Explosion and roof collapse at Chicago sewage treatment works

UPDATE: A full report on the incident was posted by the MWRD on December 13, 2018. (Link goes to press release; here is a link to the full engineering report) Summary: “hot work” (in this case, cutting with an oxyacetylene torch) ignited vapors, probably under the manhole they were trying to open, causing an explosion in the sludge blending tanks below the floor of the Gravity Belt Thickener room. The sub-floor explosion caused the subsequent structural failure of the building.

CHICAGO — Metropolitan Water Reclamation District (MWRD) officials this morning reported an “explosion” at the Calumet Water Reclamation Plant (WRP). A subsequent press release called the event, which occurred about 11 a.m., a “roof collapse”. A fire spokesman said that ten workers were removed from the building; two had been pinned or buried by debris. Fortunately, no deaths have been reported, with five of ten workers released from the hospital before the end of the day. The plant remains in operation. The cause is still under investigation.

I was curious about exactly what could have happened, so I have written what I learned here.

The building and process

MWRD called the building where the explosion may have occurred, or whose roof partially collapsed, the “sludge concentration” building.

The treatment process used in Chicago is called the “activated sludge” process. Part of this process involves returning some sludge (solids that settle out after microorganisms have eaten organics in the sewage) to the aeration tanks, to keep populations of those microorganisms healthy. “Sludge concentration,” according to my reading, refers to a part of the process that ensures the return activated sludge (RAS) contains the proper number of microorganisms for efficient operation. Also, more generally, it can refer to simple thickening, or water removal.

Below is an aerial photo of most of the plant (from Google Maps) with the extent of the roof damage outlined.


Based on my limited understanding of the plant itself, the twelve larger, round tanks at the north end of the plant should be primary clarifiers. These allow heavier solids and sediment to settle out.

The next step is the long, rectangular aeration tanks. Here, sewage is infused with air to increase dissolved oxygen (DO), encouraging the growth and reproduction of bacteria and other microorganisms, which consume organic materials in the sewage.

The dark, round tanks (and some square tanks on the west side) are secondary clarifier tanks. These allow solids (sludge) to settle out after the water passes through the aeration tanks. Sludge is scraped from the bottom of the clarifiers and sent to the next step of its process, while clear, clean water overflows the top of the tank and is returned to a waterway as effluent.

This is where my understanding gets foggy when it comes to sludge. Various sources I have read say that “sludge concentration” is basically a “thickening” process — the removal of water. Both return activated sludge (RAS) and waste activated sludge (WAS, sent to landfill or consumers of biosolids) go through some thickening process. The RAS needs to be regulated carefully in terms of the populations of microorganisms present, in order to correctly feed back to the beginning of the treatment process. My guess is that the covered clarifiers immediately to the west of the building where the incident occurred today are sludge concentration tanks. Covered tanks help reduce odors.

This helps explain why the plant is still online; the sludge concentration process is required to keep populations of microorganisms healthy. I think the covered tanks are part of the return sludge concentration process. Perhaps the building in question is where waste sludge is processed for removal, and not part of the return process. It does appear to have large doors for trucks, which might be required to remove the waste sludge.

If anyone has further insight, it would be most welcome!

UPDATE (2018-09-09): The return sludge isn’t concentrated or thickened at Calumet WRP. Rather, the concentration or thickening process is only used on waste sludge, to reduce mass and volume for transport.

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