Owing to the long solids retention time and effective sludge decay, the increase in the amount of phosphorus and dissolved organic matter (DOM) in effluents is a major obstacle to the sludge in situ reduction process. In this study, the coagulants hydrated aluminum sulfate (Al₂(SO₄)₃), ferric trichloride (FeCl₃), and polyaluminum chloride (PAC) were applied for the simultaneous removal of phosphorus and DOM from the secondary effluent of a sludge process reduction-activated sludge wastewater treatment system. The results indicated that in most cases, Al₂(SO₄)₃ exhibited the best coagulation performance, followed by FeCl₃ and PAC. Three-dimensional excitation–emission matrix fluorescence spectroscopy and gel permeation chromatography (GPC) were used for the detailed characterization of the DOM after the coagulation treatment. Herein, two humic-like components and a protein-related component were identified in the wastewater samples by parallel factor (PARAFAC) analysis. The protein-related component was removed more effectively by FeCl₃ and PAC, whereas Al₂(SO₄)₃ was more effective for the removal of humic-like components. The GPC analysis showed that FeCl₃ exhibited the best removal performance for large molecules with a MW > 1000 kDa, whereas Al₂(SO₄)₃ showed a relatively good removal performance for small molecules with a MW < 2 kDa. The correlation analysis showed that for the Al₂(SO₄)₃ samples, there were significant correlations (p < 0.01) between the total organic carbon, UV absorbance at 254 nm, total phosphorus, and maximum fluorescence intensity of the three PARAFAC components; on the other hand, for the FeCl₃ and PAC samples, DOM removal was mostly correlated with the humic-like components, and the correlation coefficients among the seven variables were lower than those for the Al₂(SO₄)₃ samples.