No, in Rio this is not waste water, but "pluvial" (rain) water that gets to the beachs when the collecting system (separated from the waste water system) overflows. All waste water in Rio is adequately collected and disposed off.
Whatever the source, this article states:
"Showers found on the beach are not currently subject to regulation and, as a result; do not contain chlorine which sanitizes the water. Instead, the contaminated water harbored traces of fecal coliform, which develops in human feces and can cause serious diseases such as Hepatitis A, Salmonella, Typhoid and Gastroenteritis"
http://riotimesonline.com/brazil-new...beach-showers/
I can't imagine that this contamination comes just from rain water runoff at high rainfall seasons.
This is not to say that you personally will have problems if you swim I have not in similar conditions elsewhaer in Brazil), but it does not give me confidence.
And actually I am told by informed Rio residents that untreated black water in Rio is only pumped out to sea some distance. Nothing to prevent it coming back in on a strong tide.
Some time ago, Globo (the major Brazilian media outlet) measured the fecal coliform level in the sand at Ipanema and Copacabana and the result was alarmingly high (in parts per million) according to their TV news broadcast. This bacteria has undoubtedly been left by decades of untreated sewage being improperly dumped, and unless the beach sand is replaced, remains.
Here is the month by month report for the year to date (2013) for various local Rio beaches from the Rio de Janeiro State Institute of the Environment . Note how much red (not recommended for bathing in the sea) is on the line for Ipanema, for example:
http://200.20.53.6/meioambiente/arqu...o_corrente.pdf