Potential benefits and environmental life cycle assessment of struvite production and nitrification of source-separated human urine in buildings of dense cities

Abstract

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, (NH₄)₂HPO₄) fertilizer, which is also referred to as struvite (i.e. magnesium ammonium phosphate [MAP], MgNH₄PO₄.6H₂O). 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/m³ of the polyamide filter bags; 1.00–1.11 MJ/m³ of the acrylonitrile butadiene styrene (ABS) cartridges; and 0.29–0.78 MJ/m³ 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.

Date of Award	2016
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology