The translation of bajante really depends on what it's being used for. Not obvious to me from your context, but maybe to you if you have more info.

bajantes de pluviales are downpipes from the roof for rainwater. I believe they are called downspouts in the US.

bajante de aguas negras = waste stack

bajante de retrete = soil stack

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Note added at 21 mins (2006-08-29 16:09:11 GMT)
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Waste Stack
Vertical, drainage pipe system to which all of the plumbing, except water closets (toilets), discharge.

http://www.yourwebassistant.net/glossary/w2.htm

Soil Stack
Vertical vent which is connected to a building drain to drain the discharge from the water closets.

http://www.yourwebassistant.net/glossary/s15.htm

Downspout
The pipe or duct used to carry rainwater from a gutter to the ground. Also called a leader, this vertical portion of the gutter system, carries water away from the house, stopping water from causing basement leaks.

http://www.yourwebassistant.net/glossary/d9.htm

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Note added at 31 mins (2006-08-29 16:18:52 GMT)
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I have just found the following:

bajantes insonorizadas que reducen el ruido aéreo. y estructural producido durante las descargas de. los aparatos sanitarios. ...
www.uralita.com/.../UST0510EdificacionPreliminar.pdf

Bajantes insonorizadas con ventilación a cubierta y registros en red colgada. (determinar características y prestaciones a cumplir). ...
www.realiabusiness.net/.../ESTANDARIZACION CALIDADES/CENTRO...

which seem to confirm that we are talking about soil and waste stacks (the top end of both are left open above the roof to venitlate the drain) - Scott's Building dicitonary