Control of sulfides in gravity sewer pipelines using coated sustained-release granules: Efficiency and mechanisms

To address the challenges in sewer systems where continuous wastewater flow renders conventional iron salt dosing ineffective for long-term control, and where iron salts cause a sharp pH decline and structural weakening of pipelines, this study developed a coated sustained-release ferrous chloride granule (PMDI‑Fe²⁺). Under long‑term operation of laboratory‑scale reactors, the results showed that at a PMDI‑Fe²⁺ dosage of 0.045 % (w/v), the half‑maximal recovery time (RT₅₀) of the sulfide production rate was 9 days; at a flow velocity of 0.6 m/s, iron ions were sustainably released for 45 hours. The granules control sulfide through a dual biological and chemical mechanism: on the one hand, they elevate the oxidation‑reduction potential and zeta potential of the wastewater, disrupt biofilm structure, and reduce both the diversity of sulfur‑metabolizing microorganisms and the expression of key genes; on the other hand, they directly remove sulfide via chemical precipitation. The sustained‑release characteristic of PMDI‑Fe²⁺ avoids a sharp pH drop (the pH decreased from 6.29 to 6.03 after treatment), which is favorable for sulfide precipitation. After treatment, the soluble carbon-to-nitrogen (C/N) and carbon-to-phosphorus (C/P) ratios in the wastewater increased significantly, which may alleviate the operational burden on downstream treatment plants. At a dosage of 0.045 % (w/v), the cost of sulfide control was 1.50 yuan/kg‑S. This study validates the practical potential of coated sustained‑release granules for sulfide control in sewers.