The aim of this Theme is to develop advanced biological wastewater treatment technologies for water reuse and energy harvesting. A focus area within Theme B is anaerobic processes coupled with membrane technology.
GRAPHENE POWERS UP WATER PURIFICATION
PI: Pascal Saikaly
A new wastewater treatment system uses hollow fiber membranes coated with graphene to filter impurities from water and create hydrogen fuel. Membrane bioreactors (MBRs) have become increasingly popular for wastewater reclamation. MBRs use a mix of bacteria to digest pollutants and porous membranes to separate bio-generated solids from clean water in a single tank. Despite the small footprint of this design, it requires significant energy to aerate the reactor and provide it with oxygen – an ingredient needed for bacterial respiration and to control the sticky fouling films that build up on membranes.
Pascal Saikaly’s team in collaboration with the Advanced Membranes & Porous Materials Center recently developed an alternative anaerobic electrochemical membrane bioreactor (AnEMBR) that works without oxygen. The device contains special electroactive bacteria that release electrons and protons at the anode when they digest organic matter. By driving electrons and protons to join up into hydrogen gas at a cathode, enough clean-burning fuel is recovered to power the entire treatment process. To reduce space, the AnEMBR system contains long and porous hollow fibers that act as both catalytic cathodes and filtration membranes. Although the absence of oxygen in this technique made it challenging to control membrane fouling, hydrogen bubbles forming on the cathodes could self-scour biofilms off membrane surfaces if gas generation rates were high enough. Several features in the reactor design and operation can affect hydrogen production rates and fouling in the AnEMBR system, such as electrode spacing and operating voltages. However, the specific cathode surface area typically limits its performance, making it a key design factor.
The team produced two types of reactor configurations – a rectangular layout where the anode faced opposite the cathode and a tubular shape where the anode was stacked below the graphene membrane. Treatment trials revealed that closely spaced rectangular electrodes produced the highest amount of hydrogen bubbles and significantly delayed membrane biofouling. To delve deeper into the reactor’s preference for rectangular shapes, the researchers used genomics to analyze any microbes developing on the membrane surfaces. These tools revealed an abundance of methanogens on the tubular reactor that consumed hydrogen gas almost as fast as it was produced. Membranes in the rectangular reactors proved less hospitable to the micro-bugs. These findings should help testing of real domestic wastewater by indicating how to optimize electrode configurations for low-conductivity solutions.
AGAINST THE FLOW OF DRUG RESISTANCE
PI: Peiying Hong
Many wastewater treatment facilities rely on membrane bioreactors (MBRs) that use bacterial communities to consume and break down contaminants that make the water unsafe for reuse. This research examined how specific pollutants in water affect these bacteria and identified a potential edge for one particular class of MBRs. The microbes within MBRs can be oxygen consuming aerobic bacteria or anaerobic bacteria that do not require oxygen. Anaerobic MBRs are known to have significant advantages, including stable bacterial populations that are more efficient at processing pollutants and higher cost-effectiveness.
Before this study, little was known about how the contents of wastewater, which include household chemicals, antibiotics and pharmaceuticals, affect the integrity of microbial community structures, their gene expression and the presence of antibiotic resistance genes. The research team of Peiying Hong tested two miniature MBRs with simulated wastewater spiked with defined amounts of common chemical pollutants. Both the anaerobic and aerobic communities changed over the course of exposure, with certain species becoming more or less abundant in each bioreactor. Results also showed population-level changes in the expression of genes that enable bacteria to break down biological waste, although both bioreactors remained consistent over time.
A potential concern with MBRs is that steady exposure to antibiotics in wastewater will promote the development of drug-resistance genes, which can in turn make their way into the environment. Here the anaerobic system showed a clear advantage with significantly lower antibiotic resistance gene levels compared to the aerobic system, even at similar antibiotic concentrations.
These findings provide yet another strong argument for using anaerobic MBRs. The team is now investigating other potential benefits of the technology with the ultimate aim to demonstrate that microbial communities in anaerobic systems are robust enough to adjust to high concentrations of organic micropollutants while achieving good water treatment efficacy.