Producing energy by burning waste chemicals may be good for a sustainable future. But converting waste into useful chemicals is better.”
– From the 2010 annual report, Biotechnology Department, TU Delft.
Alongside the efforts by the other biotechnology groups to establish and utilize pure cultures of (engineered) microorganisms, the Environmental Biotechnology (EBT) group relies on mixed cultures to do the job. Mixed cultures are complex populations of different types of microorganisms, each competing for dominance. “We unravel the conditions under which a microbial ecosystem performs exactly as we want, and apply this knowledge to design bioprocesses in which the desired microbial community is naturally enriched,” explains Mark van Loosdrecht, the group’s leader.
Mixed cultures bioprocesses
Mixed cultures can be obtained at lower costs, since they do not need axenic (and sterilisation) procedures and can run easily under continuous process conditions. “Important is that we do not try to control the organisms but try to understand the conditions under which they proliferate.” Van Loosdrecht illustrates this by comparing the microbial ecosystem to nature management. “We try to fundamentally understand under which conditions you can grow a certain ecosystem. Compare it to a farmer who makes sure only potatoes can grow by preventing growth of any other plants or a manager of a heath land making the conditions such that heather proliferates.”
In environmental processes retention of bacteria is often wanted. Therefore they are attached to a solid material, where they form thin biofilms, such as in the case of dental plaque. From a process point of view, biofilms are complicating elements that are well suited for a research approach combining fundamental science (physics, biochemistry), modelling and experimental aspects. A recent example is biofouling of membranes used for reversed osmosis. Using numerical models, researchers at EBT revealed that the degradation of the membrane functionality during biomass accumulation is actually due to the formation of preferential flow channels in the filtration modules.
Van Loosdrecht and his co-workers put mixed cultures to work either as environmentally friendly alternatives to current production processes, or to turn waste streams into clean water and useful materials. The defining feature of the EBT group is its integration of microbial expertise with process technology. “Because we know the process context so well, it’s easy to zoom in on the most urgent fundamental questions. And at the same time we can quickly turn our microbiological breakthroughs into processes that are ready to be applied on a useful scale by our industrial partners such as Paques, DHV and Dow.”
An example of how useful materials can be obtained from waste streams is that of polyhydroxyalkanoate (PHA). PHAs are biodegradable polymers and as such promising candidates to function as raw material for the chemical and pharmaceutical industry. Production processes using pure cultures are expensive and require sugar for the fermentation. The EBT group recently developed an alternative route to produce PHAs. “Our process is much faster and only relies on waste streams,” van Loosdrecht says. “It doesn’t require agricultural land or energy-intensive harvesting steps because there’s no sugar involved.” The coming years, the group will further study the genetics and metabolism of biopolymer production by microorganisms, and apply these findings in a demo-scale production reactor.
“Imagine,” concludes van Loosdrecht, “two separate water infrastructures in our cities. One that uses salt water for non-drinking purposes, such as to transport waste streams, and one for our drinking water purposes. The water-saving advantages of such a concept are obvious. And issues such as salt water corrosion and wastewater treatment based on a sulphur cycle are wonderful new challenges for a research group such as ours.”