Persistent Organic Pollutants POPs in Biosolids

Research on POPs is really popping.  We are learning at an accelerating rate of the fate of “persistent organic pollutants,” or the POPs, flushed down home drains and treated in systems where biosolids are destined for farm soils.

Our POP research rate may not be fast enough for some, as I learned very recently. David Lewis, long-retired from EPA but not from anti-biosolids activism, has posted to the United Sludge Free Alliance his recommendation for a new EPA Clean Soil Standard. I was reminded of just how “chemophobic” or “chemonoic” some people can be.  Dr. Lewis’s report would have you believe the threat from POPs is significant and growing. It is neither, and accumulating research shows this to be true.

We are not only learning about POPs, we are reducing POPs.  Back in the summer, the FDA issued a rule banning triclosan, triclocarban and other antibacterial POPs found in soaps and toothpaste.  We learned from Dr. Arjun Venkatesan, Arizona State University, in his presentation to the MABA Annual Symposium, that this action will remove more than half of the POPs that find their way into biosolids.  

Dr. Venkatesan has a unique vantage point to see POPs in biosolids. He is principal author of 7 science reports stemming from exhaustive analyses of the U.S. National Sewage Sludge Repository. The repository is a collection of representative biosolids samples, originating with three national surveys conducted by the US EPA two decades back, and now under the direction of Dr. Rolf Halden, Center for Environmental Security. Dr. Halden is “a noted expert in determining where in the environment mass-produced chemicals wind up…” and that expertise encompasses biosolids.

Dr. Halden has recently made the point that widespread use of industrial chemicals ensures that new questions about these chemicals will arise continually and science will be ongoingly playing catch-up to understand their potential effects on human and environmental health. Dr. Rolf Halden described the generation-long process well in Epistemology of contaminants of emerging concern and literature meta-analysis.

As environmental stewards, we biosolids practitioners necessarily need to support sound scholarship into POPs.  The chemical and biological matrix in which POPs occur is highly complex and reactive, and the science will be deeply complicated.

The European Union seems to lead the way in addressing the fate of POPs.  It has for two decades guided the testing for degradability of chemicals (OECD Degradability Testing) with a guidance document listing 7 types of tests. One test deploys a “301C sludge,” an “activated sludge precultured with synthetic sewage containing glucose and peptone.” Another is the 314 test which simulates transformations of chemicals in a sewer system.

In the US, the Water Environment & Research Foundation has led the way in POP research.  Currently at work is Dr. Drew McAvoy, the principal investigator of the fate of flame retardants and two antibiotics in biosolids. His project on  “Priority Trace Organics” appears right above “High Quality Biosolids from Wastewater” on WE&RF’s list of Recent Contract Awards.  

Those of us with an eye on biosolids may not be fully aware that POP degradability is being studied in different portions of the wastewater system.  These include the sewer system, the activated sludge process, nutrient removal processes and several kinds of digestion. Researchers are learning that unique microbial communities at work in each step can be important to the attenuation of POPs.

If we still believe the sewerage system is a mere collector, how wrong we are!  One paper shows that “biodegradation in the sewer has a substantial impact on levels of surfactants and surfactant metabolites that ultimately reach wastewater treatment plants” (Biodegradation of nonionic and anionic surfactants in domestic wastewater under simulated sewer conditions). Another researcher, hoping to track illegal drug use, complained “in sewage epidemiology, it is essential to have relevant information of the sewer system” (Effects of sewer conditions on the degradation of selected illicit drug residues in wastewater) .

The fate of POPs within the treatment plant has proved enormously difficult to characterize. Classes of compounds degrade through different mechanisms, with water solubility being a key discriminator for biosolids-borne or effluent-borne POP discharges.  Plant configurations vary, with the cycling of aeration, anoxic and anaerobic processes apparently having great influence on POP degradation. One article pointed out that “biotransformation parameters are impacted by in-situ carbon loading and redox conditions (Factors impacting biotransformation kinetics of trace organic compounds in lab-scale activated sludge systems performing nitrification and denitrification).   An early review of this topic, Refinement of biodegradation tests methodologies and the proposed utility of new microbial ecology techniques, explained: “We were also able to compare various processes and pointed out activated sludge with nitrogen treatment and membrane bioreactor as the most efficient ones.”  It seems we may one day learn how to design our treatment plants specifically to increase POP degradation.  Hold that thought!

Our choice of biosolids stabilization technologies makes a big difference. I won’t get into the effects of digestion here, which are significant in their own right, but I will admit that, for POP reduction, composting is one of my favorite stabilization methods. A large body of research shows how robust composting is for POP degradation.  The abundance of compost/POP research is in part because composting applies not only to biosolids but to the much vaster supply of animal manures, which like biosolids contain POPs. The good news is that fairly rudimentary composting techniques yield good results:  “low-level manure management, such as stockpiling, after an initial adjustment of water content may be a practical and economical option for livestock producers in reducing antibiotic levels in manure before land application ( Antibiotic Degradation during Manure Composting).

Compost/POP research also stems from the concern that compost is a consumer product, with a direct exposure pathway to humans. But compost has consistently shown strong results in mitigating risks.  For one paper (Organic Micropollutant Degradation in Sewage Sludge during Composting under Thermophilic Conditions)  concluded “concentrations of all 12 micropollutants decreased during composting, and degradation was statistically significant for 7 of the 12 micropollutants.”

Much of our Biosolids/POP research has been with the pathway of biosolids to soil to micro/macro fauna (including humans). Understanding the gaps in the research record on these pathways was an early WE&RF concern.  Trace Organic Chemicals in Biosolids-Amended Soils: State-of-the-Science Review provided the backdrop to the work that Dr. McAvoy now has underway.  

For the most part, the research record for many types of biosolids-borne POPs re-affirms the capacity of the land treatment system, consisting of soil minerals, organic matter and in synergistic relationships with microbial communities, to trap and degrade a wide range of POPs.  From among many dozens of recent journal articles in this domain, one review article, Plant uptake of pharmaceutical and personal care products from recycled water and biosolids: a review, concluded: “Field studies showed that the concentration levels of PPCPs in crops that were irrigated with treated wastewater or applied with biosolids were very low.”  In another pertaining to potential human health risks, “our assessment indicates that the majority of individual PPCPs in the edible tissue of plants due to biosolids or manure amendment or wastewater irrigation represent a de minimis risk to human health” ( Human health risk assessment of pharmaceuticals and personal care products in plant tissue due to biosolids and manure amendments, and wastewater irrigation ).  Nevertheless, in recognizing the enormous complexity of biological and chemical systems, “highlighting the significance of contaminant and soil properties in influencing risk assessment,” we must inevitably turn to risk modeling: A quantitative risk ranking model to evaluate emerging organic contaminants in biosolid amended land and potential transport to drinking water.  

Modeling may seem to be the poor cousin to comprehensive analytical investigations, but it may be the great way forward. Imagine a future in which you “dial-in” options for source control, sewer maintenance, in-plant processes, biosolids stabilization methods and land treatment protocols, all of which, when taken together, accomplish nearly complete removal of POPs from pathways of human and environmental exposure. To accomplish so grand an endpoint, however, means making POP degradation an intentional and central goal of biosolids management, not an incidental consequence.

How about we ”’POP’ the clutch” and accelerate our research into the fate of POPs through the entire treatment system and DRIVE THE POP OUT OF BIOSOLIDS.