Wastewater treatment system recovers electricity and filters water

Valuable resources can be recovered from wastewater using dual-function electrodes that also filter water in a microbial electrochemical system, shows the lab of Professor Zhen (Jason) He from the Department of Energy, Environmental and Chemical Engineering in the McKelvey School of Engineering at Washington University in St Louis.

Collaborating with Egyptian researchers from the Institute of Graduate Studies and Research at Alexandria University in Egypt, Washington University PhD candidate and first author Fubin Liu conducted bench-scale trials, with results published in the journal Environmental Science: Water Research & Technology.

Professor He said the question of whether wastewater is full of ‘waste’ is a matter of perspective, adding that the organic materials in wastewater can provide energy in a number of ways. The waste materials in wastewater are full of organic materials which, to bacteria, are food.

“Bacteria love them and can convert them into things we can use,” Professor He said. “Biogas is the primary source of energy we can recover from wastewater; the other is bioelectricity.

“We can also recover nutrients like nitrogen or phosphorus for fertiliser. We can use it to feed plants. It’s only when we don’t use it, then it becomes waste.”

Methods harnessing bacteria to produce energy from wastewater already exist, but do so at the expense of the water, which could be filtered and used — if not for drinking, for ‘grey water’ purposes such as irrigation and toilet flushing.

Professor He’s lab took the two processes — filtration and energy production — and combined them, integrating the filtration system into the anode electrode of a microbial electrochemical system.

The system is set up like a typical microbial fuel cell, a bacterial battery that uses electrochemically active bacteria as a catalyst. In this type of system, the bacteria are attached to the electrode. When wastewater is pumped into the anode, the bacteria ‘eat’ the organic materials and release electrons, creating electricity — the permeable anode filters the water.

The anode is a dynamic membrane made of conductive, carbon cloth. Together, the bacteria and membrane filter out 80–90% of organic materials, which leaves the water clean enough to be released into nature or further treated for non-potable water uses.

Professor He said that a mixed culture of bacteria was used, but they had to share one feature — the bacteria had to be able to survive in a zero-oxygen environment.

“If there was oxygen, bacteria would just dump electrons to the oxygen not the electrode,” Professor He said. “If you cannot respire with the electrode, you’ll perish.”

The bacteria that did survive and respire with the electrode were selected for the system.

The amount of electricity created by the process is enough to help offset the substantial amount of energy used in a typical water treatment plant.

“In the US, about 3–5% of electricity is used for water and wastewater activity,” Professor He said. Considering the usage by a local municipal plant, Professor He believes his system can reduce energy consumption significantly.

“Typically, the process consumes about 0.5 kW/h of electricity per cubic metre,” he said. “We can reduce it by half, or more [than] that.”

Professor He explained that the primary goal of the system isn’t electricity production — it’s wastewater treatment and nutrient recovery.

The Egyptian researchers are interested in using similar technological platforms for water desalination.

Image caption: Professor He’s lab shows valuable resources can be recovered from wastewater using dual-function electrodes that also filter water in a microbial electrochemical system. © 2021 Washington University in St. Louis. Original article by Brandie Jefferson.