Sewer expansion and urbanisation in developing countries: Evidence from Brazil

A version of this article first appeared in VoxDev.

In many developing countries, rural residents can double or triple their incomes by moving to cities. Even within cities, those living at the periphery often see large increases in income by moving closer to the city centre, where higher-paying formal sector jobs are available. And yet, the urban share of the population in many developing regions is far below that of more developed countries. This leaves us with the question of why cities in developing countries are not growing faster.

One possibility is that these cities have such low living standards that a doubling or tripling of income is not enough to compensate for the forgone pleasures of rural life. Indeed, according to the World Bank, about one-third of the residents of developing world cities lack access to basic sanitation facilities, and about the same share live in slums. Given the high population densities in these cities, it is easy to see why people might prefer to stay in the countryside.

How do sewer systems affect the population density in cities? 

Taking this into account, our new study on sewers in developing countries examines whether improving access to residential sanitary sewers can allow cities in developing countries to accommodate more people by making higher population densities more tolerable. By extension, we assess whether greater access to sewers can facilitate the movement of people from the countryside to the city and, therefore, the higher incomes associated with urban jobs.

We investigate the effects of sewer access on neighbourhood population density, literacy, and mean income in a sample of 92 cities in Brazil, Colombia, South Africa, Tanzania, and Jordan. Since we expect that the construction of sewers is responsive to population density, we assume that policymakers would not sewer undevelopable land. Simply comparing the population density in places with sewers to places without is also uninformative, as it compounds the fact that places that receive sewer service are different from those that don’t, owing to whatever changes result from sewer construction.

Comparing differences in sewer access across drainage basins

To resolve this problem, we compare neighbourhoods on opposite sides of drainage basin divides. A drainage basin is an area where all rainfall (or sewage) will drain to a common point. Figure 1 illustrates the drainage basins around Cascavel, Brazil.

Figure 1: Drainage basins around Cascavel

From this definition, it is clear that water and sewage must flow away from the boundary of a drainage basin divide. These divides are local high spots, meaning that sewage outside a drainage basin with sewer service must often travel uphill to cross the divide and reach a central sewer network. This is not easy to accomplish, so crossing a drainage basin divide increases the cost of sewer service, reducing its availability. 

Figure 2 illustrates sewer access in a neighbourhood of Cascavel, along with basin divides. This clearly shows the decrease in sewer access that occurs when crossing from the drainage basin containing central Cascavel and its sewer system to an adjacent basin. 

Figure 2. Sewer access across neighbourhoods in Cascavel

Even though drainage basin divides are important for sewer construction, they are usually such minor landscape features that they are almost unnoticeable. Thus, if we see differences in population across basin divides, we can be confident that it is because of differences in sewer access. Hence, our strategy is to compare population density and other outcomes for nearby neighbourhoods on opposite sides of a basin divide.

Sewer systems increase urban density and access to city centres

We estimate that providing sewer access to an additional 1% of households in a census tract causes an increase in population density of about 6%. This increase takes place without precipitating an influx of wealthier, better-educated migrants, suggesting that improving sewer access in low-income neighbourhoods does not displace residents from their homes.

We conduct two exercises to assess the importance of this effect. In the first, we provide sewer access to an additional 1% of households living in the densest parts of the city to measure how much this increases the density around an average resident. We find that this has a nearly as large, but opposite, effect as constructing a single highway through an average US city. 

In the second, we simulate providing universal sewer access to all neighbourhoods within walking distance of the city centre. The resulting increase in central population means that, in many cities, about the same share of the population gains walkable access to the central city as those that gained access to central Bogota because of the Transmilenio BRT system, one of the most successful Bus Rapid Transit systems in the world. This suggests that, in many developing country cities, providing sewer access to a small fraction of households can be as important for how the city is organised as major improvements to transportation infrastructure.

Prioritising sewers to improve urban productivity

Evidence suggests that sanitary sewers can cause dramatic improvements in health and reduce infant mortality. Our results posit that their benefits are even larger. Improved sewer access can allow cities to accommodate many more people, who can then take up high-paying urban jobs. These results suggest that, as they juggle the many problems that cities face, policymakers in developing countries would benefit from increasing the priority given to the provision of residential sanitary sewers.