Pretreatment of bakery wastewater by coagulation-flocculation and dissolved air flotation

The pretreatment of wastewater from a large-scale bakery was studied. In the coagulation-flocculation reaction, it was found that both alum and FeCl₃ were effective in the jar tests. When at coagulant dosage of 90 to 100 mg/l, 55 % of COD and 95 to 100% of SS could be removed. The optimum pH was at 6.0. In addition, the removal of SS was affected by pH more significantly, while the removal of COD was not affected in the pH range of 6.0 to 8.0. In the DAF experiments, 48.6% of COD and 69.8% of SS were removed in 10 min at a pressure of 4 kg/cm², recycle ratio of 0.3 l/min, and pH of 6.0. Upon the addition of 100 mg/l of alum, the removal efficiency of COD did not increase while SS removal increased to 82.1%. It was found that 5-min flocculation time did improve the COD removal while it had little effect on SS removal. Flocculation for longer than 5 min did not enhance the flotation performance. Similar phenomena were observed when FeCl₃ was used as the coagulant, except that flocculation had an insignificant effect on COD and SS removal. It was also found that FeCl₃ was relatively more effective than alum. In summary, both coagulation-flocculation and DAF were efficient for the pretreatment. The advantages and disadvantages were discussed.