Your discussions on toilet technology (Nature 486, 185; and Nature 486, 186–189; 2012) should have mentioned an important aspect of sewage disposal — the problem of contamination with antibiotics and antibiotic-resistant organisms.

Processes for treating waste water provide perfect mixing pots for bacteria carrying genes that confer antibiotic resistance. These genes can come from many environmental sources and are often conveyed on mobile genetic elements. Ways must be urgently sought to prevent the genetic recycling and dispersal of antibiotic-resistance genes and of resistant organisms.