Composting toilets and sewers

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.)

Update on sustainable toilets in Central America

I have recently returned from Central America, where I was training extensionists to build and maintain dry, urine-diverting toilets. See page four of Sustainable Harvest International's spring newsletter for a summary of my work there.

These toilets keep pee and poop separate. The pee, which is sterile, can be diluted and used as a liquid fertilizer. The poop is treated over time with heat from a solar collector, which destroys germs. Then it can be used to improve soil. How cool is it that with a little concrete, wood, and metal you can turn a farmer's toilet into a fertilizer factory?

Origins

A little over a year ago, I was giving a presentation about composting toilets at the Common Ground Country Fair. I mentioned the remarkable fact that the amount of manure a person produces over the course of a year contains enough fertilizer to grow a year's worth of food for that person. One gentleman, who owned a composting toilet, stood up and told me he thought I was full of it, and that there couldn't possibly be that much fertilizer in human manure.

I replied with several reasons it had to be so--most notably that since elements can't be destroyed, every gram of nitrogen, phosphorous, and potassium that we eat in the form of food will exit our bodies as manure. The food is digested, and the molecules are rearranged, but the elements are all still there in the same quantity. If we treat that manure to destroy germs and then return it to the soil, we return to the farm all the plant nutrients we took away when we harvested the food.

A farm loses nutrients when food leaves the farm, and in order to keep the soil from becoming impoverished, the farmer needs to bring nutrients back onto the farm to replace what is lost. These nutrients most often come in the form of chemical fertilizer or animal manure from another farm. Though the equation is not precise, generally speaking we can say that the amount of nutrients being added to the farm as fertilizer needs to equal the amount of nutrients leaving the farm in the harvested food. Otherwise, the amount of nutrients that are available in the soil to feed growing plants will steadily decline and yield will drop.

If we can return to farms, in the form of treated human manure, all the nutrients we take from the farms, in the form of food, then we can potentially eliminate the need for chemical fertilizers. This would be a huge step toward sustainability!

The numbers add up, and the theory is there to say it would work, but the man arguing with me was not convinced. I realized that nothing I could say to him was going to change his mind, and that he would not believe me unless I could show him the manure pile, the plot of ground, and the harvested food.

But that is as it should be. The world has seen too many ideas that look great on paper or in the lab, but which fizzle when they brush up against reality. If this human manure business was really such a good idea, where could a person see it in action? Where could we see a person-year of manure supplying the fertility to grow a person-year of food?

There are loads of "humanure" evangelists in the United States, growing vegetables from composted human manure, but it is usually a garden patch, not a farm, and almost nobody keeps records of inputs and outputs. China has a 4,000-year history of depending on human manure as a primary fertilizer, and at least one wonderful book delves deeply into the nutrient balances involved, but it's too far away for most folks to visit.

What we needed was a place right here, where the whole cycle happened in one place: eating, excreting, composting, fertilizing, growing crops, harvesting food, and eating once again. If all this could happen in a well-documented manner, with testing to keep track of the quantities of nutrients all along the way, we could show clearly whether recycled human manure could in fact replace chemical fertilizers as the the primary source of fertility in modern agriculture. If we did it publicly and transparently, we could win over not just to the skeptical gentleman at the fair, but also regulators, agronomists, reporters, politicians, students, farmers, doctors, chefs, and conservationists.

Thus was born the Poopstitute.