Anaerobic Supernatant Research Articles

Struvite crystallization: a feasible and reliable way to fix phosphorus in anaerobic supernatants

The possibility to fix phosphate in sludge handling liquors supplied from a carbon–nitrogen wastewater treatment plant through the crystallization of struvite was investigated in this paper. The anaerobic supernatant used for the experiments was obtained from the dewatering section (belt-press) of a 85000 equivalent inhabitants civil wastewater treatment plant, with a combined carbon oxidation and nitrogen nitrification-denitrification and anaerobic digestion of sludge. The experimental results showed that a natural aging with phosphate precipitation can be observed in a few days with a different percentage of struvite and hydroxyapatite in relation to the liquor composition; moreover supersaturation curves play a major role in the interpretation of the process. Finally, the FBR plant managed in batch mode assures a good phosphate removal through crystallization on quartz sand in about 100 min.

Phosphate removal in real anaerobic supernatants: modelling and performance of a fluidized bed reactor

Phosphate removal in anaerobic supernatant coming from a centrifugation sludge station of an A2O process is studied. A fluidized bed reactor is employed to crystallize phosphate as hydroxyapatite or struvite using only air stripping to reach the supersaturation pH. The classic composition of supernatant (alkalinity 3550 mgCaCO3/l, PO4 139 mg/l, Mg 24 mg/l) does not require any addition of chemicals for phosphate removal. Seventeen runs are performed in a bench scale FBR obtaining very high conversion and removal efficiency and phosphate loss in the effluent ≤3.5%. The use of Ca or Mg enriched supernatant has no meaningful influence on efficiency, but it determines the prevalent salt formed between MAP or HAP. Efficiency can be related to pH and sand contact time in a double saturational model. The half efficiency constants: 0.075 h for t and 7.75 pH, have an important role in the process knowledge and optimization of plant design. Exhaust sand analysis indicates the same composition at the top, bottom and mean of the sand bed (39% mol MAP and 61% mol HAP). This result together with the high half efficiency constant for contact time indicate that the phosphate growth on the bed is not competitive. Finally, the phosphate release from the plant is studied. Results show a weak release rate, equivalent to 2.8-10% d−1 phosphate as MAP, obtained at an operative pH range of 8.1-8.4.

Phosphate removal in real anaerobic supernatants: Modelling and performance of a fluidized bed reactor

Phosphate removal in anaerobic supernatant coming from a centrifugation sludge station of an A2O process is studied. A fluidized bed reactor is employed to crystallize phosphate as hydroxyapatite or struvite using only air stripping to reach the supersaturation pH. The classic composition of supernatant (alkalinity 3550 mgCaCO3/l, PO4 139 mg/l, Mg 24 mg/l) does not require any addition of chemicals for phosphate removal. Seventeen runs are performed in a bench scale FBR obtaining very high conversion and removal efficiency and phosphate loss in the effluent ≤3.5%. The use of Ca or Mg enriched supernatant has no meaningful influence on efficiency, but it determines the prevalent salt formed between MAP or HAP. Efficiency can be related to pH and sand contact time in a double saturational model. The half efficiency constants: 0.075 h for t and 7.75 pH, have an important role in the process knowledge and optimization of plant design. Exhaust sand analysis indicates the same composition at the top, bottom and mean of the sand bed (39% mol MAP and 61% mol HAP). This result together with the high half efficiency constant for contact time indicate that the phosphate growth on the bed is not competitive. Finally, the phosphate release from the plant is studied. Results show a weak release rate, equivalent to 2.8–10% d−1 phosphate as MAP, obtained at an operative pH range of 8.1–8.4.