Environmental impacts of phosphorus recovery from a “product” Life Cycle Assessment perspective: Allocating burdens of wastewater treatment in the production of sludge-based phosphate fertilizers
Graphical abstract
Introduction
Phosphorus (P) is an essential resource since it is vital for the development of plants, animals and humans. P is also a key component of mineral fertilizers, since ca.148 million t of phosphate rock are used per year, and 90% of global demand for P is for food production (Cordell et al., 2009). With the rapid growth of world population (estimated at 9 billion people by 2050), increasing demand for food and therefore for fertilizers is expected worldwide (Sorensen et al., 2015; Steen, 2006). However, P is a non-renewable resource that cannot be replaced by another element in fertilizers. P in mineral form can be found highly concentrated in reserves of phosphate rocks. These rocks are found almost worldwide, but ca. 86% of them were controlled by only six countries in 2016 (Morocco/Western Sahara (71.4%), China (4.7%), Algeria (3.1%), Syria (2.6%), Brazil (2.4%), and South Africa (2.1%)); thus, their availability is subject to high geopolitical risks (USGS, 2018). Moreover, P extraction from phosphate rocks is projected to peak around 2030. Afterwards, extraction will decrease, and global reserves should start to run out within 75–100 years, exhausting reserves of phosphate rocks by the end during the 22nd century (Rosemarin et al., 2009). In addition, the quality of phosphate rock will decline, increasing its price drastically. In the past 20 years, its price increased by 273% due to the increasing costs of extraction, processing and shipping (The World Bank, 2017). One direct impact will be an increase in the cost of producing food (Cordell et al., 2009; Rosemarin et al., 2009).
In 2017, phosphate rocks and P were added to the European Union's (EU) list of critical raw materials (European Commission, 2017). A raw material is considered critical when its supply risk and economic importance exceed a given threshold. The EU supply of P and phosphate rock depends completely on imports since they are not produced or mined, respectively, in the EU. Supply risk can be reduced by increasing the end-of-life recycling input rate (EOL-RIR) and the substitution potential (i.e. the ability to replace a critical raw material with a non-critical one). Since there is no substitute for these materials, supply risk can be reduced only by increasing the EOL-RIR of the ratio of recycling from waste feedstock to EU demand for a given raw material, the latter equal to primary and secondary material supply inputs to the EU. The EOL-RIR is estimated at 17% for phosphate rock and equals zero for P.
It is therefore extremely important to explore any potential supply of P given these constraints. P can be recovered or reused from several sources, including human excreta. Nearly 98% of ingested P ends up in wastewater and accumulates in sewage sludge (Kalmykova et al., 2015), making it an attractive resource for P recovery. Sludge can contain both mineral and organic P and be spread directly on soil as an organic fertilizer (Houot et al., 2014). Due to several constraints (presence of heavy metals and organic pollutants, social acceptability, etc.), however, new technologies have been developed to extract and recover this dissipated P. Sludge-based phosphate fertilizers can be used safely on agricultural soils. According to Egle et al., 2015, Egle et al., 2016, the most efficient P recovery technologies occur before and after anaerobic digestion and from sewage sludge ashes, mainly in the form of magnesium ammonium phosphate (struvite, NH4MgPO4∙6H2O) or calcium phosphates.
One unsolved question remains the overall environmental impacts of recovering this dissipated P compared to extracting phosphate from rocks. Some studies have assessed environmental impacts of sludge used as phosphate fertilizer using Life Cycle Assessment (LCA) (Sena and Hicks, 2018). Johansson et al. (2008) and Linderholm et al. (2012) compared four alternative options for handling sludge, with use of its P as fertilizer on agricultural soils. Bradford-Hartke et al. (2015) compared environmental benefits and burdens of recovering P as struvite from dewatering return liquors in four centralized and two decentralized systems. In these comparative LCAs, recovering P from sludge was seen more as an alternative waste treatment than as sludge-based fertilizer production; thus, sludge was considered to have no environmental burdens. In this context, using supercritical water oxidation to recover P appeared to be the best option for Johansson et al. (2008). In contrast, direct spreading of sewage sludge on soil was the option with the lowest energy use and greenhouse gas emissions for Linderholm et al. (2012), due to the beneficial association with nitrogen in sludge. For Bradford-Hartke et al. (2015), recovering P using struvite precipitation resulted in positive environmental impacts due to energy and chemical use being offset by operational savings and avoided fertilizer production.
The critical review of Pradel et al. (2016), however, emphasized that if sludge treatment is specifically designed to produce sludge-based fertilizers with high added value, sludge can no longer be considered as waste but rather as a coproduct of the wastewater treatment plant (WWTP). This assertion was shared by several authors who also started to question the “zero burden assumption” (Cleary, 2010; Holden; Oldfield and Holden, 2014; Oldfield et al., 2018) by considering that the status of “waste” is subjective and questionable, especially if it has a high nutrient or energy-recovery potential. However, most LCA studies dealing with P recovery still consider sludge as a waste (Sena and Hicks, 2018). To raise awareness of the need to consider upstream production of waste-based products in LCA, we performed LCA of a case study of sludge-based fertilizer production.
This study aimed to assess, using a “product” LCA perspective, whether recovering dissipated P by producing sludge-based phosphate fertilizer can be a suitable alternative to producing mineral fertilizers from phosphate rocks. To reach this goal, four scenarios of the production of sludge-based phosphate fertilizers were compared to the production of phosphate fertilizer from phosphate rocks. The goal and scope definition, inventory data and characterization method used for each scenario are presented in the material and methods section. Four points are then discussed in the results: the consequence of allocating part of the environmental burdens of wastewater treatment to sludge production, the unequal balance between consumed resources and recovered P, the ability to reduce P depletion if the recovery rate of diffuse P is improved and the difficulty assessing P resource depletion in LCA characterization methods.
Section snippets
Materials and methods
LCA is a four-step procedure based on international standards (ISO, 2006a, ISO, 2006b). The first step consists of defining the goal and scope of the study, i.e., the system to be studied, its boundaries, functions, and the related functional unit (the reference all the inventory data are related to), the allocation methods used and the assumptions made. The second step is the Life Cycle Inventory (LCI) during which all the inputs (raw material, energy) and outputs (emissions) related to each
Environmental impacts of sludge-based phosphate fertilizers compared to those of mineral phosphate fertilizer
Results are expressed per kg of P produced as struvite (S1-BioAcid, S2-Crystal, S4-Gifhorn), “Rhenania phosphate” (S3-AshDec) or TSP (Sref). Gross impacts and net impacts (gross impacts minus avoided impacts) are shown in Fig. 3a and b, respectively. When comparing gross impacts of the production of 1 kg of P from diffuse sources to those of concentrated sources, the sludge-based fertilizer scenarios have higher environmental impacts in all categories than Sref (Fig. 3a). Scenario S2-Crystal
Conclusion
The results highlight that producing 1 kg of P from phosphate rock has lower environmental impacts than producing 1 kg of P from wastewater sludge. The low yields of P recovery associated with a low P concentration of sludge and need for large amounts of energy and reactants to recover P are responsible for the higher environmental impacts of sludge-based scenarios. However, their environmental impacts could be deceased if a good compromise is found between P recovery efficiency and the
Acknowledgements
The authors kindly thank their colleagues Jean-Pierre Canler and Guillermo Baquerizo from the MALY Research Unit and Marie-Line Daumer from the OPAALE Research Unit at IRSTEA for providing LCI data on the WWTP and the P recovery processes. The authors would like to thank Michael Corson for proofreading the English of this paper as well as the two anonymous reviewers for their valuable comments. Part of this study was financially supported by ONEMA (French National Agency for Water and Aquatic
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