More about the science
Membrane biofilm reactor
In the membrane biofilm reactor (MBfR), we deliver hydrogen gas (H2) to microorganisms by allowing it to diffuse through the walls of special membranes that prevent H2 bubbles from forming. The microbial community grows naturally as a biofilm on the outer wall of the membranes, by removing electrons from H2 and transferring the electrons to one or more oxidized contaminants in the water.
The scientific breakthrough behind the MBfR is the discovery that bacteria that oxidize H2 are able to reduce almost any oxidized contaminant.The oxidized contaminants include nitrate and many new water contaminants whose harmful effects have recently been discovered. These include perchlorate, chromate, selenate and trichloroethene (TCE). Reducing these contaminants renders them harmless or easily removed from the water by common watertreatment methods. This breakthrough is translated into an effective, reliable, efficient and safe technology.
Biofilms refer to the assemblages of ‘microorganisms and biologically produced macromolecules attached to a surface'. Biofilms can be found everywhere that moisture is in prolonged contact with a surface.
Biofilms can be categorized as good or bad. Good biofilms are responsible for nutrient cycling in nature and for biodegradation of pollutants in water treatment technology, such as the MBfR. Engineers often wish to develop a dense and firmly anchored biofilm in treatment technologies. Bad biofilms, or ‘biofouling,’ develop on surfaces in which microorganisms are not desired. Biofouling in a water line may interfere with the system’s circulation or may harbor pathogens and is a major source of infection in humans and plants.
During the formation of biofilms, microorganisms usually encase themselves in an extracellular polymeric substances (EPS) or a slime matrix, which improves microbial adhesion to the surface and to other cells. EPS in biofilms protect microbial cells from biocides and also increase tolerance to physical and chemical disturbances. EPS clearly plays a role in making the biofilm firmly attached or easily removed.
The development of biofilms – good or bad, strongly or weakly anchored -- depends on the interactions among microbiological, architectural, and mechanical properties. These phenomena are studied with state-of-the-art experimental measurements and with mathematical modeling. Through this integrative research, we can understand how factors like the water-flow velocity, microbial species and substrate availability make the biofilm strong or weak, thick or thin, and simple or complex.
Advanced oxidation processes (AOPs), such as photocatalysis, have wide-ranging applicability, are not susceptible to process upsets from toxic inputs, but generally are not cost-effective nor practical options for many real situations.
By combining these two technologies in a process called photobiocatalysis, we take advantage of the benefits of each, while minimizing their drawbacks. Traditional work on coupled chemical-biological treatment focused on sequentially coupled systems, or those that have the chemical and biological processes in separate stages, but these systems suffer from the indiscriminate nature of advanced oxidation, which results in a large range of products, including those that are too oxidized, toxic themselves, or unavailable for biodegradation. This situation could be improved by combining the two operations into a single-stage, called intimate coupling, whereby bacteria are in close proximity to advanced oxidation, and can therefore remove biodegradable products as they are formed, focusing chemical oxidant on the non-biodegradable fraction.
We achieve intimate coupling by using a photocatalytic circulating-bed biofilm reactor, or PCBBR, which exploits biofilm carriers to hold and protect the bacteria from harmful advanced oxidation and toxic compounds, but places the bacteria as close as possible to the advanced oxidation so that the biodegradable products are removed as soon as they are produced, focusing the chemical oxidant on the non-biodegradable fraction.
The PCBBR technology offers the potential to efficiently and thoroughly treat many toxic wastewaters including those contaminated with halogenated aromatics, endocrine disrupting compounds, munitions, and a wide range of harmful industrial inputs including pharmaceutical wastes and textile dyes, resulting in a cleaner water being discharged to the environment, a healthier environment, and a healthier world for us to live in.