A visual overview of activities at the plant.
I was very humbled by our visit to the Columbia Waste Treatment Plant in North East Portland. The plant manages the waste of 614,000 Portlanders which amounts to about 120 million gallons per day in the dry season and 300 million gallons per day in the wet season. 99% of the water coming in to the plants is rain water or potable water and the other 1% is human excrement, food wastes and various commercial wastes. The main problem with water borne sewage is that it doesn’t matter how little of the actual contents are waste, it all looks and smells like sh*t water. The work of the Columbia waste treatment plant is to separate the waste from the water in order to reuse the wastes as nutrients for dry pasture land in Eastern Oregon and return clean water to our local watershed. In the end they are able to separate much of the water from the waste to create what are called ”biosolids” which are treated solid waste that only contain 80% water. It was hard for me to look at all the energy and engineering that is put into this process to get the waste from 99% water to 80% water. If there’s one thing I learned it’s that it’s really hard to separate waste from water and keep the EPA happy.
The engineers we met with were very kind and gave generously of their time and expertise to show us how they return such vile stuff safely to our ecosystem. The awesome thing about engineers at Columbia Boulevard’s plant is that they’re very aware of how ridiculous it is to combine storm runoff with sewage, but they’re also very realistic. They’re very aware that it’s hard to retrospectively change a system that relies on 2,000 miles of underground pipe without which we couldn’t even make it through our morning routines. I was really excited that one of the engineers is interested in putting in a graywater garden now that it’s legalized thanks to the hard work of Recode.
Aeratation Tank at Columbia Boulevard Treatment Plant
When trying to imagine a world where all waste is composted and returned to the soil it’s humbling to remember that even if we assume that 1% of the liquid coming to the plant is waste, that’s still about 1.2 million gallons of waste every day that needs to be dealt with. In other words, if our sewage wasn’t waterborne the city of Portland would fill up one Olympic pool every 28 days with sewage. If we were trying to compost that waste it’d have to be mixed with an Olympic pool’s worth of sawdust or other carbon matter. It’s a lot of sh*t, but not quite as large as I imagined.
The Columbia Waste treatment plant is constantly trying to improve it’s performance and it was fascinating to see what they’re tried over the years. At one point they tried composting the sewage sludge into Class A biosolids (they currently only create Class B biosolids) using aerobic decomposition. It was pretty much a gigantic mechanized version of our home composter. They found that composting sludge on a large scale required very particular types of carbon feedstock to provide enough carbon (for the microbes) and enough structure to keep each batch aerated. Since the sludge is 80 to 90% water when it enters the compost the wood chips need to be pretty large. When you’re shopping around for an Olympic pool’s worth of wood chips to be delivered consistently every month there’s not many suppliers, maybe one or two. They found that they weren’t able to find a supplier willing to screen the material to remove smaller wood chips. The composter was actively aerated and depended on the wood chips to bulk up the sludge enough to create an environment for aerobic bacteria. Nowadays the composter is filled with wood chips and used as a biofilter to process any odor in the exhaust air from the process.
Thanks again Dept of Environmental Services for all the help!
