Engineers from the University of Hawaii at Manoa have discovered that microorganisms in wastewater grease traps may actually aggravate fat, oil and grease (FOG) deposits in sewers—the very problem the traps are designed to reduce.

Grease interceptors are commonly installed between kitchen wastewater drains and sewer pipelines with the intent of trapping and removing FOG before it enters sewer lines. But a new study by Associate Professor of Civil and Environmental Engineering Tao Yan indicates that microorganisms within these grease traps could produce long-chain fatty acids that can form hardened FOG deposits in downstream sewer pipes.

Often known as "fatbergs," the hardened FOG deposits are comprised mostly of calcium salts of long-chain fatty acids, rather than simply fats and cooking oils. They cause severe problems for sanitation agencies and, in worst-case scenarios, can lead to sewer overflows and system degradation.

Associate Professor of Civil and Environmental Engineering Tao Yan works in his lab. Image credit: University of Hawaii at Manoa. “We are interested in the biological processes that contribute to sewer systems deteriorating and in developing engineering solutions that can improve the sustainability of sewer infrastructure and help protect public health and the environment,” says Yan.

Yan and postdoctoral researcher Xia He replicated the conditions within grease interceptors—where microorganisms from food waste can thrive—in their laboratory. They found that the process increased the concentration of fatty acids within the traps and in the water that would flow into a sewer pipeline.

Furthermore, the fatty acids generated by the microorganisms were mainly unsaturated. These unsaturated fatty acids produce stickier deposits and are thus a hindrance to sewer upkeep, as they are harder to dislodge and corrode concrete pipes.

In light of these findings, Yan is advising grease interceptor manufacturers to consider the potential effects of microbial activities to maximize the effectiveness of their designs.