Water recycling gives a town or factory another use for water it has already collected. Treated wastewater can supply irrigation, industrial processes and, with the appropriate treatment and oversight, drinking water. Each use can reduce demand for fresh supplies. The energy case needs a separate calculation: a recycling system consumes electricity, and its value depends partly on what source of water it replaces and how far the water must travel.

A 20-foot shipping container is about 2.2 tonnes of steel. A 40-foot one is closer to 3.7. Making that steel the conventional way, from iron ore in a blast furnace, releases roughly two tonnes of CO2 for every tonne produced. So a retired 20-footer holds something like four tonnes of carbon that has already been spent. Give the box a second job and you don’t spend it again. That is the whole pitch for container buildings, and it holds up.