Phosphorus (P) and nitrogen (N) are important human nutrients. N is readily available in the
atmosphere and can be harvested through the Haber Bosch process. P, however, is a non-renewable
resource. The current rate of P rock mining to produce mineral fertilizers to feed the
increasing world’s population is unsustainable and expected to deplete the existing reserves in
the next 30–50 years. Given the dwindling P ores, an alternative resource of P, namely P
recovered from source-separated human urine has been extensively investigated for substitution
of diammonium phosphate (DAP, (NH
4)
2HPO
4) fertilizer, which is also referred to as struvite
(i.e. magnesium ammonium phosphate [MAP], MgNH
4PO
4.6H
2O). This is due to human urine
containing high loads of P and N between 50% and 80%, re...[
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Phosphorus (P) and nitrogen (N) are important human nutrients. N is readily available in the
atmosphere and can be harvested through the Haber Bosch process. P, however, is a non-renewable
resource. The current rate of P rock mining to produce mineral fertilizers to feed the
increasing world’s population is unsustainable and expected to deplete the existing reserves in
the next 30–50 years. Given the dwindling P ores, an alternative resource of P, namely P
recovered from source-separated human urine has been extensively investigated for substitution
of diammonium phosphate (DAP, (NH
4)
2HPO
4) fertilizer, which is also referred to as struvite
(i.e. magnesium ammonium phosphate [MAP], MgNH
4PO
4.6H
2O). This is due to human urine
containing high loads of P and N between 50% and 80%, respectively, of municipal wastewater
though it represents a mere ~1% by volume. Normally, more than 90% of P and approximately
20% of N in source-separated urine can be recovered as struvite.
At the same time, water must be preserved, particularly in view of the rapidly increasing water
demands for food production and municipal uses as populations and urbanization expand
worldwide. Waterless urinals (WUs) and urine diversion toilets (UDTs) have existed since the
1990s primarily as measures to support water conservation initiatives. However, with the advent
of urine separation projects, they are being increasingly marketed around the world. Moreover,
seawater can be treated by simple methods to supply water for toilet flushing as has been
practiced in Hong Kong for over 50 years, saving up to 30% of municipal freshwater
consumption.
Hence, by combining new types of sanitary units for water saving, urine separation for P
recovery, and seawater toilet flushing, clear benefits can be achieved. However, the potential
amounts of resources that can be recovered, costs, and additional environmental impacts of the
implementation in dense buildings of large cities have not been comprehensively investigated.
This study shows that urine separation can be implemented in each building and then P
recovered as struvite by external magnesium addition or using seawater from seawater toilet
flushing. In order to evaluate the urine separation and P recovery system in a building, it is
calculated the potential amounts of P recoverable, freshwater saved, total cost, and
environmental impacts of the large-scale production of struvite from source-separated human
urine in both typical residential and office buildings. The results show that the net struvite
production can cover DAP fertilizer consumption in many countries, and the net freshwater
saving can be 21–34 L/(person·d) and 23–68 L/(person·d) respectively, using freshwater toilet
flushing and seawater flushing.
The economic viability of the implementation in five dense cities, namely Cairo, Hong Kong,
Jakarta, Moscow, and New York City indicates 100% probability of capital investments recovery
(CIR) in less than 60 years for Cairo; 100% for Jakarta; 100% for Moscow; and 35% for Hong
Kong (Cairo, Jakarta, Moscow also yield 89–98% probability of CIR in less than 30 years)
subject to the sensitive parameters being considered and their associated uncertainties, 1) the 2–3
hours per day for labour (considering an efficiency of 500 L urine/[labour d]), 2) the 20%
uncertainty in labour wage (based on the country minimum wage), 3) the OC of WWTPs of 45–65 USD/person for typical WWTP with nutrient removal, 4) the 20–30% savings in the OC of
WWTPs (based on the costs of energy, e.g. for nitrification, and chemicals for P removal), and 5)
the price of UDTs of 819–1331 USD/unit (based on the average price of Eco-flush UDT). Of
these, the labour hour and labour wage are the most sensitive . Hence, future interests in struvite
production from urine should include efforts towards reducing the costs of the production
process. For instance, implementing the project in public places offers the advantage of yielding
50–500% less costs than residential buildings as shown in this study.
Furthermore, these benefits can be achieved with less than 1% additional life cycle energy
consumption and environmental emissions when compared with conventional residential and
office buildings. Most of the life cycle energy consumption is due to the high embodied energy
of 0.46–3.49 MJ/m
3 of the polyamide filter bags; 1.00–1.11 MJ/m
3 of the acrylonitrile butadiene
styrene (ABS) cartridges; and 0.29–0.78 MJ/m
3 of the fiberglass tanks and reactors. At the same
time, only five of the selected nine life cycle impact indicators can potentially be considered, viz.
global warming (GWP); depletion of abiotic resources (DARP); marine aquatic ecotoxicity
(MAETP); freshwater aquatic ecotoxicity (FAETP); and human toxicity (HTP). Besides, the
choice of an environmental-friendly material such as filter paper to replace the polyamide of the
filter bags can significantly reduce the environmental emissions. Hence, based on all the results
of this study, it is recommended to implement urine separation and P recovery in buildings of
dense cities.
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