To view the article abstracts from this months research update follow this link: APRIL 2019 RESEARCH UPDATE.

The Biosolids and the Bees

Sometimes, rarely, but sometimes, I am grateful for Rolf Halden.  Trying to decide what to do for the Research Update this month, browsing through journals, but not getting inspired, I opted to do a Google Scholar search to see what Halden has been up to.  And, sure enough, he delivered. 

We have all heard about bees dying.  Nobody likes dead bees.  Everyone understands the critical role of pollination in us getting to eat.  Likely many of us were upset that the EPA refused to ban the pesticides that have been responsible for bee death.  But Rolf Halden is the one we can count on to tie this tragedy to biosolids.  It turns out that Fipronil is one of the pesticides responsible for bee mortality.   This library will take you through the basics of Fipronil and then its behavior in wastewater treatment, reclaimed water and finally biosolids.  How appropriate to have a library about bees as we welcome spring.

The first article goes into details about the relationship between bee mortality and this pesticide.  They note that mass bee deaths started occurring in France during the 1990s, at the same time that two new insecticides were introduced: Imidacloprid, a neonicotinoid, and fipronil.  Both compounds are added as seed dressings, are then taken up by the plant into leaves and flowers.  The bees are exposed when feeding on the flowers of treated plants.  The concentration of this pesticide in the nectar and pollen is typically under 6 ppb which is below the LD50, more like the LD5.  The authors explain that imidacloprid does not bioaccumulate in the bee tissues.  In contrast, fipronil has a much higher toxicity and also bioaccumulates.  Both of these factors contribute to the observed carpet of dead bees that has been observed and both point the finger at fipronil as the likely culprit. 

Fipronil is an insecticide used on seed coatings.  Next step is to see what happens to it in agricultural soils.  For that we go to the second paper.  Here the authors look at, first, how to analyze soil and plant tissue for the stuff, and, second, they look at the fate of the compound and its metabolites in soil and plants.  The authors start by talking about how useful this compound is.  It targets the GABA receptor which controls how chloride ions get in and out of nerve cells.  It kills lots of different kinds of bugs, including those that suck and chew (that comes into play soon).  This study was written before the bee paper and notes that the compound can potentially harm birds, fish and marine invertebrates too.  Metabolites of the compound include fipronil-desulfinyl (stable and more toxic), produced through interaction with light, and sulfone and sulfide versions of the parent compound that are produced by biological transformation.  These are also more toxic than the original compound.  Here is the chemical structures:

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The study found that the compound tends to decompose more quickly in higher pH soils with a significant portion of the decomposed product transforming into fipronil-sulfone.  Degradation was not complete for any soil for the 60-day period of the trial, though concentrations of the parent compound had significantly decreased.   The degradation on the peanut plant itself were much faster with less formation of metabolites.  At the end of the study, concentrations in the soil and plant with low to high application rates of the insecticide ranged from 0.014 to 0.15 mg/kg in the soil and 0.06-0.3 mg/kg in the peanut straw.  Concentrations in the peanut shell and kernel were much lower, but I bet that you could find some in your Skippy Smooth or Super Chunk.

Typically, agricultural pesticides and herbicides are only found at very low concentrations, if found at all, in municipal wastewater systems.  Use for agricultural crops means you find these compounds in farm fields more often than you do in urban areas.  This is where the sucking and chewing comes back to bite us, so to speak.  The fipronil, however, is also used to kill cockroaches, termites and ants and for flea and tick medication for dogs.  Concentrations in the topical stuff you use on your dog can be up to 10% fipronil.  Both of those end uses are highly common in urban areas.  In fact, we use the tick stuff for our dog every summer. 

The next three papers then follow this compound through the wastewater process.  The third paper is an older one (2015) that follows fipronil in the treatment plant.  Halden is a co-author on this, and the plant that was studied was in the SW U.S. and included a wetland treatment system.  The paper starts by noting that the compound and its metabolites are found pretty much wherever people look for it.  A survey in Orange County, CA that tested urban waters found the compound in over 70% of the samples tested, at concentrations that exceeded the aquatic toxicity limits.  This study tested influent, effluent and biosolids at the plant with a detection limits 0.5 to 0.77 ng/L in water and 0.02 to 0.24 ng/g in the biosolids. 

Pretty much what came into the plant went out; no real change in concentration occurred during the treatment process.


Most of it went out in the effluent and didn’t partition to the biosolids. The biosolids-only pathway accounted for 9% of the total mass.

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The effluent that was discharged to the wetland for a final polishing did see removal of the compound and its metabolites.  It is not clear if the compound partitioned to the wetland sediments. 

The 4th paper measured concentrations in reclaimed water and effluent from treatment plants in North Carolina. The flea and tick season is longer in N.C. for dogs, and sure enough you can confirm that by testing the waters.  Much of the influent mass of the compound goes out in the effluent.  The authors were able to clearly see the impact in waters below effluent discharge points. 

The final paper is the Halden paper that got me started in the first place.  In this paper, Halden’s team sampled biosolids that had been archived over the course of 15 years.  A total of 109 biosolids were sampled collected from 2001 and 2015/6.  Both the parent compound and metabolites were measured in the biosolids.  The compound was found wherever they looked.  The concentrations ranged from 0.2 to 385 ug/kg or ppb.  There was more fipronil in the samples collected in 2006/7 and 2015/16 than in the samples collected in 2001.  Concentrations though have held steady in the period from 2006/7 and 2015/16.  They observed that people like their dogs and don’t like cockroaches; in other words, proximity to agriculture had no relationship to fipronil concentrations in the biosolids.  Treatment processes had no impact on the concentrations either, with the exception that anaerobic digestion resulted in a higher conversion to fipronil sulfide. The researchers also noted the prevalence of this compound in effluent.  To drive home the point, they approximated how much fipronil was present in all biosolids produced in 2015/16 at a total of 1140 kg, or a little over a ton.  To put that into perspective, one study I found said that sales of the two compounds discussed above, fipronil and Imidacloprid, made up about 30% of the total worldwide sales of insecticides in 2008, sales valued in the billions of dollars with quantities of compound in the thousands of tons. 

Clearly, biosolids are just a minor area of impact here.  That is not to say that this compound is innocuous.  To the contrary.  The clear approach, taken by many countries, is to ban the compound, not the biosolids.  Maybe sometime soon our EPA will have the courage to do the same.