From Pruned:

"In the sewage treatment process, these extraordinary womb-like structures break down the organic solid matter in the wastewater into more stable materials. With additional processing, some of these byproducts are turned into fertilizers. These digesters also generate biogas with a high proportion of methane that can used to power the machines. In fact, in large treatment plants, they can produce more electricity than the installations require. The egg shape makes this process more efficient. Compared to their more conventional cylindrical counterparts, they require less energy, maintenance and space. That they are aesthetically pleasing is probably just a happy coincidence."