Households are an important source of nutrient loading to surface water. Sewage systems without or with only primary wastewater treatment are major polluters of surface water. Future emission levels will depend on population growth, urbanisation, increases in income and investments in sanitation, sewage systems and wastewater treatment plants. This study presents the results for two possible shared socioeconomic pathways (SSPs). SSP1 is a scenario that includes improvement of wastewater treatment and SSP3 does not include such improvement, with fewer investments and a higher population growth. The main drivers for the nutrient emission model are population growth, income growth and urbanisation. Under the SSP1 scenario, 5.7 billion people will be connected to a sewage system and for SSP3 this is 5 billion. Nitrogen and phosphorus emissions increase by about 70% under both SSP scenarios, with the largest increase in SSP1. South Asia and Africa have the largest emission increases, in the developed countries decrease the nutrient emissions. The higher emission level poses a risk to ecosystem services.

INTRODUCTION

Eutrophication represents the most prominent aquatic ecosystem management problem in rivers, lakes and estuaries worldwide (EEA 2012; OECD 2012). Eutrophication results from the discharge of the nutrients nitrogen (N) and phosphorus (P) to surface water, primarily from wastewater of households and agriculture (Bouwman et al. 2005). Eutrophication resulting from nutrient loading first became evident in lakes and rivers in the form of excessive growth in macrophytes and floating algal scums (Butcher 1947), and occurrence of toxic cyanobacteria. It can negatively affect ecosystem services of drinking water provision (Zhang et al. 2010) and tourism. Coastal seas are affected by decreased oxygen concentrations (hypoxia) due to the decay of algal biomass, and the production of toxins by algae and bacteria can cause fish kills (Diaz & Rosenberg 2008; Li et al. 2009). Over the past decades, major changes have occurred in many countries in N and P emissions from households to surface water (Seitzinger et al. 2010). Population growth, urbanisation and increase in prosperity resulted in increasing nutrient emissions. Rising incomes generally induce increasing protein consumption, increasing N and P excretion, while P emissions also increase due to the use of P-based laundry and dishwashing detergents. As seen in many developed countries in the last 50 years, economic growth often leads to expansion of sewer systems; however, the building of wastewater treatment plants often lagged behind, subsequently resulting first in an increase of the nutrient discharge to surface water and later a decline of the emissions. Different levels and combinations of sanitation and wastewater treatment can be distinguished, each having different consequences for nutrient emissions and water quality (Table 1). Many countries in Sub-Saharan Africa and South-East Asia either have no improved sanitation at all or improved sanitation without being connected to a sewage system. Septic tanks have a positive impact on the local health when the sludge is removed and treated, but the effects on the environment and health are negative if that sludge is dumped untreated onto landfill sites or in surface waters (Miller & Parker 2013). Countries with sewage systems do not always have any kind of wastewater treatment. In transboundary catchments, international agreements on wastewater treatment are sometimes necessary to improve the water quality (EC 2000; EEC 1991). An example is the river Rhine with a bad water quality in the 1980s. Since then, wastewater treatment systems have been subsequently improved from primary (mechanical) wastewater treatment to secondary treatment and now tertiary treatment (Eurostat 2014), with a considerable improvement of the water quality.

Table 1

The impact on water quality of the different phases in sanitation and wastewater treatment

Sanitation phase Nutrients/organic pollution Water quality effects 
No access to improved sanitation Recycling in agriculture as fertiliser, landfill Local and minor effects 
Access to improved sanitation, no sewage system Waste or septic sludge landfilled or discharged to surface water Major widespread negative effects 
Access to improved sanitation, connection to sewage system, no wastewater treatment Nutrients directly discharged to surface water Major widespread negative effects 
Access to improved sanitation, connection to sewage system and primary or secondary wastewater treatment Larger fraction of nutrient removal with secondary treatment, the removed nutrients are stored in sewage sludge Fewer negative effects under an increase in wastewater treatment 
Access to improved sanitation, connection to sewage system and tertiary treatment plants Nearly all nutrients are removed from effluent, with potential for re-use of P in sewage sludge Limited effects 
Sanitation phase Nutrients/organic pollution Water quality effects 
No access to improved sanitation Recycling in agriculture as fertiliser, landfill Local and minor effects 
Access to improved sanitation, no sewage system Waste or septic sludge landfilled or discharged to surface water Major widespread negative effects 
Access to improved sanitation, connection to sewage system, no wastewater treatment Nutrients directly discharged to surface water Major widespread negative effects 
Access to improved sanitation, connection to sewage system and primary or secondary wastewater treatment Larger fraction of nutrient removal with secondary treatment, the removed nutrients are stored in sewage sludge Fewer negative effects under an increase in wastewater treatment 
Access to improved sanitation, connection to sewage system and tertiary treatment plants Nearly all nutrients are removed from effluent, with potential for re-use of P in sewage sludge Limited effects 

In this study we explore scenarios for global future nutrient emissions from households and industries, and the development of the construction of sewage systems and wastewater treatment. The consequences of different possible developments in demography, urbanisation and economic growth for the period until 2050 were examined with socioeconomic scenarios (O'Neill et al. 2014). The results are used to calculate nutrient loads on a global scale, risks to aquatic ecosystems and scenarios for environmental studies (Stehfest et al. 2014).

MATERIAL AND METHOD

Model description

Nutrient emissions from households and industries in urban areas, as well as from urban livestock where relevant, are calculated using a global country-scale model (van Drecht et al. 2009; Morée et al. 2013). Figure 1 presents different pathways of household emissions of N and P as used in the model. The dominant pathway of nutrients was, a century ago, reuse in agriculture; nowadays the dominant pathway is discharge to surface water or removal from the effluent water. Human excretion depends on food protein intake per capita. Proteins are assumed to have a N content of 16% and a P content of 1.6%. The model also accounts for P emissions from the use of P-based detergents for laundry machines and dishwashers, and the extent to which P-free detergents are used, e.g. by regulations. The removal of N and P in primary treatment plants is 10%; in secondary treatment this is 45% for P and 35% for N and for tertiary treatment this is 90 and 80%, respectively. New technology makes it possible to remove more than 95% of the nutrients (de Kreuk et al. 2005).

Figure 1

Different pathways of human emissions of N and P from households.

Figure 1

Different pathways of human emissions of N and P from households.

Input data for the model include urban population, connection to sewage systems and presence and type of wastewater treatment. Households are divided into three main groups: (i) households in urban and rural areas that are connected to a sewer system; (ii) households in urban areas that are not connected to a sewer system, where wastewater or septic sludge is discharged directly to surface water through open sewers, accounting for some retention; (iii) households in rural areas that are not connected to a sewer system, where human waste is assumed to be deposited in pit latrines or septic tanks with no direct connection to surface water, or recycled on agriculture land. Only the first two groups are relevant for the global nutrient emissions. Data were collected of each country for sewage connection rates, wastewater treatment levels and protein consumption. The country-based data were taken from different sources, especially from the Joint Monitoring Programme (WHO & Unicef 2013), supplemented with data from other sources (van Drecht et al. 2009; Miller & Parker 2013; Eurostat 2014). The per capita protein consumption of most countries was taken from FAO (2012). The model calculated the emissions for each country. To present the results, the country data were aggregated into the seven regions defined by the World Bank (World Bank 2014). This aggregation was used because it has only seven regions and has a main contrast between developed countries and the other regions. (Ligtvoet et al. 2014).

Scenario description

The scenarios used in our analysis were based on the shared socioeconomic pathways (SSPs) as described in O'Neill et al. (2014). SSPs are reference pathways describing plausible alternative trends in the evolution of society and ecosystems and includes assumptions about future demographics, economic development and degree of global integration. We used two contrasting scenarios, i.e. SSP1 and SSP3. These two scenarios were selected because they describe contrasting developments in population, economic growth, environmental policy and technology development and transfer (Table 2; O'Neill et al. 2012).

Table 2

Key elements of SSP1 and SSP3 relevant to wastewater and sanitation

 SSP1 SSP3 
Population growth Medium Rapid 
Migration rate Medium Low 
Urbanisation Planned Unplanned, more slumps 
Growth per capita Medium (low income countries), fast (high income countries) Slow 
Inequality across regions Convergence of incomes, but retaining diversity Large 
Inequality within countries Becoming more equitable, less stratification Large 
Consumption Low growth in material consumption Material-intensive consumption is important 
Environmental policy Stringent and effective Weak 
Technology development and transfer Rapid Slow 
 SSP1 SSP3 
Population growth Medium Rapid 
Migration rate Medium Low 
Urbanisation Planned Unplanned, more slumps 
Growth per capita Medium (low income countries), fast (high income countries) Slow 
Inequality across regions Convergence of incomes, but retaining diversity Large 
Inequality within countries Becoming more equitable, less stratification Large 
Consumption Low growth in material consumption Material-intensive consumption is important 
Environmental policy Stringent and effective Weak 
Technology development and transfer Rapid Slow 

The SSP database (https://secure.iiasa.ac.at/web-apps/ene/SspDb/dsd?Action=htmlpage&page=welcome) provides country-level data for population, urbanisation and GDP levels. The first step to calculate the sewage and treatment rate was to calculate the distinction between improved or not improved sanitation. This fraction per country is calculated using the global integrated sustainability model (GISMO; Hilderink et al. 2008). This fraction is modelled separately for urban and rural populations, applying linear regressions with GDP per capita, urbanisation rate and population density. The next step was to calculate the sewage connection rate as part of the improved sanitation rate per country. The relation between GDP per capita and the connection rate and wastewater treatment rate was based on the averages of GISMO regions; both relations aggregated to GISMO regions had R2 of 0.8. The GISMO regions classify countries into 27 groups in respect to sanitation and wastewater. The relation between GDP and sewage connection rate and wastewater treatment rate was used to calculate the rates projected for 2050. Countries with a higher or a lower rate in 2010 than calculated with the GDP relation or with a higher rate than the urbanisation rate were corrected. The average nutrient removal of different levels of wastewater treatment was used to define the relation with GDP, also for the GISMO regions. The average nutrient removal was translated to a percentage of primary, secondary or tertiary treatment.

RESULTS

The two pathways show different developments in population, urbanisation and GDP with different impacts on sewage systems and wastewater treatment (Figure 2). Globally, the number of people connected to a sewage system is projected to increase to 5.7 billion people in SSP1 and 5 billion people in SSP3. However, due to large population growth, the number of people without a sewage system also increases in SSP3, to 5 billion people.

Figure 2

The total population classified as lacking a sewage connection, with connection but lacking wastewater treatment, and with sewage connection and primary, secondary or tertiary treatment for 1970, 2010 and 2050 for SSP1 and SSP3.

Figure 2

The total population classified as lacking a sewage connection, with connection but lacking wastewater treatment, and with sewage connection and primary, secondary or tertiary treatment for 1970, 2010 and 2050 for SSP1 and SSP3.

In SSP1, the combination of urbanisation, decreasing inequality within countries and the implementation of stringent and effective environmental policies lead to an improvement to sewage systems and wastewater treatment. In the poorest countries an increase in the sewage systems that are partly connected to primary treatment systems is projected. In East Asia, economic growth leads to sewage connections for the urban population with mainly tertiary wastewater treatment plants. Less economic growth in South Asia results in limited improvements to the sewage systems, which are insufficient to compensate for the growth in population. In developed countries, primary and secondary treatment plants are upgraded to tertiary treatment plants, although the rural population will not always be connected to a sewage system. In SSP3, the population in the mega cities in the poorest countries primarily increases with extended slum areas, while sewage systems are constructed only in the wealthiest districts. In South Asia, the number of people with a sewage connection is projected to increase from 160 million in 2010 to 925 million by 2050, which represents only an increase from 10 to 34% of the population of that region. In Africa and Asia most of the wastewater is discharged untreated and the absolute number of people without treatment increases with the population growth. The emissions of nutrients to surface water increase globally in both SSPs by about 70%. In most regions nutrient discharges in SSP3 exceed those in SSP1 (Figures 3 and 4). However, in South Asia and Sub-Saharan Africa nutrient discharge levels are higher in SSP1 than in SSP3, as the increase of their urban population is faster than the improvement of wastewater treatment or sewage systems (OECD 2012; Ligtvoet et al. 2014). Although the GDP increases more in SSP1 than in SSP3, the urbanisation and construction of sewage systems without wastewater treatment will result in a higher emission.

Figure 3

N emissions to surface water for seven world regions for 1970, 2010 and 2050 for SSP1 and SSP3.

Figure 3

N emissions to surface water for seven world regions for 1970, 2010 and 2050 for SSP1 and SSP3.

Figure 4

P emissions to surface water for seven world regions for 1970, 2010 and 2050 for SSP1 and SSP3.

Figure 4

P emissions to surface water for seven world regions for 1970, 2010 and 2050 for SSP1 and SSP3.

The nutrient emissions to surface water of the developed countries in SSP1 will decrease between 2010 and 2050 as a result of the construction of tertiary wastewater treatment plants. Emissions decrease only slightly in Europe, as most urban people are already connected to a sewage system with tertiary wastewater treatment (EEA 2012). In Latin America and northern Africa, emissions are projected to increase, due to a combination of increases in population and urbanisation and the construction of sewage systems. In East Asia, mainly China, large investments in sewage systems can hardly compensate for the urbanisation and increase in welfare. In SSP3, with less improvement to wastewater treatment systems, emission levels will increase.

CONCLUDING DISCUSSION

The emissions of nutrients to surface water are projected to increase globally by about 70% between 2010 and 2050, with the largest increases in the poorest regions, i.e. Sub-Saharan Africa and South Asia. Increasing urbanisation and an expanding sewage system with only partly primary treatment increases the nutrient discharge to surface water in these regions. In East Asia, major investments in sewage systems and wastewater treatment plants are necessary to compensate for the growth of the urban population and the growth in income. Without these investments, the emissions will increase. Although the present load of nutrients to freshwater and coastal seas is already too much with negative effects to the ecosystems, the load of nutrients will further increase. As a result, ecosystem services, such as drinking water, aquaculture, fishery and tourism will increasingly be affected by the deterioration of the water quality. In contrast, many developed countries currently have nearly 100% sewage connection and to a large extent secondary or tertiary treatment. A future increase in tertiary treatment will further decrease nutrient discharge and improve the water quality. The analysis shows that policies aiming at reducing inequality between and within countries and improving human health, as well as environmental policies, can make a large difference even in the medium term. Investments in sewage systems will have an increase of the nutrient load when this is not combined with investments in wastewater treatment. Urban areas will expand in the coming 40 years, which will be more planned in SSP1 or unplanned with large slum areas in SSP3. Investments in sewage systems are expensive and construction is difficult in existing urban areas. Sewage and wastewater treatment plants are long-term investments and built for 30 or more years. In many regions, an increase of nutrient emissions from investments in sewage systems without wastewater treatment will precede the improvements of the water quality by the construction of wastewater treatment plants. The load of nutrients to surface water and also the cumulative load will increase with corresponding risks to ecosystems.

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