by Elaine Cheung
“Use poison to cure poison.” This isn’t just a Chinese old wives’ tale, but an emerging approach being used to tackle one of the modern world’s greatest issues: availability of clean water. The largest portion of water consumed in American households comes from toilet and shower usage. This poses a problem for maintaining renewable water sources, as wastewater is particularly difficult to clean. Sewage contains a high pollutant load, so its recycling process necessitates many steps in order to sufficiently treat the water for reuse. Surprisingly, poisonous bacteria are emerging as an important tool for cleaning polluted water.
Water shortages necessitate recycling
It’s no surprise that as the world population has grown, fresh water availability has become increasingly scarce. In order to prevent impending water shortages, most countries have laws mandating the treatment of liquid waste so that it can be reused and safely pumped back into our environment. Depending on the scarcity of water in the area, treated wastewater can be recycled for use in agriculture, maintenance of wetlands and other natural habitats, and even as drinking water in places with limited water supplies. Although wastewater contains only 0.1% contaminants, it is this small fraction that can pose serious health risks if not removed.
Challenges to water purification
There are many ways in which a water supply can be contaminated (see Figure 1), including but not limited to contamination with toxic chemicals and/or metals (e.g. lead accumulation from old plumbing), improper disposal of drugs, gasoline leaks, etc. That said, the greatest threat to public safety in terms of consuming treated wastewater is the fear of biological contamination such as disease-causing microorganisms like cholera or Legionella. It might come as a surprise then, that one of the most promising solutions to the problem of water contamination relies heavily on our key culprit: bacteria.
Harnessing the power of microorganisms
The purification of wastewater must, inevitably, start with filtering out particulates and other solid debris. An increasingly popular technique for this wastewater filtration step involves the use of biofilms, communities of bacteria that grow as films and stick to surfaces (Figure 2). Since microorganism removal is one of the main concerns in water treatment, it may seem counterintuitive to purposely introduce bacteria to wastewater. However, biofilms exhibit many properties that make them ideal for filtration and, therefore, water treatment.
Biofilms can be made up of only one species of bacteria or many different species living together. Dental plaques, for example, are a common example of a biofilm and often contain up to 500 different bacterial species. Biofilms get a bad rap in the news, as they are often implicated in the contamination of medical devices and hospital-associated infections. However, when used as a type of biological membrane, biofilms can be beneficial and are remarkably good at filtration. The intricate webbing of sugars produced by the bacteria rivals the effectiveness of man-made filters. Biofilms have an added benefit of active decomposition, as certain bacteria can actually help consume sludge by-products and particulates that may clog filters.
From an economical point of view, biofilms are also far superior to other wastewater solutions, such as the use of activated carbon filters, reverse osmosis, or distillation. Since they are self-renewing, biofilms can be cheaply produced by seeding the right bacteria on plastic scaffolds. Given the proper combination of moisture and nutrients, biofilms can form on almost any surface. They require much less energy consumption than traditional mechanical pumps and don’t need the same costly cleaning procedures.
Biofilm based water purification
Since they can be produced quite easily, biofilms have become the driving force behind the development of new, large-scale water purification systems. Instead of the traditional systems for wastewater treatment, water treatment plants are moving to the use of fixed-bed biofilm reactors (SFBBRs), which rely upon biofilm water filtration. Key to the function of SFBBRs is the ability of the biofilm to degrade solids that can clog filtration. Even more impressive is that this biofilm is composed of a consortium of bacteria that one could find in just a handful of dirt. Researchers at the Sam Houston State University have tested and shown that this system is not only capable of cleaning wastewater just as well as the old system, it also does it more efficiently, leaving behind only 10% “sludge,” a bothersome byproduct of wastewater treatment. Typical septic systems take longer to set up (30 days vs. 24 hours for SFBBR) and leave behind up to 50% sludge.
Future directions and applications
Biofilms are also being considered for the development of portable water purification systems. All of the components needed to establish a mini water treatment center with this technology can fit inside one standard 20-foot shipping container. This system is already being tested by the Army to provide drinkable water for troops stationed in rugged terrain. In addition, one of the greatest issues that occurs in the aftermath of earthquakes and other natural disasters is water contamination. This is especially topical in light of the recent destruction done by Hurricane Dorian; the need for an effective, portable water purification system has scarcely been more urgent. The potential uses for biofilm-based water filtration are innumerable; once optimized, water purification stations could be used not only as an aid during natural disaster crises, but as a backup during main water supply contamination, and as an affordable water source for undeveloped regions throughout the world.
Elaine Cheung is a second-year graduate student in the Biological and Biomedical Sciences program at Harvard University.
For More Information:
- See this recent review for a more detailed description of the mechanics behind biofilm-mediated depollution
- This paper discusses the potential use of algal biofilms in the treatment of wastewater
- For a more detailed view of how we can incorporate biofilms into preexisting membrane filtration systems, take a look at this page
This article is part of our special edition on water. To read more, check out our special edition homepage!