Tuesday, 5 May 2009

Rio de Janeiro

I spent last Saturday looking at low-cost sewerage schemes in the state of Rio de Janeiro with my good friend (and colleague for over 25 years) Augusto Sérgio Guimarães. We went to the town of Paracambi, located in the flat lands just below the mountains separating the states of Rio and São Paulo − so the well-known orographic effect is active and, when it rains, it rains very heavily and parts of the town often become flooded. Two solutions were tried. One was to collect the all the wastewater discharges into the local stream in a sanitary sewer running along one side of the stream (all the discharges were on this side), treat the wastewater in a septic tank and anaerobic filter, the effluent from which went into the stream; stormwater continued to be discharged directly into the stream − not necessarily the ‘best’ solution but certainly a ‘good’ one and at least the stream wasn’t receiving totally untreated wastewater any more. The other was an innovative low-cost combined sewerage scheme: both domestic wastewater and stormwater go into the same sewer and into a septic tank and anaerobic filter (with the effluent going into the stream), but during intense rain the septic tank and anaerobic filter are by-passed and the combined wastewater goes directly to the stream.

In one part of Paracambi we also saw something else very interesting: a badly designed conventional sewerage scheme. Why badly designed? Well, it was designed only for domestic wastewater but most households (which had no problems before the scheme was put in) discharge rainwater from the roofs into the sewer − so, when it rains, the sewers overflow into the street! Hardly an improvement. The engineers who designed the scheme just hadn’t realised what the householders were doing and so, in ignorance, designed a scheme which simply made matters worse. A lesson for all of us: work with the people and take the local situation into account.