Activated sludge

This would be a great name for a metal band, but it happens to be the key process used in treating urban wastewater... i.e., sewage. An affection for sewage treatment runs deep in my veins (bowels?), as my father, a retired professor of chemical engineering, specialized in researching wastewater treatment for many years (in fact, he wrote the book on it). Among his favorite aphorisms were, "Water, water, every where, nor any drop to drink" and "It may be shit to you, but it's bread and butter to me." Anyway, effective and efficient treatment of wastewater probably deserves a place among the most important advances of human civilization.

On our sustainability tour in Spain we visited one of Seville's EMASESA treatment plants. Our friendly and knowledgeable guide Enrique Toro did his best to explain what was happening, given the language barrier. One interesting thing they do at EMASESA is to use an anaerobic process to obtain methane gas from the sludge. This provides a large percentage of the energy for the plant.

The real action takes place in the activated sludge reactors, where the bugs, as they are known in the business, eat the crap out of the crap, quite literally. Maintaining a healthy culture of bugs is a major task of the plant's employees. Here's where it all goes down:

After the sludge does its work, it settles to the bottom and the clear water flows from the top, in this case in large very humid geodesic domes:

The resulting treated water is not of drinkable quality but is more than clean enough to release into the Guadalquivir River. Old dead sludge that is not consumed by the next generation of bugs, known as "bug bones", is composted for fertilizer. Neat!

Most people don't appreciate how much of modern life depends on the deployment of bacteria and other microorganisms. It's not just the human biome, which is essential to health (90% of your cells are non-human), but the role of biological decomposition, fermentation and related processes. Up next: fermentation on a grand scale...