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Long-Term Assessments of Microconstituents Fate in Biosolids-Amended Soils

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Land application of biosolids is the most cost-effective option for many municipalities. Farmland application of biosolids is a common practice in the United States and worldwide as it provides tremendous savings in fertilizer costs to the farmers. However, concerns continue to be raised in the news media and by the public about the safety of this practice because biosolids may contain traces of pharmaceuticals and personal care products (PPCPs) that enter the wastewater stream from various domestic and industrial sources.

Large quantities of PPCPs are consumed in the United States and worldwide each year. In 2006, the United States and Canada alone accounted for about half of the worldwide sales of pharmaceuticals. Since the early 1960s, triclocarban (TCC) and triclosan (TCS) have been added as antibacterial agents to many consumer products such as hand soaps, toothpastes, creams, detergents, etc. Nonylphenol polyethoxylates, parent compounds of 4-nonylphenol (4- NP), have been in use for more than 40 years as detergents, emulsifiers, wetting agents, and dispersing agents in many household and industrial products. In the early 1970s, the use of flammable materials in toys and electrical equipment and synthetic fibers in sofas and curtains led to the widespread use of polybrominated diphenyl ethers (PBDEs) as flame retardants.

A large portion of PPCPs used by humans ends up in wastewater through washing and direct disposal or via excretion with urine and feces either as metabolites and conjugated compounds or as parent compounds. Although a majority of the PPCPs that enter the wastewater stream are destroyed in sludge processing, some recalcitrant PPCPs and their metabolites pass through the treatment process intact. A majority of these organic contaminants are lipophilic, show high affinities for organic carbon and, thus, preferentially partition into biosolids.

Many commonly used PPCPs and their metabolites are frequently detected in biosolids worldwide. The levels of 4-NP, TCC, and TCS in anaerobically digested biosolids in the U.S.ranged from 4.8 to 1388 mg kg-1, 0.23 to 80 mg kg-1, and 0.23 to 61 mg kg-1, respectively. Recently, the United States Environmental Protection Agency (USEPA) conducted a Targeted National Sewage Sludge Survey (TNSSS) and reported significant levels of PBDEs, TCC, and TCS in the majority of biosolids samples analyzed.

Results from short-term laboratory and field studies suggest half-lives of 4-NP, PBDEs, TCC, and TCS in biosolids-amended soils to range from less than a month to a few years. However, information on concentrations of these organic contaminants in soils receiving continuous and long-term application of biosolids is scarce. It has been well documented that diverse microbial communities flourish in biosolids-amended soils. We hypothesize that enhanced biological activity in soil following long-term application of biosolids promotes degradation of recalcitrant PPCPs in biosolids-amended soils.

We evaluated concentrations of 4-NP, total PBDEs (?PBDEs - sum of BDE-47, -99, -100, -153, and -154), TCC, and TCS in soil samples from long-term field plots that received annual applications of biosolids from 1973 to 2002. The annual application rates were 0, 16.8, 33.6, and 67.2 Mg dry biosolids ha-1 and the cumulative biosolids loadings were 0 (Control), 504, 1008, and 2016 Mg dry biosolids ha-1.

All four organic contaminants evaluated in this study were detected at μg kg-1 levels in the biosolids-amended soil. The concentrations of contaminants in the surface soil layer (0 – 15 cm) increased with increased cumulative loadings of biosolids. However, the soil concentrations of all the contaminants decreased sharply with depth, and the concentrations in soil samples from the 30 – 60 cm and 60 – 120 cm depths were identical, regardless of the cumulative loadings of biosolids. The differences in soil concentrations of these compounds among various cumulative loadings of biosolids were more pronounced in the top 0 – 15 cm soil layer and less pronounced in the 15 – 30 cm soil layer. Because of the high levels of cumulative loadings of biosolids in the amended plots, approximately the top 0 – 30 cm of soil is dominantly comprised of biosolids.

It could require over 100 years to reach the cumulative loadings employed in this study if the biosolids were applied at the USEPA recommended agronomic rates.

At the greatest cumulative loading of 2218 Mg dry biosolids ha-1, the concentrations of TCC, TCS, total PBDEs, and 4-NP in the 0 – 15 cm soil samples were 1251, 52, 658, and 8834 μg kg-1 dry soil, respectively. Contaminant soil concentrations in the 15 – 30 cm samples were much less: at least in the soil surface TCC = 371, TCS = 25, total PBDEs = 105, and 4-NP = 1840 μg kg-1, and the concentrations in the 60 – 120 cm samples were even less: TCC = 23, TCS = 19, total PBDEs = 4.2, and 4-NP = 68 μg kg-1. Similar decreasing concentration with depth trends occurred in other plots receiving cumulative loadings of 554.5 and 1109 Mg dry biosolids ha-1. The data suggest limited mobility of biosolids borne TCC, TCS, total PBDEs, and 4-NP in biosolids-amended soils.

Although the concentrations of, TCC, TCS, 4-NP, and total PBDEs in soil were greater in the biosolids-amended plots than in the Control plots, the contaminants had no detrimental effects on the soil biota. Indeed, microbial community studies showed that the microbial populations were more diverse and much more biologically active in the biosolids-amended plots than in the Control plots.

Keywords: 4-NP; PBDEs; Pharmaceuticals and personal care products; TCC; TCS; biosolids; long-term fate

Document Type: Research Article


Publication date: 2009-01-01

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