Global Significance of Substrates for Nitrate Removal in Denitrifying Bioreactors Revealed by Meta-Analysis

Abstract

Denitrifying bioreactors (DNBRs) are widely used to reduce excess nitrate from agricultural drainage. Their performance depends on the physical and chemical properties of the substrate. Common substrate types have been partly reviewed in previous studies. However, few studies have attempted to determine a generalized pattern for the role of substrate type in nitrate removal. This study summarizes 41 types of substrates using a dataset collected from 63 peer-reviewed articles, which include 219 independent DNBR units. The substrates are classified into four groups: ① natural carbon (NC), such as woodchips; ② non-natural carbon (NNC), such as biodegradable polymers (e.g., polycaprolactone (PCL), and waste products (e.g., cardboard); ③ inorganic materials (IMs), such as non-carbon materials (e.g., iron oxide); and ④ multiple materials (MMs), such as a mixture of the above materials. These materials are compared and evaluated through a meta-analysis of nitrate removal rate (NRR; N removal (g∙m−3∙d–1)) and nitrate removal efficiency (NRE). This study reviews substrate performance (NRR and NRE), potential mechanisms, pollution swapping, and cost analysis. Our analysis indicates that woodchips and corncobs are the most cost-effective substrates among NCs. In a comparison of all the studied substrates, MM substrates are recommended as the optimal substrates, especially woodchip-based and corncob-based substrates, which have great potential for improvement. This analysis can assist in optimizing the design of DNBRs to meet the environmental, economic, and practical requirements of users.

Keywords

• Denitrifying bioreactor
• Substrate
• Nitrate removal
• Meta-analysis
• Environmental impact
• Cost analysis