One of the things people like best about composting toilets is that they save water. This year, that hardly seemed like a big concern where I live, as New England recorded one of its rainiest summers on record. So with this abundance of water, do composting toilets make sense?
My boilerplate answer is that there is great value how a composting toilet recycles the nutrients from human manure into a form that can be used safely in agriculture. But what if you live in a city or in town and have nothing constructive to do with the compost?
To answer that question, let's look at a conventional sewage plant, and talk about what happens if you flush your manure down the toilet and into the sewer. In particular, let's look at the small municipal sewage plant that I toured yesterday to prepare for a workshop I am giving at College of the Atlantic.
This plant uses three main stages to treat incoming sewage before discharging it to the ocean. First, the sewage flows into an activated sludge tank where microbes feast upon the organic matter in the sewage. Then it goes to a clarifier, where the microbes settle to the bottom and are sent back to the activated sludge tank. The clarified water then flows into the chlorination tank, where chlorine kills most of the remaining germs. Finally, the treated water flows into the ocean.
Of these stages, the activated sludge tank interests me the most, and so I've included a photo of it. In this tank, microbes greedily devour and digest the organic matter in the sewage. As they do so, they use a vast amount of oxygen to support their metabolism. If they were left alone, the microbes would use up all the oxygen present in the water and then become dormant, essentially going on strike until more oxygen was available. To prevent this work stoppage, (and the septic stench that would ensue,) the sewage plant is designed to pump great quantities of air into the water, with devices like enormous aquarium bubblers.
The amount of air the plant operators pump into the water is exactly matched to needs of the microbes, and the needs of the microbes are dictated by the quantity of organic matter in the sewage. More organic matter in the sewage means that more air needs to be pumped into the water, and that takes more electricity. Therefore, by keeping the organic matter from manure out of the sewer, composting toilets reduce the amount of electricity the sewage plant needs to expend pumping air.
In short, it's hard to clean up a mess once we have made it. If we dump our manure into water, it takes a lot of energy to get it back out. This is energy we can save if we use composting toilets, which keep that organic matter out of the sewers and out of the mouths of those voracious microbes. So even if water is abundant and you don't have a place to use the compost, if you are hooked to a sewer, you can save energy by putting your manure into a composting toilet instead.
(How much energy? How does it compare to the energy used by the fan in a composting toilet? Sounds like another blog posting.)
