A Mechanism And An Enhanced Method Of Phosphorus Release During Anaerobic Fermentation Of Sewage Sludge Containing Iron-phosphorus Compounds

Iron（Fe）is omnipresent in wastewater treatment plants（WWTPs）and significant amounts of phosphorus（P）can be bound with Fe,resulting in the Febound P compounds（FePs）in the waste activated sludge（WAS）.However,most FePs are relatively inert,making P tightly fixed in sludge during anaerobic fermentation.Little P is thought to be dissolved after fermentation of FePs-containing sludge.In this thesis,the P forms in the sludge of full-scale WWTPs were investigated.Then,the P release potential of different FePs during anaerobic fermentation and the effect of FePs on WAS fermentation were studied.Finally,a novel strategy for effective P release and recovery from FePs-containing sludge via co-fermentation with protein-rich biomass（PRB）was proposed.At first,the P forms in the primary sludge,secondary sludge（SS）,concentrated sludge and dewatered sludge of 8 WWTPs in Shanghai were investigated.8 WWTPs all used the process of biological P removal combined with chemical precipitation.Aluminum（Al）salts were dosed in 7 WWTPs,and a mixture of Feand Al salts was dosed in 1 WWTP.The results show that the P concentration in the sludge samples was4.39-28.62 g/kgdw（dw:dry weight）and the average was 19.31 g/kgdw,which was higher than the average P concentration in sludge in China（15.70 g/kgdw）.The P forms in sludge were affected by various factors such as the type of coagulant,dosing position and treatment process.Inorganic P was the dominant P fraction for all the sludge samples,accounting for 74.3%-94.1%of the total P,and the content of organic P（5.9%-25.7%）was relatively small.In addition,FePs and Al bound P compounds（Al Ps）were the dominant inorganic P for all the sludge samples,accounting for 10.2%-47.3%and25.0%-65.2%of the total P,respectively.The high FePs and Al Ps proportions in the sludge samples originated from the use of Feand Al coagulants for P removal.Soluble and weakly-adsorbed P accounted for 3.5%-29.1%of the total P.The content of Ca bound P compounds was relatively small,accounting for 1.0%-14.5%of the total P.According to the results of metagenomic analysis,15 species of phosphorus accumulating organisms（PAOs）,of which the sum of relative abundances was 3.8%-15.5%,were found at genus level in the SS samples.Ca.Accumulibacter,Dechloromonas,Pseudomonas and Gemmatimonas were the dominant PAOs.However,the abundances of key enzyme genes（ppk1,ppx,adk,pst,ATPase,pha A,pha B,pha C）in polyphosphate metabolism were similar in all sludge samples,accounting for 1.29%-1.37%.Therefore,there was no significant positive correlation between the relative abundance of PAOs and the organic P concentrations in the sludge samples.Then,five species of FePs（114 mg Fe/g TSS）with amorphous or crystalline structures were added to WAS for anaerobic fermentation,aiming to investigate effects of FePs forms on P release and the performance of WAS fermentation.Five species of FePs were amorphous FePO₄·3H₂O（AS-FePO₄·3H₂O）,amorphous FePO₄·4H₂O（AC-FePO₄·4H₂O）,monoclinic FePO₄·2H₂O（M-FePO₄·2H₂O）,hexagonal FePO₄（H-FePO₄）and orthorhombic Fe₅（PO₄）₄（OH）₃·2H₂O（O-Fe₅（PO₄）₄（OH）₃·2H₂O）.During the anaerobic fermentation,Fe（III）Ps were reduced to Fe（II）Ps by iron-reducing bacteria.The amorphous Fe（III）Ps species were faster to be reduced than the crystalline species,due to the larger specific surface area.The Fe（III）reduction rate of H-FePO₄was faster than that of M-FePO₄·2H₂O,due to the lower crystal field stabilization energy.FePO₄·n H₂O was reduced to vivianite and part of orthophosphate（PO₄-P）was released.After 7 days of anaerobic fermentation,the actual PO₄-P release accounted for57.5%-62.1%of the theoretical PO₄-P release in sludge samples with FePO₄·n H₂O,because 37.9%-42.5%of the released PO₄-P reacted with Ca and Mg to form precipitates again.O-Fe₅（PO₄）₄（OH）₃·2H₂O as an iron hydroxyphosphate was transformed toβFe₂（PO₄）O-like compounds without PO₄-P release.In addition,FePs had an adverse effect on the anaerobic fermentation of WAS.Compared with the control sludge,the specific hydrolysis rate constant and volatile fatty acids（VFAs）yield decreased by 38.4%and 41.9%,respectively,for the sludge sample with AS-FePO₄·3H₂O,which dropped the most;and by 4.4%and 8.0%,respectively,for the sludge sample with O-Fe₅（PO₄）₄（OH）₃·2H₂O,which dropped the least.Bacteroidales and Clostridiales as acidifying microorganisms were the most affected bacteria by FePs.The sum of relative abundances of Bacteroidales and Clostridiales was reduced by 18.6%for the sludge sample with AS-FePO₄·3H₂O.Finally,a novel strategy for effective P release and recovery from FePs-containing sludge via co-fermentation with PRB was proposed.Four PRBs（silkworm chrysalis meal（SCM）,fish meal（FM）,corn gluten meal（CGM）and soya bean meal（SBM））were used for co-fermentation.All of the PRBs were added at an equivalent total chemical oxygen demand（TCOD）(0.81 g TCOD_PRB/g TCOD_sludge).Compared with the control sample,the soluble PO₄-P increased by 88.3%,48.3%,77.1%and 33.3%;the VFAs increased by 5.3,4.4,3.7 and 2.9 times in the sludge samples with SCM,FM,CGM and SBM,respectively.SCM with strong surface hydrophobicity and loose structure favored the hydrolysis and acidogenesis processes.Sulfide produced by SCM could react with FePs to form FeS and promote P release.The P release rate increased from 3.4%（control sample）to 27.7%by co-fermentation with SCM.Due to high concentration of ammonia against VFAs,the p H was maintained near neutral,and thus prevented the dissolution of metal ions.With the addition of magnesium salt to the fermentation supernatant and p H adjustment to 8.5,struvite precipitates with a purity of 90.2%were obtained.The main impurity was magnesium phosphate.The metagenomics analysis indicated that Holophaga,Geothrix,Alistipes were the main acidifying microorganisms enriched in the sludge samples with SCM and FM;Bacteroides was the main acidifying microorganisms enriched in the sludge samples with CGM and SBM.The gene abundances of phosphate acetyltransferase and acetate kinase increased by 117.7%and 52.2%,respectively,in the sludge sample with SCM,which increased the most.Therefore,VFAs production was significantly promoted.This study provides an environmentally-friendly approach for P recovery from FePs-containing sludge,and contributes to P recycling.