The researchers studied what happens to wastewater treatment plant sludge when it's not given enough oxygen. They left samples of sludge without air for different amounts of time and found that after 24 hours, the sludge's ability to break down waste stopped completely. Leaving the sludge without air for longer periods also led to a decrease in its ability to remove harmful substances from the water. The researchers suggest not leaving sludge without oxygen for more than 24 hours to prevent damage.