Whether the sewage is full of “garbage” is a question of perspective.
“Why is this a waste?” asked Zhen (Jason) He, a professor in the Department of Energy, Environmental, and Chemical Engineering at the McKelvey School of Engineering at Washington University in St. Louis.
“These are organic materials,” he said, and these can provide energy in a number of ways. Then there is the other valuable resource in wastewater.
His lab has developed a system that collects both, filtering wastewater while creating electricity. The results of laboratory-scale trials were published on May 6 and featured as a cover article in the journal Environmental Sciences: Water Research and Technology.
The wastes contained in the sewage are full of organic matter which, for bacteria, is food.
“Bacteria love them and can convert them into things we can use,” he said. “Biogas is the main source of energy that we can recover from wastewater; the other is bioelectricity.”
There are already ways to capitalize on bacteria to generate energy from wastewater, but these methods often do so at the expense of water, which could be filtered and used otherwise – if not for drinking. – for “gray water” purposes such as irrigation and flushing.
His lab took the two processes – filtration and power generation – and combined them, integrating the filtration system into the anode electrode of a microbial electrochemical system.
The system is configured like a typical microbial fuel cell, a bacterial battery that uses electrochemically active bacteria as a catalyst whereas a traditional fuel cell would use platinum. In this type of system, bacteria are attached to the electrode. When wastewater is pumped through the anode, bacteria “eat” organic material and release electrons, creating electricity.
To filter that same water, however, requires a different system.
His lab combined the systems, developing a permeable anode that acts like a filter.
The anode is a dynamic membrane made of conductive carbon fabric. Together, the bacteria and the membrane filter 80% to 90% of organic material – leaving the water clean enough to be released into nature or treated for non-potable uses.
He used a mixed culture of bacteria, but they had to share one characteristic: the bacteria had to be able to survive in an oxygen-free environment.
“If there was oxygen, the bacteria would just dump electrons into the oxygen and not into the electrode,” he said. “If you cannot breathe with the electrode, you will perish.”
To find the good bacteria, it relies mainly on nature.
“It’s not 100% natural, but we select those who can survive in this condition,” he said. “It’s more like ‘technical selection’,” bacteria that survived and breathed with the electrode were selected for the system.
The amount of electricity created is not enough, for example, to power a city, but it is theoretically enough to help offset the substantial amount of energy used in a typical American water treatment plant.
“In the United States, about 3% to 5% of electricity is used for water and wastewater related activities,” he said. Considering the use by a local municipal factory, he believes his system can significantly reduce power consumption.
“Typically, the process consumes around 0.5 KWH of electricity per cubic meter,” he said. Based on lab-scale experiments, “we can cut it in half or even more.”
But the main objective of the He system is not the production of electricity, it is the treatment of wastewater and the recovery of nutrients.
“Bacteria can convert this organic material into things we can use,” he said. “We can also recover nutrients like nitrogen or phosphorus for fertilizers. We can use them to feed the plants. It is only when we do not use it, then it becomes waste.
“Wastewater is a resource in the wrong place.”