Harnessing the power of silicon: Enhanced coagulation and aging control in secondary effluent treatment with polyaluminum silicate chloride (PASiC)

The secondary sedimentation tank in wastewater treatment facilities is critical for separating suspended solids and biomass from treated water, producing effluent suitable for discharge or further treatment. This study fabricated poly-aluminum silicate chloride (PASiC) through silicic acid modification of polyaluminum chloride (PACl) as the precursor matrix. The coagulation efficacy of PASiC in treating secondary sedimentation tank effluent was systematically investigated, with particular focus on the impact of ageing processes on floc structural stability.Experimental findings indicated that PASiC effectively reduced organic matter in effluent; however, the silicon component can form fine suspended particles that were challenging to settle, which slightly reduced turbidity removal efficiency. Specifically, the addition of silicon increased supernatant turbidity by 0.2 to 0.5 NTU compared to the control. During floc aging, PASiC proved effective in reducing and stabilizing residual aluminum in the supernatant, with silicon-modified PASiC achieving a residual aluminum reduction exceeding 0.1 mg/L. Notably, PASiC with a basicity of 1.0 and a Si/Al ratio of 1:10 (PACl 1.0 -1:10) yielded flocs with improved resistance to aging, as indicated by slower changes in the humification index and fluorescent active substances in the supernatant. It can be reasonably deduced that the incorporation of silicon facilitates the formation of a more resilient network configuration through its complexation with aluminum, and reinforces the hydrogen bonds. These findings provide a theoretical basis for mitigating floc aging challenges, such as sludge return in wastewater treatment processes.