Nitrous oxide (N2O) emissions from human waste in 1970–2050

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ScienceDirect Nitrous oxide (N2O) emissions from human waste in 1970–2050 Maryna Strokal1 and Carolien Kroeze1,2 Nitrous oxide (N2O) is an important contributor to climate change. Human waste is an important source of N2O emissions in several world regions, and its share in global emissions may increase in the future. In this paper we, therefore, address N2O emission from human waste: collected (from treatment and from sewage discharges) and uncollected waste. We review existing literature on emissions and emission factors, and present region-specific estimates of N2O emissions and their past and future trends. We show that human waste may became an important source of N2O emissions in the coming years as a result of increasing urbanization. About two-thirds of the global emissions are from uncollected waste, and about half from South Asia. We argue that more research is needed to improve emission factors. Addresses 1 Environmental Systems Analysis Group, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands 2 School of Science, Open University of The Netherlands, Heerlen, The Netherlands Corresponding author: Strokal, Maryna ([email protected], [email protected])

Current Opinion in Environmental Sustainability 2014, 9–10:108–121 This review comes from a themed issue on System dynamics and sustainability Edited by Carolien Kroeze, Wim de Vries and Sybil Seitzinger For a complete overview see the Issue and the Editorial Received: 27-2-2014; Revised: 6-9-2014; Accepted: 11-9-2014 Available online 10th October 2014 http://dx.doi.org/10.1016/j.cosust.2014.09.008 1877-3435/# 2014 Elsevier B.V. All rights reserved.

takes place under anoxic conditions, when bacteria convert oxidized forms of N to gases such as nitric oxide (NO), N2O and dinitrogen (N2). N2O is an intermediate product in denitrification. If it is not reduced to N2, it may escape to the atmosphere. Important controlling factors of N2O release during these processes are availability of reactive N (Nr) and of dissolved oxygen [8]. The availability of Nr in the environment has been increasing since the first discovery of biological N fixation (converting nonreactive N2 to reactive N) at the end of 19th century, allowing people to produce more food [9]. Because of the food demand by a growing world population, agricultural production has considerably increased. The growing population also led to increasing urbanization. Thus agricultural production, urbanization, and the availability of Nr in the environment are interconnected [9–13]. Increased availability of Nr resulted in increased emissions of N2O to the atmosphere through enhanced denitrification [13,14]. This results in increasing atmospheric N2O concentrations. Stocker et al. [15] show that N2O concentrations increased by 20% between 1750 (271 ppb) and 2011 (324 ppb). This may lead to global warming [15]. The fifth IPCC Assessment [15] reports that around 40% of the global N2O emissions into the atmosphere are from anthropogenic sources. In 2006 anthropogenic N2O emissions were estimated at 6.9 Tg N2O-N (ranging from 2.7 to 11.1 Tg N2O-N) [15]. Various studies [2,8, 15–19,20] indicate that agricultural activities such as livestock and crop production are dominant sources of N2O, because fertilizer use increases the availability of Nr. Around 60% of the global anthropogenic N2O emissions in 2006 were from agriculture [15]. Fossil fuel use and biomass burning (including biodiesel) are other important sources [6,15,21–23], accounting for around 20% of the global anthropogenic N2O emissions [15].

Nitrous oxide (N2O) is one of greenhouse gases (GHGs) contributing to climate change [1–5]. It has an atmospheric lifetime of more than 100 years [2] and a global warming potential of 300 relative to carbon dioxide (CO2) [6]. Moreover, N2O is a main contributor to ozone depletion in the stratosphere [5,7]. Bacterial denitrification and nitrification are important sources of atmospheric N2O. During nitrification bacteria convert ammonium to oxidized forms of nitrogen such as nitrate (NO3 ) and nitrite (NO2 ) under aerobic conditions. Denitrification

Human waste is also a source of N2O. The fifth IPCC Assessment report [15] indicates that human waste accounts for about 3% of the global anthropogenic N2O emissions. Both collected human waste and uncollected human waste are sources of N2O (Figure 1). It can be released from collected human waste, in particular during wastewater treatment. Uncollected human waste can be a source of N2O after disposal on land, where it leads to increased denitrification. In the future, human waste may become a more important source of N2O than today because the population will continue to grow generating more waste [24]. Dietary preferences of people may

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Introduction

Nitrous oxide emissions from human waste in 1970–2050 Strokal and Kroeze 109

continue to change in the future, influencing protein intake. In China, for example, people have been changing their preferences towards more animal products [25,26]. This increases the demand for fertilizers in food production, and, as a result, the Nr availability in the environment. Human waste is a source of pollution particularly in urban areas. The impact of urbanization on eutrophication has been increasing worldwide [24,27,28]. Nitrogen (N) inputs from human waste to aquatic systems have been increasing worldwide [20,24,27], as well as on regional scales such as China [29,30], South America [31], Africa [32], Indonesia [33] and the Black Sea region [34,35]. There is also evidence of increasing trends in pathogen concentrations in urban surface waters [36,37]. These increasing trends are the result of an increasing population generating more waste. The amount of N ending up in aquatic systems depends largely on the extent to which the population is connected to sewage systems and on the effectiveness of N removal during waste water treatment [24,27]. These factors are also important determinants of the N2O emissions from human waste. This paper, therefore, addresses global N2O emissions from human waste. First we overview recent studies on N2O emissions from human waste and discuss emission factors that are used to estimate these emissions. Second,

we explain how global and regional N2O emissions can be quantified, taking into account uncertainty. Third, we present estimates of global N2O emissions and compare these with other studies. Finally, we elaborate on past and future trends in N2O emissions resulted from regions namely America (North and South), Africa, Europe, and Australia, Asia (North and South), Oceania and Oceans (see Figure 2 for their locations).

N2O emissions from human waste and emission factors The fifth IPCC assessment report [15] indicates that about 0.2 (range: 0.1–0.3) Tg N2O-N originated from human excreta in 2006 (see also Table 4). This estimate is, however, not well justified. Here we distinguish between two types of N2O emissions that are associated with human waste: collected human waste and uncollected human waste. Both are sources of N2O directly (i.e. emissions from wastewater treatment plants (WWTPs) or induced by N inputs to soils after waste disposal to landfills) and indirectly (i.e. after leaching and runoff to groundwater or surface waters such as rivers and estuaries) [24] (Figure 1). Collected waste

Collected human waste is often transported to treatment facilities, where part of the nitrogen is removed. In general three types of nitrogen treatment can be distinguished

Figure 1

Total N in human waste

Components for flows of total N in human waste N flows N2O emissions

Collected

N2O

Removed during treatment

Treatment N2 O

N2O N2O

Untreated

Residues

N2O

Uncollected waste

Treated

SOIL

SURFACE WATER: RIVERS, ETUARIES

GROUNDWATER

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Schematic overview of the fate of nitrogen (N) in human waste and the associated nitrous oxide (N2O) emissions. www.sciencedirect.com

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110 System dynamics and sustainability

[20]. In primary treatment about 10% of the N is removed by a mechanical-oriented treatment focusing on removing large materials. Secondary treatment is more focused on removing organic matter while organisms are added to convert organic nitrogen to mineral forms (i.e. ammonia, nitrate and nitrite); this may reduce the N content of sewage influent by 35% mainly through denitrification. Finally, in tertiary treatment N removal may be up to 80% by advanced techniques [24]. During removal processes N2O can be produced as an intermediate product of nitrification and denitrification. Local experiments [38] revealed that biological denitrification is mainly responsible for N2O emissions during waste treatment. The amount of N2O production depends on several factors, including the concentrations of dissolved oxygen, pH and the concentrations of Nr in influents [38–40,41,42]. Kampschreur et al. [41] concluded on the basis of experiments that N2O emissions are generally higher under low dissolved oxygen concentrations during both nitrification and denitrification processes, high concentrations of nitrite during both nitrification and denitrification, and low chemical oxygen demand:nitrogen ratios during denitrification. The IPCC Guidelines 2006 [43] include a default emission factor for N2O from wastewater treatment plants of 3.2 (range: 2–8) g/person/year (Table 1). This factor, however, was derived from a single experimental study [38] conducted in New Hampshire, USA in the beginning of the 1990s. This study quantified annual emissions from a WWTP at about 35 kg N2O emissions per year; this was a WWTP applying secondary treatment and collecting wastewater from about 10 thousand people. Kampschreur et al. [41] estimated that 3.2 g/person/year represents approximately 0.035% of N load, assuming a waste production of 16 g N per person per day. This percentage is within the range of 0.035–0.05% of the N input to WWTPs derived from literature review [42]. Results of about 30 laboratory-scale (using artificial wastewater) and fullscale (using real wastewater) experimental studies are available in literature [41,42] (see Table 1). These studies show a considerable range in N2O emissions among WWTPs. Moreover, laboratory-scale studies suggest a considerably higher N2O production during treatment (ranging from 0 to 95% of the N load) compared to fullscale studies (ranging from 0 to 14.6% of the N load) (Table 1). The large difference between laboratory — and full-scale studies illustrates the fact that laboratory conditions do not always represent real conditions. The variability in measured N2O emissions among full-scale studies is possibly associated with different treatment approaches that differ in controlling N2O factors (i.e. concentrations of dissolved oxygen and nitrate) [41]. For example, the range in N2O formation from 12 WWTPs in the US with biological N removal is as large as 0.003– 2.56% of the N influent to these WWTPs [44]. Another study [45] reported N2O emissions ranging from 0.06 to 25% of the N influent (3.5% on average) for seven WWTPs Current Opinion in Environmental Sustainability 2014, 9–10:108–121

with biological nitrogen removal in Australia. For WWTPs with nitritation–anammox processes N2O production was measured at 2.3% of the N influent (on average) for Rotterdam (the Netherlands) [46] and at 0.4–0.6% of the N influent for Switzerland [47] (see Table 1). Kimochi et al. [48] conducted an experiment showing that N2O production can be minimized from WWTPs in case of complete nitrification and denitrification, which can be achieved at dissolved oxygen levels of 5 mg/L during aerobic treatment (during nitrification) combined with 60 anoxic minutes (inducing denitrification). Tho¨rn and So¨rensson [49] reported on the effects of pH on N2O production: N2O emissions were maximum at 5.5 pH and minimum at 6.5 pH or higher. From the above it may be clear that N2O production during wastewater treatment differs largely among WWTPs making generalizations difficult. The uncertainties in default emission factors for N2O production from WWTPs that can be used for global estimates are, therefore, large. Foley et al. [45] concluded that the emission factor in the IPCC Guidelines 2006 may underestimate N2O emissions. A more systematic monitoring of N2O emissions and investigation of the dominant processes and factors responsible for these emissions is needed to improve the existing emission factor of IPCC. Some N remains in the sludge of WWTP after treatment [50]. This N can still be a source of N2O. For instance, when this waste is landfilled, or applied as a fertilizer to land, or incinerated, it may give rise to N2O emissions, mainly as a result of denitrification [51]. Generally, most of the sludge is landfilled and/or applied to agricultural fields. Small quantities are incinerated. In Europe, for instance, 37% of the sludge produced during treatment is applied to agriculture, 40% is landfilled, 11% is incinerated and the remainder is recycled in other ways (i.e. disposed to sea) [50,52]. However, these percentages vary among countries. For instance, in Greece, Ireland and Portugal most sludge is landfilled, and no sludge is incinerated. In contrast, in Denmark, France, Belgium, Germany and USA approximately 15–25% of the produced sludge is incinerated [50,52]. According to the 2006 IPCC Guidelines [43] 900 g (wet weight) and 990 g (dry weight) of N2O is emitted during incineration per tonne of produced waste. These default emission factors were derived from national inventories of Germany, Japan, and Austria [43]. Tsujimoto et al. [53] performed an experiment with waste disposal to landfills in Osaka city in Japan and report emissions of 40.2 g N2O per day from active landfills, and 7.8 g N2O per day from closed landfills. Bo¨rjesson and Svensson [54] estimated that 1.6% of N was emitted as N2O from municipal landfills in South-East Sweden during the first two years of waste disposal. They concluded that this is close to the default emission factor in the IPCC Guidelines 1996 (1.25% of the N load or www.sciencedirect.com

Nitrous oxide emissions from human waste in 1970–2050 Strokal and Kroeze 111

Table 1 Emissions factors (EF) for N2O emissions that can be attributed to human waste N2O emission from

Emission factor: range (units)

Reference

Remark

Collected human waste Wastewater treatment plants (WWTPs, direct emissions)

3.2: range 2–8 (g/person/year)

IPCC Guidelines 2006 [43]

Range 0–14.6 (% of N load, full-scale) Range 0–95 (% of N load, lab-scale)

Kampschreur et al. [41]

Range 0.01–0.08 (% of N-influent)

Kimochi et al. [48]

2.3 (% of N-influent)

Kampschreur et al. [46]

3.5: range 0.6–25 (% of N-influent)

Foley et al. [45]

Range 0.4–0.6 (% of N-influent)

Joss et al. [47]

Range 0.003–2.59 (% of N-influent)

Ahn et al. [44]

Based on a study [38] for one WWTPa in New Hampshire, USA with secondary treatment. This EF was derived as: the total N2O emissions from a studied WWTP (35 kg N/year) divided by population producing human waste that is treated at that WWTP (10,925 people). This EF represents about 0.035% of the total N load of a WWTP [41] assuming: 100 g protein/person/ day and 0.16 g N/g protein. According to [42] this EF factor ranges 0.035–0.05% of the N inflow. Experimental studies Review of 28 experimental studies with different treatment processes. Full-scale experiments take real wastewater from WWTP. Most of labscale studies use artificial wastewater. For fullscale, the range of 7 studies is given, published during the period of 1995–2008. For lab-scale, the range of 21 studies is given, published during the period of 1992–2008. WWTP in Japan in which treatment is based on intermittent activated sludge process (full-scale study). The conversion ratio of N2O-N is 0.03– 0.11% of N removed or 0.43–1.89 g/person/year. This study is summarized in [41,42]. WWTP in Rotterdam, the Netherlands with a fullscale two-reactor nitritation–anammox process. This study is summarized in [41,42]. Seven full-scale WWTPs in Australia with biological nutrient removal. This study is summarized in [42]. Three WWTPs and five reactors in Switzerland with nitritation–anammox treatment (full-scale). This study summarized in [42]. 12 WWTPs with biological nutrient removal in the United States (full-scale). This study summarized in [42].

1: range 0.2–12 (% of sewage effluent)

IPCC Guidelines 1996 [55]

0.5: range 0.05–2.5 (% of sewage effluent)

IPCC Guidelines 2006 [43]

1.25: range 0.25–2.25 (% of N inputs to soils) 1: range 0.3–3 (% of N inputs to soils)

IPCC Guidelines 1996 [55] IPCC Guidelines 2006 [43]

EF1 in IPCC Guidelines 1996 [55].

2.5: range 0.2–12 (% of N leaching and runoff) 0.75: range 0.05–2.5 (% of N leaching and runoff) 1 (% of N Leaching and runoff)

IPCC Guidelines 1996 [55] IPCC Guidelines 2006 [43] Nevison [70]

The sum of default EF for groundwater (1.5%), for rivers (0.75) and for estuaries (0.25). The sum of default EF for groundwater (0.25%), for rivers (0.25%) and for estuaries (0.25%). Overall EF for leaching and runoff, which is the sum of default EF for groundwater (0.1%), for rivers (as in IPCC 1996) and for estuaries (as in IPCC 1996).

Wastewater effluent to surface waters (indirect emissions)

Uncollected human waste Soils (direct emissions)

Surface waters and ground water (indirect emissions)

a

EF6 in the IPCC Guidelines 1996 [55]. It is assumed that very small amount of N is removed during treatment, thus all human waste from WWTPs are discharged to surface waters (rivers, estuaries). This default EF is the same as the sum of default EF for rivers (0.25%) and for estuaries (0.25%) in IPCC Guidelines 2006.

EF1 in IPCC Guidelines 2006 [57]: N that is added to soils via applications of fertilizers, crop residues and from mineralization of mineral soils.

WWTP = Waste Water Treatment Plant.

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112 System dynamics and sustainability

0.0125 kg N2O-N/kg N inputs to soils) [55]. However, N2O emissions from landfills that can be attributed to sewage sludge are highly uncertain because of the timedelay in the anaerobic decomposition of waste residues. For example, Barton and Atwater [39] indicated that anaerobic decomposition of wastes in landfills may take over 50 years, delaying N2O production. This implies that N2O emissions may be small during the first years after waste disposal but increase in later years with increasing anaerobic decomposition. Another complicating factor is that these landfills do not only contain sewage sludge but also solid domestic waste. The way sewage sludge is managed varies largely among countries [24,56]. This makes difficult to quantify sewage-related N2O emissions from landfills. The IPCC Guidelines 2006 do not provide an emission factor for N2O from waste disposal [43]. The N in sewage that is not removed during treatment, nor remains in the sludge, is most often discharged to surface waters. The efficiency of N removal during treatment largely determines the amount of nitrogen that is discharged [24,27] (see also Figure 1). Also this N is a source of N2O. N cycling in aquatic systems results in losses of N2O from rivers and estuaries (Figure 1). The IPCC Guidelines 2006 [43] account for N2O emissions from aquatic systems that are induced by increased N inputs to rivers. These emission factors are in fact used for the ‘indirect’ emissions from fertilized soils, but can also be applied to N inputs to rivers from sewage. This default IPCC emission factor was 1% (or 0.01 kg N2O-N/kg sewage N) in the 1996 Guidelines, but has been revised downwards to 0.5% (or 0.005 kg N2O/ kg N effluent) in 2006 (Table 1). Both default factors fall within the uncertainty range of 0.05–2.5%. The 2006 emission factor equals the sum of emission factors for rivers (0.25%) and for estuaries (0.25%). This factor is based on limited data and assumes that N2O production takes place in surface waters during nitrification and denitrification (Table 1). In a recent review, Ivens et al. [57] concluded that emission factors for surface waters are highly uncertain. Uncollected waste

Uncollected human waste may be a considerable source of N2O emissions for regions where the majority of the population is not connected to sewage systems, which is the case in many tropical countries [24,27]. The fate of uncollected human waste differs among world regions, but it likely may end up on land (i.e. as a fertilizer). Part of it may leach or runoff to water bodies [24] (Figure 1). The IPCC Guidelines 2006 [43] distinguish between uncollected wastewater that is treated in, for example, septic tanks, and uncontrolled waste entering surface and ground waters without any treatment (see also Table 1).

Quantifying N2O emissions from human waste and sensitivity analysis

human waste. Our estimates of N2O emissions from collected waste include emissions from WWTPs and from surface waters as a result of N effluents from WWTPs discharged to surface waters. We calculated N2O emissions from uncollected human waste that can end up in terrestrial systems and then leached to groundwater and/or discharged to surface waters. We, however, do not account for N2O emissions during incineration of the sewage sludge because of data scarcity, and because the percentage of the sludge that is incinerated is generally low (see [50]). N2O emissions from each of these categories were calculated according to formulas given in Box 1. N fluxes were calculated using information from Van Drecht et al. [27] who prepared this information as model inputs to the Global NEWS (Nutrient Export from WaterSheds) model [27,58,59]. These model inputs include the total population (inhabitants), population connected to sewage systems (inhabitants), the N removal during treatment (fraction; based on treatment types namely primary, secondary and tertiary) and the total production of N human waste in watersheds (Tg) (see Figure 2 for global values and Figures 3–5 for regional values). These inputs are given for over six thousands river basins in the world (Figure 2a) and they are available for 1970, 2000 and 2050 (Figure 2b). For 2050 the inputs were derived based on scenarios of the Millennium Ecosystem Assessment [27,60]. In this study we used the Global Orchestration (GO) scenario as a basis. This scenario assumes a globalized world in socio-economic development with reactive management towards environmental problems [27,60]. In this world more people will be connected to sewage systems in 2050 than in 2000 (i.e. approximately 70% increase globally, Figure 2b). The efficiency of N removal in WWTPs during treatment will increase because of, for example, shifting towards better treatment systems (i.e. from secondary to tertiary) (see Figure 2b). The production of human waste will increase in the future globally (Figure 2b) and regionally (Figures 3–5) because of a growing population. The production of human waste differs among the world regions because scenarios account for differences in

Box 1 Quantifying N2O emissions from collected and uncollected human waste Collected human waste WWTPs = total N in human waste (Tg)
sewage connected population (fraction)
N removal during treatment (fraction)
emission factor (fraction) Sewage effluents = total N in human waste (Tg)
sewage connected population (fraction)
(1 N removal during treatment (fraction))
emission factor (fraction) Uncollected human waste = total N in human waste (Tg)
(1 sewage connected population (fraction))
emission factor (fraction)

We quantified global (for 2000) and regional (for 1970, 2000, 2050) N2O emissions from collected and uncollected Current Opinion in Environmental Sustainability 2014, 9–10:108–121

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Nitrous oxide emissions from human waste in 1970–2050 Strokal and Kroeze 113

Figure 2

(A)

Africa Australia Europe North America North Asia Oceania Oceans South America South Asia

River basins

(B) Total area (106 km2) Total population (109 people) Population connected to sewage (fraction)* N removal in treatment (fraction)** Total N in human waste (Tg)***

1970 133 3.5 0.23 0.12 (0-0.47) 8.9

2000 133 5.8 0.29 0.24 (0-0.80) 18.7

2050 133 7.7 0.49 0.45 (0-0.80) 42.2

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Geographical location of regions (shown by different colours) (a), and the main characteristics of the total studied area (b). The characteristics of individual regions can be found in Figures 3–5. Data are from Global NEWS [27,58]. The results for 2050 are based on a Global Orchestration scenario assuming globalization in terms of socio-economic developments, and a reactive approach towards environmental management [59,60]. * The population connected to sewage systems was calculated as the number of people with sewage connection divided by the total population. ** N removal reflects the average for regions; minimum and maximum values are shown in parentheses. *** The total production of human waste was calculated based on protein N intake as a function of economic development (details can be found in [27]).

protein intake among these regions as a function of economic development (see Van Drecht et al. [27]). We used different emission factors to calculate N2O emissions from different categories (see Table 2). For N2O emissions from WWTPs we assumed an emission factor of 1% (range: 0–2%). It should be noted that N2O emissions differ considerably among WWTPs (see Table 1). Our emission factors are in line with a recent UNEP report [20] in which emission factors to calculate global N2O emissions from WWTPs are assumed to be 0.043% (range: 0.05–2.5%) for primary and secondary treatment, and 0.6% (range: 0–2%) for tertiary treatment. These assumptions are based on studies such as Law et al. [42], Kampschreur et al. [41] and Ahn et al. [44]. We decided to take the 1% emission factor within the UNEP range (0–2.5%) and applied it to all human waste, treated and untreated. The 1% emission factor is in line within other emission factors for biogenic emissions (e.g. from soils) as recommended by IPCC [43].

0.5% (range: 0.05–2.5%). N2O emissions from uncollected human waste are calculated as the sum of emissions from managed soils (1% of N inputs to soils, with a range of 0.3–3%) and emissions from surface waters and groundwater (0.75%, with a range of 0.05–2.5%). These are the 2006 IPCC emission factors (Tables 1 and 2).

For N2O emissions from sewage effluents discharged to surface waters we used the 2006 IPCC emission factor of

We analysed the sensitivity of global and regional N2O emissions to uncertainties in emission factors. To this end, we calculated global and regional-specific N2O emissions (sum of collected and uncollected) in 2000 using emission factors and their ranges given in Table 2. The results (Table 3) show a large range in N2O emissions. Regional emissions range from 0.3 Gg (as N2ON; Oceania, Oceans, Australia) to 96 Gg (as N2O-N; Europe) of N2O emissions except for South Asia. South Asia emitted around 130 Gg of N2O-N with an uncertainty range of 24–421 Gg. Global N2O emissions range from 39 Gg to 759 Gg (see next section). These results illustrate the large uncertainties in N2O estimates associated with uncertainties in emission factors. Clearly, more research is needed to narrow down the uncertainties in emission factors.

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114 System dynamics and sustainability

Table 2 Emission factors (EFs, %) used in the sensitivity analysis of N2O emissions from collected and uncollected human waste. Table 1 gives an overview of the EFs from existing literature N2O emissions from

Low

Default

High

Collected human waste WWTPs

0

1

2

0.05

0.5

2.5

Uncollected human waste Soils Surface and ground waters

0.3 0.05

1 0.75

3 2.5

Sum (soils + waters)

0.35

1.75

5.5

Wastewater effluents to surface waters

Global N2O emissions from human waste A few estimates of global N2O emissions from human waste exist for the 1990s, indicating a source strength of 0.2–0.3 Tg N2O-N/year [1,6,61] (see Table 4). These estimates are based on an IPCC emission factor of 1996 for human sewage (0.01 kg N2O/kg sewage N) [55]. A recent study UNEP [20] estimated emissions from sewage at 0.16 Tg N2O-N in 2010 with an uncertainty range of 0.02–0.77 Tg N2O-N. This UNEP report is one of the first studies presenting N2O emissions from different treatment systems (primary, secondary and tertiary), sewage effluents and rural areas globally. We estimate global N2O emissions at 0.24 Tg N2O-N with an uncertainty range of 0.04–0.77 (Tables 3 and 4), which is in line with existing literature [1,6,20,61]. About 80% of these global emissions originate from uncollected human waste: 0.18 Tg N2O-N with an uncertainty range of 0.04–0.6 (Table 4). And the remainder of these emissions (about 20%) originate from collected human waste including emissions from WWTPs and from N effluents discharged to surface waters. Global N2O emissions from WWTPs are 0.024 Tg N2O-N/year (about

Default EF is assumed in this study (see text) Low and high EFs are according to [20] Default EF is from volume 4 of IPCC guidelines 2006 [43] Low and high EFs are the ranges from volume 4 of IPCC guidelines 2006 [43] Default EFs are from volume 4 of IPCC guidelines 2006 [43] Low and high EFs are the ranges from volume 4 of IPCC guidelines 2006 [43] These EFs are used in this study

10% of the total waste-related emissions) with an uncertainty range of 0–0.048. However, when using the 2006 IPCC emission factor (3.2 g/person/year), global N2O emissions from WWTPs are calculated at 0.0051 Tg N2O-N/year (about 2% of the total N2O emissions). Global N2O emissions from sewage N effluents to surface waters are estimated 0.029 Tg N2O-N/year with an uncertainty range of 0.003–0.15 (Table 4). Emissions from uncollected human waste are relatively small (Table 4). It should be noted that the terrestrial emissions from uncontrolled waste are based on emission factors that in fact apply to agriculture. For agriculture, several estimates of global N2O emissions exist [2,14,16,23,61], as well as for national or regional emissions (e.g. [17,62,63,64,65]). However, these estimates rather focus on fertilizers than on uncollected human waste. More´e et al. [24] were the first to publish global and countryspecific estimates for N inputs from human waste to agriculture (i.e. recycling of human waste in agriculture). Their study can serve as a first step towards determining countryspecific N2O emissions from uncontrolled human waste.

Past and future trends in N2O emissions from regions

Table 3 Results of a sensitivity analysis: global and regional N2O emissions from human waste (sum of collected and uncollected) in 2000 (Gg N2O-N), calculated on the basis of default emission factors, as well as emission factors reflecting the low and high end of the range (see Table 2 for emission factors, and Figure 2b for regions). EF is emission factor. Regions

Remark

EFLow

EFDefault

EFHigh

3.0 2.8 2.3 1.6 0.3

30 24 13 9 2.6

96 76 47 29 10

Europe North America South America North Asia Oceania, Oceans, Australia Africa South Asia

5.3 24.1

28 130

90 421

Global

39

236

769

Current Opinion in Environmental Sustainability 2014, 9–10:108–121

We present N2O emissions from collected and uncollected human waste by region (Figures 3–5). We estimate increasing trends between 1970 and 2000 in N in human waste and the associated N2O emissions for all regions between 1970 and 2050 (Figures 3–5). These trends are driven by an increasing population and the effectiveness of N removal in treatment. In contrast, there is a considerable difference among regions in N2O emissions from collected and uncollected human waste. North America and Europe have similar patterns in N2O emissions (see Figure 3). In 1970 these regions produced about 1.5 (North America) and 2.5 (Europe) Tg of total N in human waste that resulted in around 15 (North America) and 20 (Europe) Gg N2O emissions (as N2O-N) from www.sciencedirect.com

Nitrous oxide emissions from human waste in 1970–2050 Strokal and Kroeze 115

Table 4 Global emissions of N2O originating from human waste (Tg N2O-N/year). EF = emissions factor N2O emission from Collected human waste Wastewater treatment plants

Wastewater effluent to surface waters

Uncollected human waste Soils, surface waters and ground water

Total

N2O emissions

Reference

0.024 (0–0.048)

This study

About 0.020

UNEP [20]

0.0051

Based on IPCC [43]

0.029 (0.003–0.15) About 0.030

This study

1.9 (1–9) 1.5

Seitzinger and Kroeze [71] Kroeze et al. [72]

0.18 (0.04–0.6)

This study

About 0.12

UNEP [20]

0.24 (0.04–0.77) 0.16 (0.02–0.77)

This study UNEP [20]

0.22 0.22 (0.04–2.6)

Based on IPCC [43] Mosier et al. [73]

0.3

Syakila and Kroeze [1] Stocker et al. [15]

0.2 (0.1–0.3)

UNEP [20]

Remark

Based on an assumption: EF = 1% of N treated. Source of the estimated human waste flows: Global NEWS for 2000 [58]**. An estimate is based on an EF of 0.043% (range: 0.035–0.05%) for primary and secondary treatments according to [42] and based on an EF of 0.6% (range: 0–2%) for tertiary treatment according to [41,44]. A large part of the calculated N2O emissions is calculated from tertiary treatment. Based on an EF of IPCC Guidelines 2006. Source: Global NEWS for 2000 [58]*. Based on an EF of IPCC Guidelines 2006 (Table 1). Source: Global NEWS for 2000 [58]***. Estimate is based on an EF of 0.5% (range: 0.05–2.5%) from IPCC guidelines 2006. Total N2O emissions from rivers and estuaries in 1990, sewage effluents are part of the total N effluents to surface waters. Total N2O emissions from rivers and estuaries in mid 1990s, sewage effluents are part of the total N effluents to surface waters.

Based on the sum of EF of IPCC Guidelines 2006 for leaching/ runoff (0.0075 N2O of kg N) and for soils (0.01 kg N2O of kg N inputs to soils) (Table 1). Source: Global NEWS for 2000 [58]****. Estimate is for 2010 and includes human emissions in rural areas, leakage from sewer to groundwater and discharges from nonsewered population. EF for WWTPs 0.01 (our assumption, see text). Estimates are based on EFs of 0.043% (range: 0.035%-0.05%) for primary and tertiary treatment, and EFs of 0.6% (range: 0–2%) for tertiary treatment. Values are for 2010. EF for WWTPs is 3.2 g/person/year [43]. Based on EF of IPCC Guidelines 1996 (1% of N sewage N). They consider human waste production from total population in 1990. Estimate is based on an EF of 1% of N sewage produced.

Table 1 gives an overview of EFs from the literature, and Table 2 gives EFs used in this study. Ranges in N2O emissions are indicated in parentheses. Estimated as follows: the total population connected to sewage systems (with treatment) multiplied by an emission factor (3.2 g/person/year). ** First the amount of treated N in human waste was estimated for each river basin as follows: N in human waste from people connected to sewage systems with treatment (kg N) multiplied by the fraction of N removal in treatment (0–1). We considered the 6081 Global NEWS river basins, 840 of them (covering about 60% of the total area of 6081 basins) have part of the population connected to sewage systems with treatment. The total amount of treated N from those river basins is 2.4 Tg, originated from about 1.6 billion people connected to sewage systems. Second we estimated N2O emissions as: the total treated N (2.4 Tg
N) multiplied by 0.01 (1% emissions factor). *** Total N effluents from sewage systems multiplied by 0.005 (0.5% emission factor of IPCC Guidelines 2006). **** Uncollected waste was estimated using the total N in human waste produced by population not connected to sewage systems. *

human waste. About 20–25% of these emissions are from treated N effluents entering surface waters. Between 1970 and 2000 N2O emissions from Europe increased by about one-third, and N2O emissions from North America increased by about two-thirds. These increases over time are caused by increases in the production of human waste. About 20% of these emissions are from N effluents to surface waters and the rest from WWTPs and uncollected waste, where WWTPs dominate. From 2000 onwards N2O emissions may continue increasing. In the future, more N2O may be emitted from waste

treatment than in 2000. In North America and Europe most people are connected to sewage systems (i.e. around 65–75% of the population is connected in 2000) and in these regions N removal in WWTP is relatively efficient (i.e. around 35–40% of N is removed in 2000; Figure 3). The total population increased by about 10% in Europe and by 50% in North America between 1970 and 2000. For the future, the total European population may stay at the level of 2000 and North American population may further increase. For the future, we project that more than two-thirds of the total population will be

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Current Opinion in Environmental Sustainability 2014, 9–10:108–121

116 System dynamics and sustainability

Figure 3

Nitrogen (N) in human waste

Nitrous oxide (N2O) emissions

Main characteristics

5.0 4.0

2050

Total N2O emissions (Gg N2O-N)

6.0

2050

Total N2O emissions (Gg N2O-N)

Total N in human waste (Tg)

Europe Uncollected human waste Removed during treatment Surface waters (untreated) Surface water (treated)

3.0 2.0 1.0 0.0 1970

2000

Total N in human waste (Tg)

North America 6.0 5.0 4.0 3.0 2.0 1.0 0.0 1970

2000

75 60 45

Uncollected human waste Removed during treatment Surface waters (untreated) Surface water (treated)

1970

2000

2050

Total area (106 km2)

10

10

10

Total population

0.58

0.65

0.65

0.62

0.76

0.86

0.06 (0-0.25)

0.35 (0-0.76)

0.54 (0-0.78)

2050

(109 people) Population connected

30

to sewage (fraction)* 15

N removal in treatment (fraction)**

0 1970

2000

2050

75 60

1970

2000

Total area (10 km )

23

23

23

Total population

0.30

0.45

0.64

0.55

0.64

0.76

0.32 (0-0.47)

0.37 (0-0.53)

0.54 (0-0.70)

6

45

2

9 (10 people)

Population connected

30

to sewage (fraction)* 15

N removal in treatment (fraction)**

0 1970

2000

2050 Current Opinion in Environmental Sustainability

Total nitrogen (N) in human waste (Tg) and the associated nitrous oxide (N2O) emissions (Gg N2O-N) for Europe and North America (see Figure 2 for their locations), as well as main characteristics of these regions in 1970, 2000 and 2050. The results for 2050 are based on a scenario assuming globalization in terms of socio-economic developments, and a reactive approach towards environmental management [59,60]. Data on area, population and N removal are from Global NEWS, Scenario GO [27,58]. The total production of human waste was calculated based on protein N intake as a function of economic development (details can be found in Van Drecht et al. [27]). * The population connected to sewage was calculated as the number of people with sewage connection divided by the total population. ** N removal reflects the average for regions; minimum and maximum values are shown in parentheses. See Figure 1 for categories of N in human waste and N2O emissions. The default emission factors used to quantify N2O emissions are presented in Table 2.

connected to sewage systems in 2050 with better treatment. This may generate more N2O emissions from treatments unless treatment processes will be devoted to reduce these emissions. South America, North Asia and areas of Oceania, Oceans and Australia (see Figure 2 for their locations) emit smaller amounts of N2O than other regions (Figures 3– 5). In 1970 around 1.3 Gg of N2O (as N2O-N) was emitted from Oceania, Oceans and Australia, and 5 Gg of N2O (as N2O-N) was emitted from each South America and North Asia (Figure 4). The majority of this N2O was from uncollected human waste except for areas of Oceania, Oceans and Australia, where untreated surface waters contributed to one-third of N2O emissions. By 2000 the total N2O emissions were higher than in 1970 because of increased discharging of N to surface waters from sewage systems after treatment. These trends resulted from an increasing population, and an increase in sewage connection since 1970 (except for areas of Oceania, Oceans and Australia where no large changes observed in the number of people connected to sewage systems). The N removal efficiencies in WWTPs Current Opinion in Environmental Sustainability 2014, 9–10:108–121

in these regions, however, are not as high as in North America and Europe (Figures 3 and 4). Between 2000 and 2050 N2O emissions may further increase with larger increases in South America (Figure 4). This is because the population is projected to increase generating more human waste in the future. Meanwhile, the N removal efficiencies in WWTP may become higher, leading to N2O emissions from removed N during treatment. More people will be connected to sewage systems, consequently increasing N2O emissions from discharge of human waste after treatment to surface waters. The African region occupies about 30 million km2 which is more than that of other regions (Figure 5). However, this region emitted less N2O (about 10 Gg N2O-N; Figure 5) than Europe and North America in 1970 (Figure 3). Between 1970 and 2000 total N2O emissions tripled as a result of a fast increasing total population (Figure 5). Most N2O originates from uncollected human waste because only 12% of the total population has a sewage connection, and the treatment was poor in 2000. By 2050 this region may emit more N2O to the atmosphere than Europe and North America (Figures 3 and 5). The total population may www.sciencedirect.com

Nitrous oxide emissions from human waste in 1970–2050 Strokal and Kroeze 117

Figure 4

Nitrogen (N) in human waste

Nitrous oxide (N2O) emissions

Main characteristics

6.0 5.0 4.0

Uncollected human waste Removed during treatment Surface waters (untreated) Surface water (treated)

3.0 2.0 1.0 0.0 1970

2000

2050

Total N2O emissions (Gg N2O-N)

Total N in human waste (Tg)

South America 75 60 45

Uncollected human waste Removed during treatment Surface waters (untreated) Surface water (treated)

1970

2000

2050

6 2 Total area (10 km )

18

18

18

Total population

0.18

0.32

0.45

0.25

0.45

0.69

0 (-)

0.06 (0.01-0.10)

(0.16-0.40)

(109 people) 30

Population connected to sewage (fraction)*

15

N removal in 0 1970

2000

2050

treatment (fraction)**

0.30

2050

Total N2O emissions (Gg N2O-N)

2050

Total N2O emissions (Gg N2O-N)

Total N in human waste (Tg)

North Asia 6.0 5.0 4.0 3.0 2.0 1.0 0.0 1970

2000

Total N in human waste (Tg)

Oceania, Oceans, Australia 6.0 5.0 4.0 3.0 2.0 1.0 0.0 1970

2000

75 60

1970

2000

2050

Total area (10 km )

19

19

19

Total population

0.11

0.15

0.16

0.27

0.31

0.56

0.03 (0-0.10)

0.26 (0-0.42)

0.51 (0-0.71)

6

45

2

9 (10 people)

Population connected

30

to sewage (fraction)* 15

N removal in treatment (fraction)**

0 1970

2000

2050

75 1970

2000

2050

6 2 Total area (10 km )

11

11

11

45

Total population

0.15

0.27

0.38

30

Population connected

0.25

0.24

0.52

0.02 (0-0.28)

0.15 (0-0.55)

0.35 (0-0.72)

60

9 (10 people)

to sewage (fraction)* 15

N removal in treatment (fraction)**

0 1970

2000

2050 Current Opinion in Environmental Sustainability

As Figure 3 but for South America, North Asia, Oceania, Oceans and Australia.

reach 1.4 billion by 2050, one-third of which may be connected to sewage (Figure 5). N removal efficiencies in treatment are projected to still be poor compared to the other regions and thus N2O emissions from treated human waste will not be large. In contrast, about 80% of the total N2O emission may come from uncollected sewage (Figures 3–5). South Asia is the largest emitter of N2O to the atmosphere among the other regions (Figures 3–5). In 1970 this region emitted about 50 Gg of N2O (as N2O-N; see Figure 5), which is equivalent to the total N2O emissions from Europe or from North America in 2050 (see Figure 3). However, the total area of this region is similar to the total areas of North America (Figures 3 and 5). The main difference is that this region is densely populated than the other regions with total population of 1.8 billion in

1970 from which only 7% are connected to sewage systems. This explains that N2O emissions originated mainly from non-connected areas in 1970 (Figure 5). By 2000 N2O emissions increased by a factor of 2.5, where non-connected areas are still dominant contributors to these emissions. This increase is associated with increased population (about 3 billion in 2000), but with more people connected to sewage systems. N removal in treatment is absent in some areas (min value is 0) and high in other areas (0.80). On average N removal is equivalent to North Asian region (Figures 4 and 5). From 2000 onwards N2O emissions may continue increasing considerable in line with increasing population (about 4 billion people in 2050), higher sewage connection and better treatment efficiencies (Figure 5). This may increase N2O emissions to the atmosphere from treatment processes.

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Current Opinion in Environmental Sustainability 2014, 9–10:108–121

118 System dynamics and sustainability

Figure 5

Nitrogen (N) in human waste

Nitrous oxide (N2O) emissions

Main characteristics

6.0 5.0 4.0

Uncollected human waste Removed during treatment Surface waters (untreated) Surface water (treated)

3.0 2.0 1.0 0.0 1970

2000

2050

Total N2O emissions (Gg N2O-N)

Total N in human waste (Tg)

Africa 75 60 45

Uncollected human waste Removed during treatment Surface waters (untreated) Surface water (treated)

1970

2000

2050

6 2 Total area (10 km )

30

30

30

Total population

0.34

0.76

1.39

0.08

0.12

0.32

0 (-)

0.03 (0-0.10)

0.18

9 (10 people)

Population connected

30

to sewage (fraction)*

15

N removal in treatment (fraction)**

0 1970

2000

(0-0.40)

2050

25.0 20.0 15.0 10.0 5.0 0.0 1970

2000

2050

Total N2O emissions (Gg N2O-N)

Total N in human waste (Tg)

South Asia 300 250 200

1970

2000

2050

Total area (106 km2)

23

23

23

Total population

1.84

3.16

4.08

0.07

0.17

0.41

0.04 (0-0.10)

0.16 (0-0.80)

0.42 (0-0.80)

9 (10 people)

150

Population connected

100

to sewage (fraction)*

50

N removal in treatment (fraction)**

0 1970

2000

2050 Current Opinion in Environmental Sustainability

As Figure 3 but for Africa and South Asia.

We show how N2O emissions vary among the regions because of different urbanization characteristics (see Figures 3–5). IPCC [66] also show the different patterns in N2O emissions from human sewage among regions. We calculate that over half of the total N2O (the sum of collected and uncollected) is emitted from South Asia, about 13% from Europe, 12% from Africa, 10% from North America, 6% from South America and the rest from North Asia (4%) and from Oceania, Oceans and Australia (1%) in 2000 (see Figure 2a for locations of the regions). Gupta and Singh [67] indicated that about 50% of global N2O emissions from wastewaters originated from Indonesia, the United States, India and China in 2000. Various global studies [20,24,27,59] also demonstrate similar trends in urbanization development among the regions. A most recent study [24] indicates a higher number of people connected to sewage systems for North America and Europe that led to higher N discharges to surface waters in the past, however, improved N treatment resulted in a decrease of N to surface waters in 2000. This implies that more N is treated leading to higher N2O emissions to the atmosphere from treatment. We show that treatment is a dominant contributor to emissions of N2O for these two regions and will continue to be so in the coming decades assuming no improved treatment to decrease emissions. USEPA [68] report an Current Opinion in Environmental Sustainability 2014, 9–10:108–121

increase in N2O emissions (12%) from human sewage in the United States between 1990 and 1997, but they do not distinguish between treatment and N effluents. Efficient treatment with minimal production of N2O might be a wise solution to reduce N2O emissions from North America and Europe. Desloover et al. [40], UNEP [20] and IPCC [66] discuss the mitigation strategies for N2O production and emissions from treatment. For example, it was experimentally demonstrated that during active aeration more N2O is emitted [69]. Desloover et al. [40] indicate that reducing the size of bubbles or aeration intensity may reduce N2O emissions. For Africa and South Asia uncollected human waste is a major source of N2O emissions and thus mitigation measures in treatments may not help to reduce N2O from these regions unless more people will have access to sewage facilities. Similar situation is for South America and North Asia.

Conclusions In this paper we addressed N2O emissions from human waste. Today, the contribution of human waste to global anthropogenic N2O emissions is lower (around 3%) than that of agriculture (around 60%). However, this may change in the future. We distinguish between two types of human waste: collected and uncollected. Collected human waste is treated (causing direct N2O emissions) and the rest is discharged into surface waters (causing indirect N2O emissions). Uncollected human waste may www.sciencedirect.com

Nitrous oxide emissions from human waste in 1970–2050 Strokal and Kroeze 119

stay on land (causing direct N2O emissions) and/or discharged to surface waters (causing indirect N2O emissions). We reviewed existing literature in relation to N2O emissions from these types of human waste. We presented the current knowledge and discussed emission factors used to estimate global N2O emissions. Our study adds to the current knowledge in several ways. First, we presented a sensitivity analysis for regional N2O emissions to get more insight in the uncertainties in emission factors used to calculate these emissions. Second, we presented our estimates of global N2O emissions from collected and uncollected human waste (with the uncertainty range), and compared them with existing global estimates. Third, we presented past and future trends in region-specific estimates of N2O emissions from collected and uncollected waste. Global emissions related to human waste amount to 0.24 (range: 0.04–0.77) Tg N2O-N for 2000. N2O emissions from human waste have been increasing worldwide since the 1970s and may continue increasing in the coming decades, as a result of urbanization. More than two-thirds of these global emissions are from uncollected human waste. South Asia is a major contributor to the total emissions. In Europe and North America the waste-relate emissions largely originate from wastewater treatment, while in other regions uncollected waste and sewage discharges to surface waters dominate. It is important to consider regional differences in policies to reduce N2O emissions. In some regions, for instance South Asia, human waste is a major contributor to coastal water pollution, making it interesting for policies aimed at reducing greenhouse gas emissions. Many emission estimates are based on generic emission factors for wastewater treatment plants (WWTPs) and from N effluents discharged to surface waters. However, these factors are relatively uncertain. More research is needed to better understand the controlling factors of N2O emissions from WWTPs and also from uncollected waste. This information is essential in determining N2O emission factors that can be used for global and national emission estimates.

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