Phosphorus release in aerobic sludge digestion.

Area of Science:

• Environmental Engineering
• Wastewater Treatment
• Biogeochemical Cycles

Background:

• Phosphorus release from sludge is a critical factor in wastewater treatment and nutrient recovery.
• Understanding the mechanisms governing phosphorus release is essential for optimizing sludge digestion processes.
• Secondary sludge from conventional and biological nutrient removal (BNR) processes exhibit different characteristics.

Purpose of the Study:

• To examine phosphorus release during aerobic sludge digestion.
• To elucidate the governing mechanisms of phosphorus release under various operational conditions.
• To compare phosphorus release from conventional and BNR sludges.

Main Methods:

• Experiments conducted at room temperature (22 ± 2°C) using secondary sludge.
• Varied pH control (4.5–7.8) and aeration schemes: fully aerobic (3–4 mg/L DO), low DO (0.2–0.8 mg/L), and cyclic.
• Analysis of released phosphorus concentrations and calculation of phosphorus released per unit volatile solids digested (ΔP/ΔVS).

Main Results:

• Released phosphorus concentrations ranged from 20–80 mg/L (conventional sludge) and 60–130 mg/L (BNR sludge).
• Lower pH (<6.0) significantly promoted inorganic phosphorus precipitate dissolution.
• Fully aerobic digestion resulted in higher phosphorus release compared to low DO or cyclic aeration.

Conclusions:

• Phosphorus release is influenced by pH, aeration intensity, and sludge type.
• Inorganic phosphorus precipitates are sensitive to pH, while polyphosphate hydrolysis is minimal under aerobic conditions.
• Aerobic digestion contributes to phosphorus release primarily through volatile solids digestion and inorganic precipitate dissolution.