Response Mechanism Of Phosphorus Accumulation Organisms (PAOs) To Ferric Salts Stress In Wastewater Treatment System

Ferric salts were widely used in wastewater treatment to enhance phosphorus removal,in this paper,ferric salts dosage of 0?control group Q1?,0.02 mM?micro dosage Q2?and 0.25 mM?common dosage Q3?were added to SBR reactors which were operated in A/O/A mode.We comparatively studied the removal efficiency of system,the characteristics of sludge and the bacterial communities and their evolution in the whole system,especially the variation of PAOs,The results obtained were as follows:The average removal rates of C,N and P in Q1,Q2 and Q3 were NH₄⁺-N:96.1%,95.8%,97.3%,TN:74.5%,76.5%,71.1%,COD:89.5%,93.1%,92.7%,TP:74.3%,83.8%,86.5%,respectively.The addition of ferric salts had no effect on NH₄⁺-N removal,Microdosage of ferric made TN removal rate increased by 2.0%,COD removal effiency increased by 3.6%,TP removal rate increased by 9.5%.Common dosage of ferric improved TN removal rate by 3.4%,made COD removal effiency increased by 3.2%and TP removal rate increased by 12.2%.The more iron salt dosage,the more MLSS and MLVSS increased,the better sedimentation performance of activated sludge.The results of SEM and EDS showed that:As the common dosage addition of ferric salts,a large number of cocci appeared in pairs or four superimposed groups,the proportion of P on the 100th day in Q1,Q2,Q3 was 1.54%,1.88%and 5.13%respectively.The quantity of total EPS in Q2 were 60.87 mg/g,the ratio of PS/PN were increased.from 1.38 to 2.17,the quantity of total EPS in Q3 were decreased by 10.16mg/g compared with Q1,especially,PN decreases greatly.3D-EEM showed that the addition of iron salt increased tryptophan and protein substances in EPS and decreased humic acid substances.XRD results showed that the sludge with ferric salts addition contained the same form of phosphorus:FeFe₂?PO₄?₂?OH?₂-4H₂O,which may be the main form of ferric salts and phosphorus combination.The proportion of chemical phosphorus removal was 15.4 mg/g?26.4%of total phosphorus removal?in Q2 and31.9 mg/g?42.2%of total phosphorus removal?in Q3.The diversity and richness of bacterial community were improved by microdosage of ferric salts addtion;however they reduced by adding common dosage ferric salts.Proteobacteria accounts for more than 70%of sludge in all stages.With the increase of ferric salts concentration,AOB increased slightly,while NOB was inhibited.Microdosage of ferric salts increased the ratio of DNB and DNPAO in the system,and the number of bacteria forming bacterial micelles.After adding iron salt,Accumulibacter belongs to Phosphorus Accumulating Organisms?PAOs?were increased on the whole,other PAOs like Pseudomonas were disappeared,Dechloromonas decreased dramatically.The abundance of PAOs in Q1,Q2,Q3 were1.47%,2.66%and 1.22%,respectively.It indicated that addition of micro dosage ferric salts promoted the PAOs and the addition of common dosage ferric salts inhibited it.The maximum phosphorus release of PAOs in Q2 increased from 13.75 mg/L to 19.98mg/L,the removal rate of total phosphorus increased from 74.5%to 83.8%,the maximum phosphorus release of PAOs in Q3 decreased from 13.75 mg/L to 6.71 mg/L.Combined with the effects of ferric salts on the treatment performance of the system,sludge performance and microbial population,the addition of trace iron salt?0.02 mM FeSO₄?is more efficient and economic.