Stability of P Saturated Water Treatment Residuals under Different Levels of Dissolved Oxygen

Abstract

Water treatment residuals (WTRs) are effective phosphorus (P) immobilizers that have been used in constructed wetlands (CWs). In CWs, dissolved oxygen (DO) levels vary from location to location and fluctuate over time. Therefore, this work accessed the stability of P saturated ferric and alum water treatment residuals (FARs) under low (<1 mg/L), medium (2-4 mg/L), and high (5-8 mg/L) DO levels. In the experiments, which had a 40-day duration, three stages of P release from the P saturated FARs were observed: an initial rapid P desorption stage, followed by a P re-adsorption stage, and a P desorption balance stage. The strongest bonding between P and FARs occurred at the low DO level. A limited amount of Fe and Al was released from the P saturated FARs. Interestingly, the P in the FARs tended to transform from the Al bound P to the Fe bound P, and this transformation was stronger at lower DO levels. However, no more than 1.12% of the total P in the P saturated FARs was desorbed under any of these DO levels. Therefore, FARs can be considered as a safe P adsorption medium for CWs.

Comparison of metal lability in air-dried and fresh dewatered drinking water treatment residuals

“…[2] The high P adsorption capacity of WTRs can remain stable under various conditions, [3][4][5] and the adsorbed P is not readily desorbed. [6,7] WTRs have also been used as a soil amendment to reduce P loss, [8] as a substrate in constructed wetlands to remove excessive P from wastewater [9] and as a stabilizer to control P release from sediments. [10] The reuse of WTRs as metal adsorbents is another area of interest for investigators.…”