SUMMARY: ORGANIC CHEMICALS
Chemicals in commerce represent about 90,000 specific organic chemicals that are candidates for societal wastes. If plastics/polymers are excluded, there are relatively few organic chemical classes that are persistent in terrestrial systems of land application. Biodecomposition is the major assimilative pathway for organic chemicals in reuse land application systems and thus is a priority topic for investigation and understanding in order to meet environmental goals for land application systems. The rates of biodecomposition of parent compound and intermediate product(s) are necessary to determine the quantitative assimilative capacity of a land application system. Soil incorporation of waste/chemicals is generally found to enhance biodecomposition. A comparison of the biodecomposition rate with the application (or loading) rate must be made in order to ensure that sufficient land will be utilized to safely assimilate the waste and chemicals of concern (COCs). Most important for evaluation of the beneficial reuse of organic chemicals and risk assessment is the condition where application rate exceeds the assimilative rate for a given COC. Under this condition, scientific studies are needed to assure acceptable risks and reuse objectives.
An adequate risk assessment of all COCs is critical for the safe and effective design, operation, and management of a sustainable land application system. A systematic approach is necessary to identify COCs among the approximately 90,000 organic chemicals in commerce. Information needed includes: toxicity and dose response; transport potential; environmental stability; analytical capability in the matrix of interest; persistence in waste streams; plant uptake; and environmental fate. A life-cycle analysis is recommended for COCs and should include residuals resulting from product use and disposal, and predicted behavior in land application systems. However such information for the majority of chemicals in commercial use (estimated at 90,000) has not been adequately developed. There is a need to identify a subset of chemicals that are relevant for consideration for land application system (based on mass production, toxicity, and presence in residues applied to land treatment systems). Procedures for the judicious selection of surrogate chemicals to represent classes of organic chemicals with regard to persistence, toxicity, potential migration, or plant uptake aspects of risk assessment need to be identified, promulgated, and applied. Data gaps that are identified for surrogate chemicals need to be addressed. When potential risk is predicted to be unsatisfactory, then source reduction, pollution prevention, and substitute chemical approaches are recommended to reduce the amount of chemical or residue that will be produced.
Real world performance of full-scale land application systems needs to be better documented. Specific information that is needed includes biodecomposition, runoff (field rainfall simulators are useful tools), leaching/downward migration, climate effects, and plant uptake. From the very limited amount of information available concerning leaching and runoff from land treatment systems, risk pathways do not indicate that this a a major limitation. However additional specific information is needed and, to accomplish this, field monitoring methods and instrumentation that provide more rapid, less expensive, and more accurate information are required. The inclusion of short-term bioassays may be appropriate to indicate the relative decrease (or increase) in toxic response to a test organism as a function of treatment time and/or method. Predictive models need to be developed that incorporate site, climate, waste, and chemical information that are useful for application to the design, operation, and monitoring of reuse land application systems. Especially important is the performance of land application sites that have a history of use for non-hazardous waste assimilation, and relevant and appropriate control sites.
Comparison of the fate of specific organic chemicals present in biosolids, effluents, composts, manures, etc. is influenced by the waste organic matrix that affects soil water, plant uptake, and partitioning of chemical among soil phases (air, aqueous, solid, SOM, and NAPL). Information is needed to address the effect of waste matrix on the fate and behavior of individual organic chemicals with regard to biodegradation rate, sorption, volatilization, and plant uptake in waste/soil systems. Although some experimental information currently exists, there is not sufficient information to determine the effect of waste matrix on the soil/waste treatment mechanisms of biodegradation, sorption, volatilization, and plant uptake.
Because biodecomposition is the major assimilative pathway for organic chemicals, including pharmaceutical, household, and surfactant chemicals, and understanding of the mechanisms, pathways, fate, and control approaches is critical for the safe and effective utilization of reuse land applications systems. Information is necessary to describe and measure bioavailability, humification, metabolite generation and fate, potential for reverse transformation to a parent compound, and the effect of aging on the biological binding of organic chemicals to soil organic matter, including microorganisms.
Since the major mass of non-polar persistent organic pollutants (POPs) initially resides in the soil organic matter (SOM) phase of a soil system, the SOM controls the amount and percentage of POP in the water and air phases in a soil system that contains solid, aqueous, and air phases. When a non-aqueous phase liquid (NAPL) is added to the system, for example, oil, solvent, or liquid hydrocarbon, or an exogenous solid or semisolid is added, for example, manures, sludges, composts, etc., then the NAPL or solid/semisolid phase generally dominates with regard to associated POP. The chemical, physical, and biological characterization and reactions of SOM and non-aqueous phases within a land application system need to be better understood in order to determine the influence of non-aqueous phases on biodegradation, bioavailability, and humification of POPs. The phenomenon of bio(re)generation where, for example, 4-nonylphenol (4-NP), a breakdown intermediate from polyethoxylates, is produced and has potential biological activity as an endocrine disruptor needs to be assessed for COCs. 4-NP is one of the most frequently detected organic chemicals in biosolids at concentrations as high as 1400mg/kg. 4-NP, ethynylestradiol, estradiol, and estrone demonstrate hormonal activities. 4-NP is a biodegradation metabolite of the surfactant nonylphenolethoxylate (NPE), and is more persistent and less polar (less soluble) than the parent compound. The fate of 4-NP in soil/waste mixtures is of environmental concern, especially the biodegradation rate and the potential for plant uptake. The major soil fate pathway for 4-NP appears to be mineralization, and for the steroidal estrogens the major pathway is formation of non-extractable soil-bound residues. These issues related to biodecomposition are especially important with regard to long-term loading of POPs and the goal of sustainable reuse land application.
There is also a need to characterize and evaluate waste composting as contrasted with land application, as well as the sequential utilization of composting following by land application. Composting and land application processes are not identical, have different design, operation, and monitoring criteria, and different requirements for performance. Guidance on the utilization of these two technologies and the incorporation of composting into the land application process for long-term sustainable reuse is needed.
There is a need for POP chemical information related to bioaccumulation with regard to (1) terrestrial plant uptake, (2) transport to surface water with eroding soil particles and exposure to aquatic life, (3) accumulation in soil fauna, e.g., earthworms and transfer to birds during feeding. Bioaccumulation will depend on fate and bioavailability in a soil environment, and the rate of biodegradation. Kow cannot be used as a predictor of bioaccumulation for compounds that transform because the enzymatic reaction of a compound is not related only to hydrophobicity.
Finally, the limited technology choices for biosolids and other wastes means that acceptance of some risks and the occurrence of some benefits will continue to characterize land application practices. Within the context of waste management, including landfill, incineration, and beneficial reuse), land treatment contributes directly to emerging goals of sustainability.