Multifaceted benefits of magnesium hydroxide dosing in sewer systems: Impacts on downstream wastewater treatment processes

Abstract

Magnesium hydroxide [Mg(OH)<inf>2</inf>] is a non-hazardous chemical widely applied in sewer systems for managing odour and corrosion. Despite its proven effectiveness in mitigating these issues, the impacts of dosing Mg(OH)<inf>2</inf> in sewers on downstream wastewater treatment plants have not been comprehensively investigated. Through a one-year operation of laboratory-scale urban wastewater systems, including sewer reactors, sequencing batch reactors, and anaerobic sludge digesters, the findings indicated that Mg(OH)<inf>2</inf> dosing in sewer systems had multifaceted benefits on downstream treatment processes. Compared to the control, the Mg(OH)<inf>2</inf>-dosed experimental system displayed elevated sewage pH (8.8±0.1vs 7.1±0.1), reduced sulfide concentration by 35.1%±4.9% (6.7±0.9mgSL⁻¹), and lower methane concentration by 58.0%±4.9% (19.1±3.6 mgCODL⁻¹). Additionally, it increased alkalinity by 16.3%±2.2% (51.9±5.4 mgCaCO<inf>3</inf>L⁻¹), and volatile fatty acids concentration by 207.4%±22.2% (56.6±9.0 mgCODL⁻¹) in sewer effluent. While these changes offered limited advantages for downstream nitrogen removal in systems with sufficient alkalinity and carbon sources, significant improvements in ammonium oxidation rate and NOx reduction rate were observed in cases with limited alkalinity and carbon sources availability. Moreover, Mg(OH)<inf>2</inf> dosing in upstream did not have any detrimental effects on anaerobic sludge digesters. Magnesium-phosphate precipitation led to a 31.7%±4.1% reduction in phosphate concertation in anaerobic digester sludge supernatant (56.1±10.4mgPL⁻¹). The retention of magnesium in sludge increased settleability by 13.9%±1.6% and improved digested sludge dewaterability by 10.7%±5.3%. Consequently, the use of Mg(OH)<inf>2</inf> dosing in sewers could potentially reduce downstream chemical demand and costs for carbon sources (e.g., acetate), pH adjustment and sludge dewatering.