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    plant 3 served as a control without an anaerobic tan/* The -AS of the (BPR plant was withdrawn directly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