This journal article is indexed by SCOPUS and others.

1. Applied Mechanics and Materials Vols. 110-116 (2012) pp 2043-2048
© (2012) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.110-116.2043
Wastewater Treatment for a Sustainable Future: Overview of
Phosphorus Recovery
S. Muryanto1,a and A.P.Bayuseno2,b
1
Office of Research and Department of Chemical Engineering, UNTAG University, Bendhan Dhuwur
Campus, Semarang 50233, INDONESIA
2
Centre for Waste Management and Mechanical Engineering Graduate Program, DIPONEGORO
University, Tembalang Campus, Semarang 50275, INDONESIA
a
email: technologypark28@yahoo.com.au, bemail: apbayuseno@gmail.com
Keywords: crystallisation, phosphorus, renewable, struvite, wastewater
Abstract. Intensified agriculture in response to the growing population has led to excessive nutrient
discharges to natural waters causing environmental problems in the form of eutrophication and its
associated risks. Treatment options for this adverse effect include removal and recovery of soluble
phosphorus by chemical precipitation, biological uptake, and struvite crystallisation. Chemical
precipitation is the most common method due to its simplicity, but the chemical requirements can be
prohibitive and the removed phosphorus is less reusable. Biological uptake requires less chemicals
but the process is complex and prone to seasonal variations. Phosphorus removal and recovery from
wastewater by struvite crystallisation is an attractive option since the crystallisation process converts
phosphorus into struvite crystals, i.e. phosphate minerals which have proved to be good fertilizer,
hence potentially reduces fertilizer production and the subsequent greenhouse gas emissions.
Moreover, struvite crystallisation helps prevent scaling of wastewater treatment facilities. A number
of struvite crystallisation projects utilising primarily agricultural wastewater is already operational at
industrial scale.
Introduction
Intensified farming in response to rapid population growth demands extensive use of fertilizer, and
over-application of phosphate fertilizer to soil has been frequently encountered. As a consequence,
waste streams from agricultural activities contain considerable amount of soluble phosphate.
Discharging such nutrient-rich streams to surface waters results in eutrophication which is unsightly
and lethal to aquatic life, as well as polluting water supplies [1].
Nutrient-rich wastewater also causes operational problems for wastewater treatment (WWT)
facilities due to crystalline scale deposited on the surface of equipment and parts such as pumps,
valves, pipes, and separating screens [2]. The major component of such deposits is in many cases
magnesium ammonium phosphate hexa-hydrate (MgNH4PO4.6H2O), commonly known as struvite,
which precipitates spontaneously out of the solution once its solubility limit is exceeded. Due to its
low solubility in water, struvite may accumulate and strongly attach to the equipment extensively
rendering the equipment failure and substantial financial loss [2, 3].
A number of legal initiatives were put forward to solve the problems of wastewater, such as the
EEC directive of 1991[4]. One important aspect of such initiatives is the requirement that wastewater
should contain phosphorus (P) at a level considered safe for the environment.
Removal of excessive P from wastewater is also an indirect means of securing a sustainable future.
P can be recovered from organic wastes, particularly those originated from agricultural streams [5],
and subsequently converted into struvite, a potential slow-release fertilizer, and thus contributes to
reducing greenhouse gas emissions (in fertilizer production, transport, and so on).
By the middle of this century, the world is predicted to face a formidable challenge to provide food
and other vital resources for the estimated nine billion people [6]. This tough challenge will have to
take into serious consideration the availability of essential nutrients for crops, especially P, which is
not a renewable resource. With the current global rate of P utilisation, which is mainly for fertilizer
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 180.246.64.123-03/10/11,17:41:13)

2. 2044 Mechanical and Aerospace Engineering
production, high quality phosphate rock as the main source of P is predicted to last within the next 50
to 100 years [6]. Therefore, efforts to remove and recover P from wastewater as a substitute for
phosphate rock deserve serious attention. This paper discusses three methods of P removal and
recovery from wastewater, namely chemical, biological, and struvite crystallisation, respectively.
P removal through chemical methods
The common practice of WWT involves precipitation processes followed by separation of the
precipitation products into solid and liquid fractions. The liquid portion would then be discharged into
the sea or fresh water bodies. Since excess concentration of P in the liquid effluent gives rise to
environmental nuisance such as eutrophication, WWT plants need to lower the P concentration down
to an acceptable level. This is commonly achieved by chemical precipitation using either chloride or
sulphate salts of various metal ions (such as iron and aluminium), or lime to form phosphate
precipitates.
The WWT facilities utilising chemical precipitation methods may well spend substantial cost for
chemical dosing. However, if the chemical precipitation technique is combined with struvite
crystallisation (to be described later) the requirement for chemicals can be reduced. Britton and Baur
[7] reported that the cost spent for aluminium sulphate dosing in their WWT plant decreased by more
than 20% after struvite crystallisation unit was added into the existing plant.
A typical P removal using chemical method is by direct injection of precipitants into the inffluent
line as depicted in Fig. 1. The chemicals injected react with the nutrient in the wastewater inside the
clarifying tank. With the chemical method, concentration of P in the effluent can be reduced to as low
as 0.7 ppm [8]. For optimum P removal using iron (Fe2+ or Fe3+), the pH of the streams should be low,
i.e. about five [9]. Higher pH promotes the formation of a loose precipitate of Fe(OH)3 with the
consequence that the soluble phosphates (which should be precipitated as solid FePO4) will largely
remain in the water and hence most of the P unremoved. As suggested by Parsons and Smith [9], both
pH level of the liquid waste and agitation speed for the thorough dispersion of the added chemicals
are crucial for the attainment of optimum P removal. For higher pH, rigorous mixing is required since
the rate of precipitation of FePO4 is much slower than that of Fe(OH)3.
Chemicals
Clarifying Tank
Influent Effluent
P sludge
Figure 1 Schematic representation of a typical chemical P removal
Chemical precipitation is a fairly reliable and common method of P removal. However, the cost of
chemicals can be prohibitive and the volume of the sludge generated after precipitation is higher than
that before the treatment. Keplinger and co-workers [10] estimated that for one mole of P in the liquid
waste, approximately two moles of alum must be added in order to achieve the acceptable discharge
content of one mg P/liter (= 1 ppm). Due to the presence of metal ions, the sludge is also less
biodegradable. Moreover, various contaminants in the waste such as heavy metals and pathogenic
bacteria may contaminate the soil and hence, the food chain.

3. Applied Mechanics and Materials Vols. 110-116 2045
P removal through biological methods
Another method of P removal (and recovery) from wastewater is through biological means,
whereby the organic matter in the wastewater is anaerobically digested and converted into CH4
(methane gas) and CO2 by the action of selected groups of bacteria, such as Acinetobacter spp.,
Microlunatus phosphovorus, Lampropedia spp., and Rhodocyclus [11]. These microorganisms also
incorporate the P content in the wastewater into their cells, hence they are termed
phosphorus-accumulating organisms (PAOs) [9]. These PAOs are then removed together with the
sludge. The advantages of biological methods include less chemical consumption and less volume of
treated sludge, as well as the production of methane which can be used as biogas. Of particular
interest is the enhanced biological P removal (EBPR) process where most of P is accumulated by the
bacteria and converted into polyphosphates. The EBPR process consists of anaerobic and aerobic
zones through which the PAOs and wastewater are circulated to encourage P uptake and release. The
growth and activities of the bacteria, hence the uptake and release of P, are highly dependent on the
type and quantity of organic substance in the waste, especially volatile fatty acids (VFAs). In many
cases, optimum removal of P can only be achieved if organic substance such as acetate is added into
the waste prior to EBPR processing [9].
The EBPR can be regarded as a two stage process, i.e.an anaerobic followed by an aerobic phase
(see schematic diagram of an EBPR in Fig. 2). During the anaerobic phase, polyphosphate chains in
the cells of the PAOs are hydrolysed into orthophosphates (PO43-) and released from the cells, a
process which generates the necessary energy for the PAOs to convert VFAs in the waste into
poly-β-hydroxybutyrate (PHB), and to store the PHB in the cells. Then, during the second stage -
owing to the availability of oxygen - the stored PHB is metabolised, providing energy for cell growth
and for PO43- uptake from wastewater.
Among the drawbacks of the EBPR facilities are the difficulties in maintaining a stable and
reliable operation. These difficulties could materialise when local conditions change, such as
fluctuations in flow rates and compositions of the wastewater due to heavy rainfalls and so on. Thus a
stable operation of EBPR can be difficult to achieve.
Influent
Clarifier
Anaerobic digester Aerobic digester
Return activated sludge Sludge
Figure 2 Schematic representation of an EBPR
P recovery through struvite crystallisation
Recovering P from wastewater through struvite crystallisation is technologically and
economically promising. Struvite (MgNH4PO4.6H2O) is a phosphate mineral which consists of
equimolar amounts of magnesium, nitrogen and phosphorus, respectively. Therefore, it is chemically
similar to common fertilizer. Struvite has proved to be excellent fertilizer, especially for
container-grown nurseries, horticulture and turf [12, 13]. In addition, struvite has a low solubility in
water so that over-application of it does not cause root burning of crops and losses due to leaching.
Furthermore, recovery of P from wastewater through struvite crystallisation can alleviate the burden
of scale formation and build-up in WWT facilities. Finally, this type of P recovery contributes to
reducing environmental impacts such as greenhouse gas productions, through the reduction in
fertilizer manufacture, transport and so on.

4. 2046 Mechanical and Aerospace Engineering
Figure 3 Schematic diagram of a biological treatment coupled with an FBR to recover P as
struvite fertilizer (Courtesy of Multiform Harvest Inc. Seattle, Washington,USA,
www.multiformharvest.com)
Figure 4 Detailed description of the FBR (Courtesy of Multiform Harvest Inc.
Seattle,Washington,USA, www.multiformharvest.com)

5. Applied Mechanics and Materials Vols. 110-116 2047
Struvite crystallises as white orthorhombic crystals according to the following reaction:
Mg2+ + NH+ + HnPO4n-3 + 6 H2O MgNH4PO4.6H2O + nH+
Depending on the pH of the solution, the values of n can be 0, 1, or 2. Obviously, the
crystallisation process is highly dependent on pH. As can be seen from the above equation, the
struvite crystallisation process prefers high pH values, thereby the reaction proceeds to the right.
Basically, three struvite crystallisation techniques have been widely tested [14], and these are (1)
selective ion exchange, (2) crystallisation in stirred reactors, and (3) crystallisation by air agitation.
The third method is carried out either in a fluidized bed reactor (FBR) or an air-agitated column.
The FBR technique is by far the most attractive and promising method, and a number of FBR
projects are already operational at industrial scale [12, 13]. A typical FBR is shown in Fig. 3, where
filtrate from an anaerobic stage, which is at this stage rich in P and nitrogen (N) due to anaerobic
digestion, is pumped up through the base of the inverted cone-shaped FBR (see details of the FBR as
shown in Fig. 4).
Since struvite crystallisation is highly dependent on pH, and that common wastewater is in general
lacking in magnesium ions, two inlets were made near the base of the FBR, to supply NaOH (to
increase pH) and MgCl2 (for magnesium ion addition) solutions. MgSO4 can also be used in lieu of
MgCl2, but the later is preferred due to its higher solubility in water. In addition, the presence of SO4
ions in water may interfere with struvite formation, especially if calcium ions are also present, i.e. the
formation of calcium sulphate, a non-reuseable product. Having sufficiently provided with the three
struvite-building components (Mg, N, P), and pH adjustment, struvite rapidly crystallises out of the
filtrate. Once struvite crystals formed, they are carried to the top of the FBR by the continuous upflow
stream, and start agglomerating with each other to form pellets. Owing to their high density and the
conical shape of the column, these heavier pellets move downward to the base of the column while
the small struvite crystals are suspended. A straightforward additional process is usually required to
produce bigger size struvite pellets for easier handling.
Figure 5 (left) An FBR treating 50,000 gallons/day of dairy effluent, (right) struvite
granules as slow release fertilizer (diam. 1 to 2 mm) (Courtesy of Multiform
Harvest Inc. Seattle,Washington,USA, www.multiformharvest.com)

6. 2048 Mechanical and Aerospace Engineering
Conclusion
Intensified agricultural activities in response to the growing population have led to eutrophication
of water bodies and its associated environmental risks. A number of initiatives are already in place to
prevent eutrophication, especially to remove P from waste streams and recover it for beneficial use.
Among the three methods commonly applied to remove P, struvite crystallisation is the most
promising. This method has three fold benefits: (1) effectively removes P from waste streams, (2)
converts P into potential fertilizer, hence reduces fertilizer manufacture and greenhouse gas
emissions, and (3) helps prevent scaling problems in WWT plants. Converting P into struvite
fertilizer has gained much interest since it is technologically and economically feasible.
References
[1] L.E. de-Bashan and Y.Bashan: Water Research, 38 (2004), pp. 4222-4246.
[2] M.I. Ali and P.A. Schneider: Chemical Engineering Science, 61 (2006), pp. 3951-3961.
[3] L.Shu et al.: Bioresource Technology, 97 (2006), pp. 2211-2216.
[4] Anonymous: Urban Waste Water Treatment Directive- European Commission on Environment,
Council Directive of 21 May 1991 concerning urban wastewater treatment (91/271/EEC) (1991).
on-line://ec.europa.eu/environment/water/water-urbanwaste/directive.html (accessed
04/04/2011).
[5] J.Wang et al.: Journal of Environmental Engineering, (Oct.2005), pp. 1433-1440.
[6] D.Cordell: The Story of Phosphorus. Sustainability implications of global phosphorus scarcity
for food security, a joint PhD thesis of University of Technology Sydney, Australia, and
Linkӧping University, Sweden, 2010.
[7] A.Britton and R.Baur: Journal AWWA, (Sept. 2010), pp. 117-118.
[8] D. Baetens: Enhanced Biological Phosphorus Removal: Modelling and Experimental Design,
PhD thesis, Ghent University, Belgium, 2000.
[9] S.A. Parsons and J.A. Smith: Elements, 4 (2008), pp. 109-112.
[10] K.Keplinger et al.: SCOPE Newsletter, 70 (Feb.2008), p. 3.
[11] T.Mino: Biochemistry (Moscow), 65(3) (2000), pp. 341-348.
[12] Anonymous: How the Cone Technology Works, on-line: www.multiformharvest.com (accessed
12/02/2011).
[13] R.Prasad: Creating Value from Waste, on-line: www.ostara.com (accessed 14/01/2011).
[14] K.S. Le Corre: Understanding Struvite Crystallisation and Recovery, PhD thesis, Cranfield
University, the UK, 2006.