Effects of Organic Chemicals in Sludge Applied to Soil

1. Introduction

Wastewater sludges contain a vast number of hazardous substances including high volume chemicals such as surfactants, which are used in commercial household detergents, and contaminants like e.g. phthalates, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs). In Denmark, the levels of xenobiotic chemicals in sludges have raised concerns about the current use of sludge and other organic waste products as fertilizers on arable land. The present knowledge on the behaviour of sludge-bound xenobiotic chemicals in the soil environment is relatively scarce. It is very difficult to assess the exposure of humans to contaminants in sludge-treated agricultural soil and the associated risk. Humans may be exposed to the more recalcitrant sludge-bound contaminants via, e.g., drinking water, crops, milk and meat. Data necessary to estimate the importance of these exposure routes, i.e. leaching of sorbed contaminants in soil, uptake of the contaminants in edible parts of plants, and uptake and metabolism of the contaminants in domestic animals, are very limited. Another objective is to ensure conditions for a sustainable land use by protecting the functioning of the soil system. Relatively taken, the organisms living in the soil receive the highest concentration of sludge-bound contaminants. The toxicity of sludge-bound chemicals to soil-living organisms has not been examined in detail, and the more abundant data on aquatic toxicity merely indicates inherent toxic properties of a chemical that might lead to toxic effects in soil. However, adverse effects of a toxicant in soil are only expected if the chemical is present in a form, which renders it available to soil-living organisms.

The Danish regulation of the application of waste products (Ministry of Environment and Energy 1996) defines certain cut-off values for the maximum concentrations of organic contaminants and heavy metals in sludge to be distributed on agricultural land. The organic chemicals included and the 1997-cut-off values specified by the Order are: Linear alkylbenzene sulfonates, LAS (2,600 mg/kg dry matter), nonylphenol including nonylphenol ethoxylates with 1 and 2 ethoxylate units (50 mg/kg dry matter), PAHs (total concentration of nine selected compounds, 6 mg/kg dry matter), and di(2-ethylhexyl) phthalate, DEHP (100 mg/kg dry matter). At the year 2000, the cut-off values will be reduced to 1,300 mg/kg for LAS, 10 mg/kg for NPE, 3 mg/kg for PAHs and 50 mg/kg for DEHP.

The purpose of this study was to examine the biodegradation and the toxicity to soil organisms of some of the chemicals included in the legislation described above. Biodegradation laboratory experiments were performed with intact sludge samples that were mixed with soil as this approach provides a realistic bioavailability of the sludge-bound contaminants. A parallel set of experiments were conducted with suspensions of sludge in order to examine the potential biodegradability of the chemicals at a higher concentration level. Tests focusing on the relations between biodegradation and toxicity of sludge-bound contaminants to soil-living organisms were performed by using the reproduction of the springtail Folsomia candida and bacterial nitrification in soil as the end points.