is thickened, anaerobically digested and dewatered in decanter centrifuges. The chemical composition and physical characteristics of the digested sludge were determined during the experiments. Experimental set-up Reactor design and operation The digested sludge was treated in a batch reactor with a diameter of 20 cm and a height of 100 cm. The reactor was filled with 20 L of digested sludge and CO2 was stripped up to a pH set point value of pH = 7.4, 7.6, 7.8 and 8.0, using a fine bubble aerator operated with an airflow of 500 L/h. Magnesium chloride (33% (weight) solution) was added instantaneously at the sludge surface. The applied magnesium to phosphate ratios of Mg:PO4 = 0.5, 1.0, 1.5 and 2.0 are expressed as the molar magnesium concentration directly after dosing, i.e. the sum of the dosed magnesium and the background magnesium concentration, divided by the molar orthophosphate concentration as measured before dosing. As protons are released during the formation of struvite, the addition of magnesium led to a sudden pH decrease. After magnesium addition, CO2 was stripped from the digested sludge until the pH set point was again reached, to mimic a full-scale reactor that is operated at continuous pH. At that point, a sample was taken in the middle of the well-mixed reactor from a pre-installed sampling tap. The first flow of digested sludge was drained off to gain a representative sample. Sample analysis The samples were analysed on dry solids content (DSC) using a Sartorius MA35 DSC measuring device, and on concentrations of the components orthophosphate (PO4 –P), ammonium (NH4 –N) and magnesium. To measure the component concentrations, part of the sample was centrifuged, filtered over a 45 μm nylon Syringe filter, diluted if necessary and analysed using a Dr. Lange LASA 50 cuvette test apparatus. To determine the dewaterability before treatment, a 200 g test sample was prepared using the digested sludge sample and cationic polymer solution (0.21%) in such a way that 10 g of polymer was available per kg of dry solids. Digested sludge and polymer were intensively mixed for 3 s in a blender with a timer connected to its power supply. The mixture was instantaneously brought on a woven synthetic filter with minimal filter resistance. The amount of percolated water was measured with a balance (accuracy = 0.01 g) after 5, 10, 15, 20, 30, 40, 50, 60, 90 and 120 s. The dewaterability after treatment, i.e. CO2 stripping and/or MgCl2 dosage, was determined in the
same way and with the same mass of sludge and polymer as before treatment. This was done to assure that a change in dewaterability was not caused by a change in polymer addition. Calculation of free ionic concentrations and supersaturation Under supersaturated conditions, struvite is formed by a 3− chemical reaction between free Mg2+ , NH+ 4 and PO4 ions, with the incorporation of six H2 O molecules. In the solution, magnesium, ammonium and phosphate are present in different forms. For the calculation of struvite solubility, at least the following dissolved ionic species are 2− 3− to be considered [16]: H3 PO4 , H2 PO− 4 , HPO4 , PO4 , + − + + 2+ MgH2 PO4 , MgHPO4 , MgPO4 , MgOH , Mg , NH4 and NH3 . Considering additional soluble and solid species such as carbonate, bicarbonate and calcium phosphates increases accuracy as well as complexity of the calculation.[1] In this paper, other soluble and solid species were neglected. For each combination of pH and component concentrations that were measured during the experiments, the free ionic concentrations of magnesium [Mg2+ ], ammonium 3− [NH+ 4 ] and phosphate [PO4 ] were calculated in MATLAB by solving the system of thermodynamic equations as described by Ali and Schneider.[10] The value used for the ionic strength (I = 0.02 mol/L) was adapted from example calculations in digested sludge.[11] During the experiments the temperature of the sludge decreased from ±30◦ C to ±24◦ C. In the MATLAB model, a constant temperature of 3− 25◦ C was assumed. The product of [NH+ 4 ] and [PO4 ] was 2+ plotted against [Mg ] to obtain a graphical presentation of the equilibria reached in the experiments. Using the system of thermodynamic equations as described by Ali and Schneider,[10] the observed minimum struvite solubility product pKS0 was calculated. For the formation of pure struvite, equimolar amounts of magnesium, ammonium and orthophosphate are used. As ammonium was abundant and magnesium was added during the experiments, the efficiency of struvite formation was represented as the observed magnesium/orthophosphate removal ratio CT ,Mg : CT ,PO4 . Using the observed struvite solubility product, the average observed magnesium/orthophosphate removal ratio and the system of thermodynamic equations as described by Ali and Schneider,[10] curves were constructed in MATLAB that predict the final orthophosphate concentration as a function of pH and magnesium to phosphate ratio. For the construction of these curves, the initial orthophosphate concentration was considered equal to the average orthophosphate concentration as measured in the untreated digested sludge in the different experiments. The NH4 –N concentration was assumed constant and was calculated as the average concentration of all NH4 –N measurements before and after treatment. It was assumed that this simplification was permitted because the relative removal of NH4 –N was
Environmental Technology Table 1.
Overview of the experiments.
Experiment no. 1 2 3 4 5 6 7 8 9
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Description
Volume (L)
pH set point
Mg:PO4
Aeration (no dosing) Reference Mg:PO4 variation Mg:PO4 variation Mg:PO4 variation pH variation pH variation pH variation Stirring
20 20 20 20 20 20 20 20 0.4
7.5, 7.75, 8.0 7.6 7.6 7.6 7.6 7.4 7.8 8.0 7.0
– 1.5 0.5 1.0 2.0 1.5 1.5 1.5 1.5
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expected to be small as compared with the relative removal of orthophosphate and magnesium. This was verified after completion of the experiments. Overview of the experiments Table 1 presents an overview of the experiments. In experiment 1, the influence of pH increase without magnesium addition was investigated by aerating the sludge and analysing the samples taken at different pH values. Experiment 2 was used as a reference. In experiments 3–8, the pH and the magnesium to phosphate ratio were varied according to Table 1. In experiment 9, it was investigated whether stirring the sludge leads to better dewaterability results than using the bubble aerator. A small amount of digested sludge (0.4 L) was stirred in a beaker using a magnetic stirrer (800 rotations per minute), while magnesium was added to reach a magnesium to phosphate ratio of 1.5. The sludge was stirred for 30 min, after which it was analysed in the same way as the samples from other experiments. To investigate whether costs savings on polymer could be achieved, dewaterability tests were performed on treated sludge from the reference experiment using different polymer dosing ratios. Results Sludge dewaterability We evaluated the effect of pH change only (Figure 1), the effect of varying magnesium additions (Figure 2), the effect of magnesium addition and varying pH adjustments (Figure 3) and the effect of polymer dosage (Figure 4) on digested sludge dewaterability. The dewaterability before treatment is the average dewaterability of the raw digested sludge as found in the nine experiments. Error bars represent the standard deviation. An increase in pH by CO2 stripping without magnesium addition led to a deterioration of the dewaterability (Figure 1). This was also observed visually, as the sludge/polymer mixture, directly after intensive mixing, showed a strong floc formation at pH = 7.5, while at pH = 8.0 no floc formation was observed. At a constant pH of 7.6, achieved by CO2 stripping, the addition of magnesium led to an improvement in dewaterability (Figure 2). At a
Figure 1. Dewaterability of digested sludge after CO2 stripping to varying pH values, without magnesium dosing.
Figure 2. Dewaterability of digested sludge after magnesium addition at varying magnesium to phosphate ratios and CO2 stripping to pH = 7.6.
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Figure 3. Dewaterability of digested sludge after magnesium addition (Mg:PO4 = 1.5 mol/mol) and CO2 stripping to varying pH values, or after magnesium addition (Mg:PO4 = 1.5 mol/mol) and 30 min of stirring.
magnesium to phosphate ratio of 0.5, only a slight improvement was found. A magnesium to phosphate ratio of 1.0 led to a further improvement. Higher magnesium to phosphate ratios (1.5 and 2.0) led to the same dewaterability results as a magnesium to phosphate ratio of 1.0. At a constant magnesium to phosphate ratio of 1.5, lower pH values led to a better dewaterability (Figure 3). For all pH values, the dewaterability after treatment improved as compared with the initial dewaterability. Stirring the sludge instead of using the bubble aerator led to the best dewaterability result. A decrease in polymer dosing led to a deterioration in dewaterability (Figure 4). Adding 80% of the standardized amount of polymer (that was added before treatment) to the treated sludge of the reference experiment resulted in a dewaterability slightly below the average dewaterability before treatment.
Struvite formation The formation of struvite was investigated by evaluating the chemical equilibria that were reached during the experiments. Table 2 presents the pH values and concentrations of magnesium, ammonium and orthophosphate that were measured in the experiments. Using these data, the theoret3− ically available concentrations [Mg2+ ], [NH+ 4 ] and [PO4 ] were calculated. Figure 5 presents the product of [NH+ 4] 2+ and [PO3− ] as a function of [Mg ]. The trend line through 4 the plotted values represents the observed struvite solubility curve and corresponds with an observed struvite solubility 0 product (pKS;observed ) of 12.41. The average observed magnesium/orthophosphate removal ratio (CT ,Mg : CT ,PO4 ) was 0.84 mol/mol. No
Figure 4. Dewaterability of the treated digested sludge from the reference experiment at varying polymer dosing ratios.
clear correlations were found between CT ,Mg : CT ,PO4 and pH, and between CT ,Mg : CT ,PO4 and Mg:PO4 . The relative removal of ammonium was only 3% on average while the relative removal of orthophosphate and magnesium was 85% and 49% on average, respectively. Using the 0 observed struvite solubility product (pKS;observed = 12.41), the observed average magnesium/orthophosphate removal ratio (CT ,Mg : CT ,PO4 = 0.84), an initial orthophosphate concentration of 318 mgP/L and a constant ammonium concentration of 871 mgN/L, curves were constructed in MATLAB that predict the final orthophosphate concentration as a function of pH and Mg:PO4 (Figure 6). Discussion Sludge dewaterability In our experiments an increase in pH without magnesium addition led to a deterioration in sludge dewaterability, which is in agreement with earlier research by Shao et al.,[17] who suggested that the deterioration in dewaterability at high pH values is caused by the shifting of proteins from the pellet and tightly bound extracellular polymeric substances (EPS) fraction to the slime fraction of the sludge, retaining a higher degree of bound water. Previous studies [12,18] emphasized the important role of divalent cations, such as magnesium, in sludge dewaterability. Therefore, it was expected that the improvement in sludge dewaterability resulting from struvite formation was predominantly caused by the increase in magnesium concentration. In a comparative study by Sobeck and Higgins [18] it was concluded that the divalent cation bridging (DCB) theory explains the role of cations best. According to the DCB theory, divalent cations bridge the negatively charged groups present on the EPS that are
Environmental Technology Table 2.
pH and component concentrations measured during the experiments.
Before treatment (average) After treatment (experiment 1–9)
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Experiment no.
pH (−)
PO4 –P (mg/L)
NH4 –N (mg/L)
Mg (mg/L)
– 1 2 3 4 5 6 7 8 9
7.1 – 7.6 7.6 7.6 7.6 7.4 7.8 8.0 7.0
318 – 13 137 62 26 25 10 8 58
900 – 853 880 787 885 855 794 819 –
25 – 142 28 79 332 179 155 151 182
Figure 5. Equilibria reached during the experiments. Dosing magnesium (a) leads to a higher magnesium concentration and hence to a lower negative logarithm of the magnesium concentration. Reaction of magnesium, ammonium and phosphate to struvite (b) decreases all component concentrations and hence increases the negative logarithm of these concentrations. Based on these equilibria, the observed 0 struvite solubility product was calculated (pKS;observed = 12.41).
Figure 6. Prediction of the final orthophosphate concentration as a function of pH and Mg:PO4 for pKS0 = 12.41, CT ,Mg : CT ,PO4 = 0.84, PO4 –Pinitial = 318 mgP/L, NH4 –N = 871 mgN/L and T = 25◦ C.
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produced by microorganisms. This bridging helps to aggregate and stabilize the matrix of EPS and microorganisms, thereby promoting floc formation, hence improving sludge dewaterability.[12] In our experiments, adding magnesium chloride to the digested sludge at a constant pH (pH = 7.6) led to an improvement in sludge dewaterability. However, at magnesium to phosphate ratios above 1.0 mol/mol no significant further improvement was observed. This implies that the improvement in sludge dewaterability might have been caused by a decrease in orthophosphate concentration or by precipitant formation rather than by an increase in magnesium concentration. Moreover, it was observed that the addition of magnesium chloride could not compensate for the need of polymer addition. As the working principle of (cationic) polymer addition [11] is similar to the working principle of divalent cations according to the DCB theory, the persistent need for polymer addition was considered an additional proof that the working principle of struvite formation for better sludge dewaterability differed from the working principle of polymer addition and that the mechanisms of these working principles can work parallel to each other. In line with the above, the best dewaterability result was found using a magnesium to phosphate ratio above 1.0 mol/mol (in this case Mg:PO4 = 1.5 mol/mol) at the lowest pH (pH = 7). This relatively low pH was achieved by stirring the sludge instead of using the bubble aerator.
dewaterability result (Mg:PO4 = 1.5 mol/mol, pH = 7), around 80% of orthophosphate was removed.
Struvite formation Many researchers report on the value of the standard struvite solubility product, with pKS0 values ranging from 12.6 to 13.36.[19] These values, which are determined exclusively in chemically well-defined matrices and at relatively low ionic strengths, are higher than the negative logarithm of the observed struvite solubility product as derived from 0 our experiments (pKS;observed = 12.41). This means that in our experiments less struvite was formed than expected based on known pKS0 values. Similar results are described in research on calcium carbonate precipitation during anaerobic digestion by Langerak et al.,[20] who concluded that the difference between the actually prevailing solubility product and the thermodynamic value can be attributed to kinetic inhibition of crystal formation by wastewater constituents. As the magnesium/orthophosphate removal ratio CT ,Mg : CT ,PO4 was below 1 mol/mol (0.84 mol/mol on average), the experimental results suggest that the formed precipitate did not solely consist of struvite. Phosphate could have also been removed by other reactions, forming different precipitates such as hydroxyapatite (Ca5 (PO4 )3 OH),[21] which has a relatively high pKS0 value of 57.5.[22] At all process settings, a significant orthophosphate removal was achieved, varying from 57% (Mg:PO4 = 0.5 mol/mol, pH = 7.6) to 97% (Mg:PO4 = 1.5 mol/mol, pH = 8.0). At the process settings that led to the best
References
Conclusions In this paper, the influence of magnesium concentration and pH on sludge dewaterability was investigated and was discussed in relation to the efficiency of struvite formation. The main conclusions are shown below: An increase in pH without magnesium addition led to a deterioration in digested sludge dewaterability. At a constant magnesium to phosphate ratio of 1.5 mol/mol, lower pH set points led to better sludge dewaterability results. The addition of magnesium to digested sludge improved the sludge dewaterability. However, magnesium to phosphate ratios above 1.0 mol/mol did not further improve dewaterability and the addition of magnesium could not compensate for the need of polymer addition. Therefore, it seemed that the working principle of struvite formation for better sludge dewaterability differed from the working principle of polymer addition and that the mechanisms of these working principles can work parallel to each other. The best sludge dewaterability result was found at pH = 7.0 and a magnesium to phosphate ratio above 1.0 mol/mol (in this case Mg:PO4 = 1.5 mol/mol), while stirring the sludge instead of using a bubble aerator. At these settings, an orthophosphate removal of about 80% was achieved.
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