Sewage water can be used to boost agricultural yields, as Tierramérica has reported, but can also contaminate crops and soil with polycyclic aromatic hydrocarbons (PAHs). Such was the finding of the environmental unit of Brazil's agricultural research agency, Embrapa.

PAHs are organic pollutants, some are carcinogens, which can be absorbed by some plants, said Lourival Paraíba, an Embrapa researcher who coordinated a study that showed contamination of maize that had been fertilized with sewage mud.

Continued use of this sort of fertilizer can contaminate the soil for a long time, even decades, he said.

The agronomic advantages of the mud are evident – it is high in nutrients – but it is necessary to inform farmers and consumers about the risks of PAHs, chemical compounds “that nobody can see,” so that people can decide on agricultural use of sewage, he told Tierramérica.