The submarine outfall is a system for collecting and releasing domestic sewage into the sea. Does it mean that all that dirt is thrown into the water and that’s it? No, not exactly this. First, the sewage goes through several stages of treatment and when it is dumped, it has already lost more than 90% of the fecal coliforms it had. The rest of the cleaning, then, is up to the dilution and natural dispersion of domestic waste in the immensity of the sea. It is clear that, if certain precautions are not taken, the area where the treated sewage is discharged may suffer some kind of ecological imbalance. “This region always ends up having an environmental impact, especially on the bottom of the sea, where the discharge takes place. If the correct dilution does not occur, the oxygen level in the water can drop and affect small plants and animals that live in suspension in sea water”, says engineer Edward Brambilla, from the Basic Sanitation Company of the State of São Paulo (Sabesp). . Therefore, before installing a submarine outfall, the sanitation company responsible for the city needs to carry out a series of studies on the region, analyzing everything from the size of the local population to the behavior of the sea currents in the area. It is also necessary to obtain a license from government bodies that take care of ecological issues. “The projects must comply with the resolutions of the National Council for the Environment (Conama)”, says biologist Claudia Lamparelli, from the Environmental Sanitation Technology Company (Cetesb), in São Paulo. As each region has its own characteristics, the length and depth of submarine outfalls vary greatly. The one in Ilhabela (SP), for example, is 250 meters long, against the more than 4 kilometers in the outfalls of Santos (SP) and Ipanema beach, in Rio de Janeiro. There are about 20 submarine outfalls throughout Brazil.
Onshore pre-treated sewage is discharged into the sea several kilometers from the beach
1. The structure involving an underwater outfall begins with the collection of domestic sewage from a coastal city. All this dirt is collected and pumped to a pre-conditioning station (EPC), where preliminary sewage treatment will be carried out at an impressive rate of 3,500 liters per second.
2. In EPC there are three cleaning steps. First, the sewage passes through grates that retain plastic soft drink bottles, pieces of wood, etc. Afterwards, a set of sieves removes finer residues — particles larger than 1.5 millimeters cannot pass through them. Lastly, sandboxes filter out substances like oils and greases.
3. Before going to the emissary piping, the pre-treated sewage still goes through a disinfection stage. The entire downpour continues to move through tanks where it receives chlorine applications. Thus, 98% to 99% of the total fecal coliforms that were in the collected sewage are eliminated
4. The emissary piping captures the sewage treated by the EPC and goes underground. Once ashore, an embankment can help protect buried pipe. This embankment sometimes extends out to sea. In Santos (SP), for example, it extends 500 meters into the sea and has become a leisure area
5. Once the landfill is completed, the outfall still extends for a long distance — in Santos it is another 4.5 kilometers. The steel pipe is 1.75 meters in diameter and is fixed to the ocean floor with concrete rings, at depths that reach up to 15 meters.
6. In the last meters of the outfall are the diffusers, holes through which the sewage is released into the sea water. The region where the diffusers are located is called the mixing zone and is constantly monitored to see if the outfall is not causing any environmental impact.
7. The released sewage is called plume. After a few seconds in contact with sea water, its concentration is reduced in the order of 100 times. If the plume is not diluted enough when it reaches 300 meters from the beach, it is necessary to improve the sewage treatment or increase the length of the outfall.