Cleaning up our act

Separating water from sludge is a fundamental step in any sewage treatment system and getting this process right can substantially reduce costs and environmental impacts.

That’s exactly what Carleton University engineer Banu Örmeci is doing. She is in the midst of getting patents in several countries for a new technology that efficiently “dewaters” sewage sludge.

This sludge is in the process of being dewatered at a wastewater treatment plant in North Carolina. (Banu Örmeci)

This sludge is in the process of being dewatered at a wastewater treatment plant in North Carolina. (Banu Örmeci)

Sewage sludge is a semi-solid slurry that after being treated for pathogens is disposed of in one of three ways: it is spread as fertilizer on farmers’ fields, incinerated, or landfilled.  The less water sludge contains the lower its volume and weight, which reduces the cost of hauling it to fields or landfill sites. In the case of incineration, less water means less fossil fuel is needed to dry and burn it.

Perfecting the dewatering system can also reduce the use of chemical compounds designed to separate water from sludge. These compounds are called polymers — substances whose molecules are made of long chains of repeating groups of atoms.

Polymers used in dewatering sludge are toxic to aquatic ecosystems and suspected of causing adverse health effects in humans. That’s important because water separated from sludge is released back into the environment. While this water is treated for pathogens, the polymers can’t be removed.

“There is no other option at the moment,” says Örmeci “But we are looking at alternatives.”

Solid, dewatered sludge is either incinerated or used as fertilizer on agricultural land. (Banu Örmeci)

Solid, dewatered sludge is either incinerated or used as fertilizer on agricultural land. (Banu Örmeci)

In the meantime, her technology lets plant managers determine exactly how much polymer is needed to do the job, minimizing its concentration in both solid sludge and discharged water.

Her system relies on a spectrophotometer, an instrument that measures how much light a substance absorbs, which in turn lets researchers know how much of that substance is present. In this case, she is measuring how much light polymers absorb in water retrieved from sludge. This gives her an accurate, and immediate, reading of the concentration of these polymers in the water.

Plant managers can then react immediately: if the reading is too high, they can dial down their polymer use. If it is too low, they can boost it.

Getting on the right wavelength

While water treatment operators use spectrophotometers to detect concentrations of organic matter, they don’t currently use them to detect polymers. That’s because their spectrophotometers measure the absorbance of light at a wavelength of 254 nanometres and polymers don’t absorb this wavelength. But Örmeci’s research showed that they do absorb wavelengths of 191 nanometres.

“The nice thing about this system is there is already an instrument that exists. We just have to modify its wavelength,” she says.

More importantly, she can use the modified spectrophotometer to determine the relationship between how much polymer is needed to dewater sludge and its minimum concentration in treated water released to the environment.

She and her team perfected the system at three wastewater treatment plants in the United States and are now working with GreenCentre Canada, a federally funded program that aims to move university research to the marketplace. They are also in talks with an international wastewater company to commercialize their technology.

“We started this just three years ago,” says Örmeci. “We now have both lab-scale and full-scale results, and the project is a great example of how fundamental research can lead to new and innovative applications and products.”

 

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