Key Takeaways & Executive Findings
- •• • Iron sulfide-based autotrophic denitrification achieves stable pH and reduces by-product formation (e.g., sulfate, nitrous oxide), with a hydraulic retention time of only 0.5–2 h to purify secondary effluent to Class IV surface water standards, enabling efficient simultaneous nitrogen and phosphorus removal. • • SAD eliminates the need for external carbon sources, reducing operational costs and sludge production compared to heterotrophic denitrification, as evidenced by pilot-scale studies (e.g., sulfur-limestone biofilter achieving high nitrate removal). • • The use of pyrite (FeS2) as an electron donor in autotrophic denitrification has been shown to effectively remove nitrate and phosphate simultaneously, with studies demonstrating enhanced performance in constructed wetlands and vertical-flow biofilters. • • SAD technology aligns with the 'Dual Carbon Strategy' by offering a low-carbon approach for deep nitrogen removal, addressing the bottleneck of meeting stringent nitrogen and phosphorus discharge standards in wastewater treatment plants.
Abstract
Sulfur autotrophic denitrification (SAD) has attracted increasing attention due to its low cost, no need for external carbon sources, and low sludge production. This review systematically examines the reaction principles and key material elements of various electron donors for SAD, including elemental sulfur, sulfide, thiosulfate, and iron sulfide. It discusses recent research progress on different SAD processes and the influence of environmental factors. A comparative analysis between heterotrophic denitrification and SAD highlights SAD's advantages in reaction rate, secondary pollution, and cost-effectiveness, underscoring its promising application prospects. Notably, iron sulfide-based autotrophic denitrification maintains stable pH and produces fewer by-products (e.g., sulfate, nitrous oxide). When developed into an aggregate sulfur concrete system, it can purify nitrogen and phosphorus from secondary effluent standards to Class IV surface water standards within a hydraulic retention time of only 0.5–2 hours, addressing the contradiction between SAD reaction rate and engineering demands. This enables efficient simultaneous nitrogen and phosphorus removal, making it viable for groundwater remediation, advanced wastewater treatment, eutrophication control, and deep nitrogen removal. The national 'Dual Carbon Strategy' (carbon neutrality and peak) positions SAD as a promising method for wastewater treatment plants to meet increasingly stringent nitrogen and phosphorus discharge standards.
1. Introduction
Sulfur autotrophic denitrification (SAD) has emerged as a promising alternative to conventional heterotrophic denitrification for nitrogen removal from wastewater, particularly in scenarios where organic carbon is scarce. Traditional heterotrophic processes require external carbon sources, incurring significant operational costs and producing substantial sludge. SAD leverages inorganic sulfur compounds as electron donors, offering cost savings, reduced sludge generation, and elimination of carbon addition. However, early SAD systems faced challenges such as slow reaction rates and the accumulation of sulfate and nitrous oxide as by-products, limiting their practical application.
This review critically re-evaluates the advantages and limitations of SAD by examining various electron donors—elemental sulfur, sulfide, thiosulfate, and iron sulfide—and their reaction mechanisms. Notably, iron sulfide-based systems demonstrate superior performance: they maintain stable pH, produce fewer secondary pollutants, and when engineered as aggregate sulfur concrete, achieve rapid purification of secondary effluent to Class IV surface water standards within 0.5–2 hours. This breakthrough addresses the historical bottleneck of SAD reaction rates, enabling simultaneous nitrogen and phosphorus removal. The findings underscore SAD's viability for groundwater remediation, advanced wastewater treatment, and eutrophication control, aligning with the national 'Dual Carbon Strategy' for sustainable wastewater management.
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WANG Fuqiang, JIANG Yanbo, TANG Shihui, FAN Mengying, YANG Haotian, MA Zhengyu, GUAN Yuntao, LI Ruihua (2026). Re-analysis of the Pros and Cons of Sulfur Autotrophic Denitrification Technology. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025010306
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Frequently Asked Questions
What are the key operational parameters for iron sulfide-based autotrophic denitrification to achieve Class IV surface water standards?
Iron sulfide-based autotrophic denitrification, when developed as aggregate sulfur concrete, can purify secondary effluent to Class IV surface water standards with a hydraulic retention time (HRT) of only 0.5–2 hours. This process maintains stable pH and produces fewer by-products such as sulfate and nitrous oxide, enabling efficient simultaneous nitrogen and phosphorus removal.
How does sulfur autotrophic denitrification compare to heterotrophic denitrification in terms of cost and sludge production?
SAD eliminates the need for external carbon sources, significantly reducing operational costs. It also produces less sludge compared to heterotrophic denitrification, as autotrophic bacteria have lower biomass yields. Pilot-scale studies have demonstrated effective nitrate removal with reduced secondary pollution, making SAD a cost-effective and environmentally friendly alternative.
What are the main challenges in scaling up sulfur autotrophic denitrification from laboratory to pilot or full-scale applications?
Challenges include managing by-product accumulation (e.g., sulfate), ensuring consistent electron donor supply, and optimizing reactor design to maintain high reaction rates. However, recent developments using iron sulfide minerals and aggregate sulfur concrete have addressed these issues by achieving rapid treatment within HRTs of 0.5–2 hours, demonstrating feasibility for engineering applications.
Can sulfur autotrophic denitrification be integrated into existing wastewater treatment plants to meet stricter nitrogen and phosphorus discharge standards?
Yes, SAD can be integrated as a tertiary treatment step. For instance, iron sulfide-based systems have been shown to purify secondary effluent to Class IV surface water standards, which are more stringent than typical secondary effluent limits. This integration supports compliance with evolving regulations and aligns with the 'Dual Carbon Strategy' by providing a low-carbon nitrogen removal option.
What are the environmental implications of using different sulfur electron donors in autotrophic denitrification?
Different donors have distinct impacts. Elemental sulfur and sulfide can lead to sulfate accumulation and pH fluctuations, while iron sulfide (e.g., pyrite) maintains stable pH and produces fewer by-products. The choice of donor affects secondary pollution and operational stability, with iron sulfide offering a more sustainable option for simultaneous nitrogen and phosphorus removal.
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