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Nanoparticles and Biosolids
Greg Kester, Biosolids Program Manager,California Association of Sanitation Agencies, March 11, 2013
Several recent articles have appeared on nanoparticles (silver, zinc, and cerium) that intimate nanoparticles would have adverse effects on plants grown in biosolids-amended fields. I checked with nanoparticle expert colleagues at US EPA and have reviewed the papers in question as well as several co-authored by US EPA folks. Two of the papers are credible (both by Lombi et al) and two are flawed (Coleman et al., Priester et al)). My summary of them follows:
The Colman paper (“Low Concentrations of Silver Nanoparticles in Biosolids Cause Adverse Ecosystem Responses under Realistic Field Scenario” ) mixed nanoparticles with biosolids just prior to land application. Therefore, the particles did not undergo any typical sludge treatment and did not become an integral component of the biosolids matrix. This is somewhat analogous to the old flawed metal salt pot studies used as the basis for the originally proposed 503 rules. Behavior is very different when the nanoparticles are subjected to actual sewage sludge treatment as they would be when entering a wastewater treatment plant.
The Priester (“Soybean susceptibility to manufactured nanomaterials with evidence for food quality and soil fertility interruption” ) research was essentially a salt effect study in which the researchers added nanoparticles (ZnO and CeO2) directly to soil without any biosolids and erroneously reported observations as though they would hold true when introduced within a biosolids matrix. The researchers hypothesized that nanoparticles would wind up in soil through biosolids application or from diesel exhaust (for CeO2) but discounted any changes that would occur to the nanoparticles by going through a treatment process and becoming an inherent part of the biosolids matrix. Furthermore, the researchers added unrealistically high concentrations of nanoparticles (up to 1,000 ppm CeO2 and 500 ppm ZnO) in order to see any negative response.
Two papers by Lombi et al, (“Fate of Zinc Oxide Nanoparticles during Anaerobic Digestion of Wastewater and Post-Treatment Processing of Sewage Sludge” and “Transformation of four silver/silver chloride nanoparticles during anaerobic treatment of wastewater and post-processing of sewage sludge” ) subjected nanoparticles (zinc and silver) to bench-scale anaerobic digestion and composting or long term storage. These papers found that all nanoparticles studied converted through treatment to silver or zinc sulfides which are much less plant available, non-reactive, and much less toxic. Interestingly, one of the authors of the Coleman paper (Kim et al 2010) evaluated the biosolids from the wastewater plant with the highest silver concentration (856 mg/kg) from the US EPA Targeted National Sewage Sludge Survey (2009) and found all sllver was present in the form of silver sulfide.
While more work will always be recommended on these emerging constituents, the papers getting the headlines do not merit undue concern, and the earlier papers represent a much more robust experimental design and pertinent and valid conclusions. This really highlights the need for the wastewater profession to review and evaluate all such claims of adverse effects from constituents in biosolids and to communicate it effectively. Surprisingly, it seems that peer reviewed publications are no longer held to the standards as was historically the case.
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