Composting Toilets – The Next Sustainable Frontier

June 24, 2014

These living ecosystems return sewage to nature.

Most facility managers wouldn’t bat an eye at installing low-flow fixtures, irrigation controls, or rainwater harvesting to conserve water. But are you daring enough to trade your traditional toilets for composting ones?

It goes without saying that restrooms generate hundreds of pounds of unmentionable waste each day, but few building professionals view all those flushes as carrying away a commodity. Enter the composting toilet.

Potable Benefits
One of the disadvantages of municipal sewage treatment is that biological materials are processed along with industrial waste and stormwater runoff, explains Denis Hayes, CEO of the Bullitt Foundation. When the waste water is purified, it leaves behind a sludge that can retain many heavy metals and toxicants.

Composting toilets, however, capture human waste upstream rather than downstream. Unlike an outhouse or the pit toilets found in state parks, these dry systems use vermiculture and anaerobic digestion to turn collected materials into a nutrient-rich soil amenity. This ensures excrement isn’t contaminated by potentially worrisome compounds and the resulting biosolids can be safely used for farm, landscape, and garden applications.

The low-tech solution is also relatively hands off for busy facility managers – it takes years before enough compost is generated to fill the collection bins. In the meantime, about one gallon of liquid byproduct (leachate) is produced for every 20 uses, according to manufacturer Clivus Multrum, which can be used in constructed wetlands or carried away by a septic hauler.

The biggest benefit is water savings. Only a few ounces are needed for some systems to flush, whereas others are completely waterless. Models that use vermiculture require some irrigation, but you can acquire that for free from rainwater. All the while your facility is saving thousands of gallons of potable water each year.

Composting toilets are poised to become more common in commercial buildings with the rise of net-zero water. Learn about real-world applications found in the Moorhead Environmental Complex and the Bullitt Center.

Toilets Teach a New Generation
The Stroud Water Research Center is no stranger to composting toilets. Situated in rural Pennsylvania near White Clay Creek, the research campus installed waterless toilets in 1996. When it came time to build the Moorhead Environmental Complex, it was a no-brainer to replicate their earlier success, says Dave B. Arscott, assistant director and research scientist.

The LEED Platinum facility sports four composting toilets and one waterless urinal that connect to two polyvinyl plastic bins in the basement. The toilets have no basin or flushing mechanism – materials simply drop down a vertical chute. The system also includes:

  1. Vermiculture – Worms may be a staple in science classrooms, but these industrious creatures also turn waste into compost. Worms can process 90% of collected solids and liquids, Arscott explains.
  2. Compost – To ensure the worms have a suitable habitat, the bins are balanced with a bulking agent. The facility uses a combination of wood chips, newspaper, and kitchen scraps.
  3. Ventilation – Negative air flow must be maintained in the system to control odors and encourage aerobic digestion. A fan operates continuously and vents air through the roof.
  4. Moisture – Worms need a small amount of dampness to thrive. A humidity sensor keeps track of bin conditions and rainwater is routed through spray irrigation when needed.
  5. Drainage – Too much water, however, can drown out a worm colony. A pump is used to separate liquids from solids while maintaining the bacteria and worm community.

Once a month, staff check on material levels, replenish worms if needed, and confirm irrigation and drainage are still working, notes Arscott. Without a basin, there’s also little daily cleaning required.

No harsh chemicals are used lest they kill off the worms. While the drop structures are far wider than a traditional toilet trap, they can be sprayed down with a garden hose from a nearby janitorial closet, Arscott says. This is done once every four months or as needed.

Located in a teaching area of the facility, close to 4,000 students pass through the restrooms each year. The new models have been such a success that the older composting toilets from 1996 have been retrofitted with vermiculture as well.

The site uses a septic and well water system so it was never on the city grid in the first place. The Stroud campus is certified as its own wastewater treatment plant, with constructed wetlands that are approved to service 2,000 gallons per day. The liquid waste from the toilets is processed in this manner.

In operation for two years, it will likely be another two years before enough composting materials have accumulated to require removal. “We haven’t crossed the bridge yet of who will take it down the road, but a traditional septic hauler could manage it if need be,” Arscott adds.

Urban Setting Ripe for Net-Zero Water
Stuffed to the brim with sustainable features, the Bullitt Center is the tallest building to install composting toilets. To meet net-zero water objectives stipulated in the Living Building Challenge, the six-story office uses a nearly waterless version in its restrooms.

A sensor detects the presence of a user and delivers a coating of foam and two tablespoons of water to the bowl. The slippery mixture then falls through a vertical pipe to the 10 composters located in the basement.

“It’s unsophisticated engineering – a straight chute connects the toilets to the composters, letting gravity do all of the work,” explains Hayes.

The compost bins are filled with wood chips and contain a spindle with a series of tines, which is manually spun for aeration by a building engineer. A fan also pulls air through the system and out through the roof. The center expects a year and a half will pass before the first batch of compost needs to be removed.

The Bullitt Center is working with Seattle to become an independent water district. Combined with other water saving strategies, the composting toilets allow the building to be exempt from sewer hookup fees. Hayes estimates this will result in an $80,000 savings, which should cover the investment costs for the toilets.

Flush with Success
Given their plumbing requirements, composting toilets are best for new construction, additions, or major renovations. Ensure there’s space in a sublevel that’s large enough for bins, as well as appropriate access for removal. The Moorhead complex has a walkout basement, for example, and the Bullitt Center directs leachate to a vacuum pump accessible from its alleyway.

Choose models that are NSF/ANSI certified to avoid code issues and use only compostable toilet paper. As most restroom users are unfamiliar with these systems, dispel any concerns by posting instructional signage.

“Composting toilets mirror natural processes and produce a valuable material, not a source of pollution,” Hayes stresses. “This model of waste diversion is possible, feasible, and affordable.”

Jennie Morton [email protected] is senior editor of BUILDINGS.

About the Author

Jennie Morton

A former BUILDINGS editor, Jennie Morton is a freelance writer specializing in commercial architecture, IoT and proptech.

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