SECTION IV Potential Hazards, Precautions, and Regulations of Compost Production and Utilization © 2001 by CRC Press LLC CHAPTER 16 Heavy Metal Aspects of Compost Use Rufus L Chaney, James A Ryan, Urszula Kukier, Sally L Brown, Grzegorz Siebielec, Minnie Malik, and J Scott Angle CONTENTS I II III Introduction A Industrial Pretreatment Improves Quality of Biosolids and Composts B Nutrient Supply from Biosolids and Composts C Defining High-Quality Biosolids and Composts for Sustainable Use Risk Assessment Methodology for Contaminants in Beneficially Used Biosolids and Composts A Pathway Risk Assessment Used for the U.S EPA Section 503 Rule B U.S Limits on Contaminants in Biosolids and Composts C Hidden Safety Factors in Pathway Calculations D Phytotoxicity of Trace Elements E Phytoavailability of Applied Trace Elements over Time F Labels May Confuse Risk Communication G Soil Cadmium Risk to Humans H Food Consumption Rates vs Cadmium Risk Potential I Are Soil Microbes Protected by the U.S EPA Section 503 Rule? J Using the U.S EPA Section 503 Biosolids Limits for Other Composts and Organic Amendments Some Future Directions for Composts and Biosolids in Agriculture A Remediation of Metal Toxic Soils Using Composts and Biosolids © 2001 by CRC Press LLC B Lime-Induced Manganese Deficiency IV Summary References I INTRODUCTION Composts prepared from municipal solid waste (MSW), biosolids (municipal sewage sludge), food processing wastes, manures, yard debris, and agricultural byproducts and residues are increasingly available for agricultural use Utilization of composts as fertilizers and soil conditioners provides benefits from nutrients, from organic matter, from biodegradation of organic matter, and from organisms in the composts Remarkable benefits have been identified in new approaches for control of plant diseases by use of composts in media or in field plantings, and in revegetation of disturbed soils and mine wastes Production of composts provides an important cost saving to cities, industries, and agricultural users, and allows recycling for beneficial use of more of society’s discards Although some compost products continue to be poorly manufactured, scientists have discovered improved manufacturing methods, methods to monitor or evaluate composts inexpensively, and criteria for quality control of compost products Because production of composts will offer large cost savings to both urban and agricultural areas, such composts will be available at relatively low cost to horticultural industries for use as fertilizers, soil conditioners, and when prepared properly, potting media components Although many benefits are possible from use of composts, these products must be safe for sustainable agriculture for their use to be permitted by governments These products also must reliably supply nutrient and organic matter benefits to become competitive in the marketplace The potential presence of pathogenic organisms, heavy metals/trace elements, potentially toxic synthetic organic compounds (compounds that are not normally biosynthesized are referred to as “xenobiotic” compounds), and possible element imbalance in composts have caused concern to some potential compost users Some believe that because the concentration of zinc (Zn) or copper (Cu) in composts is higher than found in background soils, these materials must not be utilized on soils However, practicing horticulturists and researchers have used high-quality organic matter/compost products for decades without adverse effects (Andersson, 1983; Chaney and Ryan, 1993; de Haan, 1981; Mays and Giordano, 1989; Sanderson, 1980; Woodbury, 1992) Boron (B) phytotoxicity was observed when high rates of MSW-compost were used in media in the 1970s, but changes in glue formulations removed this possible adverse effect of MSW-composts (Chaney and Ryan, 1993; Sanderson, 1980) How could such high benefits be observed so often if the metals and other constituents were so dangerous? In brief, the logical flaw, in presuming that metals in composts must cause adverse effects in the future, is the focus on total concentrations, when phytoavailability of microelements is well known to vary as a function of source Similarly, biosolids, pet excreta in yard debris, and manures contain pathogenic organisms, but proper composting generates products that comprise no human or plant pathogen risk It is not biosolids and composts that should cause concern, but whether these products © 2001 by CRC Press LLC meet enforceable standards of acceptable quality composting technologies, maturity of composts, and composition of composts that are to enter the marketplace of organic amendments and media components In 1970, when modern interest in the safety of utilizing organic byproducts and composts on cropland began its rapid increase, biosolids were often highly contaminated with metals and xenobiotics Few MSW composts were available for use, and only a few of these were mature composts ready for use in crop production Substantial efforts were undertaken in many countries to conduct research to characterize the potential for adverse effects from use of composts and biosolids so that regulations could be developed to protect soil fertility and food-chain safety In the U.S., these efforts culminated in the development of the U.S Environmental Protection Agency (U.S EPA) Clean Water Act Section 503 Rule on land application of biosolids (U.S EPA, 1989a, 1993), hereafter called the 503 Rule Such U.S rules are “proposed” for public comment, to allow errors and omissions to be identified, and other data to be provided to the U.S EPA to improve the scientific basis of the rule Errors were found and questions were raised after the U.S EPA prepared the first Proposed Draft 503 Rule (U.S EPA, 1989b) Therefore, the scientific community thoroughly evaluated data from many experiments to develop improved risk assessment models to protect soil fertility and food-chain safety during use of composts and biosolids Development of the final corrected 503 Rule is discussed later A Industrial Pretreatment Improves Quality of Biosolids and Composts Fortunately, pretreatment of industrial wastewaters has allowed most municipalities to produce biosolids and composts with low concentrations of metals and synthetic organic compounds (Table 16.1), reducing the potential for adverse effects The median concentrations of metals in biosolids have fallen substantially over the last 25 years When pretreatment of industrial sources is complete, biosolids still contain significant levels of Zn, Cu, and some other elements because such elements are in foods (hence in human wastes) and food wastes, or are leached from the pipes which carry water to and in our homes Interestingly, the need to keep lead (Pb) in drinking water at low concentration at the home tap to protect children is requiring many municipalities to treat their water to reduce the corrosion of water pipes This improvement in drinking water treatment to reduce risks from Pb in water transmission systems has reduced Zn, Cu, Pb, and cadmium (Cd) levels in biosolids formed during treatment of domestic wastewaters These management options have made it possible to attain biosolids and composts with reduced concentrations of metals and xenobiotics Some have argued that only products which are as low in contaminants as possible should be allowed to be used in agriculture Although common sense dictates that avoidable metals should be avoided, costs involved with avoidance of metals in biosolids or composts become an issue At some point, the increase in benefits from lower concentrations are less than the costs associated with reduction It is especially difficult to define concentration limits in composts for the elements that are naturally present in all soils and foods; it is even more difficult to so for those metals that are micronutrients for © 2001 by CRC Press LLC Table 16.1 Range of Contaminant Concentrations Reported for Biosolids Before Pretreatment Enforcement Historic Reported Range Element Zn Cd Cd/Zn, % Cu Ni Pb As Hg Cr PCBsy Min Max 101