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    REFIXATION OF PHOSPHATES RELEASED DURING BIO-P SLUDGE

    HANDLING AS STRUVITE OR ALUMINIUM PHOSPHATE

    ABSTRACT

    Phosphate release and phosphate fixation during sludge treatment of waste activated sludge was

    investigated with a

    enhanced biological phosphorus removal, sludge treatment, polyphosphate, struvite

    !TR"#$CT"!

    %ost of the new or expanded wastewater treatment plants in &ermany are designed for the so

    called enhanced biological phosphorus removal process '(BPR)* n contrast to conventional plants, the

    phosphorus content of the activated sludge solids from this process reaches values of up to + *

    Phosphorus can be bound in the activated

    -AS, 'ii) the estimation of the amount of P.release and the resulting P.feedbac/ during sludge

    stabilisation, and 'iii) the investigation of physico.chemical P.fixation mechanisms in stabilising systems*

    %AT(RA0S A!# %(T1"#S

    The pilot plant 'PP) consisted of two continuous flow activated sludge systems both operated with

    settled domestic sewage '2igure 3)* Plant 4 has been operated with an anaerobic 5one for (BPR, whereas

    plant 3 served as a control without an anaerobic tan/* The -AS of the (BPR plant was withdrawn directly

    from the activated sludge tan/ to prevent anaerobic conditions prior to sludge treatment* Thic/ening of the

    sludge was carried out with a centrifuge, a flotation unit or by gravity thic/ening* Thereafter, the thic/ened

    sludge was mixed with primary sludge and pumped into the stabilising system that consisted of an

    anaerobic.mesophilic digester 'A%S) and aerobic.thermophilic stabilisation 'ATS)* The stabilising reactors

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    were operated in parallel at different retention times 'A%S6 37 to 89 days: ATS6 8 to 34 days),

    temperatures 'A%S6 87;C: ATS6 79 to 7.m

    filtration) and Ptot were determined* P">.P concentration represents the so.called dissolved reactive

    phosphate '#RP) and the difference between Ptotand P">.P is called the nonreactive phosphate '!RP)*

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    (lementary analyses of P, Ca, %g, , Al and 2e in the sludge samples were performed by means of

    atomic absorption spectrometry 'AAS) with a Per/in (lmer 4399* Soluble Ca4, %g4, and !awere

    analysed by ion chromatography with a #ionex SP 4999* Al8was determined by a colorimetric method

    using chroma5urol S* .ray diffraction analysis were performed using a ST"( powder diffraction system*

    2or energy dispersive .ray spectroscopy a Doel DS% 87 scanning microscope and a Tracor 7799 were

    used* -ith this system the element distribution of the samples could be visualised for a total of E elements

    at the same time* All other analyses were performed according to #(F ?8@*

    R(S$0TS A!# #SC$SS"!

    Type and mechanisms of P.binding in -AS

    #uring the 4.year experimental period, the P, %g, , Ca, 2e and Al contents of the -AS from the

    (BPR plant were determined wee/ly* 2rom a correlation analysis, it was found that magnesium and

    potassium were significantly correlated on an G 9*93 level with phosphorus* This indicates that poly.P

    formation, which usually is accompanied by an upta/e of these cations, has ta/en place* The linear

    regression between the cations and the phosphorus content of the -AS is s/etched in 2igure 4* 2rom this

    graph a molar upta/e ratio of 9*887 % %g %.3P and 9*47E % %.3 P can be calculated which agrees

    well with values reported in the literature 'e.g. ?3>, 3@)* !o correlation between P and Ca, 2e or Al was

    found* Conse=uently, the amount of physicochemically fixed phosphorus in the -AS of the (BPR plant

    was very low under the operating conditions used in this study*

    Although these dependencies provided a strong indication that at least part of the phosphorus is

    fixed as poly.P, it was not possible to calculate the exact amount of poly.P storage* To =uantify the amount

    of poly.P, P.fractionations were used* 2igure 8 shows the results of the periodically performed

    fractionations of the -AS from the (BPR plant* As it can be seen from this figure, the maHor part of total

    phosphorus is recovered as !a"1.!RP* n -AS from plants with (BPR, this fraction usually consists of

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    organic phosphorus and poly.P, whereas at plants with iron or aluminium precipitation the maHor part of

    precipitated phosphorus is also found in this fraction* A differentiation between the different P.species is

    facilitated if the counterions are considered* Potassium gives especially valuable indications toward

    P.binding in the !a"1.fraction* Because of the former upta/e in the course of poly.P synthesis, potassium

    is expected to be released simultaneously with poly.P during the al/aline extraction* Because potassium

    usually participates only to a small degree in precipitation or adsorption reactions in wastewater and sludge

    treatment, it can be assumed that high potassium levels in the extracts are mainly the result of poly.P

    hydrolysis* Therefore, we loo/ed for a dependence between potassium and !RPI#RP concentrations in the

    different extracts* 2or the !a"1.!RP fraction, this dependence is also depicted in 2igure 8* 2rom this

    graph it can be seen that !a"1.!RP and potassium are very closely correlated* This clearly demonstrates

    that for the -AS from the pilot plant, the maHor part of phosphorus in the !a"1.!RP fraction can be

    assigned to poly.P* 2urthermore, for the other fractions, a similar correlation between #RP and potassium

    was found 'data not shown)* n all, a poly.P content of 79 to +9 of total P could be calculated assuming

    an exchange ratio between phosphorus and potassium of 9*8> % %.3P*

    P.release and P.fixation during sludge stabilisation

    Because of the elevated temperatures in anaerobic.mesophilic 'T G 87;C) or aerobic.thermophilic

    'T G 79 to

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    that a shift from the !a"1.!RP fraction in -AS towards the !a"1., 1Cl. and to a lesser extent to the

    original.#RP fraction has occurred* The former two fractions mainly consist of physicochemically fixed

    phosphorus, whereas the latter fraction represents the soluble phosphate in the stabilising system*

    The same result, that is, a complete release of poly.P, was obtained performing potassium balances

    for the stabilising systems ?7@, assuming that potassium is released during poly.P hydrolysis and does not

    participate in precipitation reactions and remains, therefore, in soluble form*

    Although these experiments provide evidence that stabilising -AS from (BPR plants with A%S or

    ATS causes a rapid hydrolysis of poly.P, only a part of the released phosphate remains in solution* n our

    experiments the amount of soluble P">.P depended mainly on the total P.concentration in the stabilising

    system, which primarily reflects the amount of poly.P in the inflow to A%S or ATS* At total P

    concentrations in the stabilising system of 3,999 to 3,799 mg l.3Ptot, which is common for large wastewater

    treatment plants, the amount of soluble phosphate accounts for not more than 49 of Ptot, whereas at

    excellent (BPR conditions with a total P concentration of up to >,999 mg l.3, the amount of P">.P

    increased to 8E of P tot*

    2rom the results obtained so far it seems clear that the difference between released phosphorus and

    the soluble phosphorus concentration observed during stabilisation was mainly fixed by physicochemical

    mechanisms* To estimate the amount of physicochemical phosphorus fixation, some of the possible

    counterions for precipitation andIor adsorption reactions were examined further* n view of their high

    amounts in stabilised sludge, aluminium, magnesium, and calcium should be the most li/ely counterions for

    physicochemical fixation of phosphorus*

    Beside the sludges from the pilot plant, different stabilised sludge samples from large wastewater

    treatment plants with or without (BPR, which are described in detail in %aterials and %ethods, were also

    included in the investigations* They were examined towards possible interactions of magnesium, aluminium

    and calcium with phosphate*

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    %agnesium is affected by sludge stabilisation in two ways6 2irst, because of the degradation of

    organic material a part of the physiological magnesium is dissolved, and second, magnesium is released in

    the course of poly.P hydrolysis* n view of the high ammonium concentrations in stabilising systems, a

    precipitation of magnesium in the form of %g!1>P"> < 14" 'struvite) seems to be the most li/ely

    reaction to occur* n fact, struvite was found in most of the sludge samples as was demonstrated by .ray

    powder diffractometry and energy dispersive .ray spectroscopy '(#S)* This is shown in 2igure 7 for a

    digested sludge sample from the (BPR pilot plant* The diffraction pattern 'A) of the sludge agrees well

    with the theoretically expected pattern for struvite and, furthermore, (#S shows 'B) that phosphorus

    and magnesium are closely correlated in the sample*

    2urthermore, all sludge samples were examined by a se=uential dilution procedure in which

    dissolution of precipitated solid phases is achieved through progressive dilution of the sludge sample* The

    results of these tests are summarised in 2igure < by correlating the amount of released magnesium with the

    released phosphate in the course of the se=uential dilution* 2or the (BPR sludges a surprisingly high

    correlation between phosphate and magnesium release was found* To verify that the observed release

    behaviour was mainly due to the dissolution of struvite, digested sludge from the pilot plant, which was

    supplemented with phosphate and magnesium to induce struvite precipitation 'struvite formation was

    proved by .ray diffraction), was also investigated with the se=uential dilution test* A comparison with the

    (BPR sludges reveals a nearly identical release behaviour which provides further evidence that the

    released amounts of magnesium and phosphate in the (BPR sludge samples are due to the dissolution of

    struvite solids* 2rom the se=uential dilution test the amount of struvite in the original sample could easily

    be determined using the total phosphate and magnesium release during the dilution procedure* The amount

    of P.fixation in form of struvite was highest in the (BPR sample from the pilot plant '8+ of Ptot) and was

    usually in the range of 49 to 89 of P tot?>@ &reater deviations from the predicted release behaviour were

    only found in the sludge samples from the plant with simultaneous precipitation, which is obviously due to

    a dissolution of iron phosphate*

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    Because of the substitution of detergent phosphates with 5eolites in &ermany 'e.g. 5eolite A6

    !a34'Al"4)34'Si"4)34K 4+ 14"), sludges from wastewater treatment plants usually show relatively high

    aluminium concentrations 'in the sludge samples6 3+ to 87 mg Al g.3TS)* Therefore, aluminium was also

    considered as a possible counterion for phosphate precipitation or adsorption in stabilised sludge* Although

    in none of the sludges indications for crystalline aluminium solids were found, acidimetric titration of the

    sludge samples reveals a significant participation of aluminium in phosphate fixation as is shown in 2igure

    +* Below p1 8 to p1 8*7, the release behaviour of both aluminium and phosphate is very similar, whereas

    greater differences for the phosphate release exist at higher p1 values which are mainly due to the release

    of other solid phases 'e*g* struvite)* 2or the (BPR sludges from plants with the main stream process, the

    amount of P fixed by interaction with aluminium, was calculated as 87 to 74 of P tot* Although the exact

    mechanisms of phosphate.aluminium interactions are not clear yet, we believe that phosphate is mainly

    fixed by surface reactions, such as complexation or adsorption to aluminium solids*

    Calcium was also considered as one of the possible counterions for phosphate fixation in stabilising

    systems and was, therefore, further examined* Dust as with aluminium, no crystalline calcium phases were

    found in any of the sludges* Because most of the possible calcium.phosphate precipitates are acid.labile,

    acidimetric titration was used to determine the amount of possible calcium.phosphate fixation*

    n 2igure E the release of calcium is normalised to the total calcium content of the sludge samples*

    Some interesting information concerning possible interactions between calcium and phosphate could be

    obtained from this graph* 2irst, although there are =uite great differences in the total calcium

    concentrations in the sludge samples, only minor differences are found in the release behaviour normalised

    to the total concentration* 2urthermore, the behaviour of the different sludges 'e.g.primary sludge, (BPR

    sludge and digested sludge supplemented with phosphate and magnesium) is nearly the same* Second, a

    correlation between the release of phosphate and calcium was not found in any of the titrations 'data not

    shown)* Therefore, it seems li/ely that calcium did not participate in phosphate fixation reactions and was

    mainly bound by other mechanisms in the sludge samples, such as adsorption to hydroxyl surfaces*

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    n cases of high P.concentrations in sludge water, precipitation of phosphate in the centrate or

    filtrate of the dewatering facility can be necessary* n principle, all common chemicals for phosphate

    precipitation could be used but in view of the relatively high ammonium concentrations and the high

    al/alinity of the process water, precipitation with calcium can re=uire large amounts of lime* $sing iron,

    the reduction of 2e8to 2e4has to be considered*

    2igure J shows the results of P"> precipitation in sludge water using different chemicals and

    different initial phosphate concentrations* As can be seen from these figures, aluminium proved to be most

    effective on a molar base* $sually, more than E9 of the soluble phosphate was precipitated at a molar

    dosage of 3 % Al %

    .3

    P, whereas for calcium and iron an E9 elimination was achieved only when a molar

    dosage of 4 % Ca %.3P or 3*7 % 2e %.3P was reached*

    C"!C0$S"!S

    $nder the conditions of this study 79 to +9 of total phosphorus in -AS of the (BPR pilot plant

    was stored as poly.P which could be calculated on the basis of P.fractionations and potassium balances*

    Poly.P synthesis was always accompanied by an upta/e of magnesium and potassium at a molar ratio of

    9*8> % %g %.3P and 9*4< % %.3P, respectively*

    Poly.P hydrolysis during stabilising -AS was complete within the retention time of the stabilising

    systems which could be demonstrated by P.fractionations and potassium balances* 1owever, because of

    physicochemical fixation mechanisms only a part of the released phosphate remains in solution* n the

    P.fractionation a shift from the !a"1.!RP fraction of the -AS 'primarily poly.P) toward the !a"1. and

    1Cl.#RP fractions 'primarily physicochemical P.fixation) in stabilised sludge was observed* The amount

    of soluble phosphate in the stabilising system depends mainly on poly.P content in the -AS* 2rom the

    results obtained in our study, the P.feedbac/ on the average large (BPR plant, characterised by a total P

    content of not more than 87 mg P g.3TS in the -AS, was estimated to be below 49 of the influent P

    load*

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    n the stabilising system the released phosphate was fixed mainly by two mechanisms6 2irst,

    because of the simultaneous release of magnesium during poly.P hydrolysis, a part of the released

    phosphate was precipitated as struvite* Second, another fraction of released phosphate was fixed by

    interactions with aluminium, probably by surface reactions on aluminium solids* !o participation of

    calcium in phosphate fixation reactions was found*

    To prevent a possible P.feedbac/ in cases of high phosphate concentrations in the sludge water of a

    dewatering system, precipitation with lime, iron or aluminium could be necessary* 2rom our experiments

    aluminium.phosphate precipitation seemed to be most effective*

    AC!"-0(#&(%(!TS

    The financial support for this study was provided by the &erman %inister for Research and Technology

    'B%2T), grant !o* 94 -S EJ44I4*

    R(2(R(!C(S

    ?3@ Arvin, (*: ristensen, &*1* '3JE7) (xchange of organics, phosphate and cations between sludge and

    water in biological phosphorus and nitrogen removal process* Water Science and Technology, 17,

    3>+.3

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    ?7@ Dardin, !*: PMpel, 1*D* '3JJ>b) Phosphate release of sludges from enhanced biological P.removal

    during digestion* Water Science and Technology, 30, 4E3.4J4*

    ?*

    ?J@ PMpel, 1*D*: Dardin, !* '3JJ8) nfluence of enhanced biological phosphorus removal on sludge

    treatment* Water Science and Technology, 28, 4

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    parameter plant 3 plant 4volume 'm8) 39 3 4*7anaerobic 1RT3'h) . 3*7%CRT 'd) 4*< 4*9*< 39 38 87 33*