A Preliminary Study On The Formation Mechanism Of Phosphorus Recovery From Anaerobic Digestion Sludge-blue Iron Ore

“Phosphorus?P?crisis”and“P pollution”existed simultaneously and are equally serious.The best choice of mitigating these two phenomena is P recovery.Therefore,Research on P recovery in wastewater or animal waste is becoming a hot topic in the whole world.For the advantage of chemical stability and wide utilization,,the recovery of vivianite?Fe3?PO4?2·8H2O?from anaerobic digestion sludge has entered the scientific community's horizon in recent years.Thus,this topic aims to study the required environmental conditions,interference factors,separation and purification of the vivianite in anaerobic digestion systemThe literature review shows that vivianite is a biological secondary mineral,which is widely found in surface sediments of the earth and very stable(K_sp=10^-36)ferrophosphorus compound.Besides the internal factors of iron-rich and P-rich,The determination factors of formating vivianite also need appropriate reducing conditions?ORP<-300 mV?and neutral pH conditions?6⁹?.Sometimes there are internal and external conditions for the formation of vivianite in sewage or sewage treatment.Preliminary experiment of Chemical synthesis shows that the vivianite can be chemically generated in the range of pH=4.5¹0.0.Morever,the purity could be reached over 90%in the range of pH=4.5⁸.0?detected by XRD etc.?.Acidity and alkalinity can affect the crystal size of the viivanite and the growth direction of the crystal.The crystallite distribution of the vivianite is most concentrated?50⁷0?m?in weakly acidic environment.The main part of the vivianite biosynthesis experiment was conducted in three steps.Firstly,investigating the generation of vivianite in the co-digestion mode of“Fe³⁺+excess sludge”.The results shows that in the anaerobic digestion system,Fe³⁺was gradually reduced to Fe²⁺by the DMRB,and then to the hydrolysis of the PO₄^3-generated vivianite.When Fe³⁺is biologically reduced,DMRB competes with methanogens?MPB?for electron donors?organics?,thereby inhibiting anaerobic digestion.However,Fe³⁺also provides the necessary Fe element for MPB,which promotes the anaerobic digestion process;The comprehensive effect in formating vivianite is positive promotion of methane production process in anaerobic digestion.The second step of the biosynthesis experiment of vivianite is to focus on interference factors and inhibit exploration.Based on these experiments,we add"interference factor"to carry out the experiment.Some environmental ions(Al³⁺,Mg²⁺,Ca²⁺,S^2-,CO₃^2-,HA)have some inhibitory effects on Fe³⁺biologic reduction;In addition,these interference factors have a competitive effect on Fe²⁺or PO₄^3-inside the system,which in turn inhibits the production of vivianite.The experimental results showed that the inhibition degree of these factors on the formation of vivianite was:CO₃^2-<HA<S^2-<Mg²⁺<Ca²⁺<Al³⁺.The third step focus on biosynthetic vivianite in actual excess sludge AD system,then separating and purifying it from the mixed sludge.The results illustrated that vivianite can be produced in the actual excess sludge AD system with Fe³⁺.However,high concentration of Al³⁺in the system would seriously inhibit the formation of vivianite.Moreover,the effective way to separate vivianite form the sludge is the ultra-gravity centrifugal separation at owe to their density difference.With centrifuging at 2 000 rpm for 15 min,the vivianite and mixed sludge could be effectively separated.