The Mechanism Of Fe²⁺ And Vivianite Formation During Anaerobic Fermentation Of Waste Activated Sludge

Phosphorus（P）recovery from waste activated sludge（WAS）is an effective method to simultaneously solve the problem of P shortage and P pollution.Vivianite crystallization,with the advantages of natural ubiquity,high economic value and easy accessibility,is a promising method to recover P from waste activated sludge（WAS）.This study presented for the first time to improve the iron reduction and vivianite formation from WAS via acid/alkali pretreatment and iron sources adding during anaerobic fermentation（AF）.The influences of different pH pretreatment conditions and different external iron sources on iron reduction,P release and vivianite formation during AF of WAS were investigated.The optimal treatment conditions for iron reduction and vivianite formation were determined.Finally,the effect of extracellular polymers substances（EPS）on P recovery by vivianite precipitation under different initial sodium alginate（SA）concentrations（0-800 mg/L）,pH values（6.5-9.0）and Fe/P molar ratios（1:1-2.4:1）was investigated using synthetic wastewater.The main research results are as follows:（1）The iron reduction rate and efficiency were effectively promoted by acid/alkali pretreatment,and the positive effect of acid pretreatment was better than that of alkali pretreatment.Compared with the Control-Fe group,both pH 3.0 and pH 10.0 groups increased the maximum iron reduction rates by 1.9 and 1.7 times and the iron reduction efficiency by 17.5%and 12.0%respectively.The concentration of PO₄^3-in supernatant was negatively correlated with iron reduction efficiency,and the better the iron reduction efficiency,the lower the concentration of PO₄^3-in supernatant.Furthermore,the P in Loosely-P was converted to Fe-P during AF via acid/alkali pretreatment.The Fe-P proportion in the sludge in the pH 3.0 and pH 10.0 groups increased by 50.0%and 33.7%,respectively.The optimal pH pretreatment was determined to be pH 3.0.The iron-reducing bacteria were enriched after pH 3.0 pretreatment,resulting a large number of ferrous irons and the formation of Fe-P.（2）After pretreatment,the concentrations of SCOD,NH4+,protein and polysaccharide in the supernatant were increased.The protein and polysaccharide in the supernatant were reduced due to the complexation with iron ions after external iron sources dosing.（3）Both pH 3.0 and pH 10.0 pretreatment,the order of iron reduction efficiency from high to low was as follows:FeOOH>FeCl₃>Fe₂O₃.Different iron sources affect the iron reduction efficiency,and then affect the concentration of PO₄^3-in the supernatant,the conversion of Loosely-P to Fe-P,and the formation of vivianite.The addition of FeOOH and Fe₂O₃ stimulated the secretion of proteins and humic acids with electron transport capacity in EPS,enhanced the ability of electron transport,and led to the formation of electron transfer between microorganisms and iron sources,as well as direct interspecific electron transfer between microorganisms.（4）In the pH 3.0-FeCl₃ group,the hydrolysis of organic matter was significantly promoted（SCOD concentration increased）,while the SCOD,protein and polysaccharide concentration in supernatant significantly decreased in the pH 10.0-FeCl₃ group.The addition of FeOOH and Fe₂O₃ had little effect on the concentrations of SCOD,protein and polysaccharides.（5）The results showed that SA in low concentrations（≤400 mg/L）had little inhibitory effect on the P recovery rate.However,when the concentration of SA was larger than 400 mg/L,the P recovery rate decreased significantly with increasing SA concentrations.The inhibition rate of 800 mg/L SA was about 3 times as large as that of 400 mg/L SA.It was worth noting that the inhibitory effect of SA on vivianite precipitation decreased with increasing initial pH and Fe/P molar ratios.Additionally,SA has no obvious influence on the composition of products,but the morphology of harvested crystals was transformed from branches to plates or rods in uneven sizes.