8) Pergamon

Pll: 50273-J223(98)004J2-0

Wat Sci Tech.Vol. 38, No. I. pp. 21S-283, 1998.IAWQ

~ 1998PublishedbyElsevier Science Ltd.PrintedinGreat Britain.All rights reserved

0213-1223198 S19'00+ 0'00

PHOSPHATE REMOVAL IN REALANAEROBIC SUPERNATANTS:MODELLING AND PERFORMANCE OF AFLUIDIZED BED REACTOR

P. Battistoni*, P. Pavan**, F. Cecchi*** andJ. Mata-Alvarezj

··Institute ofHydraulics. Engineering Faculty. University ofAncona, 60131Ancona,Italy.. University ofVenice, Department ofEnvironmental Sciences, Dorsoduro 2137Venice. Italy... University ofL'Aquila, Department ofChemistry. Chern. Eng.and Mat, L'Aquila,Italyt University ofBarcelona, Department ofChem.•Eng> c. Martii Franque 1/6,08028Barcelona, Spain

ABSTRACT

Phosphateremoval in anaerobic supernatantcomingfrom a centrifugation sludge stationof an AfJ processis studied. A fluidized bed reactor is employed to crystallize phosphate as hydroxyapatite or struvite usingonly air stripping to reach the supersaturation pH. The classiccomposition of supernatant (alkalinity 3550mgCaCOyt. P04 139 mgll. Mg 24 mgll)docs not requireany additionof chemicalsfor phosphate removal.Seventeen runs arc performed in a benchscale FDRobtaining very high conversion and removalefficiencyand phosphate loss in the efnuent S3.5%. The usc of Ca or Mg enriched supernatant has no meaningfulinfluence on efficiency. but it determines the prevalentsalt formedbetweenMAP or HAP.Efficiency can berelated to pH and sand contact timein a doublesaturational model.The half efficiencyconstants: 0.075 h fort and 7.75 pH. have an important role in the processknowledge and optimization of plant design. Exhaustsand analysis indicatesthe same composition at the lop, bottomand mean of the sand bed(39% mol MAPand 61% mol HAP).This result togetherwith the high half efficiency constantfor contact time indicatethatthe phosphategrowthon the bedis not competitive. Finally.the phosphate releasefrom the plant is studied.Resultsshow a weakrelease rate. equivalent to 2.8-10% dol phosphate as MAP,obtainedat an operativepHrangeof 8.1-8.4.@ 1998Published by ElsevierScienceLtd.All rightsreserved

KEYWORDS

Crystallisation; fluidized bed reactor; hydroxyapatite; magnesium ammonia phosphate; phosphorus removal.

NOMENCLATURE

FBR=fluidized bed reactorHAP=hydroxyapatite, Ca5 OH (P04hN=number of cycleson FBRor sandgrains

HRT=hydraulic retention timeMAP=struvite, MgNH4P04oI2HzOQi=influent flowrate

275

276

QR= recycle flowrateSPM=phosphate release rate related to PMAPVI=stripper volumeV3=FBRtower free volumet=bed porosityll=phosphate removal efficiency

Subscripts

P. BAmSTONl tl al.

SP=specificphosphatereleaseVevolumeV2=stripperdevice volumet=contacttime on sand grainsX=phosphate conversion

FBR=FBRsectorTetotal (for HRT stripping plus FBR sectors)sesolubleo=effluentf=in small particlesdefined fines

INTRODUCTION

stripp=stripping sectorexp=cxpanded bedi=influentcry=expanded crystallization bed

Phosphate removal in anaerobic liquors represents a necessity to limit the feedback of phosphorus releasedin sludge handling or in anaerobic digestion and to improveperformances of biological P removal plants. Inthis area, fluidized bed reactors to crystallize phosphateas hydroxyapatite or struvite are diffuse (Momberget al., 1992; Fujimoto et al., 1991). All of them use a base dosage to obtain the right pH for supersaturation,but, considering the high buffer capacity of the supernatant,expensive processesare produced which worsenthe water composition.A further aspect is the necessityto add calcium or magnesiumto satisfy or overcomethe stoichiometric demand (Fujimoto et al., 1991) or to select MAP or HAP formation (Momberg et al.;1992). On the contrary, their normal presence could be used for phosphateremoval without any addition ofchemicals. The same situation is found in the removalof phosphate in the effluent of wastewater treatmentplants: a strong weighton operationalcosts is attributedto chemicals: 92% in Van Dijk (1985) pilot or 52%in Egger (1991) demonstrativeplant.The classic composition of an anaerobicsupernatant interfereswith theprocess because carbonate inhibits HAP formation (Jenkins et al.; 1971) or determines calcium carbonateco-crystallization (Seckler et al., 1990), while magnesium ions, even at low concentration, affect calciumphosphate precipitation (Abbona et al., 1990; Kibalczyc et al., 1991). An advanced analysis of calciumphosphate crystallisation on a fluidized bed and a mathematical model for process optimization has beenpresented by Seckler (1996a-b-c) even if a single passage on the bed was the main aim of the study andalkali addition inside the bed was needed. However, a high phosphate precipitation (80-100%) but a lowremoval efficiency (=50%) is obtained. In a recent paper, Battistoni (1997) demonstrated the feasibility toobtain the supersaturation pH by air stripping a phosphateenrichedanaerobic supernatant(40 mg POJl) andthe possibility to remove the phosphate only as MAP in an FBR plant. In this work, a real anaerobicsupernatant supplied from an A20 process is used to investigate phosphate crystallisation without anyaddition of chemicals.Phosphate removalefficiency was mathematically modelled to optimize the process.

PROCESSDES~ON

The process is based on the precipitation of MAP or HAP using both a basic pH by air stripping and themagnesiumand calcium ions concentration of the supernatant. The supersaturation cannot be calculated withthe ion activities and the solubility product of the salts (Seckler et al., 1996c) because carbonate andmagnesium exerted an inhibitory effect in a similar system (Battistoni et al., 1997). The FBR plant (FigureI) in the stripping zone produces supersaturation, thereforesub-micronparticles initially and then small saltparticles defined 'fines' are formed as their aggregates (Seckler et al., 1996b). Fines give 'aggregation' withsand grains and 'breakage' in relationto the recycle on the fluidizedbed (Seckleret al., 1996b).

A further contributionto phosphate removal is the moleculargrowth toward the bed of soluble phosphate. Inthis scenario, phosphate leaving the plant is attributed to residual fines (P04fo'> and soluble phosphate(P04so) ' Phosphate aggregated on sand grains (P04cry) and phosphate removal efficiency and conversioncan be determined through the total and soluble phosphate monitoring in influent and effluent of a

Phosphate removal in real anaerobicsupernatants 277

continuously fed FER plant accord ing to eq. I 5. The conversion is defined as the percentage of phosphateprecipitated. In the upward movement of the liquid inside the FER, the number of passages (N) isdetermined by the ratio between the mean hydraulic retention time in the plant and that spent in theexpanded bed, while the contact time on sand grains depends on the number of cycles and the time neededfor one single passage (Table 1).

Table I. Meaning of the main operative parameters of the FER plant

PO. ro=PO.TO • PO. oo [1] POeay =PO.Tin - PO.TO [2]PO·fix,= __0 .100 [3]PO.n.

P04..,[4] X=TJ+X, [5]TJ = P04Ti ·100

V. +V2+V,VI +V2

HRTT= QI[6 ] HRT,1ripp =~ [7 ]

V, V. -eHRTFBR = QII [8] HRT. =...!!!- [9]

exp QII

N= HRT, [ 10] t= N·HRTu.p [ 11 ]HRTm

MATERIALS AND METHODS

Materials

Anaerobic liquors are carried out by the centrate of a solid bowl centrifugation station fed with anaerobicallydigested sludges , The full scale civil wastewater treatment plant (100.000 P.E.) adopts an A20 process fornutrient removal and waste activated sludges are removed as mixed sludges from primary sedimentat iontanks. The sludge line follows a convent ional depurative scheme with gravitational thickening before andafter a mesophilic anaerobic digestion process. The chemical-physical characterist ics of liquors (Table 2) areperformed using Standard Methods (APHA 1985).

Table 2.Chemical-physical character istics of liquors

Parameters Average Min Max s.d. npH 7.7 7.4 8.0 0.1 29SCOD mgll 1320 1097 1626 223 29PO. mgll 139 125 209 16 29NH. mgll 914 558 1301 127 29Ca mgll 153 104 168 15 29Mg mgll 24 23 27 3 29HCO, mgCaCO/l 3550 2400 3850 325 29CO, mgCaCOI1 0 0 5 29CalPO. mol mol" 2.6 1.4 3.0 0.4 29MglPO. mol mol" 0.7 0.3 0.8 0.1 29NH.t PO. mol mol" 35 S.6 21 S3 29

278

Pilot plant

FBR

P. BArnSTONI et al.

'J~-----~~ .

p

__ ..•... rtf.! .•.•. ...............· .· .· .· .· .· .· .· .· .lit I

~~~

by-pall

p

FigureI. FBRpilotplant.

The FBR pilot plant is constituted of two sectors (Figure I), a stripping tank (18+3 I) and a fluidized bedreactor (ljli 0.09 m, height I m), The column is filled with 3.3 kg of quartz sand (0.21-0.35 mm) to obtain acompress bed height of 0.4 m with virgin sand. Anaerobic supernatant, as supplied from the real plant, is fedthrough a peristaltic pump, but daily renewed. One air diffuser is used for a flowrate up to 50 Jlmin. Therecycle flowrate (0.23 m3/h) remains constant for all runs allowing a fixed expanded bed height of I m. Online monitoring of pH, air and influent flowrates, temperatures (air and FBR) is performed by a data loggerwhile influent and effluent are collected through an auto sampler. In 'total' and filtered (0.45 urn) samples,both of influent and effluent, phosphates are analysed, while Ca, Mg, NH4 and alkalinity are determinedonly in 'total' samples. No differences exist between total and soluble phosphate in the influent.

Release tests

Phosphate release is studied using an FBR plant filled with exhaust silica sand (12.5% P04, 5.44 %Ca,1.25% Mg and 0.96% NH4) and tap water (flow rate 4 JIm) at different pH (6.9+8.7) as feeding. Effluentsamples are collected and analysed as in the FBR runs.

Sand analysis

Sand removed from FBR plant is air dried for 72-96h, then weighed and £ measured. Dried grains are treatedwith 2 N HCI, boiled for 30 min and Ca, Mg, NH4 and P04 are analysed in the acid solution.

RESULTS AND DISCUSSION

Anaerobic supernatants are supplied from a full-scale BNR plant. During the six months of experimentationthey conserve a rather constant composition (Table 2) with a low-medium P04 content (139±16 mg/l) withrespect to the typical concentration of liquors coming from a phosphorus release step (Popel et al., 1993).The CaIP04 molar ratio is 2.6±0.4 exceeding the stoichiometric request (1.7) for HAP formation, while the0.7±O.1 MglP04 molar ratio is insufficient to guarantee a complete MAP formation (l.0) even though astrong concentration of ammonia is present. The bench scale pilot plant under widely varying operatingconditions is studied. Influent and air flowrates are the main conditions investigated. Up to seventeen runsare performed. By feeding the reactor with real or with magnesium or calcium enriched supernatant (Table3), the strategy was to increase the CaJP04 molar ratio from 2.6 up to 5.8 and the MglP04 ratio from 0.7 toabout 1.6 to investigate how different compositions affect phosphate removal.

Phosphate removal in real anaerobic supernatants 279

Table 3. FBR operative conditionsand performances

Run CalMg CafPO. MglPO. Qi Qair pH X TJmolmol" mol mol'' molmol" IJh l/min % %

1 3.6 2.5 0.7 4 15 8.48 84.7 82.7la 1.8 2.9 1.6 4 25 8.55 88.1 86.7Ib 6.6 5.3 0.8 4 20 8.40 84.4 83.02 3.7 2.2 0.6 8 30 8.42 76.4 75.42a 1.9 2.9 1.5 8 30 8.39 76.0 75.02b 7.0 5.6 0.8 8 25 8.21 78.8 78.83 4.1 2.9 0.7 12 35 8.29 76.0 73.03a 1.9 3.0 1.6 12 35 8.32 74.3 71.73b 7.3 5.8 0.8 12 35 8.23 72.6 72.14 3.6 2.9 0.8 15 35 8.07 72.8 70.04a 1.6 2.8 1.7 15 35 8.15 71.0 69.04b 6.7 5.4 0.8 15 35 8.13 72.1 70.05 4.0 2.8 0.7 19 50 8.41 78.2 77.0Sa 1.7 2.8 1.6 19 35 7.94 59.3 58.25b 7.3 5.1 0.7 19 50 8.35 78.1 76.35c 3.5 2.8 0.8 19 40 8.10 71.4 67.95d 4.0 7.6 1.9 19 45 8.25 75.2 72.9

Each run shows a typical pH and P04 trend versus time (i.e.Run 3a. Figure 2) with an initial start-up phaselasting about 2 h. followedby a steadystate condition. The phosphate exponential decrease during the start­up phase, was interpreted by Seckler (l996b) as necessary for covering the sand with a minimumthicknessof phosphate salt which makes the subsequent salt growth easier. In our case, both the methodologyfollowed and the fact that the bed used comesfrom sevenpreviousruns, suggesta trend connectedonly withthe specularpattern of pH. Run time rangesfrom a minimum of 1 d for enrichedfeeding to a maximum of 4d for real supernatant. The mass balance demonstrates high performances in phosphate efficiency with amaximumXrat 3.5% and an averageof 1.7±O.9% (Table3).

I ,' 140pH ......pH P04

1 ........P04t (mgn)I ,ll Ii ......P04' 120

_P041

1001,4

JJ 1.. .~ 10...~1,2

-r

\60

I40

1,' 20

1, 00 0,1 0,2 0,3 0,4 0,5 0,' 0,1 0,' O,t 1

Time (d)

Figure 2. FBRplant Run 3a.

280 P. BAmSTONI et al.

10 • •HAP % •to •• ••• ................. •MAP%10

.........70 ......--.to /-.

/10

/40 /

/

<.30 //

20 //

10./ ••• • •• •

00 2 4 1 1 7

CalMg (mol/mol)

Figure3. Prevision ofsandgrainsgrowth: HAPandMAP%moJ.

Efficiency is strongly related to the operative pH, the lowest result being 58.2% at 7.94 operative pH, buteven in this case no meaningful fines increment is observed (Xr=l.1%). This means that the process up­scaling does not require a filtration step of the effluent. On the contrary, this was needed in previousphosphate crystallisation processes with alkali or chemical additions (Dijk et al., 1985; Egger et al., 1991;Fujimoto et al., 1991) to fix the high Xf obtained even at optimum conditions (i.e. Xf 20% or more, Seckleret al., 1996b). The different molar ratios adopted exert a strong control on MAP or HAP formation (i.e. all aand b runs, see Table 3); however they are the only salts formed independently of the efficiency orconversion obtained (Figure 3). In particular, the runs with real supernatant can be represented by a CalMgratio of about 3.6. They generate a MAPIHAP molar ratio of about 35/65, while a Mg enriched supernatant(CalMg=1.8) gives a MAP formation ranging from 80 to 100%. An inverse situation is observed with Caenriched supernatant. These results are different from those obtained with anaerobic supernatant comingfrom a biological N removal plant, which was used as a matrix to study phosphate removal in a FBR(Battistoni et al., 1997). Notwithstanding a higher CalP04=11 and a lower CalMg=2.l molar ratios withrespect to those in the present work (i.e. 2.6 and 3.7 respectively), only MAP formation was obtained. Thiscan be explained by the Mg concentration ~S3 mg/l exerting both a strong inhibitory effect on HAPformation and a high MglP04=S.2 molar ratio. A good mathematical model can be individuated relating theefficiency to pH and to the contact time on sand grains [eq. 12]. The double saturational model has thephysical meaning that not only the operative pH, but also the contact time characterises the processperformances. In fact, a first linear model of efficiency versus pH does not explain more than 80% of resultvariance.

pIt t,,= 100· pIt +O.1S '-t+-0-.0--7--S [ 12]

R2-O.98 SE=2.S n-17runs pH" ""pH· 7.6

At the same time, useful information can be drawn from the half efficiency constants for pH and t: anoperative pH of 7.7S,equivalent to pH· = 0.15, reduces the efficiency by a half. The same result is obtainedoperating at a contact time equal to 4.5 min (0.075 h). The contact time is calculated by N, but for the same tdifferent N are possible since e changes with phosphate growth on sand and consequently HRTexp forconstant Qr and Vexp changes [eq. 9 and 11]. In fact, £ varies from a maximum of 0.66 to a minimum of 0.37passing respectively from virgin (run 1) to exhaust sand (run 5d) as shown in Table 4. Therefore, the halfefficiency constant of 0.075 h for contact time is equivalent to 3.8+6.7 passages in the bed in relation to the

Phosphate removal in real anaerobic supernatants 281

growth level. For this reason, t is the best operative parameter to be considered for comparing results fromdifferent researches. In this work, the operative t ranges from a minimum of 0.7 h (run 5d) to a maximum of4.8 h (run I, Table 4) and it is very high with respect to its half efficiency constant. Therefore, furtherexperimental work is required for improving the validity of the model proposed. However, the model tracesthe route to optimise the process since a lower t can be possible at a higher operative pH and it is feasible onincreasing the air to influent ratio.

Table 4. Operative conditions of FBR plant

Run t E N(h)

1 + Ib 4.8 0.65 + 0.66 243 + 2452 +2b 2.3 0.6+ 0.64 124 + 1283 +3b 1.5 0.54 + 0.58 8+ 7904+4b 1.1 0.46 + 0.51 74+ 785 +5d 0.7 +0.8 0.37 + 0.44 63+ 68

The results obtained by the mass balance of the solution were checked by chemical analysis of the exhaustsand grains (Table 5). A MgINH4= I molar ratio on grains confirms the presence of MAP while the residualamount of phosphate is correspondent only to HAP, even if a 1+3% mol of CaC03 can be present. After 24working days no significative differences exist in terms of phosphate and HAP or MAP growth between thetop and bottom sand grains , in spite of the different starting and final granulometric distributions ($starting=0.3 mm, final 4>=0.4 mm; Table 5). Furthermore, the results are consistent with a uniform presencein the bed of 39%mol MAP and 61%mol HAP. All these evidences in addition to the value of the halfefficiency constant for the contact time (4.5 min) and to the fact that no difference can be noted in themathematical correlation of the results obtained using real or enriched supernatant, suggest that thecrystallization process occurs without competition.

Table 5. Exhaust sand grain analysis

BEDMEDIUM ljlmmTopBottomMean 0.4

Ca%w5.445.165.44

Mg%w1.251.181.24

12.611.912.5

NH.%w0.920.870.92

Table 6. Phosphate release from an exhaust FBR plant

Run

123

Feeding

Tap-waterTap-waterTap-water

Flowrate(lib)

444

pH

7.007.858.7

Ca:Mg:NH.:PO.molar ratios

1 : 8: 7: 7:8I : 6.9: 6.9: 7.5o : I : 1 : 0.93

This aspect can be related to the pH adopted in FBR runs, always lower than 8.55 (Table 3), while a basicreagent addition determines an operative pH range from 8.6 to 10.5 (Van Dijck et al., 1985; Egger et al.,1991; Momberg et al., 1992). To complete the process analysis, the release of phosphate from an FBR plantis investigated. Exhausted sand, after all the runs of Table 3, is employed and feeding tap water at differentpH (Table 6) is used. The pH and phosphate plots versus time (Figure 4) show that by feeding the bed at lowpH 7.0, steady state conditions cannot be reached because of the salt dissolution which determines a pHincrease in the effluent. On the contrary, the influent pH at 7.8 or more determines an immediate steady stateeven if at a higher pH.

282 P. BAmSTONI et al.

I 800P04To

RUN2RUN3

700

800

~ 500

400

300

200

100

8, 05 0

Figure4 Phosphate releasetests.

Concerning the type of salt, the MglNH4IP04 molar ratios (Table 6) demonstrate that only MAP is released,while the calcium concentration is always lower than that in the tap water used. The specific phosphaterelease rate, expressed as g P04 /kg sand d-I and calculated via a mass balance in the effluent, can bedefined as a function of pH according to eq. 13. In a similar way, even the specific phosphate release rateexpressed as g P04/kg P04MAP d-I can be related to pH according to eq. 14.

SP =88.18- 10.34pH

0=30

[ 13 ] SPM = 2128.3·249.7 pH

R2= 0.875 SE .. 39.9 0=30

[ 14]

In the operative conditions of pH ranging from 8.1 to 8.4, the SP decreases from 4.4 to 1.3 g POikg sand d­" this can be considered very low in relation to the fact that the feeding simulates a hard condition of a realplant: no presence of phosphate in the supernatant. Finally, in terms of the MAP present on the grains, SPMis clearly higher than SP. As a matter of fact, SPM reaches 40% d-I, but it represents an extreme condition:no phosphate, no air stripping. On the contrary, at the same previous operative pH of 8.1-8.4, a weak releaseis obtained: from 2.8 to 10% d-I of that present as MAP. These facts must be attributed to the differencebetween the solubility products (Kps) of MAP (Kps=12.6) and that of HAP (Kps=57.8).

CONCLUSIONS

The present work outlines many facts on phosphate crystallisation, which can be summarised in thefollowing points:

in an FBR plant the process has high performances and can be carried out without any addition ofchemicals on real supernatant of an A20 process;the use of anaerobic supernatant supplied from a centrifugation dewatering station does not requireany pre-treatment to remove the low suspended solid content;Mg and Ca ions are enough to guarantee the removal of phosphate as a mixture of MAP and HAP;the CalP04 and MglP04 molar ratios strictly influence the molar percentage of MAP or HAPformed, so even if Ca and Mg concentrations vary, only a different MAPIHAP mixture must beexpected;

Phosphateremovalin real anaerobicsupernatants 283

the low operative pH due to air stripping limits the loss of phosphate as fines and adds simplicity tothe depurative flow chart since no filtration step is necessary;contact time and pH as the main operative conditions define the process efficiency in anexperimental saturational model, but further research is needed to optimise the plant design;a moderate MAP release from the FBR is observed when no phosphate supernatant is used and theFBR plant operates in the 8.1-8.4 pH range.

ACKNOWLEDGEMENT

Authors wish to acknowledge the financial support of Regione Veneto, Italy

REFERENCES

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Egger, E., Dirkzwager, A. H. and Van der Honing, H. (1991). Full scale experiences with phosphate crystallization in acrystalactor.Wat.Sci. Tech.,23(4-6),819-824.

Fujimoto,N.• Mizouchi,T. and Togarni,Y. (1991). Phosphorous fixationin the sludge treatmentsystemof a biologicalphosphateremovalaccomplishments and needs. War. Sci. Techn. 25(12), 1471-1478.

Momberg,G. A. and Oellerman, R. A. (1992). The removalof phosphate by hydroxyapatite and struvite crystallization in SouthAfrica. Wat. Sci. Tech., 26(516),967-976.

Popel, H. J. and Jardin, N. (1993). Influence of enhanced biological phosphorus removal on sludge treatment. War. Sci. Tech.,28(1),263-271.

Seckler,M. M., Bruinsma.S. L••Roseden,G. M., van Ogh, J. C.. Delgage,1. and Egger. E. (1990). Phosphate removal by meansof a full scale pellet reactor. Int. Symp. Ind. Cryst., 18·20 September,GarmischPartenkirchen, Germany

Seckler, M. M.• Bruinsma. S. L. and Van Rosmalen, G. M. (1996a). Phosphate removal in a fluidized bed-I. Identification ofphysical processes.Wat. Res., 30(7), 1585-1588.

Seckler. M. M.• Bruinsma. S. L. and Van Rosmalen, G. M. (I996b). Phosphate removal in a fluidized bed-Il. Processoptimization.Wat.Res.•30(7),1589·1596.

Seckler,M. M., Bruinsma,S. L. and Van Rosmalen,G. M. (l996c). Calciumphosphateprecipitationin a fluidized bed in relationto processconditions:a black box approach.Wat. Res., 30(7), 1677-1685.

Van Dijk, J. C. and Braakensiek,H. (1985). Phosphateremovalby crystallizationin a fluidizedbed. Wat. Sci. Tech., 17(213), 133­142.