chocolate cake generation

Treated wastewater from one of the two wastewater plants in my city, the 1% poopwater slurry (sludge) heads down to an award-winning biosolids recycling facility. Today, I got to tour the plant and see how they make chocolate cake. I mean, compost.

The coolest part that we got to see was the gravity belt thickener used to dewater the sludge–what the plant manager lovingly called the “shit squeezer”. Behind me is the belt press, basically two conveyor belts which “thicken” the solution by squeezing out the water. Underneath it you can see a progressive cavity pump which pumps the sludge onto the press. These are not unlike the pumps we’re designing replacements for at the wastewater plant!

From milkshake to chocolate cake

This facility takes a sludge composed of 6% poop (chocolate milkshake consistency) and presses it to 18% poop (chocolate pudding consistency). They then “digest” it and combine it with yard trimmings (letting little bugs bake it, literally releasing heat) until it is about 65% poop (chocolate cake consistency). Fresh from the oven! Just kidding….
At the end, the city beneficially reuses almost all the sludge–no discharging water to the river and no disposal in landfills. Some Class B is used on ranches as land application and some Class A is turned into Dillo Dirt (aka fertilizer).

Environmentally friendly and safe for people (less than 10 coliforms/gram!). Science is very very cool sometimes. And a little gross in a chocolatey way.

Belt Filter Press Schematic Drawing from this website has a bit of a different set-up but functionally supports the same process.

This is the gravity belt thickener. They add a little polymer (sticky stuff so it doesn’t fall off the belt) and dewater the sludge to reduce the volume before it goes into the anaerobic digesters. 

The sludge is transported down in between two conveyor belts where it is compressed to squeeze out more water. Finally the sludge is transported out of the area on a conveyor belt. Excess wash water is sprayed onto the belt to break off any additional cake stuck to the bottom.

The final product.
Yum, now I really want some chocolate cake! Cake for dinner? Yes please…because I am an adult. #IdoasIwant