Benjamin: “Just how do you mean that sir?”
Some of you perhaps remember that old quote from the file The Graduate when Mr. McGuire clues in Dustan Hoffman’s character to the potential of plastics. Although you may have noticed that I made a little substitution above: plastics became crap. That’s right, it’s time to talk about what you flush down your toilet and what biotech can do with it.
I was just reading my latest copy of the Journal of Industrial Microbiology and Biotechnology when a paper title caught my eye: “Molecular insight into activated sludge producing polyhydroxyalkanoates under aerobic-anaerobic conditions.” (1) I read the article and it was mainly about how the authors are using real-time Polymerase Chain Reaction and whole genome shotgun sequencing to identify the bacterial strains (there are several) that operate in cleaning up sludge in treatment plants. I did some more reading though because the whole idea fascinated me. It seems that modern sewage treatment plants use a process known as EBPR (Enhanced Biological Phosphorus Removal) where the sludge is cycled through aerobic and anaerobic conditions. In the anaerobic stage (absence of oxygen or nitrates), bacteria known as PAOs (Polyphosphate Accumulating Organisms) take up large amounts of phosphates removing that pollutant from the waste stream. The excess biomass can then be removed and used for fertilizer or whatever. It is interesting to note though that in this anaerobic phase, the PAOs accumulate and store energy as polyhydroxyalkanoates (PHAs) which they then utilize later during the aerobic phase for growth. PHAs are a class of biodegradable plastics that are coming into industrial scale production. Currently they are produced mainly by recombinant bacteria (E. Coli) fed with corn sugars. The Mirel brand co-owned by Metabolix and ADM is the most successful and largest supplier of PHAs. Metabolix has also done some work in engineering the PHA gene into plants. These approaches though have several drawbacks. The recombinant bacteria approach requires a sterile environment for production to avoid contamination from wild strains and it is expensive. The plant based one would of course require the use of crop land taken away from food production.
Now since the strains of naturally occurring bacteria used in sewage treatment produce PHA as part of the process, why not just make a biodegradable plastics factory as part of the sewage treatment process. This solves several problems with one solution. It cleans waste streams and makes a renewable, biodegradable source of plastic in one process. Why is this not done already? It is because the amount of PHA produced is not cost effective. To be cost effective, it has been estimated that the PHA producing process needs to result in a final biomass that is 80% PHA by weight (2). Recombinant E. Coli is right around there at 76% PHA by weight. The natural bacteria used in sewage treatment only achieve 30% PHA by weight. So this process is no where near cost effective or competitive with recombinant techniques. There seems to be a huge room for improvement however. Changes in the process alone could boost PHA production significantly. Several papers exist with experiments in sludge hold times and adding acetate to the sludge to boost PHA output. The bacteria themselves have really never been studied or even specifically identified. Perhaps with future process improvements and optimization of the bacterial strains used in the EBPR process, our sewage might be used for production of renewable, biodegradable plastics.

(1) Molecular insight into activated sludge producing polyhydroxyalkanoates under aerobic-anerobic conditions. S. Ciesielski, T. Poloj, E. Kilmiuk, Industrial Microbiology and Biotechnology, 2008, 8, 805.
(2) Production of PHA by activated sludge treating municipal wastewater: effect of pH, sludge retention time (SRT), and acetate concentration in influent. A. Chua, H. Takabatake, H. Satoh, T. Mino, Water Research, 2003, 37, 3602-3611