BIOLOGICALLY-DRIVEN PHOSPHATE PRECIPITATION
IN BIO-P SLUDGES.
R. Elaine Dick*, Paul G. Devine, John P. Quinn1 and Stephen J. Allen
School of Chemical Engineering/1School of Biology and Biochemistry/*QUESTOR
(Queen’s University Environmental Science and Technology Research) Centre, Queen’s
University, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland,
UK.
*Corresponding Author: e-mail: r.dick@qub.ac.uk
ABSTRACT
Current processes for phosphate removal from wastewater are often regarded as dividing
into either biological or chemical processes. However, during phosphorus removal in
Bio-P sludges, the cellular efflux of phosphate that occurs naturally under anaerobic
conditions prior to aerobic phosphorus removal causes local changes in the solution
chemistry around the bacterial cells. If the conditions are favourable, bacteria can act as a
seed for crystallisation and precipitation reactions. Such reactions have been observed in
biological wastewater sludges.
A combination of biological and chemical processes in a Bio-P sludge may increase the
overall phosphate content of the sludges to a level that may be advantageous for
phosphate recovery. The operation of a laboratory scale fill and draw system aims to
optimise the conditions promoting precipitation of crystalline phosphates on the surface of
phosphate-releasing cells during anaerobic conditions during Bio-P removal. This paper
examines the conditions that are thought to promote such reactions, and how this
phenomenon might be exploited for phosphorus recovery.
INTRODUCTION
In municipal wastewater treatment plants where there is a requirement to remove
phosphate this may be done by either biological or chemical means. Enhanced Biological
Phosphate Removal (EBPR) exploits the ‘luxury’ P uptake that occurs when biological
sludges are cycled through appropriate anaerobic and aerobic conditions in a number of
possible configurations such as “UCT” and “Phostrip”[1]. The process of EPBR uses the
metabolism of specific bacteria, which under certain conditions accumulate large amounts
of phosphate as intracellular polyphosphate. These bacteria use the degradation of
polyphosphate to produce energy under anaerobic conditions with the release of
phosphate into the wastewater (resulting in a transient increase in the phosphate content of
the wastewater). The wastewater and activated sludge is then treated in an aerobic basin.
Here the polyphosphate bacteria use stored carbon reserves to produce energy for growth
and to replenish their stores of polyphosphate. The result is a net removal of phosphate
from the wastewater (due to replenishment of polyphosphate in extant cells and de novo
polyphosphate reserves in divided cells).
Elaine Dick et al, page 1
Dick, R.E., Devine, P.G., Quinn, J.P. and Allen, S.J.
2
In chemical phosphorus removal metal salts such as iron, (Fe2+ or Fe3+) or aluminium
(Al3+) are used to precipitate insoluble metal phosphates and to coagulate/flocculate
suspended solids. The addition of chemicals, often dosed into biological sludges,
increases sludge volumes - by up to 40% in the case of iron addition [1]. In addition,
aluminium and iron containing sludges are not compatible with current methods for
phosphate recovery [2].
Biological phosphate accumulating (Bio-P) sludges can contain 5-10% P, however, the
form of this phosphorus, locked up in an organic sludge, is not readily available for
phosphorus recovery. The ‘Phostrip’ process combines the biological accumulation with
a step where the accumulated phosphate is released into a concentrated liquor and
precipitated as a calcium salt. Although calcium produces low residual phosphate
concentrations, effective precipitation requires a high pH (>10) and precipitates of CaCO3
may also be formed. Generally, more than stoichiometric quantities of cations must be
added for the complete removal of phosphate. The two processes currently used for
phosphate recovery from wastewater are based on the chemical precipitation of phosphate
with Ca2+, or Mg2+ and NH4+ as struvite [1].
BIOLOGICALLY-DRIVEN PRECIPITATION
The presence of crystalline phosphates in sludges has been observed in a number of
studies [3, 4], and has been reported to be promoted by denitrifying conditions that also
tend to raise the pH of the wastewater [3, 4]. Vivianite, Fe3(PO4)2.8H20, has also been
found in sludges under anaerobic conditions following the addition of FeSO4 [6].
Microorganisms can specifically act as a seed for crystallisation and precipitation
reactions when conditions are favourable. The unicellular alga Chlorococcum sp. has
been shown to co-precipitate phosphate with calcite [7]. In Alcaligenes eutrophus CH34,
the mechanism of heavy metal resistance - efflux of the metals, together with locally
elevated levels of CO2 from cellular respiration resulted in precipitation of heavy metal
carbonates [8]. Research on heavy metal removal by Lynne Macaskie’s group in the
University of Birmingham showed that the cellular efflux of phosphate during organic
phosphate degradation (Citrobacter sp.) [9] or inorganic polyphosphate hydrolysis
(Acinetobacter spp.) [10] promoted metal phosphate precipitation on cell surfaces. In
each of these cases, the active metabolism of the microorganism affected the chemistry of
the solution around the cell in a way that favoured precipitation.
Using microbial metabolism in such a way, the formation of metal phosphates can occur
under conditions where the bulk solution chemistry is unfavourable. During anaerobic
conditions in Bio-P sludges the efflux of phosphate from the cells results in a locally high
phosphate concentration gradient. This can act as a thermodynamic driving force for the
precipitation reaction. Although the precipitant, for example calcium, may be present in
relatively low quantities, the excess of phosphate can help to exceed the metal phosphate
solubility product. As this happens in the locality of the cell surface, adsorption reactions
can also provide potential crystal nucleation sites. As anaerobic phosphate release is often
accompanied by a rise in the pH [12] this could also be advantageous in promoting the
formation of calcium phosphates.
The precipitation of calcium phosphates in real wastewater is complex. This has been
recently examined in detail by Maurer et al. [12] who demonstrated experimental calcium
Dick, R.E., Devine, P.G., Quinn, J.P. and Allen, S.J.
3
phosphate precipitation with formaldehyde-inactivated sludge below pH 8.0. A model
also showed favourable conditions for calcium phosphate precipitation at physiologically
achievable pH values in nutrient removal plants would be promoted by phosphate and pH
gradients due to cellular metabolism [13].
ANAEROBIC
AEROBIC
Influent wastewater
containing phosphate
mixed with EBPR sludge
P-rich
sludge
Sludge settled:
Wastewater low in
phosphate separated
from P-rich sludge
Phosphate
Polyphosphate
Phosphate
Remaining
phosphate taken up
by cells for growth
and to reform
polyphosphate
Anaerobic release of
phosphate by cells
Addition of calcium
as a precipitant
Chemistry of the solution
around cells favours
precipitation
Figure 1. Proposed operation of biologically driven phosphate precipitation.
In initial cycles crystal nucleation is likely to be the rate-limiting step; in subsequent
cycles precipitation will be more rapid. The goal is to produce a compact sludge
containing precipitated phosphates in addition to biologically sequestered polyphosphate
with a total phosphorus content higher than would be possible with either biological or
chemical phosphate removal alone.
ADVANTAGES TO COMBINING BIO-P REMOVAL AND PRECIPITATION
If it were possible to promote chemical precipitation reactions within Bio-P sludge while
at the same time avoiding the disadvantages normally associated with the addition of
chemicals to sludges, namely the formation of voluminous flocculent sludge, there are
potential advantages to be gained from combining chemical and biological P removal.
Polyphosphate forming cells have a maximum storage capacity for polyphosphate, but the
potential for surface phosphate precipitation is unlimited. It may be possible to optimise
the anaerobic phosphate release in the presence of added cations to promote the formation
of insoluble phosphate salts as crystals on the surface of phosphate-releasing cells. In the
subsequent aerobic period, in which the bacterial cells reform their quotient of stored
polyphosphate, the removal of phosphate from the wastewater would be enhanced. Over
a number of anaerobic/aerobic cycles the precipitation process would build up a
substantial phosphate content in the sludge. The sludge would thus contain two
independent forms of phosphate – polyphosphate and crystalline phosphates, and have an
enhanced content of total phosphorus which may make economic recovery of the
phosphate possible.
Dick, R.E., Devine, P.G., Quinn, J.P. and Allen, S.J.
4
It has been suggested that the presence of a precipitant may accelerate the extent and rate
of phosphate release [14]. If, as is likely, the degradation of polyphosphate is a reversible
reaction governed in part by the concentration of free phosphate, the reduction in
phosphate concentration at the cell surface by precipitation reactions may serve to
increase the magnitude of phosphate release. This in turn may have a positive effect on
BOD removal and competitive advantage by polyphosphate accumulating bacteria.
If it is intended to remove only part of the phosphate by precipitation, less than
stoichiometric amounts of cations would be added. This may reduce the risk of
competing reactions consuming the added precipitant. As only a proportion of the
phosphate is removed, the remaining phosphate would re-form polyphosphate within the
cell: the precipitated portion would increase only gradually. The ultimate phosphate
content of the sludge would thus be a function of sludge age: a high sludge age is also
desirable for EPBR. The additional phosphate precipitated within the biological flocs
may increase the total phosphate content sufficiently to potentially make phosphate
recovery attractive.
This proposed process would have particular advantages in wastewater where the BOD:P
ratio is low, especially in cases where the phosphate requiring removal is high. The
process may be compatible with mainstream operation or it may be desirable to operate
this as a side-stream process to minimise sludge volumes, interference from competing
ions, and maximise the sludge age and phosphate content.
LABORATORY-SCALE TESTS
To test the feasibility of this approach, a fill-and-draw reactor with an operating volume
of 5 litres has been set up and seeded with sludge from a full scale EBPR (Phostrip) plant
(Milcote, England). Due to the potential difficulty of maintaining biological phosphate
removal activity over long periods, our approach has been to operate the system for
experimental periods of up to two weeks duration. Active liquor can readily be
established as required from frozen sludge. This has provided a source of conditioned
sludge for batch experiments and will enable the test of different precipitants and
conditions within relatively short time periods.
POTENTIAL FOR PHOSPHATE RECOVERY
Should this approach be successful, sludge containing two independent forms of
phosphate, polyphosphate and precipitated phosphate salts, would be formed. The
amount of polyphosphate in the sludge at the end of each cycle should be relatively
constant (typically 9% P for EBPR sludges); the proportion of additional phosphate
precipitate present would increase with sludge age.
Assuming that this approach is feasible, there still remains the problem of how to recover
the phosphorus from the sludge. Research has shown that sludge incineration followed by
water elution [15] can extract biologically-sequestered polyphosphate from EBPR
sludges. However, the additional presence of calcium phosphates would likely require
chemical extraction. Thermal treatment could also be used to eliminate the organic
material present leaving an ash with a relatively high calcium phosphate content, although
Dick, R.E., Devine, P.G., Quinn, J.P. and Allen, S.J.
5
the cost of sludge incineration is high. Other suitable methods are likely to depend on the
phosphorus content achievable and the concentration of undesirable metals present in the
sludge.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Brett, S., Guy, J., Morse, G.K. and Lester, J.N., Phosphorus Removal and Recovery
Technologies, Selper Ltd., London. (1977)
Driver, J., Lijmbach, D., and Steen, I., Why Recover Phosphorus for Recycling, and
How? Environ. Technol., 20, 651-663. (1999)
Appeldoorn, K.J., Boom, A.J., Kortstee, G.J.J. and Zehnder, A.J.B. Contribution of
precipitated phosphates and acid soluble polyphosphate to enhanced biological
phosphate removal. Water Res., 26, 937-943. (1992)
Carlsson, H. Aspegren, H., Lee, N. and Hilmer, A., Calcium phosphate precipitation
in biological phosphorus removal systems. Water Res., 31, 1047-1055.
Arvin, E. and Kristiansen, G.H. Phosphate precipitation in biofilms and flocs. Water
Sci. Technol., 15, 65-85. (1983)
Frossard, E., Bauer, J.P. and Lothe, F., Evidence of vivianite in FeSO4-flocculated
sludges. Water Res., 31, 2449-2454. (1997)
Hartley, A.M., House, W.A., Callow, M.E. and Leadbeater, B.S.C., Coprecipitation
of phosphate with calcite in the presence of photosynthesizing green algae. Water
Res., 31, 2261-2268. (1997)
Diels, L., Dong, Q., van der Lelie, D., Baeyens, W. and Mergeay, M., The czc operon
of Alcaligenes eutrophus CH34: from resistance mechanism to the removal of heavy
metals. J. Ind. Microbiol., 14, 142-153. (1995)
Macaskie, L.E., Empson, R.M., Cheetham, A.K., Grey, C.P. and Skarnulis, A.J.,
Uranium bioaccumulation by a Citrobacter sp. as a result of enzymatically mediated
growth of polycrystalline HUO2PO4. Science, 257, 782-784. (1992)
Dick, R.E., Boswell, C.D. and Macaskie, L.E., Uranyl phosphate accumulation by
Acinetobacter spp. pp.177-186. In: Biohydrometallurgical Processing Volume II.
Jerez, C.A., Vargas, T., Toledo, H. and Wiertz, J.V., (eds) University of Chile, Viña
del Mar, Chile. (1995)
Bond, P.L., Erhart, R., Wagner, M., Keller, J., and Blackall, L.L., Identification of
some of the major groups of bacteria in efficient and nonefficient biological
phosphorus removal activated sludge systems. Appl. Environ. Microbiol., 65, 40774084. (1999)
Maurer, M., Abramovich, D., Siegrist, H. and Gujer, W., Kinetics of biologically
induced phosphorus precipitation in wastewater treatment. Water Res., 33, 484-493.
(1999)
Maurer, M. and Boller, M. Modelling of phosphorus precipitation in wastewater
treatment plants with enhanced biological phosphorus removal. Water Sci. Technol.,
39, 147-163. (1999)
Boswell, C.D., Dick, R.E. and Macaskie, L.E., The effect of heavy metals and other
environmental factors on the anaerobic phosphate metabolism of Acinetobacter
johnsonii. Microbiology, 145, 1711-1720. (1999)
Matsuo, Y., Release of phosphorus from fly ash produced by incinerating waste
activated sludge from enhanced biological phosphorus removal. Water Sci. Technol.,
34, 407-415. (1996)