In a shipping container next to a PepsiCo bottling plant in Fresno, California, a startup called Aquacycl is working with the food giant to pioneer new technology that can help tackle one of the lesser-known sources of climate emissions: cleaning up industrial wastewater.

“Globally, water and wastewater treatment accounts for more greenhouse gas emissions than the entire shipping industry,” says Orianna Bretschger, Aquacycl’s CEO. Most of those emissions come from the energy used to run large wastewater treatment plants, where building-sized tanks process dirty water. The traditional system also generates sludge that often ends in landfills.

Aquacycl’s modular technology helps clean up contaminated water before it flows down the drain. In some cases, it can also be reused onsite after it’s cleaned. Inside the shipping container, hundreds of microbial fuel cells, each roughly the size of a car battery, use natural, locally-sourced bacteria to break down contaminants. The system can be sized up or down depending on the volume of wastewater, with the fuel cells attached together like Legos.