Selection Of Chemicals For Phosphate Removal Combined In Biological Aerated Filter Process

Biological aerated filter (short for BAF) is an emerging wastewater treatment process with the advantages of small area occupation and steady effluent quality. However, the deficiency that there is no condition for biological phosphorus removal in BAF will lead to poor biological phosphorus removal capability in the system. To achieve the effluent standards in phosphorus, the combining chemical phosphorus removal process is in need. Phosphorus removal process for BAF includes pre-chemical phosphorus removal system and post-chemical phosphorus removal system. In this research, experiments were carried out for the selection of reagents for these two chemical phosphorus removal systems, respectively, based on the characteristics of BAF. The objective of this study was to confirm the reagents for pre- and post-chemical phosphorus removal systems with the consideration of effluent pH, removal rates of TP, SS and COD, and the operation cost. Toxic effects of the reagents for phosphorus removal on microorganism in the system were also studied.The result of pre-phosphorus removal process showed that the optimal pH of the selected iron salts and aluminum salts in pre-chemical phosphorus removal system were around 4 and 6, respectively, however, phosphorus could also be well removed with these salts in neutral conditions. The pH value must be kept over 10.5 when calcium salts was used, indicating that calcium was not suitable for this system. When the same dosage of iron salts were used to remove phosphorus, the removal efficiency of TP, COD and SS by FeCl3 and Fe2(SO4)3 were similar. When TP of effluent was less than 0.5mg/L, the COD removal rates of COD and SS were higher than 45% and 80%, respectively. However, sludge production and cost of the FeCl3 were higher than those of Fe2(SO4)3. Among all the aluminum salts used for phosphorus removal, AlCl3 had the best TP removal rate and it also had less sludge production, however, lower COD removal rate and higher cost were got; PAC had the best capabilities in COD and SS removal, while TP removal was low, indicating that PAC was not suitable for the system. Al2(SO4)3 was confirmed as the best reagent for pre-chemical phosphorus removal system with good capabilities in TP and SS removal, lower sludge production and lower cost, as well as low COD removal. In general, Al2(SO4)3 satisfied the requirements of BAF.The result of reagents selection in post-chemical phosphorus removal system showed that the calcium salts, with a lower TP removal rate after treatment, were not suitable for post-chemical phosphorus removal system, besides, the pH of the effluent needed to be adjusted. Both FeCl3 and AlCl3 had good efficiencies in high TP and turbidity removal requirements in this system, however, reagent costs were high. PFS was confirmed as the best reagent for post-chemical phosphorus removal as there was no need to adjust pH after treatment and little sludge was produced, and the turbidity removal rate was higher than 60%. With the use of PFS, reagent cost was only 0.029 Yuan/t, which was the lowest cost among all reagents, when the effluent standard in TP was achieved.The study of the impact of chemicals on microbial showed that a high dosage of iron salts and aluminum salts would have little affect on nitrifying bacteria and anti-nitrate bacteria.