The effects of varying Zn²⁺ on the activated sludge properties and its distribution patterns

Metallurgy, electroplating and pharmaceutical industries generate a large amount of wastewater containing Zn²⁺, which influences the activated sludge properties. The present work investigated the effects of Zn²⁺ on sludge properties and its distribution patterns and chemical forms under a Zn²⁺ concentration range of 0–20 mg/L. When the influent Zn²⁺ concentration was 1 mg/L, parameters indicating sludge properties such as Zeta potential (−14.80 mV), contact angle (78.93°), particle size (54.43 μm) and re-flocculation (FA, 30%) reached the optimal states. Zn²⁺ was adsorbed by the functional groups of extracellular polymeric substances (EPS) and low EPS promoted the sludge flocculation. The lower pH affected the EPS functional groups, which in turn affected the adsorption of Zn²⁺. The FeMn oxidation state Zn (2.66–6.86 mg/gVSS) predominated in the sludge and decreased as sludge flocculation increased. The results provided evidence to make full use of low concentration of Zn²⁺ to ensure wastewater treatment performance.