Sanitation, welfare economics

Preferences and constraints – when does container-based sanitation address the binding constraint to uptake?

In welfare economics, “preferences” denote which alternative goods or services someone would choose, based on the relative “utility” provided by each (more on utility another time). For example, when presented with a box of chocolates, my first choice is always a praline (P). But if only marzipan fruits (M) and brazil nut caramels (B) were left, I would choose the latter. This allows my preferences over a “set” of chocolates to be written as:


However, if I liked B and M equally, I would be “indifferent” towards them, written as:

B ~ M

Enough about chocolate. Let’s assume that I interviewed 1,000 people in an urban setting. I asked them about their preferences over various household sanitation options if there were no constraints like space or money, and I found that:

  1. People’s first choice would be a private household toilet if it were possible
  2. Preferences for other types of sanitation followed the order of categories of sanitation I set out in this post, giving the result:

Household private Household shared Communal Public Open defecation

This is called stated preference (what they told us) as opposed to revealed preference (what we observe in people’s actual choices).

Household shared was preferred to communal because of not having to leave the plot. Communal was preferred to public on the assumption it would be closer, cleaner and cheaper on average. The other preference relationships are obvious, though may not hold in all urban settings, e.g. there is a revealed preference for open defecation in some parts of India.

The interesting question is, if people aren’t able to fulfil a stated preference for a private household toilet, which constraints prevent them from doing so? Furthermore, which of those is binding? Interventions addressing other constraints will not increase demand unless they address the binding constraint.

I’ve been thinking about this every time container-based sanitation (CBS) is considered as an option in a city I’m working on. CBS comprises systems in which toilets collect excreta in sealable, removable containers for transport to treatment. Below is one example of a CBS service chain from SOIL in Haiti.


Where would CBS fit into the above preference set? The answer would depend on which constraint the household was facing towards fulfilling their preference.

So, what are common constraints to uptake of private toilets in urban areas? Three factors I’ve seen most reported are:

  1. limited space – nowhere to build a toilet on the plot
  2. limited ability to pay (ATP) – not enough access to cash or finance to purchase
  3. limited willingness to pay (WTP) – investment not perceived as worth its opportunity cost

Land tenure and tenancy status are key, and closely related to WTP. People who own their home but are squatting on land may have ATP but not WTP, because they fear making sunk investments which would be lost in the event of eviction. Landlords may not have WTP for private toilets for tenants, since they will not benefit personally (though in theory they could increase rents – evidence on this is mixed). Tenants may be reluctant or unable to invest in a landlord’s property. The ATP constraint relates not only to inability to fund the absolute CapEx cost, but also to inability to spread it over time. There are more factors besides.

To my mind, CBS seems most appropriate where space and/or ATP are the binding constraints.

  • Space: CBS overcomes this by placing the toilet in an existing room rather than a new structure
  • ATP: CBS overcomes this, partially, by making sanitation a service with a small regular fee (like water), rather than an upfront capital investment.

CBS may also have its place in some settings where WTP is the binding constraint, but then its value proposition would have to be better than the alternative. From a tenancy/tenure perspective, a CBS investment is not a sunk cost.

Where does this leave CBS in the hypothetical preference set, then? Of course it would depend on the household e.g. what their alternatives are, whether they have a suitable room, their relative preference for leaving the household building / plot to use a toilet (which could be gendered). On average, I think it might look like this:

private CBS  shared communal  public open defecation

The symbol ≽ denotes  “weak” preference (“better than or equal in value to”), as opposed to “strict” preference ( “better than”). A CBS toilet is essentially a private toilet which is not in a room constructed for the purpose. So it offers many of the advantages of private toilets over shared ones (privacy, security, convenience etc.). I suggest weak preference above (≽) because whether CBS is preferred to an on-plot shared arrangement will very much depend on the setting. The same is true for the other relationships but probably less so. Whether a private toilet is preferred to CBS will depend on other factors too – a household with space and ATP would probably prefer a specific structure not taking up an existing room (allowing those in adjacent rooms to hear and smell…).

CBS is not a silver bullet for all urban sanitation challenges. However, it does have potential in some settings, especially informal settlements where space, ATP (cost-spreading) or tenancy/tenure are the binding constraint to uptake of improved sanitation.

2 thoughts on “Preferences and constraints – when does container-based sanitation address the binding constraint to uptake?”

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