W2170: Soil-Based Use of Residuals, Wastewater and Reclaimed Water
Statement of Issues and Justification
Millions of tons of residual by-products are annually discarded as municipal (biosolids, municipal solid waste), agricultural (manure) and industrial (various sludges) waste in the U.S. Many of these residuals are disposed of in landfills or incinerated at substantial cost to our industries and the public. Reuse of residuals as soil amendments or soil substitutes could substitute beneficial agronomic and environmental uses for disposal costs. Treated liquid wastes, such as wastewater effluent, recycled water and other non-potable water sources, also present opportunities for beneficial reuse in lieu of surface water discharge or expensive treatment.Key obstacles to beneficial reuse of residuals other than the strictly regulated biosolids and treated liquid wastes have been the lack of national and state regulatory frameworks to enable use of such by-products. The most pressing need is a risk-based protocol that allows decisions to be made on the potential risk posed by contaminants in residuals and their suitability for land application or use as soil blend components. Aside from biosolids, there is not a national framework to evaluate arsenic, lead, cadmium and other trace elements in residuals for soil applications.
Recycling of residual by-products requires practical scientific knowledge to determine if and how the residual constituents can be beneficially reused without impairing the environment (soil, water, and air quality), plants grown on the amended soils, or humans and animals that consume such food, feed, and water impacted by land application. There is increasing evidence that land application of a variety of residuals may provide agronomic and environmental benefits that were either not previously well understood and/or that are critical to addressing emerging environmental issues associated with climate change. The W1170 committee members propose to continue the investigation of biogeochemical cycling of plant nutrients, the movement of trace elements into the food chain, the potential toxicity of pollutants in residuals to the soil and water ecosystems, and the long-term bioavailability of trace elements in residuals and residual-amended soils to accrue further knowledge that will promote residual recycling practices that are protective of human health and the environment. The results of such research will provide information for continuing risk assessment of the USEPA Part 503 Rule for land-applied biosolids and regulations developed at the state level for land-based recycling of residuals and effluents. Additional research will address the field and global scale effects on soil quality, plant drought response, soil carbon sequestration, and greenhouse gas emissions associated with soil-based reuse of residuals and reclaimed water. W1170 members are conducting, on both short- and long-term application sites, research whose results will enable the development of recommendations for maximizing the beneficial use on land of a variety of residual by-products.
Justification: The beneficial use of residuals requires both an understanding of potential hazards and value of the constituents in the by-products. Investigation of the behavior of potential pollutants in residuals has been the focus of recent research of the W1170 (and its predecessors, W170 and W124) multistate workgroup. Much of the research has been conducted on the behavior of nitrogen, phosphorus, metals and organics in biosolids-amended soils. This research formed the basis for the U.S. EPA Part 503 sludge rule, which is one of the few rules to include bioavailability assessments in the development of appropriate limits for critical contaminants (National Research Council, 2003). The scientific basis for this rule and the risk assessment approach used were deemed to have been valid, but recent developments in risk assessment and criticisms expressed by some scientists necessitate that the limits for some existing regulated pollutants and pollutants for which no limits were previously established be revisited. Members of the multistate workgroups have been extensively involved in the development of the Part 503 regulation and continue to be involved in the promulgation of risk assessment for other residuals (e.g., the EPA risk assessment for land application of cement kiln dust) and for other elements not initially regulated in Part 503 (e.g., barium). The scientific approach that was initially used in the development of the 503 regulations by these research groups has been applied to an expanded variety of residuals, contaminants and receptors. As the understanding of bioavailability has expanded, the group has also broadened its focus to develop linkages between a quantitative understanding of the form of the contaminant and its bioavailability. Research has also been altered to measure effects of contaminants on a range of receptors.
Two examples illustrate the altered focus of the group and its suitability for research on these broadened topics: metal and phosphorus bioavailability. In the initial development of the 503 regulations the difference between total and bioavailable metal concentration was clearly illustrated through field studies that used metals added in biosolids rather than metals added to soils as salts to determine appropriate metal loading limits. With this research as a starting point, cooperative work within the group has changed its focus to develop a quantitative understanding of the behavior of metals in biosolids amended and other soil systems.
In recent studies, the role of organic and inorganic components of biosolids in metal binding has been defined through a combination of laboratory incubations, greenhouse studies, and the use of x-ray adsorption spectroscopy (Brown et al., 2003a; Hettiarachchi et al., 2003; Ryan et al., 2003, 2004a; Scheckel et al., 2004). Biosolids and other soil amendments have been used to reduce the bioavailability of metals in contaminated systems (Brown et al 2003b; DeVolder et al., 2003; Hettiarachchi and Pierzynski, 2002). Extracts to assess bioavailability for a range of receptors have been developed and linkages between mineral form of inorganic contaminants and bioavailable fraction have been made (Basta et al., 2003; Brown et al., 2004; Ryan et al., 2004; Schroder et al., 2003). As the potential for metals to affect a range of receptors is more fully understood, research has broadened to encompass a range of measurement endpoints. The goal of this research is to evaluate function of the restored ecosystem and utilizes tools such as in vivo and in vitro assays, toxicity assays, and measures of microbial function (Alexander et al., 2003; Basta et al., 2003; Brown et al., 2004b; Schroder et al., 2003). As a result of cooperative research conducted by members of W1170, alternative in situ remedial options have been included on a number of EPA Superfund National Priority List (NPL) sites. These include use of biosolids to rehabilitate metal contaminated ecosystems. The tools developed for this research have also been applied to gain a fuller understanding of the functioning of biosolids amended soils. The sustainability of biosolids application to agricultural lands has been demonstrated by evaluating the effect of biosolids application on soil function. Potential receptors have included earthworms and soil microorganisms. While important initial research has been done in this area and implications of this research are being recognized in the remediation of contaminated sites, this type of work is still at an early, developmental state.
Initial work on biosolids and nutrients focused on determining the plant available fraction of total N. Work was done to predict the mineralization rate of organic N over time, under different management practices, and in different soils and climate regimes (Gilmour and Skinner, 1999). Application rate recommendations for agronomic crops were based primarily on meeting the N needs of the crop. Determination of the plant available N (PAN) based on characteristics of the residual being applied is still a focus of research with better approximations potentially existing for different biosolids in comparison to manures (Andraski et al., 2000; Gilmour et al., 2003; Van Kessel et al., 2000).
Initially, there was very limited research on the availability of P in biosolids amended soils as the Part 503 regulations based agronomic loading solely on N needs and utilization potential of a crop. There is a growing body of research and regulation that considers P solubility and potential for runoff from different P sources, and many states have adopted P-based loading limits for biosolids, manures and fertilizers. In addition, the recently promulgated Confined Animal Feeding Operations (CAFO) regulations, and Natural Resource Conservation Service Code 590 Practice standards for Nutrient Management require consideration of P as well as N for utilization of manures and biosolids.
A large body of cooperative research, both in communication of information and collaborative laboratory and field studies is currently underway with members of the W1170 group to evaluate characteristics of biosolids and how they alter the phytoavailable fraction of soil P, use of residuals to reduce P availability in soils, and appropriate tools to measure excess P in soil and water systems. The effect of chemical characteristics of biosolids on P availability and evaluating P availability in a range of biosolids and biosolids amended soils have been areas of emphasis within the group (Brandt et al., 2004; Elliott et al., 2002; OConnor et al., 2004; Sakar and OConnor; 2004: Wagner et al., 2008). Use of residuals to reduce the availability and leachability of excess P in soils has also been a focus of the groups research (Codling et al., 2000; Dayton et al., 2003; Ippolito et al., 2003). This research has shown that characteristics of particular biosolids including high Fe and Al concentrations are able to reduce the solubility of biosolids P over both commercial fertilizers and animal manures. In some cases this reduction is sufficient to limit the utility of biosolids as a P source, even when applied to meet the N needs of a crop. Other byproducts, including water treatment residuals, have a very high capacity to adsorb P and, thereby, reduce its potential to run off a soil surface or leach through a soil profile. These results suggest that tools are available to reduce the environmental hazards associated with excess P in soils and that nutrient management plans need to take into account the source of P in developing land application recommendations. It is still necessary to develop a fuller understanding of the longevity of the P sorption mechanisms and the role of soil properties in altering P availability. Additionally, the likelihood of loss of soil particulate matter under different tillage operations and management controls is required to determine site-specific runoff risk.
A variety of strategies are being considered/employed to reduce P transport to surface and ground waters, including consideration of new best management practices (BMPs). One such BMP is to remove dissolved P from runoff water and leachate by the land application of P-sorbing materials rich in aluminum (Al) or iron (Fe) oxide. Drinking water treatment residuals (WTR) are often rich in amorphous Fe or Al oxides due to the use of Fe or Al salts as coagulants during drinking water treatment. Several studies (Gallimore et al., 1999; Ippolito et al., 1999; Wagner et al., 2008) have demonstrated that using WTR as a P sorbent (either surface applied, soil-incorporated, or blending with high P-containing residuals) is an effective BMP to reduce P loss from agricultural land.
Research efforts, led by W1170 researchers, have resulted in methods for evaluating the sorption capacity and methods to apply WTR to reduce P loss from agricultural land (Dayton and Basta, 2005a, 2005b). A major obstacle that needs to be overcome before this BMP is accepted into practice by the agricultural community is the accepted inclusion of P sorbent strategy into the P index framework. Specific information needed is the actual reduction of P loss under field conditions from application of a standardized amount of P sorbent. The use of residuals as sorbents to reduce P loss is a promising water quality protection technology, but further research is required to appropriately characterize the P sorbing capacity of many residuals.
Both these examples illustrate the change in the focus of the group to concerns on the bioavailability of constituents in residuals and residual-amended soils where excess or potential detrimental effects, rather than deficiencies, are the focus. They also reflect the change in emphasis from a plant to an ecosystem focus. It is also clear that this type of research is in its early stages.
In addition to the cooperative research areas described above, there are concerns on the fate of residuals-borne organic chemicals in soils. Classes of such organic micro-constituents include estrogenic compounds, personal care products, and pharmaceuticals (Topp and Clucci, 2004; Xia and Pillar, 2004). A recent report by USGS (http://water.usgs.gov/nawqa/) noted the presence of a wide range of micro-constituents in streams in the vicinity of waste water treatment plants and confined animal feeding operations. The fate and persistence of these compounds during biosolids stabilization processes or following land application of residuals and non-potable water is not known. Persistence in soils and potential for ecosystem effects as result of land application of residuals and non-potable water containing these micro-constituents will require research.
To adequately protect or restore soil ecosystems, it is necessary to accurately characterize residuals suspected or presumed to be contaminated with trace elements and define the levels of trace metals in residuals and/or residual-treated soil that constitute a hazard to human and ecological receptors. Most human and ecological risk is associated with trace elements that are biologically available for absorption or bioavailable to the human and ecological receptors. Therefore, risk-based soil screening levels should be based on contaminant bioavailability not solely on the total content of the contaminant in the residual or residual-treated soil.
Human and ecological risk assessment (HERA) science continues to mature with bioavailability-based risk assessment frameworks being developed and/or considered for implementation in the U.S., Canada, the European Union, Australia and other countries. Research is needed to provide the scientific basis for risk-based methods to evaluate residuals and residual-treated soils for adoption by HERA frameworks. Research needed to evaluate contaminants in residuals includes (i) trace element speciation by advanced spectroscopic methods and wet chemical speciation methods, (ii) in vitro methods correlated with human and ecological endpoints, and (iii) novel in vivo methods.
The importance of agriculture in climate change has been recognized (Smith et al., 2007). Agriculture is both a source of greenhouse gases and a sink for terrestrial carbon sequestration. Deforestation and conventional tillage practices were the largest source of CO2 release to the atmosphere until the 1970s (Lal, 2007). By incorporating oxygen into the soil during cultivation, conventional tillage has resulted in oxidation of fixed soil carbon and its release into the atmosphere. In addition, use of synthetic fertilizers, while improving plant yield, has also had a negative impact on climate change through release of greenhouse gases (GHG). The quantity of energy required to produce both nitrogen and phosphorus fertilizers as well as to transport fertilizers to the field can be quantified to determine the amount of GHGs that are released as a consequence of their use (Brown and Leonard, 2004). The use of synthetic nitrogen has also been associated with release of N2O, a GHG with approximately 300x the global warming potential of CO2. Altering agricultural practices to minimize oxidation of soil carbon and increase rates of sequestration in soils has the potential to reduce GHG. Understanding the factors involved in transformation nitrogen that lead to release of N2O, and understanding how to minimize losses of N as N2O also has potential to reduce GHGs.
Nitrogen availability in biosolids will vary as a function of the different biosolids processing technologies as well as different soil types and climates. Research is necessary to quantify N2O release from biosolids amended soils under a range of soil types, tillage practices and climatic conditions. It is likely that research on N2O release from biosolids amended soils may result in identification of appropriate management techniques to minimize release of this highly potent greenhouse gas.
Within this larger picture, use of biosolids both for the fertilizer value of the material as well as its carbon content has the potential to alter the dynamics of GHG balance in agricultural systems. The Intergovernmental Panel on Climate Change recognized the role of municipal biosolids and manures to substitute for synthetic fertilizers as well as a source of carbon in these systems (Stevens et al., 2007). Our new understanding of the role of soils in global climate change presents new opportunities and challenges for residuals management. Biosolids can be both a tool for increased carbon sequestration (via soil carbon additions and displacement of synthetic fertilizers) and a source of GHG emissions (via N2O emissions following land application). One of the goals of future research for the W1170 group is to determine appropriate management practices for biosolids to maximize the benefits associated with these materials and minimize any fugitive emissions using climate change as a focus.
Impending water shortages and scarcity are motivating renewed interest in the use of organic residuals and the intentional reuse of degraded waters (OConnor et al, 2008). Soil application of organic amendments for field crop production may have an ameliorative effect on drought-stressed crops. Sahs and Lesoing (1985) observed higher sweet corn yields in plots amended with beef feedlot manure than those that were inorganically fertilized during drought years. Heckman et al. (1987) found that field grown soybeans fertilized with sewage sludge had increased drought resistance and nitrogen fixation than the control treatment. Zhang et al. (2005) determined that biosolids subjected to various treatment processes enhanced endogenous antioxidant enzyme activity, photochemical efficiency, and drought resistance of tall fescue.
Expansion of irrigated agriculture is one important component to address the needed growth in global food production due to population increases (Jury and Vaux, 2007), but additional sources of fresh water for irrigation are limited. While the application of treated or partially treated wastewater effluents to cropped and forested lands has long been practiced, other sources of degraded water (stormwater, irrigation return flow, graywater, and concentrated animal feeding operations [CAFO] effluents) are available to meet anticipated shortfalls. Since many of the major reuse opportunities involve water applications to soil systems (e.g., irrigation), addressing the benefits, risks, and sustainability of degraded water reuse logically fits within the scope of the proposed research project.
Water reuse, along with water conservation and water demand management, are key components of sustainable water resources development (Metcalf & Eddy, 2007). Irrigation of reclaimed water (agricultural, landscape, and recreational) continues to be a major water reuse option, although there is a tremendous untapped potential since <10% of treated wastewaters are currently reused (Miller, 2006). Besides the value of the water to satisfy the consumptive needs of the crop, a major advantage to degraded water reuse is to avoid direct nutrient discharges to water bodies and to use the soil to further treat reclaimed waters. While many of the important irrigation-related issues are identical to those when freshwater supplies are used, other parameters (nutrients, salinity and micro-constituents) become important when reclaimed wastewater is applied to soils.
Some of the greatest challenges to the expanded beneficial use of reclaimed water and treated wastewater effluent are the health implications associated with micro-constituents possessing endocrine disrupting activity (Miller, 2006). Many pharmaceuticals, personal care products, plastics, pesticides and industrial by-products possess such activity. The endocrine disrupting micro-constituents are inherent to municipal wastewaters, but information on concentrations in treated effluents is limited (Metcalf & Eddy, 2007). Endocrine disrupting compounds (EDCs) interfere with natural hormones causing reproductive and growth problems in animals, particularly aquatic organisms (Xia et al., 2005). A major concern is the ecological impacts at trace concentrations (~1 ng/L) of EDCs in surface waters from wastewater effluent discharges (Koplin et al., 2004). Based on the hydrophobicity and biodegradability of many EDCs, Heberer (2002) speculated that the compounds are adsorbed on soils and have limited transport in subsurface systems (Metcalf & Eddy, 2007). Soil organic matter increases with long-term irrigation using reclaimed wastewater (Quin and Mecham, 2005; Walker and Lin, 2008), so hydrophobic EDCs retention should be enhanced. However, some EDCs are hydrophilic and have structures that resist enzymatic attack (Metcalf & Eddy, 2007), and their persistence and impact is less certain. Field research in a turfgrass system at the University of California, Riverside confirmed that these compounds have very limited mobility, even under heavy irrigation (Wu, 2008; Personal communication). Nevertheless, there are few published studies on the presence, mobility, and degradability of EDCs at effluent irrigation sites (see Pedersen et al., 2003; Kinney et al., 2006). The impact on groundwater beneath spray irrigation sites has received very limited attention.
In summary, there is still considerable research to be conducted on bioavailability of constituents (i.e., nutrients, trace elements, and organic micro-constituents) in residuals applied to land. New ecological endpoints must be investigated in order to improve risk assessment. In addition, the use of such residuals offer opportunities for increasing agronomic productivity, enhancing environmental quality by sequestering environmental pollutants and providing alternatives to current, sometimes environmentally degrading, agricultural practices.
Back to Top