Environmental Technology, Vol. 22. pp 1355-1361
© Selper Ltd, 2001
TECHNICO-ECONOMIC FEASIBILITY OF P-RECOVERY
FROM MUNICIPAL WASTEWATERS
N. JEANMAIRE1* AND T. EVANS2
1Office
2Tim
International de l’Eau, rue E.Chamberland, 87065 Limoges Cédex, France
Evans Environment, Stonecroft, Park Lane, Ashtead, Surrey, KT21 1EU, England
(Received 21 May 2001; Accepted 31 August 2001)
ABSTRACT
The feasibility of recovering phosphates from municipal wastewater is assessed from literature, targeted interviews of water
industry experts and process modelling. It is concluded that it is technically feasible to recover up to 75% of wastewater
treatment plant (WwTP) inflow P, but today this is only attractive in WwTPs operating biological nutrient removal. The
recovered P-product is likely to be of a better quality than currently available phosphate rock. P-recovery should reduce total
wastewater biosolids production by 2-8% dry matter but where biosolids are incinerated ash production should reduce by
12-48%. It will considerably facilitate co-combustion in cement works. The impact of P- recovery on wastewater biosolids
management, and in particular the reductions in transport distances where biosolids are used on agricultural land (resulting
from lowered P:N ratios) are costed. Savings could be around UK£100 per tonne of P recovered (roughly half of UK dockside
prices of imported phosphate rock), giving a total saving of UK£450.000/year for the UK if 20% of all sewage P were
recovered. The size of the savings is related to the capacity of the WwTP and to the % land surface around the WwTP that is
available for agricultural spreading. Economic feasibility will depend on the local context, for example possible advantages
for WwTP operation (nuisance deposit avoidance, removal of return streams of P to WwTP head), advantages for biosolids
management, or national decisions to fix P-recycling as environmental objectives.
Keywords:
Phosphate, recycling, wastewater, biosolids, sewage sludge
INTRODUCTION
Over the last decade or so, considerable work has been
carried out across the world into phosphate recovery from
municipal wastewater. A literature survey was carried out to
assess the feasibility of phosphorus recovery in municipal
European wastewater treatment plants (WwTPs). In addition
33 experts in the water industry, national water regulators, the
phosphate industry, agricultural biosolids utilisation, fertiliser
and other relevant areas were interviewed. A water industry
WwTP process model and modelling of WwTP biosolids
production and management were used.
The use of WwTP biosolids as a nutrient-rich soil
improver is generally regarded as the Best Practicable
Environmental Option (BPEO). It is encouraged by the
governments of Member States and by the European
Commission, which regards the completion of nutrient cycles
and the conservation of organic matter as important
components of sustainable development (CEC, 2000). The
implications for this of recovering some of the phosphate from
biosolids (in a form that is useable by the chemical and/or
fertiliser industries) are considered for England and Wales but
could be extrapolated to other countries.
We limited our study to hypotheses of phosphorus
recovery as struvite (MgNH4PO4) or as calcium phosphates,
via precipitation or crystallisation. Other routes (aluminium
phosphates, ion exchangers, for instance) have not been
studied enough yet, so no thorough study can be carried out.
Two major principles emerge from the analysis of these
technologies: Mainstream P-recovery (processes where Precovery is applied to the WwTP inflow or discharge) and
Sidestream P-recovery (processes operated on P-rich
sidestreams within biological nutrient removal plants or
sewage sludge processing lines).
Analysis of the published literature converged with the
information given by the interviewed experts enabling the
following conclusions to be drawn. In municipal wastewater
treatment plants and especially in the countries where
laundry detergent phosphates are not used, the Mainstream
technique is difficult to implement because soluble phosphate
concentrations in the wastewater are too low. This technique
is more appropriate to industrial, agricultural or food
industry waste streams which offer higher concentrations of
phosphates.
For P to be recovered by precipitation or crystallisation
of calcium phosphates or struvites the following conditions
must be fulfilled by the liquid stream to be treated:
Is it Possible to Recover P with Current Technologies?

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P content of at least [P soluble]  10 mg l-1

Suspended Solids (SS) contents 150-200 mg l-1 in order
to avoid:
impurities in the recovered P,
clogging of reactor nozzles in crystallisers
precipitation onto suspended solids (SS) gives particles
whose density is lower than that of sand, resulting in
losses of these phosphates in reactor effluent
In order for such a stream to be found in the WwTP (or for
such a stream to be readily made by for example mixing
existing streams or passing an existing stream through a
settling tank):




The WwTP must run with the activated sludge process.
The WwTP must be equipped with biological
phosphorus removal (EBPR process or Bio-P process),
generating P-rich side and sludge streams
The sludge must undergo a second phase of release in a
second anaerobic unit. This unit may already exist on the
WwTP (anaerobic sludge digestion or non-aerated
holding tank) or may be specifically created for the Precovery (stripper tank of the Phostrip process for
instance).
After this second release, the sludge must usually go
through a separating unit (settling for instance) to lower
suspended solids levels.
Recovery of phosphates for recycling requires their
production in a recyclable form (acceptable P-content,
“pellets” or some other form which can readily be handled
and dewatered – not a sludge or slime of fines), with minimal
chemical addition. Where the above WwTP conditions are
available, there is general agreement that phosphate recovery
by precipitation is technically feasible, although further work
clearly needs doing to design reactor/ process systems which
operate reliably and economically.
The minimum WwTP capacity for P-recovery to be
feasible is probably 30.000 pe (generally considered a
minimum size for biological nutrient removal), though larger
WwTPs are more likely to justify the capital investment and
staff operating competence necessary for P-recovery.
Where P-recovery is installed in biological nutrient
removal WwTPs, recovery of up to 75% of influent P is
feasible, but lower values may be more realistic.
Technical Consequences on the Operation of the WwTP
Phosphate recovery offers several potential positive
repercussions for WwTP operation. However, there are also
direct consequences on the time necessary to operate the
Table 1.
processes of recovery, on the consumption of reagents and
energy:

Limitation of P-rich flows returning to the head of the
WwTP so that the efficiency of the biological P-removal
BOD 5
is improved through the increase of the ratio
P

Reduction of technical problems and costs incurred by
the nuisance struvite deposits which can occur in sludge
treatment lines (see eg. Williams 1999, Durrant et al.
1999, Parsons 2001, Mohajit et al. 1989). These can cause:
reduction in the diameters of pipes,
clogging of valves, press casings …,
clogging of the belts in filter belt presses,
abrasion of rotary appliances (pumps, centrifuges …)
etc
Reductions in Sewage Sludge Mass
In order to assess the effect of P-recovery on sludge
production, different calculations have been made, based on
the CIRSEE (Centre International de Recherche sur l’Eau et
l’Environnement - Lyonnaise des Eaux) model and on the
researches of Popel & Jardin 1993, 1997 and 2001 and
COMEAU 1990.
The CIRSEE model on the simulated scenarios leads to
a sludge production estimate 1.05 kg SS produced kg-1
biochemical oxygen demand over 5 days (BOD5) removed.
The calculations are based on the principles below:

Sidestream P-recovery requires an anaerobic release of
the phosphorus assimilated during the biological Premoval step.

P-release goes together with a joint release of a cation
type Mg2+ and K+, which also reduces the sludge
production. The hypotheses used to assess the
importance of the reduction are as follows:
2 to 2.4 g SS g-1 P released (personnal communication,
N. Jardin 2001)
1.6 g SS g-1 P released (personal communication Y.
Comeau, 2001)
Table 1 summarises the different results for an activated
sludge WwTP in extended aeration treating a mean effluent
5-day biochemical oxygen demand [BOD5] = 300mg l-1;
suspended solids [SS] = 250 mg l-1; % suspended volatile
(SVM) = 70%; food on microorganisms ratio (= sludge
loading) (F/M) ratio = 0.1 kg BOD5 kg-1 volatile suspended
solids/ day. The comparisons expressed in the table below
give the differences in sludge production between a WwTP
Potential for P-recovery in WwTPs : as % of total P inflow in the WwTP.
% P recovered in WwTP
% of sludge reduction (mean value - Dried Solids)
% of sludge reduction with anaerobic digestion (mean
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60 %
2,3%
3,4%
75%
3%
3,8%
90%
3,5%
5,2%
value – Dried Solids)
equipped with a biological P-removal and the same WwTP
where a P-recovery technique is used. Overall, average
reductions in biosolids production (dry solids) are estimated
at 2 – 8%.
Implications for Biosolids use in Agriculture
Nitrogen and phosphorus are often the limiting
parameters for agricultural use of wastewater biosolids.
However there are some exceptions in Europe, such as
Sweden or the Netherlands, where the limits for heavy metals
are so low that they impede agricultural use. In the UK,
nitrogen has been the limiting factor, but phosphorus is
becoming the limiting element. There are already other areas
in Europe where the soils have enough P and where P is
already a limiting factor. In this context, decreasing the Pconcentration in biosolids would mean reducing the area over
which they need to be spread and thus reducing the costs of
transportation necessary to reach these application sites.
England and Wales have data sources about soils,
farming, fertiliser use and biosolids that are amongst the most
comprehensive and detailed in the world. In some cases these
data sources are unique (e.g. McGrath and Loveland, 1992).
To develop a model to examine the sustainable use of
biosolids-P the following assumptions were made (based on
Smith et al. 2000): that the average availability of P in
Table 2.
biosolids is about 35% that of single superphosphate fertiliser;
that 20% of all municipal wastewater will be subjected to Premoval; that the P-content of biosolids is 2.6% and 5.0%
respectively for conventional and for P-removal WwTPs (dry
solids basis). Using these assumptions the average 5-year
application requirement for biosolids is 26.3, 20.3, 14.3 and 3.2
tDS ha-1 for soil P-index 0, 1, 2 and 3 respectively (data from
HMSO, 2000). These assumptions also lead to the conclusion
that the total biosolids production predicted for 2000/01 is
only sufficient to provide 2.8% of agronomic need in England
and Wales.
Table 2 shows the numbers of works in different size
classes (by population equivalent, p.e.) from which biosolids
were reported as being recycled to agriculture in 1996/97.
This was before the application of untreated biosolids was
phased out and during the investment programme to comply
with the Urban Wastewater Treatment Directive (CEC, 1991).
The advent of treatment for all biosolids has meant that
biosolids from smaller works, where they used to be recycled
directly to land, are now taken to larger works for treatment
prior to recycling. Although the total quantity of biosolids
recycling is increasing, the number of works at which it is
produced is thus decreasing. Figure 1 and Table 3 are derived
from the predictions of the quantity of biosolids that will be
used on land in 2000/01 in England and Wales (which
includes most of the increases resulting from Directive
Numbers of works in England & Wales and the biosolids production in 1996/97 (from EA, 1999).
Size of works
Small
<10k p.e.
Medium 10-150k p.e.
Large
>150k p.e.
Number of works
Total biosolids
tDS y-1
Average biosolids
per works tDS y-1
776
376
55
37980
242323
159625
48.94
644.47
2902.27
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Figure 1. Model of haulage radius for sustainable use of biosolids phosphate for different sizes of works and different intensities
of land use.
Table 3. Predicted maximum delivery radius based on the predictions of biosolids use on land in 2000/01 (E&W) and
assumptions in this paper (the figure in bold is the most likely).
Number of biosolids
production facilities
431 (= >10k p.e.)
300 (average ≈150k p.e.)
200 (average ≈230k p.e.)
100 (average ≈450k p.e.)
1673.6
2404.3
3606.5
7213.0
1991/271 implementation). They consider different sizes or
numbers of biosolids production facilities and the
assumptions and conclusions that have been made above
about using biosolids-P sustainably.
The cost of an efficient biosolids haulage operation
using maximum size delivery vehicles is about UK£0.14/t.km
(excluding loading and off-loading). The form of the biosolids
does not matter provided the road haulage vehicle is carrying
maximum payload. P-recovery would reduce biosolids’ P:N
ratios, so that sustainable application rates would increase,
and as a consequence haulage radii would decrease.
If 20% of all of the P in biosolids used on land were
recovered that would be a total of 4443 t P recovered per year.
If the average P-fertiliser equivalence of the biosolids-P was
unaltered by P-recovery (i.e. it remained the same at about
35% that of single superphosphate-P) then we can calculate
the new haulage radii and the savings in haulage. The total
savings in haulage costs for England and Wales are shown in
Table 4. The calculations used to derive Table 4 also assume
that the average %DS of the biosolids used on land will be
24%. This reflects the extra dewatering (including drying)
that has been (and is continuing to be) installed since 1996/97
with the investment in greater biosolids treatment.
Based on all of these assumptions, the total saving for
England and Wales would be in the range £217k to £1424k
depending on the number of biosolids production facilities
and the percentage of treatable land. That is £50 to £320 per
tonne of P recovered. The most likely scenario in this range is
that there will be 200 facilities treating about 5% of the total
land within their distribution radii. The total annual saving
for this scenario would be £450k, which is £101 per tonne P
Table 4.
Maximum delivery radius (km) for different
percentages of land around facility treated
Average production
per facility tDS y-1
10%
5%
1%
4.88
5.85
7.17
10.14
6.91
8.28
15.44
18.51
22.67
32.07
10.14
14.34
recovered, which is only half the market price of imported,
mined phosphate rock (£207 t-1 P) at port of entry to the UK
quoted by Gaterell et al. (2000). This is offset by the dispersed
arisings of recovered P, the smaller quantity at each recovery
site, the greater purity compared with phosphate rock and
delivery from the recovery site to the point of use.
Implications for Biosolids Incineration and Co-Incineration in
Cement Production
Sewage sludge incineration enables use of the (nonfossil) thermal energy and a reduction of dry solids by a factor
of 6. This means that the reductions in biosolids resulting
from P-recovery (see above) will then lead to a reduction in
incineration ash residues of between 12 and 48 %. If an
approximate cost of landfill is 228 ¤ t-1 ash (average 2000 price
in France) the saving could be about 70 ¤ t-1 of initial ash
production. Further work is necessary to confirm these results
given by mass calculations.
Although not very widespread, co-combustion of
sewage sludge in cement production is an interesting energy
recovery route because it does not produce waste (the ashes
are trapped and used in the making of cement). However,
high levels of phosphorus in cement lower the short-term
resistance. An OFEFP technical document (OFEFP, 1991)
recommends a limit value of 0.5 % P2O5 in cement (% of the
weight). Reducing the phosphorus content of biosolids by Precovery can address this problem, which is also dependent
on the proportion of biosolids in the charge to the cement
kiln.
Predicted total savings in haulage for England & Wales that might result from recovering 20% of the P from biosolids
(the figures in bold are the most likely).
% of area around facility available for biosolids spreading
10%
Number of biosolids production
facilities
431 (= >10k p.e.)
300 (average ≈150k)
200 (average ≈230k)
5%
1%
£000 y-1
£ tDS-1
£000 y-1
£ tDS-1
£000 y-1
£ tDS-1
217
260
318
0.301
0.361
0.442
307
368
0.425
0.510
450
0.624
686
822
1007
0.951
1.140
1.396
1358
100 (average ≈450k)
450
0.624
Other Costs, Savings, Revenues and Regulatory Issues Related
to P-Recovery
The investment and depreciation costs depend a lot on
the local context, on the choice of the P-recovery technology
and on the changes to the existing WwTP installations that
would be required in order to generate a suitable stream for
operation of P-recovery (see above).
The cost of the consumption of chemical reagents in the
phosphate precipitation reactor depends on the choice
between struvite recovery and calcium phosphate recovery.
The quantities of reagent required (to increase the pH value,
and to obtain molar calcium : P or magnesium : P ratios for
phosphate precipitation) depend on the initial properties of
the wastewater stream used, on the type of reactor and of
reagent used. Also, the price of reagents varies a lot from one
country to another and interesting alternative solutions can be
considered:
-
use of sea water as a source of magnesium (eg.
Kumashiro et al, 2001)
use of industrial by-products concentrated in
magnesium or calcium
pH can be increased by aeration if the stream used is
high in dissolved CO2
The additional labour input is estimated at between 1
and 2 person-hours/day for operation and maintenance of a
phosphate recovery reactor. The training of the staff on the
operation of the WwTP and on the safety as regards the use of
reagents depends on the prior knowledge of the staff, but is
estimated at between 2 and 10 days.
The sale of the recovered phosphates depends very
much on the context of the recovery. Values between 1235 and
2833 ¤ ton-1 of recovered P (struvite) have been reported.
However, this looks rather high when compared with the
average price of the P-rock delivered to northern Europe
which is approximately 320¤ t-1 P (Woods et al. 1999) and the
average price of P-fertilisers is 1150 ¤ t-1 P (ADEME 2001).
According to the context, the sale of the recovered
phosphates may cover part or the whole of the costs of the
Table 5.
637
0.833
1424
1.975
recovery. However, allowing for the expenses of marketing
and distribution, water industry operators do not consider the
sale of the recovered product as a business purpose but rather
as a disposal process.
The savings that can result from mitigating problems of
struvite deposition depend directly on the importance of the
problems. In the case of chronic situations, some operators
have reported annual costs of removing struvite and
associated problems at 65,000 ¤ year-1 per WwTP. P-recovery
from municipal wastewater leads to the production of a new
product that is differently considered by the parties involved
in the process: waste from the water treatment, raw material
or fertiliser? As emphasised by the phosphate industry
(Schipper et al. 2001), this question of regulatory product
classification is fundamental for the prospects of large-scale
recycling of a waste. Current regulatory status effectively
prevents the transport of the recovered product across
national boundaries within the EU, thus limiting the potential
development of recycling for example in the Benelux Northern Germany – UK (cross Channel) area.
CONCLUSIONS
This study concludes that it is technically feasible to
recover phosphates from municipal wastewater treatment
plants (WwTPs) equipped with biological P-removal, up to
75% of WwTP inflow P.
The selling price of the recovered P will be related to
that of P-rock or fertilisers and is likely to cover only the
purchase of the reagents required for the precipitation
process. The resale of the recovered P as a fertiliser
component or local marketing as a specialist fertiliser could
obtain a better price.
Phosphate recovery will bring cost savings through
reduced wastewater biosolids production and improved
biosolids management: lower P:N ratios will facilitate
agricultural use and reduce transport distances for
agricultural spreading, where P is the limiting factor.
Reductions in wastewater biosolids production are estimated
at 2 –8% and cost savings from reduced transport distances to
Summary of costs and revenue/savings related to P-recovery operation.
Expenses
Financial benefits
Investment and depreciation costs
Sale of the recovered phosphates
Consumption of reagents and of energy
Savings due to the suppression of problems of
struvite deposits
Additional labour costs / time
Possible reduction of transportation distances for
agricultural reuse.
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Specific training of the operating staff
Savings on the landfill of incineration ashes.
agricultural fields at £50 to £320 per tonne of P recovered.
Reduction of ash where biosolids are incinerated is estimated
at 12-48%. P-recovery could also facilitate co-combustion of
biosolids in cement works
The driving forces for phosphorus recovery for
purposes of recycling are thus very dependent on the
particular national context. Phosphate recovery is already
registered as a national objective in Sweden, and this notion of
recycling is expected to become a regulatory requirement. In
other countries, P-recovery answers different needs:



The quality of the recovered product is generally better
than imported mined phosphate rock, in particular as
regards the heavy metal contents.
Sustainable development in the phosphorus industry via
the recycling of a recovered product – this is regarded as
particularly attractive in the Dutch context.
P-recovery can be a way to avoid internal problems at
some wastewater treatment plants (where scaling is a
problem) and to improve biological P-removal
There are two scenarios that are likely to lead to a significant
development in P-recovery:

Local feasibility: locally, P-recovery can be a significant
solution to case-specific WwTP operation problems or to
sustainable biosolid utilisation or disposal. In this case,
the costs of P-recovery will be covered by the water
company through resulting specific local WwTp

operating savings.
National or European political decision: the arguments
of sustainable development (phosphorus recycling) or of
better quality of the recovered product may lead to Precovery being stipulated as a national or European
political objective. In this case, the consumers and/or
taxpayers will share any net costs.
In any case, the development of phosphorus recovery
strongly depends on regulations not being an obstacle to
recycling. For example if the recovered product were
classified as something other than a waste it would facilitate
the trade in and use of the material.
ACKNOWLEDGEMENTS
Tim Evans would like to thank: Jane Slater, Deputy
Director of the Fertiliser Manufacturers’ Association for
information about the trends in use of mineral fertiliser; Dr
Stephen Smith of Imperial College for information about the
mineralisation of biosolids – phosphorus in soils, the fertiliser
equivalence of different types of biosolids and for the use of
certain unpublished data (with kind permission from Thames
Water, which funded Dr Smith’s work). Nicolas Jeanmaire
would like to thank the 33 experts who provided information
for this study, in particular Norbert Jardin, Yves Comeau,
Pete Pearce and Yasmin Jaffer, Paolo Battistoni, Bernd
Henzmann, Hermann Hahn, Philippe Vioget.
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