Last spring I got a call from someone very concerned about PCBs in biosolids in Spokane, WA. Spokane, WA, is a fine town, albeit a little slow on the trends for my NYC blood. Last I heard about Spokane’s biosolids program, all of its material was going to fertilize dryland wheat -- a great use for biosolids and one this library has looked at in the past. When this guy asked about PCBs, I figured he just was slow on the trends; it will likely be a decade before he starts worrying about microbeads in Spokane. But as we talked more, it became clear that he was more up on the literature than I was. A recent paper in Environmental Science and Technology (#1 in the library) modeled sources of PCBs from aerial emissions in Chicago and concluded that biosolids drying beds operated by the Metropolitan Wastewater Reclamation District of Greater Chicago (MWRDGC) were a significant source.
Now, a few background details before we dive in here. PCB stands for polychlorinated biphenyls. That means that PCBs include a range of compounds, all with two phenyl rings and all with more than one chlorine attached to them. Depending on the number of chlorines attached, different PCBs have different properties and different toxicities. A subset of PCBs, notably the lower molecular weight ones, can be volatile or evaporate. These are legacy compounds that were used for industrial purposes. You would not find them in your shampoo or toothpaste. Because they were shown to bioaccumulate and harm sensitive species, PCBs were banned from use before the 503s were promulgated. As a consequence, concentrations of PCBs in biosolids have been decreasing over time, as has been also seen with a range of banned compounds and metals such as lead.
The second background detail is that the MWRDGC started moving towards Class A treatment processes with a portion of its biosolids a while back. It was able to achieve Class A pathogen destruction requirements by drying biosolids over time in large drying beds. In about 2005, MWRDGC was drying about 10% of its total production or 20,000 tons. More recently it has started composting the biosolids with wood waste from the City.
Knowing the background details, I started reading the recent paper. According to Rufus Chaney, the authors are well respected and typically do a good job. For this, they considered a broad range of sources of PCBs and used old literature references for the value they used for the biosolids emissions. If you read their methods section, it appears that they believed MWRDGC dries the vast majority of biosolids in the drying beds, not 20%. That is false assumption. Second problem was that they didn’t measure the PCB types or concentrations in MWRDGC’s biosolids. Instead they assumed that the human body burden of PCBs was what would be found in the waste.
The researchers had two other problems (or should I call them alternative facts?). First is that is that they are assuming the body-burden of PCBs reflect the MWRDGC PCBs, discounting the importance of residual sources that would show up in both flow from any manufacturing and from storm water. The second is that the data they are using was published in 2003 from samples collected earlier (Paper #2). So, let’s take this one step back and look at the researchers’ original sources.
It turns out that Paper #2 took air samples of PCBs and also biosolids samples. In their paper’s introduction, the authors state that the biosolids contained 1.4 1 mg/kg PCB. This was from Webber et al., a paper from 1994. Air sampling according to the table (2) in the paper seems to indicate that the biosolids drying beds were a source of PCBs. The authors point out that, for one of their sampling points, the wind directions were backwards, such that the downwind sample with the higher PCB value was actually an upwind sample. The other samples do show enrichment potentially from the drying beds. Their biosolids samples showed PCB concentrations ranging from 0.6 to 1 mg/kg, what they call similar to the results from MWRD of 0.48 mg/kg. They also measured volatilization of PCBs in the lab from a biosolids sample and say that it is right in line with the aerial samples that they observed. Sounds somewhat reasonable, except wait.
The comment on the paper (#3 in the library) says not so fast with that assumption. The comment, written by staff from MWRDGC, who admittedly had reason to pay attention, makes some excellent points. The first is that the grid used by the authors of paper #2 to identify likely places to sample was on much too coarse a scale to pick up a drying bed that is 0.1 square miles in area. They also point out issues with how the PCBs were analyzed, done in a way to include both volatile and non- volatile congeners, thereby artificially inflating the value. Issue was also raised about the likely binding of PCBs to the organics in the biosolids that would also reduce the potential to volatilize. They also raise issues about the problems associated with extrapolating from a petri dish in the lab to a program that produces 150,000 dry tons of material. All sound valid to me; sounds like you need more data. And, so, we go to paper #4.
The authors of Paper #4 do not work for the MWRDGC. They noted elevated air concentrations of PCBs in urban portions of Chicago and did a detailed sampling to determine if the biosolids drying beds were the source. This included short-term air sampling and flux chamber measurements. A total of 60 ambient air samples, 20 biosolids samples and 2 intensive sessions (3 day and 3 might) of flux chamber measures on the drying beds were conducted. While the concentrations of PCBs in the biosolids was higher than in paper #2, they detected no significant fluxes and concluded that the drying beds were not a significant source of PCBs to the air in Chicago. In other words, the authors of Paper #1 made some assumptions, as likely did the authors of Paper #2, that resulted in them placing a portion of the blame of elevated PCBs in the air in Chicago on the biosolids drying beds. These are pretty much addressed in Papers #3 and #4. More data and fewer assumptions give better results. And that leads us to Paper #5.
Paper #5 is an oldie but goodie, authored by Chaney, Ryan and O’Connor. It goes over risk assessment for organics in the part 503 and why they aren’t a big deal for biosolids. Much of the focus here is on historic organics like PCBs. Basically, no pathway exists for these organics to cause harm, and their concentrations are too low to be relevant. So, if you live in Spokane, or elsewhere, and are worried because of paper #1, take a deep breath and relax. It is not the biosolids. Most places don’t have large drying beds, and, even if they did, the PCBs do not volatilize from the biosolids in significant quantities. Most likely, PCBs are no longer even presents in significant quantities. Now, don’t get any ideas about asking me about microbeads.