Stabilising coal ash ponds using ‘biocement’

RESEARCHERS at North Carolina State University (NCSU), US have developed a technique that adds bacteria to coal ash ponds, to make coal ash easier to store and reduce the risk of coal ash spills into surface water.

Coal power plants have been one of the world’s primary sources of electricity generation. The byproduct of coal burning is coal combustion residuals, or coal ash, of which the US produces nearly 100m t/y. In the US nearly half of the coal ash produced is placed in landfills (dry storage) or impoundments (wet storage). The failure of wet impoundments, or ponds, can cause coal ash to spill into nearby surface waters, which can have significant consequences as coal ash contains heavy metal trace elements such as mercury and arsenic.

NSCU researchers found that the bacteria, Sporosarcina pasteurii, could be used to create a ‘biocement’ matrix in coal ash ponds. S. pasteurii is a urea hydrolysing bacterium which in the presence of urea and calcium produces a stiff substance that binds the surrounding solid. This substance is known as biocement.

In a 2018 study, the researchers developed an “appropriate treatment recipe and approach” for microbial-induced calcium carbonate precipitation (MICP), or biocementation, of coal ash.

In the recent study the researchers explored the effects of MICP treatment on the engineering properties of coal ash. They discovered that the process “significantly reduces the compressibility of the coal ash and creates a stiffer material,” according to Brina Montoya, Assistant Professor of Civil, Construction, and Environmental Engineering at NCSU. Hydraulic conductivity was also reduced.

Introducing urea, calcium, and S. pasteurii – typically found in soil – to coal ash ponds, could make coal ash easier to contain and make it “less likely to contribute to pond failures that discharge coal ash into surface waters,” said Montoya.

Typically, coal ash ponds are simply “unlined holes in the ground”, and so contaminates such as mercury and arsenic can leach into groundwater, which raises environmental and public health concerns. Researchers hope that in addition to stabilising coal ash ponds, biocementation might also trap toxic metals in the coal ash. This work is ongoing.

“In the near future, we will advance the treatment technique to a prototype scale to further develop its applicability for use in the field,” said Brina Montoya.

“We still have a lot to do before this technique could be implemented on a large scale, but we would welcome any industry partners who are interested in helping us fine-tune the process for use in addressing the challenges surrounding coal ash storage.”

Géotechnique: http://doi.org/c3jw

Journal of Geotechnical and Geoenvironmental Engineering: http://doi.org/c3jv