Available online at www.sciencedirect.com
Plant based phosphorus recovery from wastewater via algae and
macrophytes
Andrew N Shilton, Nicola Powell and Benoit Guieysse
At present, resource recovery by irrigation of wastewater to
plants is usually driven by the value of the water resource
rather than phosphorus recovery. Expanded irrigation for
increased phosphorus recovery may be expected as the
scarcity and price of phosphorus increases, but providing the
necessary treatment, storage and conveyance comes at
significant expense. An alternative to taking the wastewater to
the plants is instead to take the plants to the wastewater. Algal
ponds and macrophyte wetlands are already in widespread
use for wastewater treatment and if harvested, would require
less than one-tenth of the area to recover phosphorus
compared to terrestrial crops/pastures. This area could be
further decreased if the phosphorus content of the
macrophytes and algae biomass was tripled from 1% to 3%
via luxury uptake. While this and many other opportunities for
plant based recovery of phosphorus exist, e.g. offshore
cultivation, much of this technology development is still in its
infancy. Research that enhances our understanding of how to
maximise phosphorus uptake and harvest yields; and further
add value to the biomass for reuse would see the recovery of
phosphorus via plants become an important solution in the
future.
Address
School of Engineering and Advanced Technology, Massey University,
Private Bag 11-222, Palmerston North, New Zealand
Corresponding author: Shilton, Andrew N ([email protected])
Current Opinion in Biotechnology 2012, 23:884–889
This review comes from a themed issue on Phosphorus
biotechnology
As phosphorus becomes increasingly scarce, it would
seem likely that irrigation specifically for the purpose
of phosphorus recovery will increase. However, this can
be far more challenging than it initially appears due to:
1. Quality constraints requiring treatment: dependant on
the type of crop irrigated (and its subsequent use) and
the wastewater source, additional treatment stages
may be required to mitigate risks due to pathogens
and/or toxins.
2. Temporal constraints requiring storage: crops have a
seasonally variable demand for phosphorus and water.
In arid regions large storage reservoirs are required to
match irrigation to the dry season, whereas in other
regions reservoir storage is required during wet seasons
to prevent leaching to groundwater and/or runoff to
surface waterways [4,5].
3. Spatial constraints requiring conveyance:
a. In some agricultural regions where intense farming
uses imported stock feed the waste needs to be
conveyed back out of the district due to tight
restrictions to avoid localised phosphorus overloading of soils [6].
b. Conversely, for phosphorus in domestic wastewater
the challenge is the large quantity of very dilute
wastewater. For example, to move 1 kg of phosphorus requires the transport of approximately
150,000 kg of domestic wastewater. Furthermore,
the larger the population the wastewater is derived
from, the longer the distance the reticulation
system is to reach suitable agricultural land.
Edited by Andrew N Shilton and Lars M Blank
For a complete overview see the Issue and the Editorial
Available online 10th August 2012
0958-1669/$ – see front matter, # 2012 Elsevier Ltd. All rights
reserved.
http://dx.doi.org/10.1016/j.copbio.2012.07.002
Introduction
Irrigation of wastewater back to crops or pasture seems
an obvious solution for closing the phosphorus cycle.
While often feasible for agricultural wastes, it is less
common for domestic wastewater. In countries such as
Israel [1], West Africa [2] and Australia [3] irrigation
of domestic wastewater is driven by the value of the
water resource rather than the nutrient value; however,
as a consequence of this practice phosphorus is also
recovered.
Current Opinion in Biotechnology 2012, 23:884–889
Providing treatment, storage and conveyance to address
the above constraints comes at significant expense. When
water scarcity is not the key driver for implementing
irrigation to crops or pasture then, as illustrated in
Figure 1, an alternative is to use an intermediate stage
that effectively concentrates the dilute, dissolved phosphorus into a solid biomass for reuse.
There are a number of treatment options for concentrating dissolved phosphorus from the wastewater into a solid
including the use of adsorptive filters, precipitation reactions and bacterial uptake (in particular the enhanced
biological phosphorus removal activated sludge process,
EBPR). These options are covered by other reviews [7,8]
written as part of this journal issue on phosphorus. However, there is also an alternative to ‘taking the wastewater
to the plants’ by instead ‘taking the plants to the wastewater’ [9]. Algae and macrophytes, which can recover
www.sciencedirect.com
Plant based phosphorus recovery from wastewater Shilton, Powell and Guieysse 885
SOURCE
Figure 1
Wastewater
(domestic, industrial,
agricultural)
IRRIGATION
EBPR
Physical/
chemical
Focus of other reviews in
this journal issue [7,8]
REUSE
Harvesting
Application to
terrestrial crops
Use as an
animal feed
CONCENTRATION
Plants
(algae/macrophytes)
Value added
products
Current Opinion in Biotechnology
Overview of options to utilise plants to recover phosphorus from wastewater.
phosphorus in less than one-tenth of the land area
required by terrestrial crops (see Box 1), are already
widely used for wastewater treatment around the world
and are reviewed in the following sections.
Phosphorus recovery by algae
Algae are ubiquitously used for wastewater treatment in
facultative and high rate aerobic ponds around the world
because of their ability to facilitate cost effective treatment of organic carbon and pathogenic pollutants [10,11].
Unfortunately, phosphorus removal is variable and most
commonly quite low in these systems [12,13].
In facultative ponds algal assimilation is not capable of
removing all the phosphorus found in the wastewater as
these ponds are not designed to optimise biomass productivity. There is, however, potential for using high rate
algal ponds which have higher biomass productivity as
they are shallow, gently mixed and operated at relatively
short retention times. Furthermore, as reported by Richmond [14] the phosphate content of algal dry biomass
could reach up to 3.3%, which is considerably higher than
the 1% content typically achieved in ‘normal’ pond algal
biomass. This finding was recently confirmed by Powell
et al. [15] who showed that the phosphate content of a
mixed algal continuous culture grown on synthetic wastewater in the laboratory could be varied between 0.41 and
3.16% depending on the conditions they were exposed to.
The extent of this ‘extra’ storage, accumulated in the
form of polyphosphate, is a function of the light intensity,
www.sciencedirect.com
the phosphate concentration in the medium, and the
temperature [16,17,18]. While research has not yet
attempted to maximise phosphorus accumulation at full
scale, maintaining a 3% phosphorus concentration could
potentially reduce the area requirement for phosphorus
uptake from a high rate pond to just 5–6 m2/capita (Box 1)
and reduce costs for harvesting, transporting and spreading the biomass as a fertiliser by over 60% compared to
algae with a ‘standard’ phosphorus content.
Both biomass growth and harvesting must be maximised
in order to maximise phosphorus recovery. At present the
number of treatment ponds with algal harvesting is very
limited. High rate algal ponds have a higher biomass
concentration than other pond designs and are dominated
by non-motile algae which helps facilitate simple gravity
harvesting of the algal biomass. Furthermore, Park et al.
[19] found that their harvesting efficiency could be
enhanced from 60% to >85% when a proportion of the
biomass was recycled.
While the vast majority of full scale, algal based wastewater treatment systems involve suspended algae there
has also been interest in immobilisation of algae in beads
and ‘algal turf scrubbers’ [20,21]. Algae can be entrapped
into beads made of materials such as alginate, carrageenan
and chitosan to assimilate phosphorus. Phosphorus
adsorption or precipitation also occurs on the media used
to immobilise the cells. Fierro et al. [22] found that
immobilised Scenedesmus achieved 94% phosphorus
Current Opinion in Biotechnology 2012, 23:884–889
886 Phosphorus biotechnology
removal compared to the control without algae which
achieved 60%. Algal turf scrubbers are an attached growth
technology where an algal biofilm is grown on a surface
and so is easily harvested by scrappers. This technique
has been used for treating many types of wastewater
including dairy [9] and swine manure [23].
As we look out to a phosphate scarce future, algal based
phosphorus recovery could significantly expand offshore.
Around the world there are massive discharges of wastewater directly to seas and oceans. Thus installation of
booms to form offshore floating pond systems around the
naturally buoyant discharges from wastewater outfalls has
been proposed [24]. Optimising for algal growth and
harvesting from offshore systems may even present
superior economics to ‘on shore’ algal cultivation due
to the absence of the highly significant land cost component.
Phosphate recovery by macrophytes
Shifting to larger aquatic plant species, floating macrophytes such as water hyacinth (Eichhornia crassipes) and
duckweed (Lemnaceae minor) grow on the surface of ponds
whereas emergent macrophytes are grown in what are
commonly referred to as constructed wetlands. Most
common is the use of emergent macrophytes. These
plants are adapted to grow up through the water column
with their root zone and stems submerged. A recent
innovation is the development of emergent macrophyte
species supported by a floating mat so that the long roots
of these plants grow through the mat and down into the
water column providing a large surface area for biofilm
development [25,26]. Because the roots in these new
systems are kept suspended in the water column, all
nutrient requirements for the plants must be extracted
from the wastewater rather than the soil/media into which
they are normally planted in conventional constructed
wetlands [25].
Some researchers state that phosphorus removal in wastewater wetlands is predominantly due to biomass growth
[27]. Others note that there is the potential for achieving
luxury uptake in macrophytes with phosphorus contents
of up to 2.9% being reported [28,29]. It has also been
found that the contribution made by biofilms growing on
the plants can be significant and has been reported to
account for up to 31–71% of phosphorus removal. Xu and
Shen [30] described a duckweed-based system which
retained a high phosphate removal in winter despite
limited duckweed growth and attributed this to improved
protein accumulation by the duckweed and nutrient
uptake by attached biofilm of algae and bacteria.
While plants provide nutrient storage during the growing
season, at present the plants are seldom harvested from
wastewater treatment wetlands and so the nutrients are
re-released when the plant material is left to decay [31].
Indeed, most of the recent research related to phosphorus
removal in wetland treatment systems instead focuses on
the adsorption–precipitation reactions of phosphate with
Box 1 How much area is required to recover phosphorus via plants?
The area required by plants to recover phosphorus is dependent on their % phosphorus content and their areal biomass productivity. Taking
typical values of domestic wastewater phosphate content of 3.6 gP/person-day, approximate comparisons of the area of plants required to uptake
the phosphorus from domestic wastewater on a per capita basis can be made as seen below. The comparative biomass yields are indicative only
as growing conditions such as irradiance and temperature are not exactly matched. Switchgrass and maize are included as examples of relatively
high yielding terrestrial crops (both commonly proposed for bioenergy production). It could be generalised that, if harvested, macrophytes and
algae would require less than one-tenth of the area to recover phosphorus compared to terrestrial crops/pastures. The required area could be
further decreased if the phosphorus content of the macrophytes and algae biomass was tripled from 1% to 3% via luxury uptake.
Area (m2/person)
Biomass yield
tonne/ha yr
%P per dry weight
Switchgrass
8.7–12.9 [35]
0.3% [40]
Maize
9–30 [36]
0.3% [40]
Macrophytes
35–106 [37,38]
1% [28,29]
12-38
Macrophytes (high %P)
35–106 [37,38]
3% [28,29]
5-13
Algae
69–91 [39]
1% [14]
14-19
Algae (high %P)
69–91 [39]
3% [15]
5-6
340-503
340-503
146-487
146-487
Notes: The boxes are drawn schematically, they are not to scale.
In the algal and macrophyte systems, phosphorus would also been assimilated by heterotrophic bacteria which would further increase the P uptake.
Current Opinion in Biotechnology 2012, 23:884–889
www.sciencedirect.com
Plant based phosphorus recovery from wastewater Shilton, Powell and Guieysse 887
the supporting physical media the macrophytes are
planted into [32–34], rather than plant uptake per se.
However, looking to the future it is notable that a key
advantage of recovering phosphorus using floating macrophytes and, potentially floating mat emergent systems, is
that the harvesting of the biomass may be easier to
facilitate than that of suspended microalgae.
grown rather than the mixed cultures typically found in
wastewater treatment systems.
Interestingly, plant biomass even has found application as
a physicochemical absorbent with Chen et al. [50] investigating water hyacinth as an absorbent filter system for
phosphate recovery.
Conclusions
Reuse
As indicated in Figure 1, the algal/macrophyte biomass
grown to recover phosphorus from wastewater can be
utilised in several ways such as for a fertiliser or as a food
source in its own right.
In order for this biomass to replace traditional phosphorus
fertilisers the harvesting, transportation, stability, application techniques and the proportion of phosphorus availability to crops must be considered. Unlike bacterial
biomass from ‘enhanced biological phosphorus removal’
systems which quickly re-releases stored phosphorus
under anaerobic conditions, Powell et al. [41] showed
that algal biomass can retain stored phosphorus for some
days. Furthermore, with regard to its fertiliser potential,
Mulbry et al. [42] compared seedling growth using dried
algal biomass to commercial fertiliser and showed growth
at comparable levels. However, overall these issues from
harvest to application are currently quite poorly covered
by the literature for both algae and macrophytes.
Algae have been identified as making a good supplementary livestock feed, such as for chickens, due to their
high protein content [43,44]. Macrophytes from wetland
treatment systems have been used as cattle feed [45] and
there is interest in use of emergent wetland plants with
higher feed value such as Taro [46]. Plants can be used as
a food source for a range of aquatic animals including fish,
molluscs and shrimps [44]. With careful management, to
minimise any risk of contamination, aquaculture ponds
can be successfully used at the end of algal pond systems
transforming the plant biomass into fish biomass which is
easily harvested [47].
As the value of phosphorus increases we would expect to
see an increasing move to a ‘cascade’ of phosphorus
recovery where the phosphorus discharged from one
process is used for the cultivation of a following stage.
For example, Endut et al. [48] reported on the further
downstream treatment of aquaculture effluent via hydroponic cultivation of water spinach and mustard greens.
Plant biomass also has the potential to be used in other
value added products such as human food supplements,
cosmetics and extraction of high value cellular components
[49] all of which applications represent a saving of mined
phosphorus resources. However, these more specialised
products generally require specific strains of algae to be
www.sciencedirect.com
Domestic wastewater irrigation is currently more driven
by the value of the water resource rather than phosphorus
recovery. While irrigation can be expanded for increased
phosphorus recovery, providing the necessary treatment,
storage and conveyance comes at significant expense. An
alternative to taking the wastewater to the plants is to
instead take the plants to the wastewater. Algal ponds and
macrophyte wetlands are already in widespread use for
wastewater treatment and if harvested, require less than
one-tenth of the area to recover phosphorus compared to
terrestrial crops/pastures. This area could be further
decreased if the phosphorus content in the macrophytes
and algae biomass was tripled from 1% to 3% via luxury
uptake. Multiple technology options exist for cultivating
algae and macrophytes including immobilisation of algae
in beads, algal turf scrubbers, offshore cultivation systems
and floating wetland mat systems as well as new
approaches for optimising harvesting, e.g. incorporating
recycle so as to improve gravity settling of algae. However, research into alternative cultivation methods and
reuse of the biomass as a fertiliser or stock food are still in
their infancy. Further research, particularly at field scale,
that enhances our understanding of how to maximise the
phosphorus content of the biomass, improve efficiency of
cultivation and harvesting, and add value to the biomass
for reuse would see the recovery of phosphorus via plants
become an important solution in the future.
References and recommended reading
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have been highlighted as:
of special interest
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