Optimizing Struvite Production for Phosphate Recoveryin WWTP

A. L. Forrest1; K. P. Fattah2; D. S. Mavinic3; and F. A. Koch4

Abstract: Struvite, a crystalline structure comprised of ions of magnesium �Mg+2�, ammonium �NH4–N�, and phosphate �PO4–P�, iscommonly encountered in wastewater treatment plants �WWTPs� through struvite encrustation. The gradual accumulation of this crystalin pipes and fittings leads to high costs due to downtime and replacement of parts. Technologies that are used to reduce this problem areideally located in biological nutrient removal plants downstream of anaerobic digesters, as high levels of NH4–N and PO4–P typicallycharacterize anaerobic digester supernatants. In 2003–2004, two technical-scale, struvite recovery studies were conducted on-site at theCity of Penticton, B.C., Canada and the City of Richmond, B.C., Canada using a novel technology developed by the environmentalengineering group at the University of British Columbia. The results of these studies showed an average reduction of 80% in phosphateand a dense, spherical product, 5–10 times larger than any commercially available struvite to date. Overall, the recovery of struvite hasenormous commercial potential, which results in WWTP becoming more sustainable, concurrently reducing problems typically associatedwith their operation.

DOI: 10.1061/�ASCE�0733-9372�2008�134:5�395�

CE Database subject headings: Reactors; Phosphorus; Sludge; Optimization; Sustainable development.

Introduction and Background

Need for Phosphorus Recovery

Global phosphate reserves are a finite resource, occurring exclu-sively as phosphate ore. Through an increasing reliance of manyindustries on phosphate, there is a growing necessity for sustain-able phosphate management. Phosphate ore is found worldwidebut, with current extraction practices, mining is focused in threecountries: United States, China, and Morocco. It was estimated in2001, by the U.S. Geological Survey �USGS�, that the globalreserve life of phosphate ore was approximately 90 years �Robertsand Stewart 2002�. This is far different from its previous 5-yeartrend estimate, made in 1996, of 160 years �Steen and Agro1998�. Of more immediate concern to the North American fertil-izer industry is the fact that the U.S. reserve life is estimated atonly 25 years �Roberts and Stewart 2002�. The end result is that,in the next quarter century, the United States will go from being a

1Doctoral Candidate, Dept. of Civil Engineering, Univ. of BritishColumbia, Environmental Fluid Mechanics Group, Vancouver BC,Canada V6T 1Z4 �corresponding author�. E-mail: forrest@civil.ubc.ca

2Doctoral Candidate, Dept. of Civil Engineering, Univ. of BritishColumbia, Environmental Engineering Group, Vancouver BC, CanadaV6T 1Z4.

3Professor, Dept. of Civil Engineering, Univ. of British Columbia,Environmental Engineering Group, Vancouver BC, Canada V6T 1Z4.

4Research Associate, Dept. of Civil Engineering, Univ. of BritishColumbia, Environmental Engineering Group, Vancouver BC, CanadaV6T 1Z4.

Note. Discussion open until October 1, 2008. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be filed with the ASCE ManagingEditor. The manuscript for this paper was submitted for review and pos-sible publication on April 19, 2006; approved on July 13, 2007. Thispaper is part of the Journal of Environmental Engineering, Vol. 134,

No. 5, May 1, 2008. ©ASCE, ISSN 0733-9372/2008/5-395–402/$25.00.

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dominant exporter of phosphorus �P� to the largest global con-sumer. USGS mineral progress reports in 2003 have indicated adecrease in domestic production of nearly 10% from 2002 to2003 �USGS 2003�. Currently, there are five main driving forcesbehind the development of P recovery: the diminishment of glo-bal high quality phosphate reserves; the reduction in the return ofrereleased phosphorus from anaerobic sludge digestion to theheadworks of wastewater treatment plants �WWTP�; improvedsludge management in advanced WWTP �AWWTP�; control ofstruvite encrustation in biological nutrient removal �BNR� plants;and, marketability of struvite as a sustainable product.

In enhanced biological phosphorus removal �EBPR� and BNRplants, the rerelease of over assimilated P during sludge digestionhas been observed to reach levels of 10–25% �Wild et al. 1997�or higher, as suggested by other authors �Mavinic et al. 1998;Niedbala 1995�. Concentrations in the anaerobic liquor have beenreported to reach concentrations of 100 mg /L, through the diges-tion of secondary sludge �Battistoni et al. 1997�. Not a problem initself, if this liquor is returned to the headworks �as is most oftenthe case� it increases the P loading in the influent stream of theplant by up to 40% �Jardin and Popel 1994�. As the overall effi-ciency of the BNR process depends on the BOD5:P and BOD5:Nratios �BOD�biological oxygen demand�, this can cause de-creased plant performance and potential process failure �Mavinicet al. 1998�.

One of the gains of implementing P removal technologies inAWWTP are the cost benefits of better sludge management strat-egies �e.g., on-site digestion� in comparison to off-site hauling. InEBPR and BNR processes, the over assimilation of P into thebacterial cells is susceptible to rerelease when subjected toanaerobic conditions. This rerelease is directly correlated with adecrease in the amount of suspended solids �SS� produced inactivated sludge systems in the range of 1.6–2.4 g SS /g P re-leased �Jeanmaire and Evans 2001�. Although debated, it is

thought to be due to the increase in P loading to the system. It was

AL OF ENVIRONMENTAL ENGINEERING © ASCE / MAY 2008 / 395

estimated that P recovery in an AWWTP would reduce the sludgeproduction by 2–8% dry matter, translating to a decrease insludge volume of up to 49% �Jeanmaire and Evans 2001�. Thepotential for cost savings is therefore, significant.

In conjunction with high phosphate levels, high levels ofammonia also characterize anaerobic digester liquors. In the pres-ence of magnesium, this will lead to a high potential for theformation of magnesium ammonia phosphate �commonly knownas struvite or MAP�. Struvite particles form deposits that accumu-late in zones of high turbulence, such as pump impellers and pipebends, and lead to decreased flow capacity and eventual equip-ment failure. Struvite accumulation has been observed in numer-ous anaerobic digestion and postdigestion processes worldwide�Doyle and Parsons 2002�. Costly solutions, such as acid washingand glass-lined piping have been tried, although their implemen-tation tends to be impractical in terms of labor, material, andsystem downtime �Williams 1999�.

The economics of struvite recovery also become important infinal product marketing. The value of the recovered P, in the formof struvite, tends to be higher than that of phosphate ore, due tohigher quality �in terms of purity�, as well as low heavy metalcontamination. The intrinsic value of struvite is also important asit was identified as a nonburning, slow-release fertilizer as earlyas the 1960s. The disadvantage is that it is more costly to produce,e.g., USD $140–460 per tonne, compared to the USD $40–50 pertonne that is required for phosphate ore. This cost differential isoffset by the suggested market value of USD $198–1885 pertonne �Ueno and Fujii 2001; Jaffer et al. 2002�. Several fluidizedbed reactors have been in full-scale production in Japan and therecovered struvite has been sold as a fertilizer �Shimamura et al.2003� at a price of approximately USD $250 per tonne in 2001dollars �Ueno and Fujii 2001�. This market analysis demonstratesthat, if distributed successfully, WWTP can generate revenue tooffset operational costs, through struvite recovery processes, con-currently becoming more sustainable.

P Recovery at the University of British Columbia

In 1999, the Department of Civil Engineering, at the Universityof British Columbia �UBC�, initiated a P-recovery program,in collaboration with British Columbia �BC� Hydro. BC Hydrobecame involved through its interest in alternative sources ofphosphate fertilizers for lake and reservoir fertilization projects.Since then, the project team has developed a novel fluidized bedreactor design that has proven successful in recovering phos-phates in the form of struvite, from real and synthetic super-natants, on both the bench and at pilot scale �Adnan 2002�. Thisreactor design has two advantages over other commercially avail-able designs. First, the application of a fluidized bed allows forcontinuous reactor operation and self-classifying of the growingcrystals to the point of harvest. Second, struvite crystals up to5 mm in diameter have been produced; these are nearly 5–10times larger than values reported in the literature �Battistoni et al.2005; Ueno and Fujii 2001� and of much greater crystal hardnessthan previously demonstrated �Fattah 2004�. It should be notedthat the measure of hardness was a qualitative assessment, basedthe observations made of the final product.

Supersaturation Ratio

Bench-scale studies at UBC demonstrated the ability of this tech-nology to recover 80–90% of orthophosphates, in the form of

harvestable struvite crystals �Dastur 2001�. This was directly

396 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / MAY 2008

correlated with the operating pH for the reactors, for low to mod-erate concentrations of phosphate in the influent stream �Adnan2002�. Further studies found that equivalent phosphate recoveriescould be achieved at lower pH values, through the use of thesupersaturation ratio �SSR� as a controlling variable �Fattah 2004;Adnan et al. 2003�. Recall that SSR is defined as the ratio ofconditional solubility product, PS, and the equilibrium conditionalsolubility product, PS

eq �Snoeyink and Jenkins, 1980�.Several different factors are involved in estimating the struvite

formation potential. These include the analytical molar concentra-tions of each of the component ions �magnesium—Mg+2,ammonium—NH4–N, and phosphate—PO4–P�, the operating pHof the system and the conductivity. As struvite forms is equimolarin each of the constituent ions, the PS is simply defined as theproduct of these three measured concentrations. PS

eq is defined asthe equilibrium conditional solubility product

PSeq =

Ksp

��Mg+2�Mg+2���NH4+�NH4

+���PO4−3�PO4

−3��1�

where Ksp�solubility product for struvite; ��ionization fractionof each of the respective species; and ��activity coefficient forthe respective ion species. Although theoretically the value of Ksp

is constant, due to the changing component concentrations eachtime equilibrium is reached, PS

eq is highly correlated with pH.SSR is an important control variable for this P-recovery tech-

nology, as it incorporates each of the measured components of theprocess fluid. Knowing the value of SSR, it is possible to predictthe potential for struvite formation: values of SSR greater than 1indicate the potential for struvite formation; values at unity indi-cate equilibrium; and, values less than 1, reveal that the system isundersaturated and result in no potential for struvite formation.SSR is also used over the log-based equivalent of the saturationindex due to the fact that the operating range of SSR was rela-tively small in this work �SSR values less than 10�.

Final Product Quality

A problem in trying to predict the growth rates of a heterogeneoussystem is that the behavior of crystalline particles in suspensionsystems is subject to several different growth factors. Initially,particle formation results directly from primary nucleation inthe supersaturated bulk fluid �Regy et al. 2001�. These particlesthen grow, not just by incorporating molecules into the crystallattice, but also by incorporating nuclei as macrogrowth crystals�Takiyama et al. 1997�. They also have the potential to growthrough agglomeration with other particles of the same nature, aprocess known as secondary nucleation �Gomez-Morales et al.2001�. At the same time, attrition and breakage will have a nega-tive effect on the growth rates and decrease the overall size of thecrystals, especially in high shear systems �Liiri and Aittamaa2002�.

Many different models have been developed for traditionalbatch type reactors; however, there is a limited applicabilityto fluidized bed reactors. One concept that can be applied is thatof the metastable zone. This region represents a stable growthzone lying between the state of supersaturation and undersatura-tion, where primary nucleation may not occur, but secondarygrowth phenomena do �Regy et al. 2001�. It has been shown forstruvite that this metastable zone is ideal for the growth of largesize ��1 mm diameter� crystals and lies within a range of SSR

values between 3 and 5 �Adnan et al. 2003; Britton et al. 2005�.

At the upper boundary of this range the characteristics of thecrystal change dramatically becoming smaller and more brittle�Adnan et al. 2003�.

Several P-recovery processes have been in operation in Japan,Belgium, and Italy since the early 1990s; however, the small size��1 mm diameter� and brittleness of the recovered products haveresulted in low economic return. It has been concluded that theseprocesses will not provide a reasonable return on investment untilthe reach the point is reached where they can produce products inthe diameter size range of 3–5 mm, and with sufficient hardnessto be spread with a commercial fertilizer spreader �CEEP 2001�.Using these criteria, the research group at UBC applied their par-ticular technology and succeeded, in a pilot-scale experiment, tomeet these objectives �Adnan et al. 2003�.

Project Objectives

In 2003, two independent technical-scale �four times larger thanpilot scale� experiments were established, to further test thistechnology; both sludge digester supernatant and centrate wereused as reactor influent. The first of these was at the PentictonAWWTP, Penticton, B.C., Canada and consisted of two,technical-scale reactors, with a design volume 4 times larger thanthe previously established pilot-scale reactors �Forrest 2004�. Thesecond experiment was at the Lulu Island WWTP �LIWWTP�,Richmond, B.C., Canada and consisted of one reactor of a similardesign �Fattah 2004�. The fundamental difference between thetwo projects was the fact that the second used centrate as theprocess fluid, in comparison with the digester supernatant used inthe first.

As previously stated, primary goal of both of these projectswas to prove that P recovery was possible using this technologyon real process fluids on a technical scale. An equally importantgoal of this work was gaining control over the size and morphol-ogy of the resultant crystalline product. The following were thespecific project objectives that were developed:• Test the technology, on a technical-scale, on real process flu-

Seed Hopper

Reaction Zone

Active Zone

Harvest Zone

MgCl2

NaOH

Check Valve

Feed Bypass

Fig. 1. Simplified illustratio

ids. To meet this objective, a performance overview was done

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in order to identify those factor�s�, which provide the highestlevels of P removal.

• Identify the factor�s� controlling product size and morphologyof the final crystal product.

• Assess final product quality of this process and how it com-pares with other commercial available struvite.

• Quantify the impact that the P-recovery process has on sludgeproduction in WWTP.

Methodology

Process Design

Each technical-scale P-recovery reactor had a feed line from thedigester, a magnesium chloride �MgCl2� dosing line and a caustic�NaOH� dosing line to control the pH. The operating concept ofthis technology was that all the chemicals were combined at thebottom of the reactor to create the ideal SSR condition for pri-mary nucleation. The main reactor body was comprised of severaldifferent sections of increasing diameter, with a final settling zoneat the top. These upflow transitions served a dual purpose: To helpsort and classify the larger, growing struvite particles into thefaster velocity fields and to induce as much turbulence as possibleinto the fluidized bed. It was hypothesized that induced turbu-lence encourages both the primary nucleation of the crystals, aswell as the agglomeration process, thus yielding the final charac-teristic product. The crystals were harvested from the reactorthrough a pair of isolation valves located on the bottom of thereactor. Britton et al. �2005� provides a detailed explanation of theP-recovery process that is illustrated in Fig. 1.

The final settling zone, the seed hopper, was located at the topof the reactor and was designed to achieve two goals: To detainfine material and provide continuous reseeding of the reactor. Theentrapment of fine particles was achieved through gravity settling,as the velocity field in this settling zone was significantly lowerthan that of previous sections. These crystals would then start toresettle back into the main flow of the reactor, where they would

CentrateHolding Tank

Clarifier

Effluent

SludgeRecycle

pH Probe

RecycleBypass

e UBC P-recovery process

15"

6"

4"

3"

n of th

continue to agglomerate. The crystals that had grown to a large

AL OF ENVIRONMENTAL ENGINEERING © ASCE / MAY 2008 / 397

enough size would eventually settle into the harvest zone and beperiodically discharged from the reactors, via two isolationvalves. As this was a continuous process, no additional materialwas required for reactor seeding �e.g., sand�, as demonstrated inprevious studies �Battistoni et al. 1998�. The overflow was thenreturned to a clarifier, which was used as the sump for the recycleline.

As indicated in Fig. 1, four influent lines converge in the in-jection zone; the MgCl2, the NaOH, the digester feed and therecycle. Mg+2 is added to the system because typical concentra-tions in digester supernatants are too low to favor struvite precipi-tation. The NaOH is metered into the system �as controlled by thepH controller� to maintain the required value of SSR.

Sampling

Grab samples of influent and effluent were collected daily andmonitored for Mg+2, NH4–N, PO4–P, conductivity, and pH.Samples for ionic measurements were filtered through 0.45 �mmembrane filter papers. As these filters tend to clog easily, thesamples were prefiltered using Fisherbrand G6 filter papers with anominal size of 1.25 �m. Mg+2 was measured with a developedin situ field technique �Forrest et al. 2007� and subjected to post-analysis using flame atomic absorption spectrophotometry �modelSpectrAA220, Varian Inc., Palo Alto, Calif.�. Orthophosphate andammonia were measured using a flow injection method �modelQuikChem 8000, Lachat Instruments, Milwaukee, Wisc.�. Allanalyses were carried out according to Standard Methods for theExamination of Water and Wastewater �APHA/AWWA/WEF1995�. These values were then used to calculate the SSR in thebottom section of the reactor. Additionally, the flowrates weremeasured to calculate the upflow velocity in the reactor, repre-senting the velocity of flow in the harvest zone of the reactor. Thecrystal product was harvested daily or on alternate days, depend-ing on the operating regime of the reactor.

Crystal Product Analyses

The harvested struvite product was tested for composition andpurity using two different techniques; flame absorption spec-trophotometry �AA� and inductively coupled plasma massspectrophotometry �ICP-MS�. AA analyses were conducted onrandom samples taken from LIWWTP and tested for the threecomponent species of struvite, as well as, Al+3, Ca+2 andFe+2 /Fe+3, according to Standard Methods for the Examination ofWater and Wastewater �APHA/AWWA/WEF 1995�. Previous re-sults on samples taken from these sites found that potassium waspresent in trace amounts, with most of the analyses being belowthe method detection limit �Huang 2003�. ICP-MS analyses wereconducted on a series of samples taken from the PentictonAWWTP, the LIWWTP and samples of struvite crystals formedfrom on-going synthetic supernatant studies at UBC.

Results and Discussion

Performance Overview

In both experiments, reactor SSR was used as the control-ling parameter for efficient reactor performance. The SSR was,in turn, maintained by controlling the reactor pH. The averagecomponent species concentrations, in the untreated influent

streams, for the Penticton AWWTP were 91.9 mg /L PO4–P,

398 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / MAY 2008

321.9 mg /L NH4–N, and 22.4 mg /L Mg+2 �n=100� and59.3 mg /L PO4–P, 762.5 mg /L NH4–N, and 6.5 mg /L Mg+2

�n=76� for LIWWTP. Throughout both studies, P-removalefficiencies as high as 95% and above were readily achievable�Fattah 2004; Forrest 2004�. Although previous studies deter-mined removal efficiencies in terms of SSR at the inlet �Adnan etal. 2003�, it was determined that the expected SSR in the bottomzone of the reactor provided better control. The importance ofreactor SSR is increased when the feed to the reactor is dilutedwith the recycle flow from the external clarifier. Pilot-scale ex-periments at UBC determined that P removal increased with anincrease in the inlet SSR �Adnan et al. 2003�. Data collected fromLIWWTP also found similar results, using the reactor SSR. Oneimportant difference was that the increase in P removal was onlytrue until a certain SSR was reached. For the LIWWTP centrate,the value of SSR was approximately 2.8. Above this value, therewas a decreasing trend in removal efficiency. Although SSR wasdetermined to be a vital parameter for efficient reactor operation,other factors like Mg:P and N:P molar ratios, pH, upflow veloc-ity were also found to be important �Fattah 2004�.

Product Characteristics: Size, Morphology, and Quality

The desired quality and size of struvite crystals will largely de-pend on its final application and use. For struvite crystals intendedfor agricultural purposes, it is desirable to have them hard �or lessbrittle�, to enable use with current fertilizer spreading machinesand also to facilitate handling and shipping �CEEP 2001�. Anotheradvantage of harder crystals is that they tend to be denser andthus cheaper to transport in bulk. Although liquid fertilizers havebeen used to stimulate the base of the food chain, their use islimited to more accessible streams and rivers and the high cost ofmaintenance required �Ashley and Slaney 1997�. Struvite, whichis a slow-release fertilizer, has the advantage that a once-per-yearor a per-season application can be sufficient and more operation-ally efficient than other methods, such as continuous drip ormonthly application of solid granular fertilizers. The use ofsmaller struvite crystals is desired for enrichment of nutrient de-ficient water bodies—the size being important when the release

Fig. 2. Influence of SSR and Mg:P molar ratio on mean crystalsize �mm� �data taken from operation data from Penticton AWWTP�n=92��

rate is considered. An increased size would lead to a slower re-

lease rate, due to the lower surface area/volume ratio. Studies onstruvite pellets have found that the size of the pellets were influ-enced by the Mg:P molar ratio and the influent phosphorus con-centration. One study showed that aggregation was favored athigher magnesium concentrations �Bouropoulos and Koutsoukos2000�.

The average size of struvite crystals grown at PentictonAWWTP and LIWWTP were 2.0 and 3.3 mm, respectively. Thedifferences may be attributed to different operating conditionsof the reactors, such as upflow velocity, pH, and SSR. Itwas suspected that the growing matrix of the crystals, and inparticular the water hardness and other impurities of the processfluids, were also the partial cause of the size differential. Experi-ments at LIWWTP showed that it was possible to grow crystals aslarge as 5 mm under suitable conditions �Fattah 2004�. Using asimilar process principle �Phosnix� at pilot scale, 0.11 mm crys-tals, with struvite content greater than 90% were grown in Aus-tralia �Munch and Barr 2001�. The process used by Kurita WaterIndustries, located in Japan, is generally operated to achieve grainsizes of 2.0–3.8 mm �Abe 1995�.

Fig. 2 illustrates the effect of reactor SSR and Mg:P molar

Fig. 3. Scanning electron microscope images of magnified crystal�b� inside of the crystal shown in �a� ��45�; �c� surface of a crystal

ratio on the mean pellet size. Fig. 2 demonstrates a strong corre-

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lation between the mean crystal size, SSR and the Mg:P molarratio. It appears that ideal crystal growth is achieved for values ofSSR ranging from 1 to 2.5 and values of Mg:P ranging from 1 to2.5. At high levels of SSR, it appears that crystals cannot begrown for any value of Mg:P although it does appear that mod-erate size crystals can be grown for higher levels of Mg:P ratiosprovided that the SSR values are toward the lower end of therange.

Struvite is a white crystalline substance that has a distinctiveorthorhombic structure �Doyle and Parsons 2002�. The internalstructure of the pellets consists of regular PO4

3− tetrahedral, dis-torted Mg�H2O�6

2+ octahedral and NH4+ groups which are all held

together through hydrogen bonding �Abbona and Boistelle 1979�.Reports on the spontaneous formation of struvite in supersatu-rated solutions suggested that the nature of the struvite pelletsdepends on the solution pH, the solution supersaturation, theMg:P molar ratio, impurities in solution and the kinetic growthfactors �Bouropoulos and Koutsoukos 2000; Wierzbicki et al.1997; Abbona and Boistelle 1979�.

High resolution, scanning electron microscope images of

e harvested at LIWWTP: �a� surface of a 2.83 mm crystal ��45�;�; and �d� inside of the crystal shown in �c� ��300�

surfac��300

LIWWTP struvite pellets were also taken, as illustrated in Fig. 3.

AL OF ENVIRONMENTAL ENGINEERING © ASCE / MAY 2008 / 399

In order to examine the interior, the pellets were cut in half. Thesurface of the pellets is predominantly smooth �Figs. 3�a and c��,whereas, the interior consists of numerous fine crystals of differ-ent sizes. From these images, it can be seen that the individualbrick-like and rod-like struvite crystals are tightly packed,confirming the observations of others �Adnan 2002�. Crystalsharvested from Penticton AWWTP also showed similar mor-phology, with the crystal growth progressing from loosely aggre-gated to tightly packed and more solidly bound pellets �Brittonet al. 2005�.

Product Quality

For the AA analyses, 15 random samples were taken and analyzedfrom the struvite product generated from LIWWTP. The resultsare presented in Table 1. As shown, there is evidence of im-purities; however, the purity, at an average value of 96.0%, isin agreement with previous investigations conducted on syn-thetic supernatant at UBC that had an average purity of over 90%�Britton et al. 2005; Huang 2003�. To more fully characterizeheavy metal impurities, the ICP-MS analysis results were com-pared to results from two commercial sources of phosphates. Theresults are presented in Table 2.

As shown in Table 2, the heavy metal impurities of struviteare, for the most part, significantly lower in the harvested product,than from these two forms of commercially available phosphates.

Table 1. Summary of Struvite Pellet Composition Analysis

Masscontent�%�

Theoreticalvalue�%�

Measuredvalue�%�

Standarddeviation�n=10�

Mg+2 9.9 9.5 0.6

NH4–N 5.7 5.7 0.2

PO4–P 12.6 12.0 1.0

Struvite �estimated� 100 96 3.5

Al+3 0 0.06 0.02

Ca+2 0 0.07 0.06

Fe+2 /Fe+3 0 0.12 0.03

Table 2. Comparison of Varying Sources of Phosphorus �mg/L�

Contents �ppm�

MoroccoP rocks

�phosphate ore�a

GeestmeraCa3�PO

�commerrecovere

Aluminum �Al� 200 95

Cadmium �Cd� 40 �

Chromium �Cr� 357

Iron �Fe� 1,600 1,26

Magnesium �Mg� 5,700 4,20

Sodium �Na� 1,700 36

Nickel �Ni� 67

Tin �Ti� 108

Zinc �Zn� 880 31

Manganese �Mn� 10 56

Copper �Cu� 23 1

Arsenic �As� 5aJeanmaire and Evans �2001�.bHigh due to the presence of aluminum sludge treated in the treatment pc

High due to the deliberate addition of external magnesium salt for struvite form

400 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / MAY 2008

Although there is a significantly larger concentration of Mg+2

�due to the addition of external magnesium salts for maintainingdesired SSR�, the levels of impurities, such as arsenic, nickel andtin, are significantly lower, thereby increasing the marketability ofthese crystals.

Sludge Production

It has been shown that sludge production in BNR and EBPR typeprocesses is reduced in the range of 1.6–2.4 g SS /g P released�Jeanmaire and Evans 2001�. It is thought that any increase insludge production is due to the P loading entering a plant. A valueof 2.0 g SS /g P released was chosen for this study. If the P that isreleased is recovered in the form of struvite, it is possible tocorrelate the operating condition of the struvite reactors with thereduction in sludge production, as is shown in Fig. 4.

Fig. 4 shows the potential reduction in SS in the plant, corre-sponding to the operating condition of the pilot reactor. Betweenoperating SSR values of 0–3.0, fairly consistent SS reduction was

t PentictonAWWTPstruvite�n=4�

LIWWTPstruvite�n=4�

Syntheticstruvite�UBC��n=2�

1161b 203 101

0.7 0.5 0.82

1.8 2.1 3.6

307 2,018 389

161,455c 155,981c 158,550c

365 282 83.5

9.4 8.7 15.1

3.7 5.3 3.0

47.1 29 101

113 145 21

79.9 67 265

0.3 1 4

orrest 2004�.

Fig. 4. Correlation of SS reduction with operating condition of thereactor �using the influent PO4-P of the Penticton AWWTP�

mbach

4�2

ciallyd P�a

0

6

8

0

0

0

8

8

0

0

7

2

lant �F

ation �Fattah 2004; Forrest 2004�.

achieved in the range of 150–170 mg SS reduced /L. Above anoperating SSR value of 3.0, the SS reduction was greatly dimin-ished. Although the typical flow conditions of these technical-scale reactors were very small, typically 1–2.5 L /s, the potentialfor significant cost savings is evident, if this were applied to afull-scale facility.

The other indirect benefit of recovering struvite is that, byremoving the component species, it is possible to prevent, orat least reduce, the formation of struvite crystals in the entireplant. This can prevent/reduce the potential for scaling that istypically seen in many secondary and advanced treatment plantsand dramatically reduce costs and downtime involved with stru-vite clogging.

Conclusions

The primary goal of these two studies was achieved when it wasdemonstrated that this technology could work on real processfluids. An equally important goal was to control the size andmorphology of the generated crystal product. The potential forsludge reduction was also examined, in order to assess the poten-tial for economic savings. The following conclusions were drawn:• Both sets of technical-scale reactors demonstrated phosphate

removal was possible, using SSR as a partial control variablein the real process fluids.

• SSR was shown to be related to the Mg:P ratio in controllingthe crystal mean size of the harvested product.

• The size of the crystals generated at both sites had an averagecrystal mean size 5 to 10 times larger than struvite generatedfrom other research groups.

• The crystals generated from the Penticton AWWTP were dis-tinctly smaller than those from the LIWWTP. This may be theresult of impurities in the water or the chemical characteristicsof the process fluid itself �e.g., hardness�.

• For process SSR values of 0 to 3, it was shown that a potentialsludge reduction in the plant was possible in the range of150 mg SS to 170 mg SS reduced /L.

• The purity of the struvite was such that it easily qualifies as acommercial grade fertilizer. In addition, the final product waslow in impurities and was of a much higher quality than sev-eral forms of commercially available phosphorus basedproducts.

Acknowledgments

The writers are grateful for the cooperation and assistance fromthe staff at both the Penticton AWWTP, as well as the Lulu IslandWWTP, in Richmond, B.C. The technical backup provided bythe staff of the Environmental Engineering Lab, UBC, is alsoacknowledged. Financial assistance for this work originated atthe B.C. Hydro and Power Authority, the Natural Sciences andEngineering Research Council of Canada �NSERC� and StantecEngineering, Ltd., Vancouver, B.C.

Notation

The following symbols are used in this paper:Ksp � solubility product for struvite;PS � conditional solubility product;eq

PS � equilibrium conditional solubility product;

JOURN

SSR � supersaturation ratio;�i � ionization fraction of each of the respective ith ion

species; and�i � activity coefficient for the respective ith ion species.

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