Environmental Technology, Vol. 22. pp 1253-1262
© Selper Ltd, 2001
REFIXATION OF PHOSPHATES RELEASED DURING BIO-P
SLUDGE HANDLING AS STRUVITE OR ALUMINIUM
PHOSPHATE
N. JARDIN*1 AND H.J. PÖPEL2
1Ruhrverband,
2
Kronprinzenstr. 37, D-45128 Essen, Germany
Carl-Ulrich-Str. 39A, D-64297 Darmstadt, Germany
(Received 28 February 2001; Accepted 20 March 2001)
ABSTRACT
Phosphate release and phosphate fixation during sludge treatment of waste activated sludge was investigated with a pilot
plant for enhanced biological phosphorus removal, complemented by laboratory investigations of sludge samples from
different large enhanced biological phosphorus removal plants. The major part of the eliminated phosphorus in the pilot
plant was due to the storage of polyphosphate in the waste activated sludge and was accompanied by an uptake of
magnesium and potassium. Stabilising waste activated sludge from the enhanced biological phosphorus removal pilot plant
results in a hydrolysis of polyphosphate. As a result of polyphosphate hydrolysis in stabilising systems, phosphate,
magnesium and potassium are released, but only potassium remains in solution whereas magnesium and a part of the
released phosphate was precipitated as struvite. Another large fraction of the released phosphate was fixed by participation
of aluminium.
Keywords
Enhanced biological phosphorus removal, sludge treatment, polyphosphate, struvite
INTRODUCTION
Most of the new or expanded wastewater treatment
plants are designed for the so-called enhanced biological
phosphorus removal process (EBPR; for an overview of the
main types of EBPR processes see [1]). In contrast to
conventional plants, the phosphorus content of the activated
sludge solids from this process reaches values of up to 7 %.
Phosphorus can be bound in the activated sludge mainly by
three mechanisms. One is as physiological phosphorus, that
is, P for metabolism and growth. Second, in plants with EBPR,
additional phosphorus can be stored as polyphosphate
(poly-P), Men+2PnO3n+1 (n indicates the chain length of poly-P
and Me represents a metal cation). Usually, magnesium (Mg)
and potassium (K) are associated with poly-P synthesis.
Finally, a physicochemical fixation of phosphate mainly by
precipitation or adsorption can occur.
From a theoretical point of view, most of the
phosphorus eliminated in the form of poly-P should be
released during the anaerobic treatment of waste activated
sludge (WAS) [2]. In contrast, at most large plants in Europe
the soluble phosphorus concentration of digested sludge is
often very low [3-6] whereas from other plants additional
phosphorus loads of up to 100 % are reported [7-9].
In order to pursue the fate of the total phosphorus
during wastewater and sludge treatment, an EBPR pilot plant
with different sludge treatment systems has been operated
over a 2 year period. Some of the results obtained during this
study are presented in this paper with special respect to (i) the
determination of type and extent of phosphate fixation in
WAS, (ii) the estimation of the amount of P-release and the
resulting P-feedback during sludge stabilisation, and (iii) the
investigation of physico-chemical P-fixation mechanisms in
stabilising systems.
MATERIALS AND METHODS
The pilot plant (PP) consisted of two continuous flow
activated sludge systems both operated with settled domestic
sewage (Figure 1). Plant 2 has been operated with an
anaerobic zone for EBPR, whereas plant 1 served as a control
without an anaerobic tank. The WAS of the EBPR plant was
withdrawn directly from the activated sludge tank to prevent
anaerobic conditions prior to sludge treatment. Thickening of
the sludge was carried out with a centrifuge, a flotation unit
or by gravity thickening. Thereafter, the thickened sludge was
mixed with primary sludge and pumped into the stabilising
system that consisted of an anaerobic-mesophilic digester
(AMS) and aerobic-thermophilic stabilisation (ATS). The
stabilising reactors were operated in parallel at different
1253
retention times (AMS: 15 to 30 days; ATS: 3 to 12 days),
temperatures (AMS: 35°C; ATS: 50 to 65°C) and solid
concentrations (1 to 5 % total solids (TS).
The design parameters and the average operating
conditions of the EBPR and conventional pilot plant in the
first experimental year are shown in Tables 1 and 2,
respectively. Because of the very low phosphorus content of
the domestic sewage used for the experiments, the inflow of
the pilot plant was supplemented with phosphoric acid, and
in some experimental phases, with acetic acid to improve
EBPR.
For the investigation of physicochemical P-fixation
mechanisms sludge samples from different large and pilot
Figure 1.
Table 1
Flow diagram of the pilot plant.
Design parameters of the pilot plants (mean
values during the first experimental year).
parameter
plant 1
plant 2
volume (m3)
influent flow (m3 h-1)
aerobic HRT1 (h)
anaerobic HRT1 (h)
MCRT (d)
10
2.5
4
2.6
16
4
2.5
1.5
2.6
1
EBPR and conventional plants were used. The characteristics
of these sludges are summarised in Table 3.
To investigate the pH-dependent release of phosphate
an acidimetric titration was performed. For this purpose, a 1-l
sludge sample was titrated with concentrated HCl. Once a
desired
pH
was
achieved,
40
ml
of
sludge
was withdrawn and incubated for 24 h to establish
equilibrium. Usually, the acidimetric titration covers a
pH range from original pH to a pH of 1. After the incubation
period, the pH was measured again and the samples
were centrifuged at 30,000g for 10 minutes followed
by 0.45-m filtration. In the filtrate PO4-P and the major
cations were analysed.
Table 2.
hydraulic retention time based on influent flow rate
1254
Average operating conditions of the pilot plants
(mean values during the first experimental year).
parameter
plant 1 plant 2
influent effluent effluent
BOD5 (mg l-1)
COD (mg l-1)
Ptot (mg l-1)
TKN (mg l-1)
175
340
12.4
66
20
82
10.3
23
14
62
3.3
23
NH4-N (mg l-1) 48
Table 3.
19
21
Characterisation of the sludges used for the investigations of physico-chemical P-fixation mechanisms.
sludge
name
(abbreviation)
origin
DS EBPR MS
Digester sludge from a
large EBPR plant with main
stream process (PHOREDOX)
Stabilised sludge from a large
EBPR plant with main stream
process (ISAH)
Digester sludge from the EBPR
pilot plant with main stream
process (A/O)
Digester sludge from a large
EBPR plant with side
stream process (PHOSTRIP)
Digester sludge from a large
plant with simultaneous
precipitation (Fe)
Primary sludge
stab ES EBPR MS
DS EBPR MS PP
DS EBPR SS
DS Sim
PS
aTS
TSa)
[g l-1]
VS b)
[%]
pH
P
Ca
Mg
K
Al
Fe
[mg g-1] [mg g-1] [mg g-1] [mg g-1] [mg g-1] [mg g-1]
38.6
51.4
7.32
27
51.5
9.8
7.4
17.1
15.2
29.8
59.6
6.82
42.8
40.6
10
13
35
11.6
28.8
58.4
7.29
52.2
45.2
11.2
16.9
23.8
10.5
25.9
55.8
7.46
26.8
57.8
7.7
6.8
26.3
12
49.3
51.5
7.8
27.2
85.4
5.8
2
17
49.7
40.6
75.5
5.46
10
31.6
3.6
5.6
17.2
10.2
= Total solids, b) VS = Volatile solids
All sludges were examined further with a sequential
dilution test. Because of the successive reduction of total
solids concentration in the course of the dilution procedure,
the solubility behaviour of precipitated solids can be
selectively modified without changing pH. For this test, 50 ml
of sludge was filled in a 100-ml volumetric flask. Deionised
water (A. dest) was added to make up 100 ml and
subsequently, the dilution was vigorously shaken for 2
minutes. From this dilution step another 50 ml were
transferred to the next 100-ml flask, and the dilution
procedure was repeated as described until a final dilution of 1
to 1024 was reached. After an equilibration period of 24 hours
the samples were centrifuged, filtrated and analysed for
phosphorus and cations.
P-fractionations were used to differentiate between the
phosphorus fixation mechanisms. For this purpose a modified
method of Psenner et al. [10] and Uhlmann et al. [11] was used.
The fractionation consists of sequential extractions of the
sludge samples with different extracting chemicals followed
by incubation, centrifugation and analysis of the supernatant.
In the supernatant PO4-P (after 0.45-m filtration) and Ptot
were determined. PO4-P concentration represents the so-called
dissolved reactive phosphate (DRP) and the difference
between Ptot and PO4-P is called the nonreactive phosphate
(NRP).
Elementary analyses of P, Ca, Mg, K, Al and Fe in the
sludge samples were performed by means of atomic
absorption spectrometry (AAS) with a Perkin Elmer 2100.
Soluble Ca2+, Mg2+, K+ and Na+ were analysed by ion
chromatography with a Dionex ISP 2000. Al3+ was determined
by a colorimetric method using chromazurol S. X-ray
diffraction analyses were performed using a STOE powder
diffraction system. For energy dispersive X-ray spectroscopy
a Joel JSM 35 scanning microscope and a Tracor 5500 were
used. With this system the element distribution of the samples
could be visualised for a total of 8 elements at the same time.
All other analyses were performed according to DEV [12].
RESULTS AND DISCUSSION
Type and Mechanisms of P-binding in WAS
During the 2-year experimental period, the P, Mg, K,
Ca, Fe and Al contents of the WAS from the EBPR plant were
determined weekly. From a correlation analysis, it was found
that magnesium and potassium were significantly correlated
on an  = 0.01 level with phosphorus. This indicates that polyP formation, which usually is accompanied by an uptake of
these cations, has taken place. The linear regression between
the cations and the phosphorus content of the WAS is
sketched in Figure 2. From this graph a molar uptake ratio of
0.335 M Mg M-1 P and 0.258 M K M-1 P can be calculated
which agrees well with values reported in the literature (e.g.
[13, 14]). No correlation between P and Ca, Fe or Al was
found. Consequently, the amount of physicochemically fixed
phosphorus in the WAS of the EBPR plant was very low
under the operating conditions used in this study.
Although these dependencies provided a strong
1255
indication that at least part of the phosphorus is fixed as polyP, it was not possible to calculate the exact amount of poly-P
storage. To quantify the amount of poly-P, P-fractionations
were used. Figure 3 shows the results of the periodically
performed fractionations of the WAS from the EBPR plant.
As can be seen from this figure, the major part of total
phosphorus is recovered as NaOH-NRP. In WAS from plants
with EBPR, this fraction usually consists of organic
Figure 2.
phosphorus and poly-P, whereas at plants with iron or
aluminium precipitation the major 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 NaOH-fraction.
Because of the former uptake in the course of poly-P synthes
is, potassium is expected to be released simultaneously with
Dependence between phosphorus and magnesium/potassium content in WAS.
1256
Figure 3.
P-fractionation of WAS (bars indicate standard error).
poly-P during the alkaline extraction. Because potassium
Figure 4 summarises P-fractionations of both anaerobicusually participates only to a small degree in precipitation or
mesophilic and aerobic-thermophilic stabilised sludge.
adsorption reactions in wastewater and sludge treatment, it
Clearly, the NaOH-NRP fraction that comprises the major
can be assumed that high potassium levels in the extracts are
part of poly-P was reduced to below 4 % (ATS) and 6 %
mainly the result of poly-P hydrolysis. Therefore, we looked
(AMS) of total P. Considering that organic phosphorus will
for a dependence between potassium and NRP/DRP
also be found in this fraction, the poly-P content of the sludge
concentrations in the different extracts. For the NaOH-NRP
samples tends to be zero. From these fractionations it is
fraction, this dependence is also depicted in Figure 3. From
obvious that a shift from the NaOH-NRP fraction in WAS
this graph it can be seen that NaOH-NRP and potassium are
towards the NaOH-, HCl- and to a lesser extent to the
very closely correlated. This clearly demonstrates that for the
original-DRP fraction has occurred. The former two fractions
WAS from the pilot plant, the major part of phosphorus in the
mainly consist of physicochemically fixed phosphorus,
NaOH-NRP fraction can be assigned to poly-P. Furthermore,
whereas the latter fraction represents the soluble phosphate in
for the other fractions, a similar correlation between DRP and
the stabilising system.
potassium was found (data not shown). In all, a poly-P
The same result, that is, a complete release of poly-P,
content of 50 to 70 % of total P could be calculated assuming
was obtained performing potassium balances for the
an exchange ratio between phosphorus and potassium of
stabilising systems [16], assuming that potassium is released
0.26 M K M-1 P.
during poly-P hydrolysis and does not participate in
precipitation reactions and remains, therefore, in soluble
P-release and P-fixation During Sludge Stabilisation
form.
Although these experiments provide evidence that
Because of the elevated temperatures in anaerobicstabilising WAS from EBPR plants with AMS or ATS causes a
mesophilic (T = 35°C) or aerobic-thermophilic (T = 50 to 60°C)
rapid hydrolysis of poly-P, only a part of the released
stabilisation, it could be expected that complete poly-P
phosphate remains in solution. In our experiments the
hydrolysis occurs within a fraction of the usual retention time
amount of soluble PO4-P depended mainly on the total
of these stabilising systems. Using P-release kinetics [15] it can
P-concentration in the stabilising system, which primarily
be estimated that 90 % of the poly-P is hydrolysed within 1.5
reflects the amount of poly-P in the inflow to AMS or ATS.
days at 35°C and within 7 hours at 60°C. Beside this
At total P concentrations in the stabilising system of 1,000
theoretical calculation, further evidence for a complete release
to 1,500 mg l-1 Ptot, which is common for large wastewater
of the stored poly-P in our experiments was provided by
treatment plants, the amount of soluble phosphate accounts
P-fractionations and potassium balances.
for not more than 20 % of Ptot, whereas at excellent EBPR
1257
Figure 4.
P-fractionation of anaerobic-mesophilic and aerobic-thermophilic stabilised sludges (bars indicate standard error).
conditions with a total P concentration of up to 4,000 mg l-1,
systems, a precipitation of magnesium in the form of
the amount of PO4-P increased to 38 % of Ptot.
MgNH4PO46 H2O (struvite) seems to be the most likely
From the results obtained so far it seems clear that the
reaction to occur. In fact, struvite was found in most of the
difference between released phosphorus and the soluble
sludge samples as was demonstrated by X-ray powder
phosphorus concentration observed during stabilisation was
diffractometry and energy dispersive X-ray spectroscopy
mainly fixed by physicochemical mechanisms. To estimate the
(EDXS). This is shown in Figure 5 for a digested sludge
amount of physicochemical phosphorus fixation, some of the
sample from the EBPR pilot plant. The diffraction pattern (A)
possible counterions for precipitation and/or adsorption
of the sludge agrees well with the theoretically expected
reactions were examined further. In view of their high
pattern for struvite and, furthermore, EDXS shows (B) that
amounts in stabilised sludge, aluminium, magnesium, and
phosphorus and magnesium are closely correlated in the
calcium should be the most likely counterions for
sample.
physicochemical fixation of phosphorus.
Furthermore, all sludge samples were examined by a
Beside the sludges from the pilot plant, different
sequential dilution procedure in which dissolution of
stabilised sludge samples from large wastewater treatment
precipitated solid phases is achieved through progressive
plants with or without EBPR, which are described in detail in
dilution of the sludge sample. The results of these tests are
Materials and Methods, were also included in the
summarised in Figure 6 by correlating the amount of released
investigations. They were examined towards possible
magnesium with the released phosphate in the course of the
interactions of magnesium, aluminium and calcium with
sequential dilution. For the EBPR sludges a surprisingly high
phosphate.
correlation between phosphate and magnesium release was
Magnesium is affected by sludge stabilisation in two
found. To verify that the observed release behaviour was
ways. First, because of the degradation of organic material a
mainly due to the dissolution of struvite, digested sludge
part of the physiological magnesium is dissolved, and second,
from the pilot plant, which was supplemented with
magnesium is released in the course of poly-P hydrolysis. In
phosphate and magnesium to induce struvite precipitation
view of the high ammonium concentrations in stabilising
(struvite formation was proved by X-ray diffraction), was also
1258
Figure 5. X-ray diffraction pattern and distribution diagram of phosphorus and magnesium for a digested sludge sample from
the EBPR pilot plant (A: X-ray diffraction pattern; B: distribution of P and Mg from EDXS).
Figure 6.
Phosphate and magnesium release during the sequential dilution test.
investigated with the sequential dilution test. A comparison
with the EBPR sludges reveals a nearly identical release
behaviour which provides further evidence that the released
amounts of magnesium and phosphate in the EBPR sludge
samples are due to the dissolution of struvite solids. From the
sequential 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 the form of struvite was highest in the
EBPR sample from the pilot plant (37 % of Ptot) and was
usually in the range of 20 to 30 % of Ptot [17]. Greater
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.
Because of the substitution of detergent phosphates
with
zeolites
in
Germany
(e.g.
zeolite
A:
Na12(AlO2)12(SiO2)12·27 H2O), sludges from wastewater
treatment plants usually show relatively high aluminium
concentrations (in the sludge samples: 17 to 35 mg Al g-1 TS).
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 Figure 7. Below pH 3 to pH 3.5, the release behaviour of
both aluminium and phosphate is very similar, whereas
greater differences for the phosphate release exist at higher
pH values which are mainly due to the release of other solid
phases (e.g. struvite). For the EBPR sludges from plants with
the main stream process, the amount of P fixed by interaction
with aluminium, was calculated as 35 to 52 % of Ptot.
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. Just 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.
In Figure 8 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. First,
although there are quite 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. Furthermore, the behaviour of the different
sludges (e.g. primary sludge, EBPR 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
1259
(data not shown). Therefore, it seems likely that calcium did
not participate in phosphate fixation reactions and was mainly
Figure 7.
bound by other mechanisms in the sludge samples, such as
adsorption to hydroxyl surfaces.
Al3+ and PO4-P release in the course of acidimetric titration.
In cases of high P-concentrations in sludge water,
precipitation of phosphate in the centrate or filtrate of the
dewatering facility can be necessary. In principle, all common
chemicals for phosphate precipitation could be used but in
view of the relatively high ammonium concentrations and the
high alkalinity of the process water, precipitation with
calcium can require large amounts of lime. Using iron, the
reduction of Fe3+ to Fe2+ has to be considered.
Figure 9 shows the results of PO4 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. Usually, more
than 80 % of the soluble phosphate was precipitated at a
molar dosage of 1 M Al M-1 P, whereas for calcium and iron
an 80 % elimination was achieved only when a molar dosage
of 2 M Ca M-1 P or 1.5 M Fe M-1 P was reached.
CONCLUSIONS
Under the conditions of this study 50 to 70 % of total
phosphorus in WAS of the EBPR 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 uptake of magnesium and
potassium at a molar ratio of 0.34 M Mg M-1 P and 0.26
M K M-1 P, respectively.
Poly-P hydrolysis during stabilising WAS was complete
within the retention time of the stabilising systems which
could be demonstrated by P-fractionations and potassium
balances. However, because of physicochemical fixation
mechanisms only a part of the released phosphate remains in
solution. In the P-fractionation a shift from the NaOH-NRP
fraction of the WAS (primarily poly-P) toward the NaOHand HCl-DRP 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 WAS. From the results obtained in our study,
the P-feedback on the average large EBPR plant, characterised
by a total P content of not more than 35 mg P g-1 TS in the
WAS, was estimated to be below 20 % of the influent P load.
In the stabilising system the released phosphate was
fixed mainly by two mechanisms: First, 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. No participation of calcium in phosphate
fixation reactions was found.
To prevent a possible P-feedback in cases of high
phosphate concentrations in the sludge water of a dewatering
system, precipitation with lime, iron or aluminium could be
necessary. From our experiments aluminium-phosphate
precipitation seemed to be most effective.
1260
ACKNOWLEDGEMENTS
The financial support for this study was provided by
the German Minister for Research and Technology (BMFT),
Figure 8.
grant No. 02 WS 8922/2.
Course of calcium release in the acidimetric titration.
1261
Figure 9.
Summary of PO4 precipitation experiments in sludge water using different chemicals.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
Stratful, I., Brett, S.; Scrimshaw, M.B. and Lester, J.N., Biological phosphorus removal, its role in phosphorus recycling.
Environ. Technol., 20, 681-695 (1999).
Pöpel, H.J. and Jardin, N., Influence of enhanced biological phosphorus removal on sludge treatment. Water Sci. Technol.,
28, 263-271 (1993).
Seyfried, C.F. and Hartwig, P., Großtechnische Betriebserfahrungen mit der biologischen Phosphorelimination in den
Klärwerken Hildesheim und Husum. Korrespondenz Abwasser, 38, 185-191 (1991).
Baumann, P. and Krauth, Kh., Untersuchung der biologischen Phosphatelimination bei gleichzeitiger Stickstoffelimination
auf der Kläranlage Waiblingen. Korrespondenz Abwasser, 38, 191-198 (1991).
Strickland, J., Perspectives for phosphorus recovery offered by enhanced biological P removal. Environ. Technol., 20, 721725 (1999).
Williams, S., Struvite precipitation in the sludge stream at Slough wastewater treatment plant and opportunities for
phosphorus recovery. Environ. Technol., 20, 743-747 (1999).
Pitman, A.R., Deacon, S.L., Alexander, W.V., The thickening and treatment of sewage sludges to minimize phosphorus
release. Water Res., 25, 1285-1294 (1991).
Sen, D. and Randall, C.W., Factors controlling the recycle of phosphorus from anaerobic digesters sequencing biological
phosphorus removal systems. Hazard. Ind. Waste, 20, 286-298 (1988).
Murakami, T., Koike, S., Taniguchi, N. and Esumi, H., Influence of return flow phosphorus load on performance of the
biological phosphorus removal process. In: Biological phosphate removal from wastewaters. R. Ramadori (Ed.). Pergamon
Press, Oxford, 237-247 (1987).
Psenner, R., Pucsko, R. and Sager, M., Die Fraktionierung organischer und anorganischer Phosphorverbindungen von
Sedimenten. Arch. Hydrobiol./Suppl., 70, 111-155 (1984).
Uhlmann, D.; Röske, I.; Hupfer, M. and Ohms, G., A simple method to distinguish between polyphosphate and other
phosphate fractions in activated sludge. Water Res., 24, 1355-1360 (1990).
DEV, Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung. VCH Verlagsgesellschaft,
Weinheim (1960).
Wentzel, M.C., Ekama, G.A. and Marais, G.v.R., Processes and modelling of nitrification denitrification biological excess
phosphorus removal systems - a review. Water Sci. Technol., 25, 59-82 (1992).
Arvin, E. and Kristensen, G.H., Exchange of organics, phosphate and cations between sludge and water in biological
phosphorus and nitrogen removal process. Water Sci. Technol., 17, 147-162 (1985).
Jardin, N. and Pöpel, H.J., Phosphate release of sludges from enhanced biological P-removal during digestion. Water Sci.
Technol., 30, 281-292 (1994).
Jardin, N. and Pöpel, H.J., Physico-chemical fixation of phosphate in sludges from enhanced biological phosphorus
removal during stabilisation. In: 6th International Gothenburg Symposium on Chemical Treatment III, Springer Verlag,
Berlin, Heidelberg, New York, 353-372 (1994).
Jardin, N. and Pöpel, H.J., Behavior of waste activated sludge from enhanced biological phosphorus removal during
sludge treatment. Water Environ. Res., 68, 965-973 (1996).
1262