| The iron reduction efficiency is the limiting factor for the formation of vivianite crystals.Redox mediators(ROMs)are a class of compounds that can be reversibly oxidized and reduced,which can increase the electron transfer rate in the reaction and improve the rate of biological oxidation-reduction reaction.This study presented for the first time to improve the iron reduction and vivianite formation by adding ROMs during anaerobic fermentation(AF)of waste activated sludge(WAS).The influences of ROMs and ferric chloride(FeCl3)on the migration and transformation of iron and phosphorus(P),vivianite formation and release of organic matter during AF of WAS were investigated.In addition,the influence of zeolite as crystal seed on P recovery by vivianite precipitation was investigated,and the optimum condition of phosphorus recovery process was determined by response surface method.The main research results are as follows:(1)The addition of ROMs could effectively improve the rate and efficiency of iron reduction during AF.When adding 1 mmol/L of anthraquinone-2-sodium sulfonate(AQS),100μmol/L of sodium 1,2-naphthoquinone-4-sulfonate(NQS)or 5 g/L of biochar(BC)during AF of WAS,88.44%～90.90%of Fe3+was reduced,and the maximum iron reduction rate was 1.64～1.84 times that of the Control group.The addition of ROMs promoted the conversion from Loosely-P to Fe-P in WAS,but inhibited phosphorus release when BC dosage was higher than 5 g/L.After adding AQS and NQS,vivianite crystals were mixed with WAS,while some vivianite crystals were adsorbed on the surface of BC after adding BC.Adding AQS and NQS could promote the release of organic matter.The maximum concentrations of dissolved chemical oxygen demand(SCOD),NH4+,protein and polysaccharide were 4.20,2.79,4.25 and 2.87 times of those in the Control group after adding 4 mmol/L AQS,respectively.However,the concentration of organic matter was slightly lower than that of the control group after adding BC.(2)ROMs and citric acid(CA)had synergistic promoting effect on iron reduction.The iron reduction rate after adding ROMs and CA was respectively 68.94%-81.30%and 11.12%-19.25%higher than that of the Control group and the group adding FeCl3 alone,and the maximum iron reduction rate was 1.26-2.34 times and 1.02-1.89 times of the Control group and the group adding FeCl3 alone.After adding ROMs,FeCl3 and CA,the concentration of PO43-in the supernatant was less than 0.5 mg/L in the end anaerobic period.The better the reduction effect of iron,the higher the proportion of Fe-P in WAS.The addition of NQS was more conducive to the formation of larger vivianite.Adding AQS and CA significantly promoted the hydrolysis of organic matter,and the maximum SCOD concentration was 5.50 times of that in the Control group.The addition of AQS,NQS and CA could promote the release of protein and polysaccharide,and the maximum concentration of protein and polysaccharide was 1.10～2.87 times and 1.14～2.79 times of the Control group,while the concentrations of protein and polysaccharide decreased after adding BC and CA.(3)Based on the results of single factor experiments,taking zeolite particle size of 40 mesh(250-425 μm),zeolite dosage of 0.1 g/L and stirring time of 40 min as the center points of three-factor and three-level response surface method,the optimal parameters of phosphorus recovery process were determined by response surface method as follows:The dosage of zeolite was 0.12 g/L,the particle size of zeolite was 40 mesh,the stirring time was 41 min,and the optimal phosphorus recovery was 99.13%. |
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