Simultaneously enhance nutrient removal and sludge settleability through hydrocyclone-based technology in a full-scale high-inert containing activated sludge process

Abstract

Sludge densification technology through hydrocyclone is a promising solution to address the challenge related to increasing loading rate in existing municipal wastewater treatment plants (WWTPs). Although previous studies have investigated the positive effects of hydrocyclone on improving nutrient removal and sludge settleability, little is known if sludge densification technology is still function into the high inert containing activated sludge process in China. This study investigated technical feasibility and revealed underlying mechanisms to simultaneously enhance nitrogen and phosphorus removal and sludge settleability through installing a hydrocyclone-based sludge densification module in a full-scale WWTP with a designed capacity of 4 × 10⁴ m³/d. Compared to the control line without hydrocyclone, the hydrocyclone installation helped improve total nitrogen and total phosphorus removal efficiency by 16.9 % and 29.4 % (p < 0.05), with the effluent concentrations of 4.86 ± 1.08 mg/L and 0.077 ± 0.035 mg/L, respectively. The strategy of “hydrocyclone treating only half of the wasted activated sludge (WAS) to produce densified activated sludge (DAS)” successfully prevented the accumulation of inerts in the mainstream. Meanwhile, sludge settleability after densification was improved, as evidenced by a reduced sludge volume index (SVI₃₀) by 7.6 mL/g (53.2 ± 7.30 mL/g) and an increased settling velocity by 2 m/h (4.04 ± 0.60 m/h) compared to control line. Beyond selecting few large-sized DAS, the hydrocyclone also targeted small, densified flocs containing ballasting inerts (primarily Fe/Al-based hydroxides and phosphates) within high-inert containing activated sludge, further enhancing sludge settleability. The hydraulic shear forces of the hydrocyclone promoted the release of active sites from metal hydroxides, further enhancing phosphorus removal in mainstream. Meanwhile, shear forces lead to the disruption of DAS and ordinary flocs but also promoted the binding of flocs and inerts, forming densified flocs. The anoxic zones formed within DAS enabled denitrifying phosphorus-accumulating organisms (DPAOs) and denitrifiers to preferentially utilize polysaccharides of extracellular polymeric substances (EPS) for nutrient removal, thereby enhancing functional enzyme activity. These findings provide an important basis for the application of sludge densification technology in high-inert containing activated sludge process.