Principles for Setting Threshold Limits of Heavy Metal Concentrations in Municipal Solid Waste Compost: A Comprehensive Review

Document Type : Review

Authors

• Mojgan Yeganeh ¹
• Kambiz Bazargan ²
• karim shahbazi ³

¹ Research department of soil chemistry and physics and fertilizer technology,Soil and Water Research Institute (SWRI), Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran

² Research Department of Soil Chemistry and Physics and Fertilizer Technology, Soil and Water Research Institute (SWRI), Agricultural Research, Education and Extension Organization (AREEO),, Karaj, Iran

³ Research Department of Soil Chemistry and Physics and Fertilizer Technology, Soil and Water Research Institute (SWRI), Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran

Abstract

Keywords

• Municipal compost
• Heavy metals
• Quality standards
• Waste management
• Elemental balance method

Main Subjects

• ..

Introduction

The rapid increase in population, urbanization, and changes in lifestyle has led to a global surge in waste production, making municipal solid waste (MSW) management a critical challenge. Countries employ various methods—such as landfilling, recycling, incineration, and composting—to manage urban waste, depending on factors like waste composition, socioeconomic conditions, and technological capabilities. Composting, introduced post-World War II, has gained traction as a sustainable waste management strategy, with some countries composting up to 25% of their MSW. However, while compost derived from organic waste can enhance soil fertility by improving organic matter content, water retention, and nutrient availability, it also poses risks due to potential heavy metal contamination. These metals, such as lead (Pb), cadmium (Cd), and chromium (Cr), can accumulate in soil, enter the food chain, and threaten ecosystem health and human safety. Given these concerns, many nations have established stringent standards for compost quality, particularly for agricultural use. However, these standards vary widely due to differences in local conditions, waste composition, and regulatory frameworks. In Iran, composting initiatives date back to 1969, with the first national compost standard published in 2007. Recent reports of substandard compost quality and heavy metal contamination have prompted calls for revising these standards to align with scientific and environmental safety principles. 

Purpose 

This study aims to: 

Review international approaches to setting heavy metal limits in municipal compost for agricultural use. 

Propose a scientifically grounded method for determining permissible heavy metal concentrations in compost, for application in agricultural lands of Iran, ensuring environmental and human health protection. 

Research Method

The study employs a comparative literature review and data collection methodology: 

Literature review: reviewing global standards for heavy metals in compost, focusing on countries like the U.S., Germany, Canada, and the EU, which use either risk assessment or elemental balance approaches, and compare it with their national survays. Also assessing Iranian compost quality, soil heavy metal background and national standard, effect of long-term heavy metal accumulation in soil under continuous compost use was done. In addition, application rate of municipal solid waste compost in agricultural lands of Iran, was considered.

The research identified two primary approaches for setting limits: adopting foreign standards or developing localized standards. Given significant variations in waste composition, production methods, and national regulations across countries, directly applying foreign standards without local adaptation lacks scientific validity. Studies revealed that countries generally employ two main approaches to determine permissible limits of pollutants in compost. The first method is based on risk assessment and is primarily used by the U.S. Environmental Protection Agency (EPA). The second method is based on the mass balance of pollutant inputs into the receiving environment (agricultural soil), with Canada's environmental protection agency being among those that utilize this approach.

Results

This paper concluded the "elemental balance method" as the most suitable approach for Iran. In this method, permissible limits for each element are calculated based on: 1) maximum allowable soil concentration after 45 years of continuous use, and 2) an application rate of 25 tons per hectare annually. The proposed limits (mg/kg dry weight) for heavy metals in Iranian municipal compost are: Arsenic: 5, Cadmium: 10, Chromium: 150, Lead: 100, Nickel: 120, Cobalt: 25, Copper: 650, Zinc: 1300, Molybdenum: 5, and Mercury: 5. Findings highlight the need to revise current standards for chromium and nickel in Iranian soils due to their naturally high background concentrations. The study emphasizes the importance of integrated compost management - from production to application - with continuous monitoring to ensure environmental safety. Implementing these science-based, locally-adapted standards while maintaining proper application rates can enable safe agricultural use of municipal compost while protecting soil health and food safety.

Conclusion

This study demonstrates that setting national standards for agricultural compost forms an integral part of comprehensive urban waste management systems. The mass balance approach - calculating cumulative elemental loading in agricultural soils - emerges as the most scientifically valid method for determining heavy metal thresholds in Iran's context, though its precision depends critically on accurate input data. Notably, natural background concentrations of nickel and chromium exceed proposed regulatory limits in significant portions of Iran's agricultural soils, creating a paradox that demands either standard revision or explicit usage restrictions in affected areas. Parallel measures must include:

Source-segregation to reduce contaminant levels

Dynamic standard refinement through ongoing research

Strict enforcement of application rates

Stakeholder education programs

International best practices confirm that integrated approaches combining improved collection systems with advanced processing technologies can yield compost products with heavy metal concentrations below regulatory thresholds, enabling safer agricultural utilization while maintaining soil organic matter benefits.

Author Contributions

 “Mojgan Yeganeh: literature review, data collecting, original draft preparation, finalize the manuscript, Kambiz Bazargan: Conceptualization, literature review, draft preparation, finalize the manuscript, Karim Shahbazi: finalize the manuscript. All authors have read and agreed to the published version of the manuscript.” All authors contributed equally to the conceptualization of the article and writing of the original and subsequent drafts.

Data Availability Statement

Data will be available based on request from the authors.

Acknowledgements

The authors would like to thank all participants of the present study.

Ethical considerations

The authors avoided data fabrication, falsification, plagiarism, and misconduct.

Conflict of interest

The author declares no conflict of interest.