Paper for Holland Conference / draft paper for “Environmental Technology”
REM NUT ION EXCHANGE PLUS STRUVITE PRECIPITATION PROCESS
Lorenzo LIBERTI, Domenico PETRUZZELLI, Loredana DE FLORIO
Department of Civil and Environmental Engineering
Polytechnic University of Bari
V.le Turismo 8, 74100 Taranto, Italy
<l.liberti@poliba.it>
Abstract
The REM NUT process allows for simultaneous removal of phosphate and ammonium ions from
sewage by selective ion exchange and their subsequent recovery by chemical precipitation in the
form of struvite (magnesium ammonium phosphate). In this paper a revised version of the REM
NUT process, i.e., P-driven layout, is presented and its cost effectiveness compared to chemical
precipitation based on the use of ferric chloride.
INTRODUCTION
Due to depletion of good-quality phosphate rock, West European phosphate industry has fixed an
objective of using 25% recovered phosphates (probably the only recyclable detergent ingredient)
within a decade and recognised sewage and animal wastes among major alternative sources of P [1].
Current practice for industrial recycling of phosphorus from waste water treatment plants (wwtp) is
based on phosphate precipitation as either calcium phosphates or struvite pellets. The chemistry of
the two compounds is different, but integration of their recovery into sewage treatment is similar.
Calcium phosphate may be recycled into industrial processes or processed to local manufacture
fertilisers while struvite can probably be used directly as fertiliser (although further research is
needed to better substantiate its economic value) or it can be directly incorporated into fertilisers
manufacture [2].
Conventional methods for P removal at municipal and industrial wwtp are based on:

Chemical precipitation: FeCl3, Al2(SO4)3 , (AlCl3)n , Ca(OH)2 or their combination is added at
various points in wwtp inducing precipitation of insoluble phosphates, removed as waste sludge
and usually disposed-off in sanitary landfills. Economic value associated with P-laden slurry is
limited while sludge production increases significantly [3,4].

Enhanced Biological Nutrient Removal (EBNR): provision for anaerobic zone in the activated
sludge bioreactor may allow for simultaneous removal of P (beyond the need for biomass
1
synthesis as "luxury uptake") and N (through de/nitrification). P ends up into sewage sludge (it
may be incidentally recovered from supernatant liquor after anaerobic digestion of sludge) while
N is stripped out to the atmosphere as nitrogen gas [3-7].
Over the last few years, resurgent interest has been observed in industrial recovery and recycling of
phosphorus. Several water companies have built or tested P-recovery systems [8-11] and others are
currently doing so on full scale plants [12-14]. The following processes deserve mention:

Crystalactor® process [12], developed by DHV Water BV, The Netherlands, allows for forced
precipitation of calcium phosphates by addition of crystallisation adjuvants in specifically
designed fluidised bed reactor with formation of salt pellets. Crystallisation is favoured by
seeding grains (sand or anthracite) with strict control of precipitation conditions by addition of
sodium hydroxide or lime. When applied to concentrated solutions (>100 mgP/L) the resulting
high crystallisation rate provides short retention time and relatively small reactors. Although
quite complex, this technology is implemented in several full scale installations in The
Netherlands [15]

Phosnix® process [13], developed by Unitika Ltd, Japan, is based on air agitated column reactor
with complementary chemicals dosing equipment (i.e., Mg(OH)2 or MgCl2 and NaOH for pH
control to 8.5-9.5) ensuring fast nucleation and growth of struvite pellets. Like similar processes
Phosnix® deals preferentially with P-concentrated wastewater (e.g., supernatant liquor from
sludge anaerobic digestion or specific industrial streams) offering removal efficiencies
exceeding 90%. The process is currently applied in some full scale installations in Japan, where
recovered struvite is reportedly sold at 150-200 €/t [16].
The REM NUT process, presented in this paper, developed at Italy’s National Research Council,
allows for P, NH4 and K removal from dilute streams through selective ion exchange followed by
struvite precipitation in proper conditions. The process is suitable for different applications and may
be integrated with, rather than substituted to, other P-recovery technologies.
THE REM NUT PROCESS
Developed in mid 1980s for removal and recovery of phosphate, ammonium and potassium ions
from wastewater in the form of a premium quality slow-release fertiliser, i.e., ammonium and
potassium struvite (MgNH4PO4 and MgKPO4) [17,18], in its basic configuration REM NUT process
relies on two unit operations (Figure 1):

selective ion exchange for removal of nutrients (NH4+, K+, HPO4=) from wastewater and their
concentration in the ion exchangers regeneration eluate
2

chemical precipitation of nutrients from this latter in the form of struvite after addition of Mg2+
at controlled pH, while the supernatant solution is recycled.
Main features of the process are:

two ion exchange units, cationic and anionic, based on a natural zeolite (Clinoptilolite,
Phillipsite or Chabasite) and a “scavenger” strong base resin respectively, are used for selective
removal (>90%) of nutrients from wastewater to the discharge limits imposed by current
legislation according to:
Z-Na + NH4+ (K+)
2R-Cl
+
HPO4=
===
Z-NH4 (K+) + Na+
=== R2-HPO4 + 2Cl-
(1)
(2)
(Z = zeolite, R = anion exchanger)

regeneration of both ion exchangers is carried-out with neutral 0.6M NaCl brine with cyclic
regeneration make-up as low as 2BV (ion exchanger Bed Volume), or with seawater whenever
possible, through a “closed loop” technique [19]

cation and anion exchangers regeneration eluates are properly mixed, pH is raised to 9.5 (where
incidental presence of heavy metals retained by the zeolite is precipitated) and a soluble Mg salt
(e.g., MgCl2) is added to yield a virtually non toxic sterile struvite-rich precipitate (Table 1)
according to:
Mg2+ + NH4+ (K+)+ HPO4= === MgNH4(K+)PO4 (s) + H+
(3)
Additional side benefits include incidental removal of residual suspended solids, BOD, COD (i.e.,
bio-persistent refractory organics, phenol derivatives, surfactants, pesticides, endocrine disrupters,
oestrogens etc.) and microorganisms, thus minimising disinfection demand and related health
hazards associated with haloforms formation (THM, AOX) during final chlorination.
The process passed extensive laboratory study [20], pilot scale investigation [21] and two
demonstration campaigns with a fully automated 240 m3/d pilot plant (Figure 2) carried out at
West Bari, Italy, wwtp (May-Dec.1983, funded and supervised by Italy’s National Research
Council) [22,23] and at South Lyon, MI, USA, wwtp (January-May 1986, funded and supervised by
the U.S. Environmental Protection Agency) [24,25].
In spite of promising results achieved, however, REM NUT process did not reach full scale
application yet due essentially to:

unbalance between P and N in municipal sewage (P:N  1:10 on molar basis), hence in the
exchanger regeneration eluates calling for expensive addition of chemicals to yield P:N:Mg =
1:1:1 stoichiometry for struvite precipitation

high P-discharge limits enforced in several EU Countries (e.g., 10 to 20 mg P/L in Italy)

poor attitude of (waste)water industry toward innovation
3

relative abundance of natural P deposits
The turn of millennium now offers a different scenario for the last three drawbacks and promotes
resurgent interest toward REM NUT process, provided the P/N unbalance is solved, as discussed
below.
P-DRIVEN REM NUT SCHEME
The REM NUT process was revisited for removing and recovering all phosphate and just equimolar
amount of ammonium ion from sewage by selective ion exchange, leaving excess NH4+ to other
conventional treatment processes (e.g., bio(de)nitrification, BPC, air stripping etc.).
The so-called P-driven REM NUT process may assume different layouts as shown in Fig.3.
According to scheme a) in Figure 3 (full REM NUT), applicable with N:P 1:1 molar ratio and
nutrient-rich effluents (e.g., animal wastes), influent stream is processed on both ion exchange units
for quantitative recovery of phosphate and ammonium load. Scheme b) in Figure 3 (P-driven REM
NUT) is applicable with unbalanced N/P ratio as, most commonly, in non nitrified municipal
effluent (N/P 10/1). In this case approx. 1/10 of influent stream (i.e., a reverse fraction respect to
N/P ratio) undergoes NH4-selective ion exchange on the cationic unit and 9/10 is by-passed,
whereas the whole stream is processed for PO4-selective ion exchange on the anionic section.
Excess ammonium ion is admitted to conventional treatment processes (e.g., bio(de)nitrification,
breakpoint chlorination, air stripping etc). In another REM NUT scheme currently investigated [26],
excess ammonium is absorbed by Na-form zeolite added directly into the wwtp activated sludge
reactor. The mixed liquor is then submitted to fractional sedimentation to separate the mineral phase
(ammonia-loaded zeolite) from the organic phase (activated sludge) and the effluent supernatant is
admitted to the P-driven REM NUT scheme. The NH4-loaded zeolite may be conveyed to bioregeneration though nitrification-(de)nitrification and then recycled.
In all cases approx. 1:1 stoichiometric amounts of P-PO4 and N-NH4 will be retained by selective
ion exchange and precipitated as struvite, after Mg addition to ion exchangers regeneration eluate.
Effluent quality will also improve in terms of COD, SS etc.
Compared to full REM NUT process the P-driven scheme obviously requires proper design and
sizing of ion exchange sections. Under these conditions preliminary tests [26] indicated that costeffectiveness of process chemistry may largely improve as external addition of phosphates to reach
Mg:NH4:PO4 1:1:1 molar ratio for struvite precipitation is no longer required and chemicals
consumption is limited to Mg (MgCl2) and alkaline buffer (NaOH) for pH control.
Additional savings of chemicals (80%) have been estimated with seashore installations where
seawater may be used as ion exchangers regenerant and source of Mg for struvite precipitation as
indicated by preliminary tests [27]. Through proper design and arrangement of basic flow-sheet
4
other revisited REM NUT schemes may be postulated. Among others, pre-concentration of just
phosphate ion by selective anion exchange followed by recovery as Fe-phosphate from the ion
exchanger eluate was successfully tested on demonstration scale with animal wastes (pig farm) in
Northern Italy [27].
Pilot investigation is now planned to assess feasibility of the P-driven REM NUT scheme.
ECONOMIC EVALUATION
Three process schemes (Fig.4) have been compared under design data and unit costs summarised in
Table 2 for a typical 11,000 m3/d municipal installation discharging into "sensible areas" (i.e., 
1mg P/L final concentration according to EU Urban Wastewater Directive no. 271/91 [28]).
Scheme A, based on most common chemical post-precipitation of phosphorus by direct addition of
FeCl3 and cationic polyelectrolyte to secondary effluent, was assumed as reference. Alternatively,
the modified REM NUT layout was considered under two different schemes (B and C), both
typically including phosphorus pre-concentration by selective ion exchange followed by chemical
precipitation. Scheme B refers to wastewater containing only phosphate ion (e.g., nitrified sewage),
to be more efficiently precipitated as ferric phosphate from the P-concentrated anion exchanger
regeneration eluate.
Scheme C applies to wider case of effluents containing both ammonia and
phosphates (e.g., non nitrified municipal sewage), to be recovered in a 1:1 molar ratio through
selective ion exchange followed by struvite precipitation with the so-called P-driven REM NUT
layout.
Almost the same amount of ferric phosphate (i.e., 176 t/y) is recovered directly from diluted
secondary effluent (10 mgP/L) in scheme A or from the concentrated ion exchange eluate (450
mgP/L) in scheme B. This latter, in spite of additional investment and operation, provides two main
advantages respect to scheme A:

a smaller precipitation reactor may be used with a more concentrated phosphate solution

incidental leakage of Fe-phosphate fines in the final effluent, exceeding P discharge limit, is
prevented through recycle of precipitation supernatant
Additional major advantage associated with scheme C is the recovery of more valuable struvite
(285 ton/y) instead of ferric phosphate.
The resulting cost summaries for schemes A, B and C are reported in Tab.3 and compared in Fig.5.
From above data finite convenience of P-driven REM NUT process results. Relevant to this aim is
the economic return associated with recovery of struvite (still to be fully assessed in the market).
Indeed, assuming for recovered struvite an intermediate figure around 500 €/t between 800 US$/t
quoted by USEPA [24,25] and more recent value exceeding 300 US$/t [16,29] and using a 33 €/t
5
figure for Fe-phosphate conservatively corresponding to average market price of phosphate rock
[30,31]) revenues would cover >40% of O&M costs for scheme C (struvite) compared to as low as
<3% for schemes A and B (Fe-phosphate).
Although appropriate at this stage, no economic value was conservatively associated with improved
effluent quality after REM NUT treatment as well as with nutrients discharge control (related to
environmental sensitivity of areas of reference), assumed equivalent for all schemes under
comparison.
Moreover, difference in transport costs of recovered by-products was disregarded on the assumption
that they could be easily sold to local farmers or markets nearby wwtp catchment area (although
reuse of Fe-phosphate would unrealistically require a fertiliser factory next door the wwtp).
CONCLUSIONS
Legislative pressure on level of P-removal from wastewater on one side and economic drivers such
as depletion of good quality phosphate rock and related industry interests on the other, are likely to
increase general appeal of P-recovery schemes.
On these premises conventional P recovery
methods based on direct precipitation (pre, sim, post) of insoluble phosphates by addition of
polyvalent ions (Fe3+, Al3+, Ca2+ or their combination) to P-containing effluents of municipal,
zootechnical or industrial origin are now being challenged by more engineered processes with
higher efficiency and increased added value of by-products recovered.
Among these latter the so-called P-driven REM NUT process allows for removal and recovery of
almost equimolar amount of NH4 and PO4 ions from sewage by proper process design and
operation, whereas excess ammonium ion is left to conventional treatment processes (i.e.,
bio(de)nitrification, breakpoint chlorination, air stripping etc).
Pilot plant investigation with a 240 m3/d fully automated mobile plant is now planned in order to
ascertain the following issues:
1. technical feasibility and reliability of the P-driven REM NUT process
2. better substantiation of commercial and agronomic value of wastewater recovered by-products
(i.e., market trials and field testing toward conventional products, social concern on "faecal"
derived by-products)
3. recovery rate and prompt availability of by-products at wwtp to satisfy market demand on large
scale.
6
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Ann Arbor, MI,USA
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Arnot T. (2001), An integrated biological-adsorption process for phosphorus recovery. CEEP Scope Newsletter, 41,37-39
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Jaffer Y. (1999) Assessing the potentialities of full scale P recovery by struvite precipitation. MSc. Project, Cranfield Univ., UK
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performance of fluidised bed reactor. Wat.Sci.Technol.38,1, 123-131
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in anaerobic supernatants, Wat.Res.34,11,3033-3041
DHV Water BV, The Netherlands, Eur.Pat.No.1120962,1988
Unitika Ltd., Japan, Jap. Pat.No. 10-118687,1998
Pavan P., Battistoni P., Bolzonella D., Innocenti L., Traverso P., Cecchi F. (2000) Integration of wastewater and OFMSW
treatment cycles: from the pilot scale experiments to the industrial realisation. The new full scale plant of Treviso (Italy).
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769-775
Katsuura H., Ueno Y. (1998) Phosphorus recovery technologies from sewage treatment plants: P resource recovery system,
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Liberti L., Boari G., Passino R. (1989), Method for removing nutrients from wastewaters, Eur.Pat.No.114,038
Liberti L. Lopez A., (1991) Zeolites closed-loop regeneration. Proc. 1st Int.Conf.on Zeolites Science and Technology, L'Aquila,
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Liberti L, Limoni N., Passino R, and Petruzzelli D. (1980), Ammonium phosphate recovery from urban sewage by selective ion
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7
Water
Tab.1: Typical analysis of struvite-rich precipitate recovered from municipal
sewage with REM NUT process
__________________________________________________________________
MgO:P2O5:N=15:27:5
As<0.3; Cd<0.5; Cr(VI)<0.1; Crtotal<0.35; Cu<0.2; Hg<0.5; Pb<4.5; Se<0.02
Ni<2; Zn<25; Fe<200 (mg/kg)
Organic matter: <7%
Microbial charge: almost sterile
Radioactive content: nil
__________________________________________________________________
Table 2: Design data for P-removal from municipal sewage (mg/L) and unit cost of
chemicals (€/t)
__________________________________________________________________
Parameter
Influent
Effluent
Chemical
P-driven REM NUT
Precipitation
__________________________________________________________________
P-PO4
10
1
1
N-NH3
50
40-45
5
BOD
25
10-20
10
SS
80
10-15
5-10
Flow-rate
11,000 m3/d
__________________________________________________________________
__________________________________________________________________
Product
Cost
Revenue
__________________________________________________________________
Ferric chloride
250
Cationic polyelectrolyte (°)
3,370
NaCl
200
MgCl2.6H2O
500
Sodium hydroxide
650
Fe-phosphate
33
Struvite
500
__________________________________________________________________
(°) type Nymco Dryfloc 652
8
Tab.3: Cost summary for a 11,000 m3/d P removal and recovery plants based on
the three proposed schemes (see Fig.4)
______________________________________A_________B____________C____
Capital cost (€ x 103)
c1 precipitation reactor
175
20
20
c2 ion exchange columns
90
100
c3 resin inventory (*)
120
144
c4 pumps
20
13
15
c5 vessels
15
10
17
c6 bag filtration unit
15
10
7
c7 electric plant
8
10
13
c8 piping and valves
(10% c1-c7)
22.3
27.5
31.6
c9 installation
(5% c1-c8)
12.3
15
17.4
c10 instrumentation
(5% c1-c8)
12.3
15
17.4
c11 mounting and fittings (5% c1-c8)
12.3
15
17.4
c12 engineering
(10% c1-c11)
28.2
34.5
40
----------------------Total investment
310.4
380
440
Running cost (€/y x 103)
c13 amortisation
c14 operation:
power
labour
chemicals (°)
resin back-up
c15 maintenance
(10y, 5%/y)
40
(0.1 €/kWh)
(1unit x 2shifts)
(5% y c3)
(5% c1-c11)
Total O&M cost
Revenue (€/y x 103)
c16 sale of Fe-phosphate or struvite
Net cost
49.2
60
5.3
60
93.5
14.2
--------213
17.5
60
107
6
19
-------258.7
18
60
173.7
7.2
20
----------336.9
5.8
5.8
142.6
207.2
252.9
194.3
Net unit cost (€/m3)
0.052
0.063
0.048
__________________________________________________________________
(*) type Amberlite IRA 458 from Rohm&Haas Co.,USA (schemes B and C)
type Zeolyst 13x18x80 from PQ Corporation,USA (scheme C)
(°) FeCl3, PE (scheme A); NaCl, FeCl3 (scheme B); NaCl, MgCl2, NaOH (scheme C)
9
Exhausted brine
(P, K, N)
SELECTIVE
ION EXCHANGE
0.6M NaCl make-up
Renovated brine
Eutrophic Wastewater
Non euthrophic
effluent
Mg
CHEMICAL
PRECIPITATION
MgNH4PO4
MgKPO4
Struvite
Fig.1: Conceptual scheme of the REM NUT process
10
Fig.2: Outside/inside views of 240 m3/d REM NUT mobile plant
11
Scheme a) (Full REM NUT R)
N-NH4:P-PO4 = 1:1
Nutrient-rich
effluent
Qo
Z-Na
R-Cl
Scheme b) (P-driven REM NUT R)
N-NH4:P-PO4 = 10:1
Municipal
Qo
secondary
effluent
(non-nitrified)
0.9 Qo
Qo
by-pass
0.1 Qo
Z-Na
R-Cl
Tertiary effluent
Fig.3: Schematic layouts of REM NUT process
(Z-Na = sodium-form zeolite; R-Cl = chloride-form anion resin)
12
Secondary effluent
FeCl3
P.E.
Tertiary effluent
sedimentation reactor
rapid mixer
filtering
bags
Scheme A
Ferric phosphate
0.6M NaCl make-up
Nitrified secondary effluent
R-Cl
Tertiary effluent
Exhausted brine
FeCl3
Renovated brine
filtering
bags
Scheme B
Ferric phosphate
0.6M NaCl make-up
Non nitrified
secondary
effluent
R-Cl
Z-Na
Tertiary effluent
NaOH
MgCl2
Renovated brine
filtering
bags
Scheme C
Struvite
Fig.4: Flowsheets of P removal processes under comparison (flowrate 11,000 m 3/d)
A: Chemical precipitation;
B: Chemical precipitation following ion exchange;
C: Chemical precipitation following P-driven REM NUT
(Z-Na: sodium-form zeolite; R-Cl: chloride-form anion resin)
13
500000
Chemical precipitation
Ion exchange+chemical precipitation
400000
P-driven REM NUT
300000
200000
100000
0
Capital costs
(Euro)
O&M costs
(Euro/y)
Revenues
(Euro/y)
Net unit costs
(Euro/Mm3)
Fig. 5: Overall economic comparison among the three schemes proposed for P
recovery from municipal sewage
14