The role of management instruments in the diversion of organic municipal solid waste and phosphorus recycling

Organic waste constitutes approximately 30%–50% of municipal solid waste streams in developed countries (Ponsá et al. 2008; Fehr and Arantes 2015; Recyc-Québec 2015). Where conventional waste management is practiced, much of this organic waste is buried in landfills, incinerated (with the ash sent to landfill), or stabilized and used as landfill cover (Tsai 2008; Senthilkumar et al. 2014; Metson and Bennett 2015a; Zabaleta and Rodic 2015). The disposal of organic waste in landfills is associated with greenhouse gas emissions and nutrient loading to waterways (Augenstein 1992; Bulkeley and Askins 2009). Landfilling organic waste also contributes to a wasteful one-way flow of nutrients through the anthropogenic food system (Nilsson 1995; Smil 2000). This paper focuses predominantly on the organic fraction of municipal solid waste (OFMSW), which includes biodegradable waste material such as food waste and leaf and yard waste that is produced and disposed of by the public, and often managed centrally. It is sometimes referred to as bio-waste, green waste, biodegradable municipal waste (BMW), organic waste, and (or) putrescible waste. A description of OFMSW, as it pertains to this paper, along with explanations for other relevant terms, can be found in Box 1.

Landfilling and incineration of organic waste, combined with agricultural fertilization using non-renewable mineral ores, has substantially altered many nutrient cycles, depleting usable stocks of these nutrients and changing the way they flow through the environment at both global and local scales (Smil 2000; Schlesinger and Bernhardt 2013). Over the last century, global phosphorus (P) cycles have seen intense acceleration caused primarily by mining to produce agricultural fertilizer (Tilman et al. 2002; Cordell and White 2011). This acceleration has multiplied P mobilization by up to four-fold since the Green Revolution resulting in an increase of P delivery to oceans (Bennett et al. 2001; Childers et al. 2011). At the same time, ever-increasing flows of food and other raw materials into cities have led to high levels of nutrient accumulation in urban areas and their landfills (Kennedy et al. 2007; Grimm et al. 2008; Neset and Cordell 2012; Lin et al. 2014).

The acceleration and disruption of the global P cycle results in two principal problems. First, there are long-term issues of P scarcity because of the non-renewable and geographically concentrated nature of phosphate rock reserves (Cooper et al. 2011; Scholz and Wellmer 2013). Phosphorus scarcity raises issues related to food security as P is a non-substitutable plant macronutrient (Cordell et al. 2009; Childers et al. 2011). The second problem is that P runoff from agriculture and urban systems can act as a powerful aquatic pollutant, creating widespread issues related to eutrophication (Carpenter et al. 1998). This can have detrimental consequences for waterways and coastal ecosystems including declines in coral reef health and loss of communities, increased incidence of fish kills, and reductions in species diversity (Smith and Schindler 2009).

Although there is no “silver bullet” solution to the current dependence on synthetic fertilizers and the disruption of biogeochemical cycles, opportunities exist both in agriculture and in waste management; this paper focuses on the waste management aspect. Some organic waste management strategies facilitate the recycling of nutrients back to arable soil and, thus, have the potential to reduce aquatic pollution and simultaneously alleviate nutrient limitations; this can play a key role in a larger, more comprehensive solution addressing issues of altered biogeochemical cycles (Burnley 2001; Taşeli 2007; Cordell and White 2013; Mayer et al. 2016). Increased diversion of the OFMSW from landfills to more sustainable options such as biological processing is a policy goal in many regions, echoed in municipal, provincial, national, and international waste management plans and policies (Conseil fédéral Suisse 1990; European Commission 1999; Québec 2011; City of Toronto 2016). These management plans generally list several reasons for promoting organic waste diversion, including the potential to turn waste into a valuable resource through the recycling of important nutrients (e.g., plans from Metro Vancouver (2010) and the Region of Peel (2009)). Increasing the recycling of nutrients such as P through organic waste diversion can reduce demand for mined P, convert waste into a resource, and reduce total waste disposal (Nilsson 1995; Eriksson et al. 2005; Childers et al. 2011; Neset and Cordell 2012).

Alternative processes for managing urban organic waste, such as composting and anaerobic digestion (AD), may help increase the amount of P recycled and reduce pollution (Giusquiani et al. 1995; Childers et al. 2011; Agegnehu et al. 2015; Mayer et al. 2016). Biological processing (e.g., composting and AD) of the OFMSW converts waste material into nutrient-rich, biologically stable end products such as compost and digestate (Levis et al. 2010; Cordell et al. 2011; Senthilkumar et al. 2014). In the case of anaerobic digestion, biological processing also produces valuable biofuel (Khalid et al. 2011). The subsequent application of these end products to land recycles residual organics into useful resources, such as P-rich soil amendment, which can increase soil fertility, enhance agricultural carbon sinks, decrease methane emissions associated with landfilling organic waste, and reduce our reliance on chemical fertilizers (Levis et al. 2010; Dorward 2012; Neset and Cordell 2012). There are various barriers to the application of municipal organic amendments to arable land, such as the proximity of agricultural land, market demand, the presence of contaminants, mineral fertilizer substitutability, and processing and transportation costs. This study, however, focuses primarily on the factors that drive organic waste diversion and the fate of P in organic waste once it is diverted and, therefore, the potential for P recycling.

The majority of existing P recovery research has focused on wastewater flows, resulting in a lack of monitoring and data for P recycling associated with solid waste management (Jaffer et al. 2002; Elser and Bennett 2011; Cordell et al. 2012; Linderholm et al. 2012). This data gap means that the effectiveness of organic solid waste processing in real-world contexts is inadequately understood (Cordell et al. 2012; Senthilkumar et al. 2014). Much urban P research to date ignores the OFMSW diversion stream (Antikainen et al. 2005; Garnier et al. 2015), explicitly or implicitly assumes all P contained in diverted OFMSW is returned to arable land (Neset 2005; Neset et al. 2008; Senthilkumar et al. 2014), or neglects the link between diversion and application, looking solely at the land application of compost (Cooper and Carliell-Marquet 2013). Solid waste management provides an important opportunity to increase overall P recycling in a region, given that similar annual tonnages of P output have been found for solid waste as for wastewater in regional systems (Kalmykova et al. 2012), and equal per capita P output was found for sewage sludge and solid waste at a multi-national scale (Ott and Rechberger 2012); however, the flow of P through waste systems must be better understood if organic waste management is to be used as a means to increase P recycling and reuse.

Although various trends existed in solid waste diversion over time across countries and jurisdictions, in general, organic waste diversion increased with time (Figs. 1 and 2). We found a significant difference between the base year (generally 1995) and both the first (t = −3.83, p = 6.42e−04) and second (t = −5.62, p = 5.097e−06) diversion target years for European countries. Although there was a general increase in OFMSW diversion over the study period for both Europe and Ontario, diversion rates varied substantially between and within these two regions; in fact, some countries and jurisdictions experienced a decrease in organic waste diversion over the length of the study period (e.g., Malta; Fig. 1).

Along with the OFMSW diversion rate in Years 1 and 2 (i.e., 2006 and 2013 for Ontario, and base year—generally 1995—and Diversion Year 2 for Europe, see Table S1), Figs. 1 and 2 also show per capita waste generation and per capita waste diversion (in kg) for Years 1 and 2. Together, this information gives a more holistic picture of how these systems changed over the study period. Including the per capita waste values provides valuable context; for example, the figures allow us to see not only which countries have increased their diversion of OFMSW over the study period but also those that have managed to reduce their per capita waste generation at the same time—or vice versa.

This study was limited by data availability. For example, results from Ontario and existing literature (Sterner and Bartelings 1999; Bernstad 2014), indicate that infrastructure may play a significant role in OFMSW diversion rates; however, we were unable to analyze this in Europe because such data were unavailable at the national level. Further research of the role of infrastructure in Europe may provide additional insight into diversion rates. Scale might also affect which factors appear to most strongly influence organic waste diversion and recycling; however, multi-scale data were generally not available. Additionally, the European data set used for this study was not well-suited to making strong inferences outside Europe or the national scale on which it was studied. When looking at national and municipal level data, behaviours that exist at a household level and that might strongly affect diversion and reapplication rates can be missed. There are existing studies related to household behaviour for recycling and some that include organics (Sterner and Bartelings 1999; Martin et al. 2006; Bulkeley and Askins 2009), but future research integrating national and municipal data with household behavioural findings could prove extremely valuable. Despite these limitations, we feel that this study still has significant value in acting as a guide for future studies of factors influencing organic waste diversion, for acknowledging the factors that played significant roles in the areas that were studied, and for bringing to attention to issues associated with data availability. Furthermore, as the Ontario data set was much larger, it is possible to make stronger generalizations from the results of that analysis, which we consider valuable, especially in the North American context.

We showed that although existing socio-economic factors are likely to play a role in OFMSW diversion and reapplication, management options such as economic incentives and infrastructure can be leveraged to increase organic waste diversion. We also found that many regions have low P reapplication associated with organic waste management, with significant room for improvement. Information from this research can be used in conjunction with pre-existing qualitative urban- (Bulkeley and Askins 2009; Metson and Bennett 2015b) and household-level studies (Sterner and Bartelings 1999; Martin et al. 2006; Refsgaard and Magnussen 2009; Bernstad 2014; Parizeau et al. 2015) to assess organic waste diversion potential in specific regions or case studies.

Although OFMSW diversion generally increases over time, our study showed that there is broad variation between regions, and that most countries and jurisdictions recycle only a fraction of their potential P from solid waste. For example, Germany—a region with a relatively successful organic solid waste diversion program and high reapplication rates—diverted 50% of their organic solid waste in 2013 (Eurostat 2014a), and has an expected reapplication rate of 69% (Barth et al. 2008). This means that approximately 34% of the P in organic waste in Germany is recycled back to arable land. There are considerable opportunities for diverting and recycling significantly more P from organic solid waste.

Our work also leads to a better understanding of why OFMSW diversion programs in some regions may be more successful than others. Countries such as Austria or cities such as Guelph have high diversion rates (Figs. 1 and 2) and can act as models for other locations. Although Austria has a high income, they also use PAYT systems to discourage waste disposal, have high taxes on landfill and incineration disposal, and have strong legislation on the disposal of organic waste to landfill (Table S5). In line with key factors in Ontario, Guelph has sophisticated infrastructure for the collection of source-separated organics and imposes a PAYT economic incentive program (Table S10). It is also important to investigate countries such as Lithuania which, despite having the second lowest income of European countries in this study (Eurostat 2014b; UNData 2014, Table S5), had a relatively high diversion rate of 37% in 2013 (Fig. 1, Table S5). Our research indicates that the use of landfill taxes and PAYT economic incentives are likely to have contributed to this admirable diversion rate. Alternatively, some regions such as Iceland have moderate to high incomes, but relatively low diversion rates (20%) (Table S5), corroborating existing literature that suggests that economic prosperity alone does not guarantee greater waste diversion, and that diversion must also be a policy and management priority (Mazzanti et al. 2009). Research regarding the influence of the EU Landfill Directive was inconclusive. The t tests indicated a significant difference between means before and after the implementation of the Directive; however, analysis of the change in OFSMW diversion was inconclusive and lacked significant relationships. It is possible that countries with high diversion prior to the Directive continued to do well or, as evidenced by some of the individual country trends, that there is a saturation level or plateau in diversion rates.

Urban P recycling can play an important role in ensuring a lasting supply of this crucial nutrient and non-renewable resource. However, unless urban P flows are returned to arable land, many of their potential benefits are lost; thus, a clear understanding of the use and reapplication of organic waste end products is critical. Although most existing literature on P recovery focuses on flows associated with wastewater treatment (Morse et al. 1998; Jaffer et al. 2002; Antikainen et al. 2005; Elser and Bennett 2011; Cordell et al. 2012; Linderholm et al. 2012), solid waste management provides an important opportunity to recycle more P than wastewater alone, given that similar levels of P output have been found for solid waste as for wastewater (Kalmykova et al. 2012; Ott and Rechberger 2012). Here, we have demonstrated that economic incentives, local legislation, and accessible infrastructure can be effective ways to increase OFMSW diversion and P recycling to move beyond waste and toward valuable resource recovery.

We acknowledge McGill University, the Natural Sciences and Engineering Research Council (NSERC) of Canada, GHD Incorporated, Ice River Springs, and the Environmental Research and Education Foundation for their support.