Elsevier

Water Research

Volume 45, Issue 2, January 2011, Pages 852-862
Water Research

Low-cost struvite production using source-separated urine in Nepal

https://doi.org/10.1016/j.watres.2010.10.007Get rights and content

Abstract

This research investigated the possibility of transferring phosphorus from human urine into a concentrated form that can be used as fertilizer in agriculture. The community of Siddhipur in Nepal was chosen as a research site, because there is a strong presence and acceptance of the urine-diverting dry toilets needed to collect urine separately at the source. Furthermore, because the mainly agricultural country is landlocked and depends on expensive, imported fertilizers, the need for nutrient security is high. We found that struvite (MgNH4PO4·6H2O) precipitation from urine is an efficient and simple approach to produce a granulated phosphorus fertilizer. Bittern, a waste stream from salt production, is a practical magnesium source for struvite production, but it has to be imported from India. Calculations show that magnesium oxide produced from locally available magnesite would be a cheaper magnesium source. A reactor with an external filtration system was capable of removing over 90% of phosphorus with a low magnesium dosage (1.1 mol Mg mol P), with coarse nylon filters (pore width up to 160 ± 50 μm) and with only one hour total treatment time. A second reactor setup based on sedimentation only achieved 50% phosphate removal, even when flocculants were added. Given the current fertilizer prices, high volumes of urine must be processed, if struvite recovery should be financially sustainable. Therefore, it is important to optimize the process. Our calculations showed that collecting the struvite and calcium phosphate precipitated spontaneously due to urea hydrolysis could increase the overall phosphate recovery by at least 40%. The magnesium dosage can be optimized by estimating the phosphate concentration by measuring electrical conductivity. An important source of additional revenue could be the effluent of the struvite reactor. Further research should be aimed at finding methods and technologies to recover the nutrients from the effluent.

Introduction

Extensive sewer networks with large centralized wastewater treatment plants have been the common civil engineering approach to provide sanitation in urban and peri-urban areas. However, the high investment and operating costs, dependency on increasingly scarce resources (e.g. water and fossil fuels), and intensive maintenance make this system not only unattainable, but unsustainable for most areas in the developing world. To reverse the trend of reproducing inappropriate designs, there is a growing demand for new technologies, which better suit the needs of the local population and their environment (Larsen et al., 2009, Guest et al., 2009, Tilley et al., 2008a) and which emphasize the value of wastewater as a resource from which nutrients, water or energy can be recovered. The goal of extracting value from waste is to maximize the social, environmental and health benefits while minimizing the investment and operation costs (Schuen et al., 2009). Recovering resources from wastewater might allow the free market to play a role in the sanitation in developing countries by supporting small businesses that collect the waste, treat it and sell the value-added products.

The present study focuses on the recovery of phosphorus from urine. Worldwide, approximately 50–60% of the phosphorus fertilizer demand is covered by mineral fertilizer (Smil, 2000). However, mineral phosphorus, like oil, is a finite resource and supplies are expected to peak around 2030 (Cordell et al., 2009). To prevent food shortages, additional phosphorus sources must be exploited, such as human and animal excreta (ibid.). In fact, humans typically excrete 1.6–1.7 g phosphorus per day, most of which (about 60%) is found in urine (Schouw et al., 2002). Alternative sanitation concepts, which are based on the separation and collection of excreta at the source, facilitate the recycling of nutrients from faeces and urine to agriculture (Larsen et al., 2009, Jönsson et al., 2004). However, preliminary treatment of urine is needed to prevent nutrient loss by ammonia volatilization, to reduce weight (caused by the water content of urine) and to remove pathogens. Composting is a common and effective treatment for faeces (Niwagaba et al., 2009) and several treatment methods have been proposed for urine (Maurer et al., 2006), but few have been tested as thoroughly as struvite precipitation (see Ronteltap et al., 2010, Tilley et al., 2008b for an overview). Through a basic precipitation reaction, the majority of phosphorus in urine can be crystallized into a white, odourless powder called struvite or magnesium ammonium phosphate hexahydrate (MAP, MgNH4PO4·6H2O). Struvite is an effective phosphorus fertilizer (Johnston and Richards, 2004, Römer, 2006), it is compact and can be stored and transported easily.

Full-scale struvite reactors have been used for several years to recover phosphate from different solutions such as WWTP digester supernatant, swine manure or agro-industry wastewater (Forrest et al., 2008, Bowers and Westerman, 2005, Moerman et al., 2009). However, these reactors are too large and too complex for urine treatment on a small scale in developing countries. Recently, a stainless steel reactor has been manufactured commercially to produce struvite from urine in decentralized settings (Paris et al., 2007, Abegglen, 2008, Antonini et al., 2009). The reactor is equipped with a spiral pump (precipitant dosage), magnetic valves and a process unit to allow for automated operation. Antonini et al. (2009) reported that they used magnesium oxide as the precipitant and dosed it at a molar ratio of 1.5 mol Mg mol P−1. After the magnesium dose, the mixture was stirred for 30 min and then left to settle for 3 h. The precipitate was later collected in a filter bag attached to the outflow of the reactor. The phosphate removal was as high as 98%. Abegglen (2008) dosed magnesium at a molar ratio of 1.8 mol Mg mol P−1 and observed phosphate removal efficiencies higher than 95%. The reactor type used in the studies by Abegglen (2008) and Antonini et al. (2009) has proven to be very suitable for pilot studies with a reliable power supply, but the investment costs are still rather high. In our project, we wanted to build struvite reactors, which conform to the requirements of low-cost sanitation systems in Nepal, i.e. where the struvite process uses only locally available inputs, without in-depth technical knowledge, and without continuous electricity supply.

Siddhipur, a village close to Kathmandu, was chosen as the project site. The opportunity and need to implement alternative sanitation in Nepal is large, as only 27% of the population has access to improved sanitation (of which, 3% are connected to sewers) (WHO/UNICEF, 2010). Although urine-diverting dry toilets (UDDT) have been promoted heavily in recent years, the use of urine directly as a fertilizer is not common in Nepal (Water Aid, 2007).

We conducted a comprehensive study to assess the technical and economic feasibility of producing struvite from source-separated urine in Nepal. Specifically, the determination of phosphate content in the influent (urine), the economics of magnesium sources for the precipitation process, the most efficient technology for crystal recovery, the potential of flocculants to improve phosphate recovery, the role of the filter cake on the recovery of struvite, and the production of a user-friendly granulated struvite fertilizer, were investigated. This study emphasized the technology’s reproducibility by making maximum use of locally available resources.

Section snippets

Location and community

The village of Siddhipur was chosen as pilot test site because of the high number of operating UDDTs (100+) (Water Aid, 2007), the institutional strength of the Drinking Water and Sanitation User Committee, and the well-established contacts to development agencies. The majority of Siddhipur’s population engages in agriculture and the predominant ethic group in the settlement (Newar) has traditionally used human and animal excreta as fertilizers (Water Aid, 2007).

Urine sources, quantity and quality

A bicycle, modified with a steel

Urine quantity and quality

The phosphorus concentration in the stored samples (195 ± 65 mg P L−1) was considerably lower than in fresh urine (388 ± 251 mg P L−1) (refer to Table 1 for a comparison). The measured values are at the lower end of the range of literature data (e.g. 370–740 mg P L−1, Tilley et al., 2008b, Udert et al., 2003a). The low phosphate concentration in stored urine can be explained by precipitation. A valuable part of the phosphate – in undiluted urine about 30% (Udert et al., 2003b, Tilley et al., 2008c) –

Conclusions

With this project, we could show that an efficient and reliable struvite reactor can be built in Nepal with locally available materials and at a low cost (60 € for a 50 L reactor). Filtration resulted in much higher phosphate recoveries than sedimentation; more than 90% of phosphate could be recovered, using only little magnesium (dosage ratio 1.1 mol Mg mol P−1) and a simple nylon fabric filter (pore width 160 ± 50 μm). The accumulation of a filter cake helped to recover most of the struvite, while

Acknowledgements

The project was supported by Eawag discretionary funds, The Angel Fund of the Gemeinnützige Stiftung SYMPHASIS, Zürich, the Swiss Agency for Development and Cooperation (SDC) and Ingénieurs du Monde at the Swiss Federal Institute of Technology Lausanne (EPFL). The funding sources were neither involved in the study design, nor the collection, analysis or interpretation of the data. Much of this work would not have been possible without contributions from Basil Gantenbein, Edmund John Kashekya

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