Pretreatment For Biological Aerated Filter By Enhanced Coagulation

BAF(biological aerated filter)had broad application prospects in upgrading the sewage treatment plant because of high volume load, high hydraulic load and small area. However, BAF had more stringent requirements on influent quality. As while, BAF had poor phosphorus removal effect through biodegradation, chemical phosphorus removal were usually used. Therefore, the influent pretreatment of BAF was necessary to realize the goals of SS and TP removal. The process of coagulation and sedimentation for BAF was usually applied. However, because of the shortcomings of more chemical dosage and high cost in the conventional coagulation-sedimentation process, the improved process of sludge-returning enhanced coagulation was put forward to reduce the cost of pretreatment. Optimal coagulant choice, enhanced coagulation by auxiliary agents and sludge returning were researched.Based on the characteristics of BAF, the influent qualities were tried to achieve SS<60mg/L, TP<1.0mg/L. Moreover, the loss of carbon source was avoid to guarantee the demand of denitrification. Therefore, three different types of coagulants including poly-aluminum chloride (PAC), poly-aluminum ferric chloride (PAFC), and poly-ferric sulfate (PFS) were compared. The results showed that: PAC had the best effect of SS and TP. Consider the effects of subsequent BAF process, PAC was choosed as the best chemicals, and 0.16 yuan need be cost to pretreat a ton of water. Although, PFS had the least cost, PFS had the shortcomings of higher chroma in the effluent, easier hydrolyzation and severe influence to pH value to impact the nitrification. The effects and cost of pretreatment were inferior by PAFC. In addition, the states and removal mechnisms of phosphorus were researched to provide the theoretical proof for the chemicals choices of phosphorus removal.Based on the optimal coagulant (PAC), lime and PAM were the auxiliary agents, enhanced coagulations were researched. The results showed that: the lime was added about 30mg/L, the effects of SS and TP removal were increased by 5% and 10%, and about 20 mg/L alkalinity was provided. PAM was added 0.1 mg/L, the effects of SS and TP removal were increased by 10% and 5%. Exceptionally, PAM could improve the performance of floc settling, and the settling time was shortened from 30min to 12~20min.Based on the above researches, sludge returning of static and dynamic experiments were researched. The results showed that: the sludge returning had a certain extent with the increasing in SS and TP removal. The effects of sludge returning not only related to the returning amount, but also concerned to residual chemical agents in the sludge. Additionally, adding chemical sludge could improve the properties of floc settling and the settling time was shortened from 30min to 20min. The results of sludge returning showed that: when the solid ratio was about 0.8% and reflux ratio was 1%, the effects of SS, TP and COD removal were increased by 10.9%, 13.6% and 3.9% respectively. Through the comparation of static and dynamic experiments, when the TP<1.0mg/L, about 10% PAC dosages were reduced by returning 1% chemical sludge.