Study On Improving The Recovery Rate Of Vivianite From Anaerobic Digestion Of Excess Sludge

Phosphorus crisis and phosphorus removal from wastewater have caused widespread interest in phosphorus recovery from wastewater/sludge.Anaerobic digestion of excess sludge can accumulate a certain amount of recoverable vivianite（Fe₃（PO₄）₂·8H₂O）.However,the practical application of vivianite recovery can be restricted by some practical problems such as low recovery efficiency and difficult separation.Focusing on the formation mechanisms of vivianite,the study explored technical measures improving the recovery efficiency of vivianite,clarified the key facctors affecting vivianite formation,and formulated an economicaly feasible strategy for vivianite recovery.Firstly,sequencing batch experiments were established to clarify the effect of acetic acid on vivianite formation in AD system,and then to quantitatively evaluate the competitiveness for acetic acid between dissimilated metal reducing bacteria（DMRB）and methane-producing bacteria（MPB）.The results showed that there were strong competition for volatile fatty acids（VFAs）.Among them,the affinity of DMRB for acetic acid was one order of magnitude higher than that of MPB,which indicated that more acetic acid would be ultilized by DMRB.These phenomena implied that the concentration of acetic acid in AD system would perform no obvious effect on the reduction rate of Fe³⁺and thus the formation of vivianite.However,the consumption of acetic acid by DMRB resulted in the low energy conversion efficiency and then significantly decreased methane production.Therefore,the anaerobic co-digestion system of food waste and excess sludge was conducted to increase the production of VFAs in AD system,which could keep the balance of the formation of vivianite and methane production.Different dosage of food waste were added in excess sludge AD system and the results showed that the increase of VFAs could indeed not enhance the foramtion of vivianite,however the methane production efficiency occurred the siginifcant improvement from 109.68 to 139.01 m L CH₄/g VS.Then,the single-phase and two-phase AD systems were evaluated the effect of different operational mode on vivianite formation and methane production.Their production efficiency were compared in AD system with different designed parameters including sludge retention time（SRT）and p H.The results indicated that these two systems resulted in differences on the concentration of VFAs in the supernatant and methane production,but the formation of vivianite was similar at about 60%.Above all,two-phase AD system was the recommended mode with high-efficiency for the simultaneous recovery of vivianite and methane.Finally,the feasibility of vivianite recovered from liquid phase（digested supernatant）was analyzed in acidic and alkaline AD system.The results showed that alkaline AD was more favor of organics and phosphate releasing from excess sludge into digested supernatant.The phosphate in the digested supernatant accounted for about 58.7%of the total phosphate（TP）in solid and liquid phase.Moreover,the purity of vivianite（81.0%-84.0%）in the alkaline digested supernatant was higher than that in the acidic digested supernatant（71.9%-74.3%）,and the recovery efficiencies of phosphorus were 47.5%-49.3%and 36.7%-37.9%,respectively.However,it took lowest cost for vivianite recovered from the acid digested supernatant at about0.052-0.058 RMB/kg vivianite,and/or 0.842-0.939 RMB/kg P.Importantly,it is more suitable to recover vivianite from the acidic digested supernatant due to its greatest advantage of addressing the issue of vivianite separation from digested sludge.