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PHOSPHORUS RECOVERY FROM WASTEWATER BY STRUVITE
CRYSTALLISATION: A REVIEW
K. S. LE CORRE
1
, E. VALSAMI-JONES
2
, P. HOBBS
3
, S. A. PARSONS
1*
1
Centre for Water Science, Cranfield University, Cranfield MK43 0AL, UK
*
Tel: +44 (0)1234 754841, Fax: +44 (0)1234 751671
E-mail address: s.a.parsons@cranfield.ac.uk
2
Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7
5BD, U.K
3
Institute of Grassland and Environmental Research (IGER), North Wyke, Okehampton,
Devon, EX20 2SB, UK
Abstract
The present review provides an understanding of principles of struvite crystallisation and examines the
techniques and processes experimented to date by researchers at laboratory, pilot and full scale to maximise
phosphorus removal and reuse as struvite from wastewater effluents. Struvite is mainly known as a scale deposit
causing concerns to wastewater companies. Indeed struvite naturally occurs under specific condition of pH and
mixing energy in specific areas of wastewater treatment plants (e.g. pipes, heat exchangers) when concentrations
of magnesium, phosphate and ammonium approach an equimolar ratio 1:1:1. However, thanks to struvite
composition and its fertilising properties, the control of its precipitation could contribute to the reduction of
phosphorus levels in effluents while simultaneously generate a valuable end by-product. A number of processes
such as stirred tank reactors, air agitated and fluidised bed reactors have been investigated as possible
configurations for struvite recovery. Fluidised bed reactors emerged as one of the promising solutions for
removing and recovering phosphorus as struvite. Phosphorus removal can easily reach 70% or more, although
the technique still needs improvement with regards to controlling struvite production quality and quantity to
become broadly established as a standard treatment for wastewater companies.
Keywords: phosphorus removal, struvite, crystallisation technologies, fertiliser
Critical Reviews in Environmental Science and Technology, Volume 39, Issue 6, 2009, Pages 433-477
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List of content
1. Introduction 3
2. Struvite chemistry 5
2.1 Struvite characteristics ..................................................................................................... 5
2.2 Spontaneous precipitation of struvite in wastewater environments ................................. 7
2.3 Notion of solubility and solubility product ...................................................................... 9
2.3.1 Definition .................................................................................................................. 9
2.3.2. Solubility product ..................................................................................................... 9
2.4 Saturation ....................................................................................................................... 14
3. Mechanisms of struvite crystallisation 16
3.1. Struvite nucleation ......................................................................................................... 17
3.1.1. Nucleation and nucleation rate ............................................................................... 17
3.1.2 Induction time ......................................................................................................... 18
3.2 Crystal growth ................................................................................................................ 20
3.3 Parameters affecting struvite crystallisation .................................................................. 21
3.3.1 pH ............................................................................................................................ 22
3.3.2 Supersaturation ratio ............................................................................................... 24
3.3.3 Temperature ............................................................................................................ 26
3.3.4 Mixing energy or turbulence ................................................................................... 28
3.3.5 Presence of foreign ions .......................................................................................... 28
4. Phosphorus removal and recycling as struvite 30
4.1 Phosphorus removal from wastewater ........................................................................... 30
4.1.1 Current treatments ................................................................................................... 30
4.1.2 The crystallisation solution ..................................................................................... 31
4.2 Design and description of processes used for struvite crystallisation ............................ 34
4.2.1 Selective ion exchange processes ............................................................................ 34
4.2.1.1 The RIM-NUT
®
Technology ........................................................................... 34
4.2.1.2 Advantages and drawbacks .............................................................................. 34
4.2.2 Stirred reactors ........................................................................................................ 35
4.2.2.1 Operation principles ......................................................................................... 35
4.2.2.2. Advantages and drawbacks ............................................................................. 37
4.2.3 Fluidised bed reactors and air agitated reactor ........................................................ 38
4.2.3.1 Process principles ............................................................................................. 38
4.2.3.2 Examples of two typical FBR and Air agitated designs................................... 42
4.2.3.3 Limitation of FBR and air agitated processes, areas of improvement ............. 43
5. Interests in controlling and recovering phosphorus as struvite 45
5.1 Environmental impact .................................................................................................... 45
5.1.1 Potential pollution reduction ................................................................................... 45
5.1.2 Sludge reduction ...................................................................................................... 46
5.1.3 Use as a fertiliser ..................................................................................................... 47
5.2 Economics ...................................................................................................................... 47
6. Summary ............................................................................................................................. 50
References ............................................................................................................................... 52
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1. Introduction
Phosphorus (P) is the eleventh in order of abundance element on Earth; under most
conditions, it is exclusively combined with four oxygen molecules, forming the phosphate
oxyanion. Phosphorus is essential for all living organisms, as it represents the energy currency
for organisms at cell level, and its availability often controls biological productivity; for that
reason, in excess quantities it is the cause of eutrophication. Eutrophication can be described
as nutrient enrichment of surfaces waters, leading to an excessive production of toxic algae,
and is responsible for turning water green in lakes, reservoirs, rivers, coastal waters and the
marine environment in general (Burke et al., 2004).
Phosphates represent the main source of P and are commonly used in fertilisers, detergents or
insecticides. Morse et al. (1993) reported that the overdose of P in European Union (E.U)
countries water essentially comes from human sources in sewage and from live stock.
Since 1991, European legislation has approached this pollution problem by establishing a new
directive (EC Urban Waste Water Treatment Directive 91/271/EEC, UWWTD, 1991). The
removal of P in wastewater discharged to sensitive areas is now regulated and minimum P
concentrations in effluents are imposed, depending on the size of discharge (Table 1).
Table 1. Requirements of P concentration for discharges from urban waste water treatment
plant (UWWTD, 1991)
Population
(population equivalents)
Phosphorus
limit
Minimum percentage
of reduction *
10 000-100 000 p.e.
2 mg. L
-1
80 %
More than 100 000 p.e.
1 mg.L
-1
80 %
* related to the load of the influent
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The legislative pressure has lead to more discussions on how to integrate P removal processes
in wastewater treatment plants (CEEP, 1971- to present). Traditional P removal processes are
based on phosphorus fixation in activated sludge either by a biological (biological nutrient
removal, BNR) or chemical (precipitation by metal salts) method. These processes are
efficient in the sense that they can reduce the P concentration in wastewater effluents to less
than 1 mg.L
-1
(Booker et al., 1999), but they lead to the accumulation of phosphorus in
sludge, an increase in sludge volumes, and contribute by reaction with magnesium and
ammonium ions to the precipitation of magnesium ammonium phosphate hexahydrate most
commonly known as struvite.
Struvite (MgNH
4
PO
4
.6H
2
O) scale deposits are causing significant concern to
wastewater treatment plants (Doyle et al., 2003). The problem is not necessarily new, as
struvite was first observed as a crust of crystalline material in 1937 in a multiple-stage sludge
digestion system (Rawn et al., 1937). Often perceived as a nuisance affecting the efficiency of
treatment processes and causing maintenance problems, the control of struvite deposition has
been widely investigated, including the dilution of struvite crystals with water effluents
(Borgerding, 1972); preventive action by chemical dosing of iron salts (Mamais et al., 1994)
or addition of chemical inhibitors (Doyle et al., 2003; Snoeyink and Jenkins, 1980).
In the past 10 years struvite precipitation has gained interest as a route to phosphorus recovery
(Doyle et al., 2003). Its composition (nitrogen (N), phosphorus (P) and magnesium (Mg) ions
in equal molar concentrations) makes it a potentially marketable product for the fertiliser
industry, provided that its nucleation and the quality of crystals recovered can be controlled
(Booker et al., 1999). Research in struvite formation is now widespread and includes studies
towards the prevention of scaling, alternative phosphorus removal and recovery from waste
water effluents and potential exploitation to the benefit of wastewater companies and
industries as a fertiliser.
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Several studies have been carried out to assess potential methods of phosphorus recovery as
struvite at a bench and pilot scale, and few processes, integrated in treatment plants, already
exist and are effective in Japan (Ueno and Fujii, 2001), The Netherlands (Giesen, 1999) and
Italy (Cecchi et al., 2003, Battistoni et al., 2005a).
This review paper focuses on phosphorus removal and recovery by struvite crystallisation. It
provides an understanding of principles of struvite crystallisation and reviews the techniques
and processes experimented to date by researchers at laboratory, pilot and full scale to
maximise phosphorus removal and reuse as struvite.
2. Struvite chemistry
2.1 Struvite characteristics
Struvite is an orthophosphate, containing magnesium, ammonium, and phosphate in
equal molar concentrations. The general formula for minerals of the struvite group is:
AMPO
4
·6H
2
O where A corresponds to potassium (K) or ammonia (NH
3
) and M
corresponds to magnesium (Mg), cobalt (Co), or Nickel (Ni) (Bassett and Bedwell,
1933).
Struvite in the form of a magnesium ammonium phosphate hexahydrate crystallises as an
orthorhombic structure (i.e. straight prisms with a rectangular base). Table 2 summarises the
main chemical and physical properties of struvite crystals.
