Enhanced performance and mechanism of the combined process of ozonation and a semiaerobic aged refuse biofilter for mature landfill leachate treatment

A semiaerobic aged refuse biofilter (SAARB) can effectively treat mature landfill leachate (ML), but prolonged operation can lead to the enrichment of pollutants in the biofilter, resulting in severely degraded treatment performance. In this study, we constructed a combination process of ozonation and a SAARB to treat ML based on the principles of selective oxidation of aromatic organics by ozone and the preference of microorganisms for ozonation products. The results showed that the removal of organic and nitrogen pollutants became extremely poor after long-term treatment of ML using the SAARB alone. The decrease of chemical oxygen demand (COD), light absorbance at 254 nm (UV₂₅₄), NH₄⁺, and total nitrogen (TN) improved significantly after recirculating the ozonated ML effluent (OLE) into the SAARB, and the removal extents increased significantly to 63.59% (COD), 26.14% (UV₂₅₄), 92.85% (NH₄⁺), and 52.04% (TN), respectively. In addition, the recirculation of OLE enhanced the complete denitrification and tolerance to high NH₄⁺ loading by the SAARB. An analysis of the community composition of 16S_bacteria and ammonia oxidation bacteria (AOB) showed that long-term treatment of ML using the SAARB alone had difficulty enriching the dominant functional bacteria. In the OLE recirculation stage, environmental factors—such as influent organic matter species and concentration, nitrogen pollutant concentration, and pH—were changed to influence the community composition of 16S_bacteria and AOB and enrich functional bacteria (e.g., Truepera, Luteibacter, and Nitrosospira). Therefore, ozonation combined with a SAARB can remove organic and nitrogen pollutants more effectively. In particular, this can be used to solve the problem of inefficient total nitrogen removal using the SAARB alone. This study provides a theoretical reference for the efficient and stable operation of biological processes when treating ML.