Polymers – Just Hang Out
Sometimes instead of asking about those exotic parts per billion contaminants, people ask about the mundane stuff. Recently here in the Pacific Northwest, the new owners of the tree farm that uses biosolids to help trees grow bigger and taller asked what happened to the polymers used to de water the biosolids. When that question reached me, the best I could come up with was ‘humina humina humina’-- I had no idea. The different polymers used are proprietary and so it is difficult to study the fate of compounds known by brand names and not structural formulas. Luckily, Kate Kurtz from King County stepped in. She tracked down a researcher at USDA that was able to provide some information. The articles he shared with Kate constitute the library this month.
The short answer is that the compounds that make up the core of the polymers used for dewatering are the same as other compounds that have very broad applications. They consist largely of polyacrylamides or PAMs. These are used extensively in agriculture and in other industrial applications, typically to hold water and clean water. They stop working after a year or so in the environment, break up a little bit, likely don’t fully degrade and don’t do anything. In other words, polymers are nothing to be concerned about.
If you want more details, we have 5 papers for you. The first paper is a summary of uses of PAMs in agriculture and other applications. These compounds have been around for several decades and are typically used to prevent soil erosion associated with irrigation by improving infiltration and limit surface sealing. The second paper provides the chemical structure of the basic unit, talks about the different uses some more, and about how they don’t go away very quickly. The third paper tests to see if composting will help them degrade. Answer is not so much for the big compounds but that they should continue to degrade slowly in soils ‘leaving no toxic residues’. The forth paper does some field sampling on these compounds. They hang out, they are used by fungi and bacteria in soils for a place to live, but they lose their ability to hold onto water. The final paper sets a degradation rate of just under 10% per year.
In other words, these dewatering polymers work great for what they are used for in wastewater and a broad range of other applications for a short time, hang out and serve as temporary housing in soils for the local residents and gradually go away. It is nice to have a clear and simple answer.
Sally Brown, University of Washington