Favorable Operating Conditions for Obtaining High-Value Struvite Product from Sludge Dewatering Filtrate

Favorable Operating Conditions for Obtaining High-ValueStruvite Product from Sludge Dewatering Filtrate

Beni Lew,1,* Somaya Phalah,2 Chaim Sheindorf,3 Mario Kummel,4 Menachem Rebhun,2 and Ori Lahav2

1Agricultural Research Organization, Bet Dagan, Israel.2Faculty of Civil and Environmental Engineering, Technion, Haifa, Israel.

3Faculty of Engineering, Shenkar College, Ramat Gan, Israel.4Mekorot Water Company Ltd., Tel-Aviv, Israel.

Received: August 11, 2009 Accepted in revised form: July 1, 2010

Abstract

Struvite (MgNH4PO4) is emerging as a potential fertilization product for agriculture. As other phosphateminerals that can be precipitated from wastewater are known to have lower value as fertilizers, the overallfertilization quality of the solids precipitated in struvite recovery systems depends largely on the percentage ofstruvite in the mixture of precipitated solids. This work focused on determining the most appropriate operatingconditions (with a focus on pH and hydraulic retention time [HRT]) for removing >90% of the dissolvedphosphate and, at the same time, attaining a precipitate with the highest possible struvite content from typicalfiltrates of sludge dewatering belt press systems. To this end, a continuous laboratory-scale completely mixedreactor was operated at different pH values and HRTs and the precipitant composition was determined. The-oretical calculations showed that the most cost-effective MgCl2 dosage to typical Israeli belt press filtrate is*10 mM. Using this dosage, *8.0 mM of phosphate precipitated under all the operating conditions studied. Theshortest retention time (15 min) and lowest pH value (pH 7.4) applied were found to be the most favorable forthe attainment of the most homogeneous struvite precipitate (*85% struvite). At higher pH values and longerHRTs, the overall precipitated mass (in P units) was slightly higher, but unwanted calcium-phosphate andmagnesium-phosphate precipitates were observed at higher percentages alongside the struvite crystal. Thisfinding implies that the typical high pH values, previously perceived to be optimal for struvite precipitation,actually result in a less-valuable product.

Key words: belt-press filtrate; MgCl2 dosage; phosphate recovery; struvite homogeneity; struvite recovery;fertilization value

Introduction

Struvite, a white crystalline compound consisting ofmagnesium, ammonium, and phosphorus at equal molar

concentrations (MgNH4PO4 � 6H2O), is known to precipitateand clog pipes and pumps, causing operational difficultiesand increased maintenance costs in wastewater treatmentplants (WWTP) around the world. Although struvite is arecognized operational problem in WWTP, it has been alsoshown that a significant percentage of the dissolved phos-phate can be recovered from anaerobic digester supernatantsthrough controlled struvite precipitation (Battistoni et al.,2001; Munch and Barr, 2001; Ueno and Fuji, 2001; Yoshinoet al., 2003; Wu and Bishop 2004). Owing to its slight solubility

in neutral pH solutions (Bridger et al., 1961), struvite may beused separately as a cheap replacement to slow-release fer-tilizers or as a component in other commercial fertilizers(Gaterell et al., 2000; Battistoni et al., 2002; Perez et al., 2009).

Several phosphate minerals can be precipitated fromwastewater, out of which struvite is recognized as the mostpreferred fertilizer for several reasons: (1) its components arereleased at a slower rate, compared with other phosphateminerals. As a result, plants can uptake the nutrients beforethey are rapidly leached from the root zone (Munch and Barr,2001). (2) The impurities caused by heavy metals in recoveredstruvite products are 2 or 3 orders of magnitude lower thanthe respective amounts in commercial phosphate fertilizers(Booker et al., 1999). (3) Three essential nutrients (P, N, andMg) are applied simultaneously with no addition of unnec-essary counter ions (Gaterell et al., 2000).

Considering its apparent Ksp value and assuming that thetypical Mg2þ concentration in wastewater is not higher than 1or 2 meq/L, struvite precipitation can practically occur only at

*Corresponding author: Agricultural Research Organization, P.O.Box 6, 50250 Bet Dagan, Israel. Phone: þ972-39683453; Fax: þ972-34604704; E-mail: [email protected]

ENVIRONMENTAL ENGINEERING SCIENCEVolume 27, Number 9, 2010ª Mary Ann Liebert, Inc.DOI: 10.1089/ees.2009.0279

733

relatively high concentrations of both ammonium (NH4þ) and

phosphate (PO43�). This concentration combination is en-

countered only in the anaerobic sludge treatment line ofWWTP. As the Mg2þ molar concentration in this treatmentline is typically around 1 order of magnitude lower than thatof ammonium and phosphate, dosage of external magnesiumsalts is required to precipitate a significant mass of struvite(Munch and Barr, 2001; Chimenos et al., 2003; Lee et al., 2003;Nelson et al., 2003; van Rensburg et al., 2003). Moreover, bothNH4

þ and PO43� concentrations are inversely dependent on

pH, and thus it is theoretically possible to define an optimumpH value for struvite precipitation for any given water char-acteristics. However, as wastewater streams differ signifi-cantly from each other in terms of ionic strength and presenceof complexing agents and also with respect to the variety ofsolids that can potentially precipitate and compete for struvitecomponents, a wide and confusing range of pH values (frompH 8.0 to 10.7) has been reported optimal for struvite pre-cipitation (Stumm and Morgan, 1970; Momberg and Oeller-mann, 1992; Buchanan et al., 1994; Ohlinger et al., 1998; Bookeret al., 1999; Doyle and Parsons, 2002).

Recently, it has been shown that efficient phosphate speciesremoval can also be attained through struvite precipitation atpH values lower than 8.0 (Mavinic et al., 2007; Fattah et al.,2008). Mavinic et al. (2007), operating a continuous pilot-scalestruvite crystallization process with dosage of magnesiumsalts, showed that high phosphate removal (>*90%) can beattained from the supernatant of an anaerobic sludge digesteroperating at a short hydraulic retention time (HRT) of 18 minand relatively low pH (pH 7.8–8.0). These authors alsoshowed that lower and higher pH values led to a decrease inthe phosphate removal efficiency. Moreover, the purity of thestruvite generated in this narrow pH range was reported to behigh (*96%), with solid calcium carbonate being the majorityof the remaining 4%. To reach 96% phosphate removal, Mg2þ

was dosed to the water at a Mg:P molar ratio of between 1.2and 3.5. Katsuura (1998) showed that phosphorus removalefficiency increased with an increase in the Mg:P ratio andthat this phenomenon was more pronounced at pH 8.0 than atpH 9.0. Further, with respect to the effect of pH on the purityof the struvite precipitant, Hao et al. (2008) stated that pH>8.0in a wastewater stream with a significant Ca2þ concentrationmight result in an impure precipitate due to enhanced pre-cipitation of calcium-based solids.

From a thermodynamic standpoint, assuming that thewater contains Mg2þ, NH4

þ, PO43�, Ca2þ, and carbonate

species, quite a few solids have the potential to precipitate inanaerobic sludge treatment lines, depending mainly on pH;these include struvite (MgNH4PO4 � 6H2O), newberyite(MgHPO4 � 3H2O), bobierrite [Mg3(PO4)2 � 8H2O], hydroxy-apatite [Ca5(PO4)3OH], tricalcium phosphate [Ca3(PO4)2],amorphous calcium phosphate [Ca3(PO4)2 � xH2O], octa-calcium phosphate [(Ca8(HPO4)2(PO4)4 � 5H2O], monenite(CaHPO4), brushite (CaHPO4 � 2H2O), calcite (CaCO3),magnesite (MgCO3), nesquehonite (MgCO3 � 3H2O), dolo-mite [CaMg(CO3)2], and huntite [CaMg(CO3)4] (Musvotoet al., 2000; Regy et al., 2001;Celen et al., 2007).

The information that exists in the literature regarding thesolids that precipitate from dewatering systems’ supernatantsis inconsistent, which reflects the chemical complexity of en-gineered operational parameters. The solubility products ofthe minerals that can potentially form in the supernatants

from anaerobic digesters are shown in Table 1. From theminerals listed in Table 1, newberyite tends to precipitate athigh Mg2þ and total phosphate (PT) concentrations only atrelatively low pH values (*pH 6.0) (Musvoto et al., 2000);Mamais et al. (1994) reported that bobierrite has a precipita-tion rate in the order of days and its precipitation has neverbeen observed in the pH range recorded in the belt press su-pernatants (7.0<pH< 10.0); of the candidate calcium phos-phate minerals, hydroxyapatite and tricalcium phosphate cantheoretically be expected to precipitate under the conditionsprevailing in dewatering systems’ supernatants. However, ithas been observed that a number of minerals act as precursorsto the precipitation of hydroxyapatite, tricalcium phosphate,and octacalcium phosphate, such as amorphous calciumphosphate. This mineral, which precipitates first, transformswith time to a more stable form, and although the formationof amorphous calcium phosphate is a relatively fast process,the growth of the more stable minerals is very slow (a mini-mum of 1 month up to a number of years) (Ferguson andMcCarty, 1971; Musvoto et al., 2000). Celen et al. (2007) hy-pothesized, for example, that both brushite and monoetitemay precipitate in swine manure; however, only monoetiteprecipitation was observed. Musvoto et al. (2000) reportedamorphous calcium phosphate to be the only calcium phos-phate mineral that practically precipitates in domesticwastewater plants. Roques and Girou (1974) reported thatunder ambient temperatures and atmospheric pressure, cal-cite [CaCO3(s)] is thermodynamically stable under domesticwastewater plants’ conditions. Musvoto et al. (2000) also re-ported calcite precipitation downstream of anaerobic digest-ers, but only at high Ca2þ concentrations (>*110 mg/L). Atlower Ca2þ concentrations, the formation of amorphous cal-cium phosphate (which has much faster precipitation kineticsthan calcite) acts rapidly to reduce the soluble calcium con-

Table 1. Solubility Product Values (pKs at 258Cand Infinite Dilution) for Minerals That Have

the Potential to Precipitate from the Filtrate

of Anaerobic Digesters

Mineral pKs

Calcite (CaCO3) 8.22a,b; 8.5b,c

Magnesite (MgCO3) 7.46a,b; 8.2a,b

Nesquehonite (MgCO3 � 3H2O) 5.19a,b

Dolomite [CaMg(CO3)2] 16.6a,b,c

Calcium hydroxide [Ca(OH)2] 5.2a,c

Brucite [Mg(OH)2] 11.16a,c

Brushite (CaHPO4) 6.6a,b

Hydroxyapetite [Ca10(PO4)6(OH)2] 114a; 57.5b

Tricalcium phosphate [Ca3(PO4)2] 32.7b

Amorphous calcium phosphate[Ca3(PO4)2 � xH2O]

25.46d

Struvite (MgNH4PO4 � 6H2O) 12.6a,b,e

Newberyte (MgHPO4 � 3H2O) 5.8b,e

Bobierrite [Mg3(PO4)2 � 8H2O] 25.2b,e

Trimagnesium phosphate[Mg3(PO4)2 � 22H2O]

23.1e

aStumm and Morgan (1981).bMurray and May (1996).cNordstrom et al. (1990).dHoffman (1977).eTaylor et al. (1963).

734 LEW ET AL.

centration to values below the calcite supersaturation level. Incorroboration with these statements, Celen et al. (2007) re-ported that the presence of Mg2þ, phosphate, and dissolvedorganics prevented calcite precipitation in swine manure, andMusvoto et al. (2000) reported that magnesite typically pre-cipitates at pH >9.5 and that at pH <8.9 no magnesite pre-cipitation was observed. At the lower pH values theconcentration of the species CO3

�2 is typically too low for thesolution to be supersaturated with respect to MgCO3. More-over, nesqehonite was reported to precipitate only at pHvalues much higher than 10.7, which is not encountered insludge dewatering supernatants/filtrates. The conditions forthe precipitation of dolomite and huntite are not well under-stood under such conditions and attempts to precipitate themunder atmospheric conditions have not been successful(Mamais et al., 1994).

On the basis of the solubility products alone (Table 1), someof the minerals that have a positive precipitation potential(PP) in WWTP supernatants are thermodynamically morefavored to precipitate than struvite. However, Musvoto et al.(2000), after conducting a 60-h batch test, concluded that theprecipitation kinetics of struvite were twice as fast as those ofamorphous calcium phosphate and at least 1 order of mag-nitude faster than those of calcium carbonate, magnesiumcarbonate, and newberyite. Similar results were observed byBabic-Ivancic et al. (2006).

The thermodynamics under which different minerals canbe formed in WWTP supernatants and filtrates have been welldocumented. However, information regarding the best rangeof practical conditions (HRT and pH) for rapid struvite for-mation in systems where dosage of Mg salts is practiced,while minimizing the precipitation of other potential solids, isstill lacking. The objective of the present work was to definethe appropriate reactor operating conditions for attaining themost homogeneous struvite precipitation that can be feasiblyand economically formed, with the aim of producing a high-value fertilizer product or fertilization additive. To this end, alaboratory-scale continuous completely mixed reactor fedwith the filtrate of a sludge belt filter press system was op-erated at different HRTs and pH values with MgCl2 additionand pH control. The relevant components in both the aqueousphase and produced solids were analyzed to assess the pre-cipitates’ compositions.

Materials and Methods

A continuous 32.1-L continuous stirred tank reactor(CSTR), followed by a 5.7-L settling compartment, was op-erated with the aim of precipitating struvite from the filtrate ofthe sludge dewatering system in the Karmiel (Israel) domesticWWTP (Fig. 1). Fluidized bed reactors (FBR) are the mostcommon systems used for struvite recovery at both the lab-oratory and pilot scales (Munch and Barr, 2001; Kumashiroet al., 2001; Battistoni et al., 2002). However, a CSTR waspreferred in the present study because it is difficult to main-tain a constant flow rate and keep the bed in a fluidized statein an FBR, and moreover, the FBR is more susceptible thanCSTR systems to clogging by particulate solids.

As the present work was aimed at investigating the natureof the mixture of precipitants that would form in a system towhich Mg salts are dosed and that is operated at various HRTand pH values, there was no need to select a WWTP that

suffers specifically from struvite precipitation nuisances.However, the use of a real WWTP filtrate (as opposed tosynthetic waste water) was perceived to be imperative be-cause the presence and composition of organic matter is veryinfluential in terms of the solids precipitation kinetics. Theonly component that was added externally to the filtrate(apart from Mg2þ) was phosphate, whose concentration wasincreased in all experiments to around 300 mg P/L to bettersimulate P concentrations prevailing in the belt press filtratewaters in Israel. Moreover, prior to each experiment, the fil-trate was collected in a 500-L container with the aim ofmaintaining the influent characteristics as constant as possiblethroughout a given experiment.

The reactor was operated at three different HRTs: 60, 30,and 15 min with addition of 10 mM MgCl2 to the influent. Foreach HRT the reactor was operated at three different constantpH values (7.4, 8.0, and 8.4), which were attained via a com-bination of CO2 stripping (by aeration) and NaOH addition.At each operating condition, reactor influent and effluentwere monitored for around 360 min for temperature, pH,electrical conductivity (EC), alkalinity, total ammonia nitro-gen (TAN), PT, Mg2þ, and Ca2þ.

Mg2þ and Ca2þ concentrations were measured using in-ductively coupled plasma emission spectrometry. TAN andPT were measured using a colorimetric method, according tostandard methods (APHA, 1995). Alkalinity was measuredusing the Gran titration technique (Gran, 1952) to pH valuesslightly lower than 4.0, to attain the alkalinity concentrationwith H2CO3*, NH4

þ, and H2PO4� as reference species [de-

noted alkalinity (H2CO3*, NH4þ, H2PO4

�)].For the precipitate ion species measurement (Mg2þ, Ca2þ,

TAN, and PT), samples collected at the end of each experimentwere first dried by warm air (around 608C) for at least 48 h andthen 1.00 g was dissolved in a known volume of 1 N HClsolution at pH*1.0 for 24 h. A magnetic stirrer was applied toaccelerate the dissolution of the crystals.

Precipitated crystals were also analyzed for total sus-pended solids and volatile suspended solids and using X-raydiffraction (XRD). The XRD test was used to verify the pres-ence of struvite and other minerals by matching the intensityand position of the peaks produced to a database of purecrystal structures (Doyle and Parsons, 2002).

Results and Discussion

The concentrations of the relevant components in the fil-trate used in the experiments are shown in Table 2. TAN, PT,

FIG. 1. Sketch of the experimental system: 32.1-L continu-ous completely stirred reactor, followed by a 5.7-L settlingtank.

STRUVITE FROM FILTRATE SLUDGE 735

http://www.liebertonline.com/action/showImage?doi=10.1089/ees.2009.0279&iName=master.img-000.jpg&w=238&h=112

Mg2þ, and Ca2þ concentrations in the filtrate were high andtypical for Israeli WWTP, with values of 267, 310, 33, and70 mg/L (19, 10, 1.4, and 1.8 mM), respectively. As expected,there was a certain fluctuation in the measured concentra-tions, as manifested by the relatively high standard deviationvalues. To overcome the fluctuations, which could hampermass balance calculations, a 500-L sealed container was in-stalled upstream of the continuous reactor to provide constantinfluent characteristics during a given experiment. Note thatferric salts were periodically added to the anaerobic digestionunit in the plant to eliminate H2S(g) in the burnt gas. Thisaction reduced the soluble P concentrations in the filtrate. Insuch cases, P was added to the filtrate as the chemical K2HPO4

to elevate PT to 300 mg/L, to better simulate P concentrationstypically prevailing in the belt press filtrate waters.

On the basis of the filtrate composition, struvite precipita-tion was predicted to occur in the plant, and indeed, down-stream of the sludge anaerobic digester, struvite precipitatewas observed in pipes and pumps. Although the questionwhether struvite would precipitate or not can be predicted bycomparing the product of Mg2þ, NH4

þ, and PO43� concen-

trations to the apparent solubility product, the potentialamount of struvite precipitation cannot be calculated basedon the solubility product alone. The amount of struvite pre-cipitation can only be approximated based on the value of itsPP. The PP model used in the present work is the one devel-oped by Loewenthal et al. (1994) and is based on the solubilityproduct for struvite, modified for ionic strength and temper-ature, the NH4

þ concentration as a function of TAN and pH,the PO4

3� concentration as a function of PT and pH, andchanges in pH and alkalinity (H2CO3*, NH4

þ, H2PO4�) due to

struvite precipitation.On the basis of the values reported in Table 2, because of the

low Mg2þ concentration, the spontaneous PP for struvite inthe typical belt press filtrates is relatively low at *190 mg/L(1.4 mM). Many authors (Munch and Barr, 2001; Chimenoset al., 2003; Lee et al., 2003; Nelson et al., 2003; van Rensburget al., 2003) reported that the precipitates obtained at similarconditions were both low in quantity and poor in struvitequality and homogeneity.

To obtain high quantities of precipitate that is rich in stru-vite (high-quality fertilization product), MgCl2 was added tothe filtrate at the inlet to the reactor. To determine the requiredMgCl2 dosage, the theoretical PP of the filtrate was calculatedfor different MgCl2 dosages at three pH values (7.4, 8.0, and8.6). The results of these calculations are shown in Fig. 2.

As shown in Fig. 2, a sharp theoretical increase in thestruvite PP is observed when Mg2þ is added to the filtrate,ranging from 1.0 to around 10.0 mM, for the three pH valuesstudied. However, beyond a dose of 10.0 mM, further Mg2þ

addition results in only a small increase in PP due to the dropin PT concentration to around 100 mg P/L (*3 mM) followingthe 10 mM MgCl2 dosage, which makes PT the limiting com-ponent for further struvite precipitation. A dosage of 10.0 mMMg2þ was therefore used in all further experiments.

Struvite precipitation

In all the experiments (regardless of the different HRTs andinitial pH values), a decrease in Mg2þ, TAN, PT, and Ca2þ

concentrations was observed in the reactor effluents with timein the first*60–100 min into the experiment and then stabilizedand remained constant until the end of the experiment(360 min), indicating that steady-state conditions can be ap-proached relatively quickly under all the operational conditionsstudied. This reduction in the aqueous phase was attributed toeither precipitation (Mg2þ, TAN, PT, Ca2þ) and/or stripping tothe atmosphere (TAN). Possible TAN removal through nitrifi-cation was assessed and disproved, that is, no nitrate or nitritewas observed in either the reactor influent or effluent, althoughthe reactor was fully aerated. The likely reason is that aerobicnitrifying bacteria are not present in anaerobic digester filtratesand the very short HRT and solids retention time applied in thedescribed experiments did not allow for the development of ameaningful nitrifying bacteria population.

Laboratory batch experiments were carried out to deter-mine the relation between the TAN concentration and HRTand the ammonia stripping rate under the conditions thatprevailed in the continuous precipitation reactor. These ex-periments were conducted with the belt press filtrate at con-stant pH. TAN concentration in the aqueous phase wasmeasured with time for 300 min. The TAN stripping rate wasfound to approximately follow the following rate equation:

DNH3(g)

Dt¼ 8:1 · 10� 4 · ½NH

3(aq)� · HRT (1)

where HRT is the hydraulic retention time (in minutes) andthe NH3(aq) concentration (M) is calculated based on themeasured TAN concentration (at ‘‘steady state’’) and pH ac-cording to the following equation:

NH3¼kN · TAN

kNþ ½Hþ � (2)

where kN is the dissociation constant for the ammonia/ammonium system (pH 9.25 at standard conditions). The kN

value used in Eq. (2) was adjusted for ionic strength andtemperature effects. Strictly speaking, as all the batch andcontinuous experiments were performed at ambient temper-ature (228C–258C), Eq. (1) is only valid for this temperaturerange.

Table 2. Average Composition of the Filtrate

of the Belt Press Dewatering System

in the Karmiel Wastewater Treatment Plant

Parameter Units Value (n¼ 20)

TAN mg/L as N 267� 40PT mg/L as P 310� 7.5a

Magnesium (Mg2þ) mg/L 33� 11Calcium (Ca2þ) mg/L 70� 8pH 7.61–7.79Alkalinity

(H2CO3*, NH4þ, H2PO4

�)mg/L

as CaCO3

1379� 224

Total COD mg/L 248� 26TSS mg/L 157� 39VSS mg/L 53� 36EC ms/cm 3.55� 0.38Nitrate (NO3

�) mg/L as N 11� 4

aConsidering periodically K2HPO4 addition.TAN, total ammonia nitrogen; PT, total phosphate; COD, chemical

oxygen demand; TSS, total suspended solids; VSS, volatilesuspended solids; EC, electrical conductivity.

736 LEW ET AL.

As previously mentioned, a fluctuation in the reactorinfluent was observed between experiments, which made itdifficult to compare the effluent species concentrations atsteady-state conditions observed at the different reactor op-erational conditions. To overcome these influent fluctuations,it was decided to compare species concentration disappear-ance from the aqueous phase and precipitation composition.

The concentration of species that disappeared from theaqueous phase was determined from the difference betweenthe influent and effluent concentrations at steady-state con-ditions [taking into consideration the expected NH3(g) strip-ping, using Eq. (1)]. These results, along with the compositionof the precipitates obtained in the various experiments, areshown in Table 3. The normalized ratios between TAN, Mgþ2,PT, and Caþ2, derived in a given experiment from both theaqueous phase removal data and from the dissolution of theformed solids, are shown in brackets in Table 3. For a givenexperiment, the ratios show close similarity between dataderived from the aqueous phase and that obtained from the

dissolution of the precipitates; in particular, the TAN:Mg2þ

ratio is very close to 1:1 throughout the pH values studied atan HRT of 15 min.

Ca2þ showed an approximately constant concentration andthe lowest removal from the aqueous phase at all the HRTand pH values studied, indicating that the precipitate com-prised some calcium minerals, but at a low percentage (*5%of the precipitated mass). Of the three species that comprisethe struvite molecule, NH4

þ (in practical terms: TAN) showedthe lowest (and also constant) removal at all the HRT and pHvalues studied. As NH4

þ does not precipitate as any othersolid under the investigated conditions, struvite formationwas assumed to be equal to the TAN removed (excludingNH3(g) stripping), that is, around 8.0 mM in the aqueousphase, a value similar to that obtained in the theoretical PPcalculations as function of 10 mM MgCl2 dosage (Fig. 2). Thisvalue of struvite precipitation is around five times higher thanthe value calculated when no Mg2þ was added to the reactor(8.0 vs. 1.4 mM). Moreover, the removal of 8.0 mM of struvite

FIG. 2. Theoretical struvite PPas a function of MgCl2 (Mg ad-dition at pH 7.4, 8.0, and 8.6). PP,precipitation potential.

Table 3. Volumetric Mass (Normalized Ratios in Brackets) of Species That Were Removed

from the Aqueous Phase, and Corresponding Precipitate Composition in Precipitation Experiments

Performed at Different Hydraulic Retention Times and pH Values

HRT (min) pH Phase (unit) TAN Mgþ2 PT Caþ2

15 7.4 Aqueous (mM) 8.1 (1.00) 8.7 (1.07) 8.6 (1.06) 0.4 (0.05)Precipitate (mmol/g) 7.5 (1.00) 8.1 (1.08) 9.2 (1.22) 1.7 (0.22)

8.0 Aqueous (mM) 8.7 (1.00) 8.9 (1.02) 10.8 (1.24) 0.4 (0.05)Precipitate (mmol/g) 7.8 (1.00) 8.0 (1.03) 9.0 (1.15) 1.5 (0.19)








HRT, hydraulic retention time.



from the filtrate line is expected to result in a reduction of*19% and 2.5% in the phosphate and ammonia loads on theWWTP, respectively (on the basis of the average TAN andphosphate concentrations in the raw wastewater and the fil-trate, and the fact that the filtrate line represents *1% of thetotal flow rate to the WWTP).

Effect of HRT and pH on the precipitate composition

In Table 3, at an HRT of 15 min and pH 7.4, very close TAN,PT, and Mg2þ molar removal from the aqueous phase wasobserved (in M terms), suggesting close to homogeneousstruvite precipitation. In the experiments operated with lon-ger HRTs, PT and Mg2þ were removed from the aqueousphase at a higher molar proportion than TAN, indicating thatPO4

3� and Mg2þ also precipitated as minerals other thanstruvite. These results were corroborated by dissolving theobtained precipitate. Direct determination of the percentageof the species in representative precipitate samples (throughcomplete dissolution) is much more accurate than performinga mass balance using inlet and outlet concentrations in theaqueous phase. On the basis of the results listed in Table 3(precipitate dissolution only), the struvite concentration ateach HRT and pH can be calculated based on the TAN con-centration. The results are shown in Table 4, along with theexcess PT, Ca2þ, and Mg2þ that were calculated to precipitate.

At each HRT, the struvite concentration increased with theincrease in pH; however, the increase was not very pro-nounced and the amount of NaOH necessary to increase thepH is not justified by the increase in struvite precipitant mass.Moreover, excess PT and Mg2þ in the precipitate, relative tothe quantities required to produce struvite, decreased withthe increase in pH at each HRT.

On the other hand, excess PT and Mg2þ increased withincreased HRT for the same pH value. These results can beexplained by the fact that struvite precipitation kinetics ismuch faster than that of other magnesium-phosphate miner-als, and thus a decrease in HRT did not allow enough time forother solids to precipitate, resulting in a precipitate that isricher in struvite. This conclusion was corroborated by Celen(2009), who observed that increasing the HRT beyond 10 minhad no effect on phosphate removal efficiency as struvite inthe operation of a pH 8.5 continuous-flow Mg2þ-enrichedreactor for phosphate removal from swine manure.

Under the conditions prevailing in the experiments in thepresent study, struvite, newberyite, monoetite, magnesite,calcium phosphate, and calcite are expected to precipitate,and struvite will be the first mineral to be formed (Musvotoet al., 2000; Regy et al., 2001; Celen et al., 2007). In the presentstudy, on the basis of the results shown in Table 4 and theresults observed by Musvoto et al. (2000), it appears that all theexcess Mg2þ precipitated as newberyite (MgHPO4). The re-maining excess phosphate (which did not precipitate as eitherstruvite or newberyite) precipitated, in all likelihood, withcalcium as amorphous calcium phosphate [Ca3(PO4)2 � xH2O].This conclusion was based on the ratio between the remainingexcess phosphate and calcium concentrations and also on theobservations of other researchers (Ferguson and McCarty,1971; Musvoto et al., 2000). On the basis of this conclusion, nomonoetite, manesite, and calcite precipitated (Table 5). Mostimportantly, the results shown in Table 5 indicate that shorterHRTs and higher pH values promote a precipitate richer instruvite.

XRD analysis of the precipitates formed at the differentHRTs and pH values was carried out to determine the degreeof struvite crystallization. XRD plots of the precipitates thatwere obtained at low pH values (no dependence on HRT)were practically identical, indicating a homogeneous, largelycrystalline precipitate with morphology similar to that of purestruvite. For example, Fig. 3a shows the XRD plot of theprecipitate obtained at an HRT of 15 min and pH 7.4.

In contrast, XRD plots of the precipitates obtained at highpH values (e.g., HRT of 15 min and pH 8.6; Fig.3b) showed,irrespective of HRT, a noisier pattern with reductions in peaksize and definition, thus indicating that the formed precipi-tates were comprised of much less macrocrystalline com-pounds.

Conclusions

Around 8.0 mM precipitated rich struvite can be removedfrom the filtrate of a typical sludge dewatering system byadding 10.0 mM MgCl2 to a CSTR. The resulting product canbe considered a good fertilizer. Such struvite precipitation canresult in a decrease of around 19% and 2.5% in the phosphateand ammonia loads on the WWTP, respectively.

Struvite precipitation kinetics was observed to be muchfaster than that of competing minerals that could potentially

Table 4. Calculated Struvite, Excess Total

Phosphate, Mg2þ

, and Ca2þ

in the Precipitates,

as Function of Hydraulic Retention Time and pH

HRT (min) pHStruvite(mmol/g)

Excess PT

(mmol/g, %)Excess Mgþ2

(mmol/g, %)Caþ2

(mmol/g)

15 7.4 7.5 1.7, 18 0.6, 8 1.78.0 7.8 1.2, 13 0.2, 2 1.58.6 8.2 0.9, 10 0.0, 0 1.4

30 7.4 6.7 2.4, 27 1.0, 13 2.18.0 6.9 2.4, 25 0.8, 10 2.48.6 7.0 2.2, 24 0.6, 8 2.5

60 7.4 5.7 3.8, 41 2.5, 30 2.08.0 5.7 3.7, 40 2.1, 27 2.48.6 5.2 3.4, 35 1.5, 20 1.4

Table 5. Hypothesized Struvite, Newberyite,

and Calcium Phosphate Concentrations

in the Precipitate, as Function of Hydraulic

Retention Time and pH

HRT (min) pH Struvite(mmol/g)

Newberyite(mmol/g)

Calcium phosphate(mmol/g)

15 7.4 7.5 0.6 0.68.0 7.8 0.2 0.38.6 8.2 0.0 0.3

30 7.4 6.7 1.0 0.78.0 6.9 0.8 0.88.6 7.0 0.6 0.8

60 7.4 5.7 2.5 0.78.0 5.7 2.1 0.88.6 5.2 1.5 0.5

738 LEW ET AL.

precipitate. A decrease in HRT from 60 to 15 min promotedthe formation of a precipitate that was richer in struvite. In-crease in pH values from 7.4 to 8.6 in the given experimentsresulted in a less-homogeneous precipitate, although the masswas slightly higher (in P units). XRD results showed poorer

struvite crystallization at higher pH values, irrespective ofHRT.

The results of the present study corroborated previousresults reported by Mavinic et al. (2007) and Fattah et al. (2008),who concluded that, to obtain a precipitate with a high struvite

FIG. 3. X-ray diffraction result of the precipitate collected from the continuous reactor operated at an HRT of 15 min and pH7.4 (a) and 8.6 (b). HRT, hydraulic retention time.



content, the precipitation reactor should be operated at pH<8.0 and a relatively short retention time. In the present work,the best conditions to attain both a considerable reduction inthe phosphorus concentration in the belt press filter line andthe most homogeneous struvite precipitate (85%) were foundto be Mg2þ dosage of 10 mM, pH 7.4, and an HRT of 15 min.

Acknowledgment

This research was supported by grants from MekorotWater Company Ltd.

Author Disclosure Statement

No competing financial interests exist.

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