Composting toilets could be the way of the future

This is an Inside Science story.

(Inside Science) -- Big, black wasplike things living in your toilet may sound more like a horror scene than a sanitation solution. That's certainly what people in rural Louisiana thought in the summer of 1930, when black soldier flies infested a set of newly installed privies.

"[C]onsiderable consternation often resulted when a person lifted a privy lid and was greeted by a swarm of insects resembling wasps, or when upon leaving the privy he experienced a strange creeping and buzzing sensation due to flies being confined within his garments,” wrote researchers in an account published in 1930 in the Journal of Economic Entomology. To make matters worse, hungry local chickens tore down the privies' foundations in search of larvae pupating in the surrounding dirt.

Now, nearly a century later, black soldier flies are being hailed as potential allies in the effort to clean up human feces. They are part of a move toward dry or composting sanitation, a set of controversial approaches that may be able to help manage bodily waste in places without sewers or septic tanks. Some researchers even think dry sanitation could replace water-based sewer systems in cities.

The need for sanitation alternatives

Most of the organisms that live in the human gut are harmless or even beneficial, and they die fairly quickly after leaving the body. But fecal matter also contains parasites and disease-causing microbes that have evolved to survive outside the gut, increasing their chances of being transmitted to a new host, said Joan Rose, a microbiologist at Michigan State University in East Lansing.

For example, it may take nearly two months for 99% of adenovirus 40 particles, which cause gastroenteritis, to become noninfectious. The parasitic protozoan known as Cryptosporidium has an egglike life stage that can endure more than six months at 41 degrees Fahrenheit -- just above refrigerator temperature -- and ascaris roundworm eggs can stay viable in sewage sludge for several years, hatching once they are ingested by a host.

Many sanitation systems deal with these dangers simply by isolating waste away from food and fingers until it can be transported to treatment plants. Most port-a-potties work that way too; that blue stuff in the tank is meant to disguise the look and smell of waste, not necessarily to sanitize it, said Karleen Kos, executive director of Portable Sanitation Association International in Bloomington, Minnesota.

Conditions can vary around the world, but many cities rely on wastewater treatment plants to process sewage. Such plants are usually fairly good at removing parasites, said Rose. However, approximately 800 cities in the U.S. have combined sewer systems that are designed to overflow in heavy rains. When that happens, they spill at least some raw sewage into lakes and rivers.

Many U.S. cities are working to reduce such sewage overflows, spurred in part by a December 2000 amendment to the Clean Water Act that requires cities to comply with specific sewer system guidelines. But at the same time, climate change is making matters worse by increasing the number of heavy rain events that cause overflows, said Rose.

Even when sewage makes it to the treatment plant, it takes a huge amount of money and energy to clean all that wastewater, and the process must be repeated over and over as the water is cycled back through people's toilets, said Cecilia Lalander, a researcher in environmental engineering at the Swedish University of Agricultural Sciences in Uppsala. In her opinion, the process is so wasteful that societies should work to move away from water-based sanitation systems altogether.

"I think that you should really make people understand how stupid it is to poop into clean water," she said.

Of course, current U.S. city sewer systems are still better than nothing -- and nothing is what many people around the world have. In many short- and long-term settlements, people often have no choice but to discard their waste wherever they can, which can lead to outbreaks of disease. The World Health Organization estimates that 2.3 billion people lack basic sanitation facilities such as toilets or latrines.

Even in the U.S., it's not uncommon for rural residents to rely on illegal "straight pipes" that dump untreated waste a short distance from their homes. Often they do so because there are no sewer systems available, and the soil is so impermeable that traditional septic systems back up into people's yards when it rains, said Mark Elliott, an environmental engineer at the University of Alabama in Tuscaloosa.

The need for cheap, environmentally friendly sanitation has received widespread attention in recent years, spurred in part by a list of 17 "sustainable development goals" set by the United Nations in 2015. Clean water and sanitation is goal number six, and the list lays out a series of targets and indicators to be achieved by 2030. Private companies, non-governmental organizations and academic institutions are all working to invent and improve sanitation technologies, including systems that use little or no water.

Some waterless sanitation systems work by drying and burning waste, sometimes using it as an energy source. But processing waste doesn't necessarily mean killing everything in it. It's also possible to use living organisms to break the waste down.

Toilets that use aerobic decomposers -- organisms that break down waste with the help of oxygen -- are often called "composting toilets," although Geoff Hill, director of Toilet Tech in Seattle, Washington, argues that this term is misleading. Such toilets generally aren't designed to produce true compost that you can use in your garden, said Hill. True composting kills parasites by raising the temperature, but unless you are using massive industrial composters and maintaining just the right conditions, human waste won't get hot enough to reliably kill everything that could be dangerous, he said.

Still, if done right, the approach can drastically reduce the volume of waste and turn it into what looks and smells like dark, healthy soil. The sources interviewed for this article agreed on that much -- though they disagreed on the how toilets should be designed and where they should be used.

The problem of pee

Composting toilets have been installed at many backcountry sites in national parks and other recreation areas in the U.S. since the '70s, and they have an even longer history in Europe, according to Hill. Most traditional designs deposit urine and feces in the same chamber, with users sprinkling a bulking agent such as wood shavings on top of their waste.

The Phoenix Composting Toilet is one popular model that follows this approach. It has a sealed tank divided into three sections arranged one on top of another. Someone must periodically remove the oldest waste in the bottom chamber, then rotate the tines separating the chambers to allow waste to fall into the next-lowest chamber.

By the time waste is removed from the bottom of such a toilet and transported to a treatment plant, it should in theory have decomposed into something relatively inoffensive. But when Hill studied how the Phoenix and similar toilets were performing as part of his graduate research, he was dismayed by what he found.

"It was, you know, packed pee and poo in wood chips … sometimes super sloppy, and sometimes sort of dry, but still, recognizable poo turds and extreme smells of ammonia in all cases," said Hill. He published his findings in the Journal of Environmental Management in 2013.

Hill and Lalander, who have collaborated on research projects, say that there's a fundamental problem in many of these composting toilets: A mix of feces and urine won't break down in a reasonable amount of time. That's because the urea present in urine degrades into ammonia. It's not just smelly; it kills the microbes that would otherwise break down the waste.

Hill now runs a company that sells an alternative type of toilet, putting him in direct competition with the companies selling more traditional designs. But he claims that when he conducted the research, he expected existing composting toilets to work and had no intention of going into business.

Ried Nelson, co-owner of Advanced Composting systems, the company that produces the Phoenix Composting Toilet, criticized Hill's methodology and disputed his claim that the Phoenix and similar toilets are ineffective at breaking down waste. According to Nelson, most of the toilets that are having problems have seen far more use than they were designed to handle. The toilets should not need to be emptied more than once a year unless they are used more frequently than anticipated, said Nelson. Some of the toilets are also improperly maintained, he said, or installed in cold climates without the proper heating facilities, which Nelson's company also sells.

Rick Chitwood, a mechanical engineer based in the city of Mount Shasta, California, said he is completely satisfied with the Phoenix Composting Toilet he maintains for a property called Horse Camp. His wife, Linda Chitwood, manages the property for the Sierra Club Foundation, and Rick takes care of the toilet for free, describing it as his "hobby."

It's not a hobby that would appeal to many people. Once a week he stirs up the fresh waste in the top chamber with a rake, wearing a respirator to protect himself from the strong ammonia fumes.

"When I took it over about 14 or so years ago, our toilet, because of lack of maintenance, was blocked up every other day. So on even days you could poop, on odd days you could not," he said.

Hill and Lalander say it's not just overuse and difficult maintenance that make traditional composting toilets fail. Mixed human waste has enough ammonia to slow decomposition, but not enough to kill off parasites, said Lalander.

To solve this problem, they say, you need to divert solid and liquid waste into separate chambers. Feces contains far less ammonia than urine, so decomposers can break it down rapidly. Urine is sterile, so it can be dried or allowed to seep through the soil in a drain field. Urine also makes a good soil fertilizer because of its high nitrogen content, said Lalander.

Several companies have designed toilets that divert urine from solid waste. The simplest and cheapest designs have separate channels in the front and back, but those require careful use to work properly, said Hill. Aiming is important, and men must sit. That may be fine for a committed person with a urine-diverting toilet in their home, but it's unrealistic to expect everyone to alter their behavior when using a public toilet, said Hill.

"If you put a sign up that says 'men must sit to pee' in a public toilet, people are going to pee on the sign," he said.

A French company called Ecodomeo has a different solution. With their toilets, a user defecates on a conveyor belt that slants up toward the back part of the seat, then "flushes" using a foot pedal that moves the waste up the conveyor belt before dropping it into a chamber behind the seat. Urine flows in the opposite direction, down and forward. Hill's company, Toilet Tech, imports these conveyor belt systems to the U.S. and sells them to clients such as national parks along with his own custom-built waste chambers.

So what do you do with the feces and toilet paper once the urine is separated and released into a drain field? In some cases, it makes most sense to just transport the solid waste to a water treatment plant, even if it hasn't had a chance to decompose. That's what they do at Mount Rainier National Park, said Richard Lechleitner, a maintenance worker who manages several urine-diverting conveyor belt toilets at the park's high-altitude sites. The area's high use, cold temperatures, and several other factors make it impractical to try to compost the waste, he said. The waste stinks less without urine, according to Lechleitner, and the reduced weight and volume make it easier to pack out.

But whenever possible, Hill recommends letting nonhuman organisms do most of the work before the remains are brought to a treatment plant. He sells a setup in which the waste falls into a crib that is open at the bottom, allowing worms, beetles and other small allies to crawl up from the soil and eat the waste. Someone must periodically rake the fresh waste over to the side, spray it with water, and cover it with a plastic sheet. Hill claims such a toilet can go 15-20 years without being emptied. This has yet to be put to the test; the first one was installed just five years ago at Smith Rocks State Park in Oregon.

Flies and worms

If you don't like the idea of leaving piles of feces on the ground to attract wild decomposers, you can try adding your own domestic decomposing organisms. One promising species is the black soldier fly. Black soldier flies don't feed as adults, so they won't spread disease by landing on human food. The larvae, on the other hand, eat poop and use it to build bodies rich in fat and protein. Once they grow into maggots the size of Tootsie Rolls, they can be harvested and fed to livestock, although they require some processing to remove the feces left in their guts, said Lalander. She has also found evidence that their excretions may help to sanitize waste, killing off certain kinds of pathogens such as salmonella.

The challenge is that if left to their own devices, the larvae will turn into flies. Adult black soldier flies certainly alarmed Louisiana residents when soldier flies infested their privies in 1930.

Given the nuisance involved in raising batch after batch of black soldier fly larvae, Lalander thinks the flies make more sense as a waste management strategy at central treatment facilities, rather than at isolated public toilets or in people's homes.

"You have to be a real enthusiast if you're going to have it as your fecal treatment," she said.

For the nonenthusiast, there's a simpler option: earthworms. Certain types of earthworms are ideal composting assistants because they can live out their entire lifecycle in fecal sludge.

Worms have been used in composting toilets for decades, although for most of that time there was not much peer-reviewed research guiding the practice, said Claire Furlong, a self-described toilet scientist at the IHE Delft Institute for Water Education in the Netherlands. Most of the available literature was produced by companies that sell worm toilets, so academic researchers remained skeptical.

"There's kind of a long and quiet history of worm-based sanitation," said Furlong. "But there was nothing to say that the worms actually ate the shit that went in."

Over the past few years, Furlong and her colleagues have published a series of studies demonstrating how well worms break down fecal sludge. In one study published in the Journal of Water, Sanitation & Hygiene for Development in 2014, worms helped reduce the volume of fecal sludge by 76% over 30 days, compared to 17% without worms. Furlong is currently studying whether worm or "vermicomposting" toilets can be an effective sanitation strategy for people in Myanmar.

A boon for society?

Could dry toilets and composting scale up to solve large-scale sanitation problems? Michigan State University's Joan Rose is skeptical. Efforts to introduce composting toilets in high-poverty urban areas have faced many challenges, from collection trucks that can't fit up narrow streets to the difficulty of providing enough toilets for the residents, she said. And if you can't solve those issues, others arise.

"When you're talking about a million people in a community and you're trying to compost all that waste, who's going to haul it out?" said Rose. "In some places, it's the poorest of the poor who do all the hauling. And it's women."

Mark Elliott from the University of Alabama thinks that composting toilets would also be a hard sell in rural parts of the United States, where people are used to using flush toilets even if, in some cases, the sewage just gets dumped behind their homes. And then there's the matter of maintenance.

"With composting toilets you've got to do some degree of raking or stirring, and you've got to do some type of emptying," said Elliott. "When it comes to dealing with your own waste, even if you say, 'Oh, don't worry, it's going to be compost at that point, it's not going to stink,' people don't want to do it."

Whether it's in cities or rural areas, Lalander agrees that for any sanitation system to work on a large scale, it must be simple for people to use, with centralized services that take care of things like emptying compost. But she sees no reason why such services couldn't be provided for waterless toilets, much as they are for traditional septic tanks.

And when it comes to the daily experience of using composting toilets, she said, design can make all the difference. She is particularly impressed by the pedal-operated conveyor belt toilets produced by Ecodomeo.

"In my own house, I dream of having a urine diverting, vermicomposting toilet," said Lalander. "And I would have one of those pedals."

Editor's note (June 15 2019): A previous version of this article described the cribs used by Toilet Tech as wooden, when in fact they are made of Fiberglass Reinforced Plastic. We regret the error.

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