[Flocculation characteristics of the return sludge in chemical-biological flocculation process]

The chemical-biological flocculation (CBF) process was used to treat municipal wastewater in Shanghai, and the effect of its return sludge on pollutant removal was studied through stopping chemical addition, simulating different return sludge ratio, and analyzing extracellular polymeric substances (EPS). The results show that the return sludge in CBF exhibits strong pollutant removal capability, and chemical in its return sludge can further TP removal and enhance CBF's adaptability to the influent TP impulse. After stopping PAFC (Al2 O3 : 10.8%, Fe2 O3: 1.8%) addition, COD removal efficiency is achieved at about 50%, and removal rates of PO4(3-) and TP gradually decline. It is also found that chemical addition plays an important role in promote sludge sedimentation in CBF. EPS extracted from CBF is 145.89 mg/g, while EPS from CEPT is only 17.24 mg/g, which indicates that there is strong biological flocculation behavior in CBF. With the increase of return sludge ratio, TP and COD removal efficiencies decrease in CEPT, which reveals that chemical in waste sludge has poor flocculation capability, while CBF return sludge exhibits good flocculation behavior due to its biological flocculation. Chemical flocculation and biological flocculation work collaboratively in CBF process.