Effect of coagulant on acidogenic fermentation of sludge from enhanced primary sedimentation for resource recovery: Comparison between FeCl3 and PACl

- PACl and FeCl3 perform similarly for chemically enhanced primary sedimentation (CEPS).
- Fe coagulant shows no inhibitory effect on organic hydrolysis of CEPS Fe-sludge.
- PACl coagulant has an inhibitory effect on the organic hydrolysis of CEPS Al-sludge.
- FeCl3 is more favored than PACl in CEPS use for organic and P recovery from the sludge.

FeCl3 and PACl as coagulants in chemically enhanced primary sedimentation (CEPS) were compared in terms of their efficiencies in removing pollutants from wastewater and their effects on the acidogenic fermentation of CEPS sludge for resource recovery. PACl was found to be more effective than FeCl3 for removing suspended solids by CEPS, with around 20% higher removal efficiency. However, the coagulated Al-sludge experienced more difficulty and had lower efficiency than Fe-sludge in organic hydrolysis and acidogenesis. The batch fermentation results showed that FeCl3 dosed at 10-30 mg Fe/Lsewage had little influence on sludge hydrolysis and volatile fatty acid (VFA) production, whereas an obvious inhibitory effect was observed for PACl in organic hydrolysis of the sludge. The specific hydrolysis rate constant (Kh,P) for sludge fermentation decreased from 0.0321 for the sludge without PACl to 0.017 for the Al-sludge obtained at a dosage of 24 mg Al/Lsewage. Compared to the Al-sludge, the Fe-sludge had a much higher VFA yield and significant PO4-P release during the sludge fermentation, which is attributed to the reduction of Fe(III) to Fe(II) under anaerobic conditions and the resulting disintegration of sludge flocs. By simple pH adjustment of the fermented Fe-sludge supernatant, up to 31% of the phosphorus in wastewater can be recovered in the form of vivianite as P fertilizers. VFAs produced in the supernatant are valuable organic carbon resources that can be recovered for beneficial uses.

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