For many, residual ash from municipal solid waste combustion had been seen as a problem. Frank Roethel, a chemical oceanographer, saw it as an opportunity.

During his research in the 1980s, Roethel found a way to stabilize the ash and successfully constructed artificial reefs. In 1990, with the permission of state regulators, he built a structure on the campus of Stony Brook University in New York. The Boathouse was made out of 14,000 ash bricks and has been standing ever since.

"People have claimed it was going to be a problem for the longest time, but it never precipitated into a problem," said Roethel.

The bricks were made using 350 tons of municipal solid waste (MSW) ash and some natural sand to create a smoother aesthetic appearance. Years of monitoring have shown no sign of toxic emissions or leaching. Roethel said the only elements that leached out were potassium and sodium and the bricks have actually gained strength over time.

In European countries, such as the Netherlands and Denmark, it has been common practice to use ash as an aggregate in cement and asphalt for years. While in the U.S. this type of structure was considered so uncommon that it garnered the attention of Ripley's Believe it or Not! Now, a recent wave of interest from industry and the academic community has led to promising new advances that could change that.

Do regulations stifle reuse?

Many companies extract ferrous metals from their ash, but the Environmental Protection Agency (EPA) estimates that only comprises 15% of the overall ash being generated. Less than 5% of the remaining amount is recovered for other uses, such as landfill cover. That means the majority of the 9 million tons of MSW ash generated every year in the U.S. is still being landfilled.

The lack of reuse activity has little to do with regulation. The EPA's 2015 Coal Combustions Disposal rule doesn't apply to residual MSW ash. Approval of beneficial use projects is up to state agencies, though many are hesitant due to concerns over the potential environmental effects of heavy metals and other hazardous elements.

The majority of these elements are found in fly ash rather than bottom ash. In order to meet EPA standards for non-hazardous waste landfills most companies mix their fly ash with bottom ash to stabilize the material. Yet the bottom ash makes up the majority of MSW ash generated and provides the best structural properties.

Roethel said that separating ash hadn't been a priority for many U.S. combustion companies, especially while they were focused on adapting to more stringent emissions standards. Recently he's seen a renewed interest.

"Their costs for disposal and management of ash is not trivial. That's one of the few pieces where they can really realize significant improvements in their bottom line," he said.

As more companies expand their metal recovery operations they're making greater efforts to separate their fly and bottom ash - as is common in Europe - and explore the properties of their material.

Research is currently underway at City College's Earth Engineering Center in New York to help companies with that process. While fly ash is stabilized by bottom ash when the two are mixed together, more information is needed on how that material will behave in a landfill over time.

Deepak Kumar Sharma - a research associate at the center - has been exploring different methods for stabilizing the ash and potentially removing toxins from it.

"Making it a non-toxic product at the end, that will be the first step in utilizing ash as a very viable product in other processes," he said.

If the combined ash could be vitrified into a solid then it might also have structural potential. This wasn't the initial goal of the research, but it could end up being an interesting finding. Sharma presented at the North American Waste-to-energy Conference last year and hopes to publish his research this fall.

Marco Castaldi, director of the Earth Engineering Center, said that the weak market for bottom ash - still the most viable structural option at this point - is the main thing holding it back from other uses. As long as landfill tipping fees remain low, and standard aggregate material for cement and asphalt is cheaper than treating ash, the industry won't make a shift. Though he does see potential as interest grows in zero waste and closed loop systems.

"For every ton of ash that you use that's a ton of material that you didn't have to mine," said Castaldi.

Seeing is believing 

In addition to research, Castaldi said more demonstration projects like Roethel's Boathouse are what's really needed to help prove this technology and bring it to the forefront. Many environmental groups are still concerned about the long-term effects of using MSW ash for infrastructure purposes and want to see more proof.

One demonstration project that has been getting a lot of attention lately is in Pasco County, Florida. In 2014, a 1,000-foot series of roadway test sections were built at the West Pasco Landfill using bottom ash. The ash was used as a partial aggregate replacement in sections of concrete and asphalt pavements, and for the base layer of one roadway section.

Working with a wide range of government, industry and academic stakeholders, a team of engineering students from the University of Florida conducted research on the project. Justin Roessler, currently a Ph.D. candidate, was in charge of overseeing research and said the results were very promising. Extensive groundwater monitoring data showed no issues. The roads were shown to be structurally sound and potentially cost-saving. Preliminary estimates indicate the county could save $50,000-$100,000 per mile of two-lane road made from MSW ash.

The next step is selecting a location for another roadway section. The engineering team has also been working with Miami-Dade, Hillsborough and Palm Beach counties on beneficial ash reuse projects as well. Pasco County is even interested in mining ash it has already landfilled.

Roessler said that while the project has been successful so far, it still hasn't changed the current market realities for MSW ash reuse.

"As far as private markets I think those are still to be developed," he said. "One of the things that we need to start to focus on is the volume of material that we can provide."

MSW ash is far less plentiful than other aggregate materials and also requires a longer turnaround time because it needs to age at a landfill for three months before being used. Though even with these constraints, Roessler projects that the field of beneficial ash reuse will continue to grow.

"Within the next five years you could be cautiously optimistic that this is going to be something that is semi-standard practice," said Roessler. "Hopefully we were the first domino to fall."