PIAHSProceedings of the International Association of Hydrological SciencesPIAHSProc. IAHS2199-899XCopernicus PublicationsGöttingen, Germany10.5194/piahs-376-83-2018Socio-Hydrological Approach to the Evaluation of Global Fertilizer
Substitution by Sustainable Struvite Precipitants from WastewaterSustainable Struvite Precipitants from WastewaterKokDirk-Jan Danield.d.kok@student.tudelft.nlPandeSaketOrtigaraAngela Renata CordeiroSavenijeHuberthttps://orcid.org/0000-0002-2234-7203UhlenbrookStefanDepartment of Water Management, Delft University of Technology,
Delft, the NetherlandsWorld Water Assessment Programme, UNESCO, Perugia, ItalyDirk-Jan Daniel Kok (d.d.kok@student.tudelft.nl)1February2018376838625July20173October2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://piahs.copernicus.org/articles/376/83/2018/piahs-376-83-2018.htmlThe full text article is available as a PDF file from https://piahs.copernicus.org/articles/376/83/2018/piahs-376-83-2018.pdf
Despite Africa controlling the vast majority of the global
phosphate it also faces the greatest food shortages – partially due to a
lack of access to the fertilizer market. A more accessible source of
phosphorus comes from wastewater flows, which is currently lost through the
discharge to open surface waters. Analysing the potential phosphorus
production of urban and livestock wastewater in meeting partial agricultural
demand for phosphorus can improve food security, reduce consumption of
unrenewable phosphorus, reduce pollution, and aid the transitioning to a
circular economy. In this study, a global overview is provided where a
selection of P-production and P-consumption sites have been determined using
global spatial data. Distances, investment costs and associated carbon
footprints are then considered in modelling a simple, alternative trade
network of struvite phosphorus flows. The network reveals potential for
increasing the phosphorus security through phosphorus recycling in
particularly the South Africa, Lake Victoria and Nigeria regions. Given
Africa's rapid urbanization, phosphorus recovery from wastewater will prove
an important step in creating sustainable communities, protecting the
environment while improving food security, and so contributing to the United
Nations 2030 Agenda for Sustainable Development.
Introduction
Phosphorus (P) is an element necessary for the development of crops as it
forms a key, structural component of DNA and RNA. It is applied in the form
of single or triple superphosphate, or mono-Ammonium or di-Ammonium
Phosphate (DAP) fertilizers which are both easy to transport, to distribute
over fields and are readily absorbed by plants. The most essential source
for the production of phosphorus fertilizer is phosphate ore. Some authors
predict a peak production of phosphate ore could occur as early as 2030
(Cordell et al., 2009), or that extractable mineral P resources will become
scarce or exhausted within the next 50 to 100 years (Steen, 1998; Smil,
2000; van Vuuren et al., 2010). This prospect threatens the food security
situation of Sub-Saharan Africa (SSA), as nearly 75 % of SSA's
agricultural soils are nutrient deficient and so already contributing
significantly to the crop yield gaps (IFDC, 2006). The immediate results of
phosphate rock depletion will be a further reduction of the accessibility to
fertilizers by small-holder and sustenance farmers that comprise areas
already coping with food shortages.
Potential to Recover
Despite the issues surrounding phosphorus demands and yield gaps, there
currently exists no financially attractive recovery technology for the
enormous phosphorus recovery potential from livestock (Schoumans et al.,
2015), while urban recovery is often also limited due to financial
constraints. The competitive position of the relatively expensive, recovered
phosphorus is improving, however, as over the past 15 years the phosphorus
price of di-Ammonium Phosphate (DAP; Index Mundi, 2017) has increased from 665 [USD t-1]
to 1552 [USD t-1]. In that same period, the price has been as high as
5217 [USD t-1] during the economic crisis around 2008 and as low as
656 [USD t-1] in 2002 (Index Mundi, 2017). Given these price trends,
and that the will, the technology and the knowledge are there to facilitate
the largescale implementation of phosphorus recovery, it is important that
investigation is done as to what such a development may come to look like.
This study aims to identify and connect those areas where there exists a
high potential for the recovery of a sustainable phosphorus products from
urban and livestock wastewater, to those areas of high agricultural demand.
Materials and Methods
Using population density maps (CIESIN, 2016; Robinson et al., 2014) and
globally generalized phosphorus excretion rates (Gilmour et al., 2008;
Barker et al., 2001; CBS, 2014), a crude mapping of phosphorus production
sites is carried out at global scale. Apart from these sustainable sources,
a total of 5.2 [mt a-1] P-production from major African mines is
included also (USGS, 2007). The demand for phosphorus is assessed through
crop harvested area maps for 6 major crops that account for approximately
50–56 % of the global phosphorus fertilizer consumption (Heffer and
Prud'homme, 2008). The P demands for maize, wheat, rice, sorghum, soy bean,
and potato are approximated for the optimal, but water-constrained, yield as
determined through the transpiration deficit method (Steduto et al., 2012).
The associated phosphorus requirement for this yield is determined through a
linear regression between yield and P-fertilizer application. These demands
are then proportionally scaled up to match the global agricultural demand of
all crops of 16 [Mt a-1]. The remaining production and demand sites are
connected to each other through a network. The creation process accounts for
generalized struvite precipitation costs by Eq. (1),
fmini=Pm×Rmp+BsSPT-Ss
where fmini is the minimum price for phosphorus for node i
[USD t-1]; Pm is the price of magnesium [USD t-1]; Rmp is the
ratio of magnesium required per ton of phosphorus; Bi fixed operational
cost [USD]; SPT is the total phosphorus production potential [t]; and
Ss is the minimum scaling cost savings from struvite precipitation, 620
[USD t-1] (Shu et al., 2006). Similarly, maximum prices that demand
nodes are able to pay are determined roughly through Eq. (2).
fmaxn=Yoptn×CanPoptn×Rn
where fmaxn is the max price for
phosphorus [USD t-1]; Yoptn is the
optimal yield [t ha-1]; Can is the
crop price in year a [USD t-1]; Popt is the
optimum fertilizer dosage rate (equal to P-demand for optimal yield) [t
ha-1]; and Rn is the ratio of fertilizer cost to total costs [–], for crop n. The
transportation costs are determined with as-the-crow-flies distances with a
land transport cost equation as a basis. In this are considered: the price
of diesel, a labour wage of 17 [USD h-1], an average velocity of 80
[km h-1], a load capacity of 60 tonnes (2 × 30), and a fuel efficiency of
0.53 [L km-1] at capacity. The model is run for a future P-supply
scenario of mine production supplemented by sustainable sources that are
introduced individually when market prices make their recovery feasible.
Dominant network pattern of potential trade in recovered P (on top
of rock based P).
Results
While the network analysis is conducted globally for the year 2005, only
outcomes relevant for continental Africa are discussed here. The network
reveals potential for increasing the phosphorus security of particularly
Rwanda, Burundi, Kenya, Tanzania, Uganda, Malawi, South Africa, and Nigeria,
through phosphorus recycling (Fig. 1).
Approximate phosphorus production potentials of 130 [kt a-1] and 1300
[kt a-1] from concentrated urban and livestock areas respectively are
determined for continental Africa. The phosphorus demand for optimal, water
restricted yield in Africa for the six major crops is approximately 1 [mt a-1], which equals 12 % of the global crop demand.
Continental wide struvite
precipitation from major urban population centres can theoretically then satisfy a maximum of 13 %
of the agricultural demand for these crops. The Lake Victoria and South
African regions show a higher than average density of fluxes in sustainable
trade. The first areas to offer competitive sustainable phosphorus lie in
these regions, entering the market around prices of 600 [USD t-1] P.
The African market in recoverables, i.e. recycled phosphorus, expands
further at 800 [USD t-1], and continues to grow in smaller amounts at
higher prices.
Discussion and Conclusion
The model offers a simple framework for network assessment of optimal global
phosphorus trade and maximum phosphorus potentials. Provided it is at global
scale and that we have only discussed outcome relevant for Africa, however,
the study is also superimposed with many generalisations and assumptions
that introduce many side notes to its validation. Inaccuracies in assessing
the urban and livestock production potentials due to generalization of
throughput figures, the inconsideration for trans-Atlantic trade, the
as-the-crow-flies distance method, assumptions of free trade, as well as the cost equation
assuming land transport costs per kilometre – disregarding cheaper costs
for shipping – all possibly contribute to an over approximation of the
market prices.
Sustainable products become more competitive with higher market prices for
phosphorus. Higher market prices are created as the price of fuel rises,
mine exploitation costs increase and total supply decreases. Sustainable
sources already have a potential to be competitive in South Africa, the
Victoria Lake region and Nigeria given the close proximity of production and
demand nodes of significant supply and demands, and their relative distance
from the world's largest phosphate mines. The introduction of recovered
products to the market will result in: (1) an increase in access to
fertilisers within Africa and elsewhere, (2) lower phosphorus cost, (3) cause
for a significant reduction in transport associated emissions, (4) stimulate
the treatment of wastewater, and (5) contribute to improving food security.
This study's produced datasets are no longer available with the updating of the model.
The complete and updated datasets are available from the corresponding author on request.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Water security and the food–water–energy nexus: drivers, responses and feedbacks at local to
global scales”. It is a result of the IAHS Scientific Assembly 2017, Port Elizabeth, South Africa, 10–14 July 2017.
Edited by: Graham JewittReviewed by: Rebecca Sindall
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