Release of phosphorus from biological nutrient removal sludges: A study of sludge pretreatment methods to optimize phosphorus release for subsequent recovery purposes

Abstract

As part of on-going studies related to development–implementation of new technologies and approaches for enhancing recovery of phosphorus from municipal wastes, the effectiveness of various methods of pretreating waste activated sludges (WASs) from a biological nutrient removal (BNR) process was investigated to determine suitable methods for maximizing phosphorus release. The additions of a strong base (NaOH), strong acid (HCl), organic acid (citric acid), and sodium acetate to waste sludge all facilitated the release of phosphate into solution. The maximum phosphate release was obtained by adding 4.9 mmole/L (~400 mg/L) of acetate to an intermittently mixed digester. Excess amounts of acetate did not further increase the phosphate concentrations. Acetate addition, which triggers carbon storage, a unique characteristic of enhanced biological phosphorus removal (EBPR) processes, releases internally stored phosphate compounds and is recommended as a method for releasing phosphates from EBPR sludges. Along with phosphates, various cations (most notably magnesium and potassium) were also mobilized and released into solution during digestion using acetate. These ions can be beneficially reused for struvite crystallization. Key words: acetate addition, caustic addition, phosphorus release, P-solubilization, sludge treatment, struvite.