It is difficult to beat microbial waste disposal systems for being environmentally friendly. Bacteria are enlisted to digest wastes, breaking them down into useful fertiliser, carbon dioxide and water, leaving very little behind that could be viewed as waste.
Even complex organic chemicals and heavy metals are being tackled by microbial digestion systems, although to varying degrees of success depending on the substance under study. Researchers are identifying organisms that can deal with PCBs, pesticides and their residues and other chemicals and are developing digestion systems that utilise these bugs.
DCU's School of Biotechnology is involved in this work and a group there has come up with a system that can deal with monochlorophenols, toxic and persistent substances that arise in pulp and paper manufacturing, pesticides, water chloridation systems and certain manufacturing processes.
Dr Brid Quilty of the School of Biotechnology has been looking at this problem for the last four years and currently has a group of seven postgraduates working with her. "We use microbes to degrade pollutants and the particular pollutants that are of interest to us are toxic chemicals," she said.
She was looking for an organism that would digest monochlorophenols, possibly as an added microbe to existing sewage waste digesters.
The first digester which used microbes to process sewage waste was developed in Britain in 1914, she said. Also known as an activated sludge system, it uses aerobic microbes that break down the waste in the presence of oxygen.
"Many toxic chemicals are not allowed into these systems because they tend to upset them," she explained. "Halogenated compounds tend to be more toxic to these organisms."
One way round the problem is to introduce a new bacterium that can deal with the toxic substance, and Dr Quilty and her group located a remarkably willing bacterial worker in a laboratory in Italy. A researcher there had identified a microbe that could deal with not one, but all three of the monochlorophenols of interest to the DCU group. "We were fortunate to get a bacterium from an Italian scientist that can degrade all three."
Dr Quilty acquired cultures of the bacterium, a Pseudomonas, and began work to optimise its ability to break down the chemicals. Helping the bacteria to do their work can be achieved in several ways, by modifying the oxygen content of the waste, adding nutrients to encourage the microbe and changing the mix of other bacteria living in the digester.
The group found that the Pseudomonas under study responded very well to a particular blend of several different bacteria. "The microbial composition is more important than other factors for this bacterium and there seemed to be a synergistic effect if the mix was right," she said.
The mixed cultures performed markedly better on the monochlorophenols than pure cultures of Pseudomonas on its own. "The results are looking quite interesting," she said, and they could have market potential in an activated sludge digester capable of handling waste streams with these chemicals.