Page Banner

United States Department of Agriculture

Agricultural Research Service

Title: Aggregate Size of Alum-Based Water Treatment Residuals Can Influence Phosphorus Sorption Kinetics and Pmax Values

Authors
item Novak, Jeffrey
item Watts, Donald

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 28, 2005
Publication Date: July 10, 2005
Citation: Novak, J.M., Watts, D.W. 2005. Water treatment residuals aggregate size influences phosphorus sorption kinetics and Pmax values. Soil Science. 170(6):425-432.

Interpretive Summary: Because of over-application of animal manures, some soils in the Coastal Plain region of North Carolina contain high levels of phosphorus (P). High rainfall rates from storms cause runoff that transports P from these soils into stream and rivers. If P concentrations are high in streams and rivers, then aquatic weed and algae growth will increase. When weeds and algae die, oxygen levels in water become low harming fish and other aquatic organisms. In severe cases, the water turns green and foul odors are created. This need not happen, however, because P transported from fields can be minimized by adding amendments to soils that are capable of binding high amounts of P. Water treatment residuals are a possible amendment; they are a byproduct produced during drinking water purification. We obtained water treatment residuals from a drinking water purification plant in North Carolina. Initially, the water treatment residuals were in large chunks, so before testing, they were ground and sieved into large and small-sized particles. Particles of different size ranges were mixed into soils with high amounts of P, and after a predetermined amount of time, soil P concentrations were measured. We found that residuals mixed into P-enriched soils reduced soil P concentrations. The greatest reductions in soil P concentration were found using very-fine water treatment residuals. This means that for better reductions in soil P concentrations, residuals should be ground to smaller size particles.

Technical Abstract: Drinking water treatment residuals (WTRs) are used as a soil amendment to minimize off-site P movement and increase a soil's phosphorus (P) sorption capacity. The P sorption characteristics of WTRs have been examined using isotherms determined in 24-h long incubation experiments with 2-mm sieved WTRs. However, aggregate size may affect sorption kinetics and P sorption maxima (Pmax) values. We hypothesize that finer-sized WTRs aggregates will have higher kinetic sorption rates and Pmax values than coarser-size aggregates. The objectives were to determine WTRs aggregate size effects on kinetic rates of P sorption and their Pmax values. A WTR sample, collected from a North Carolina water treatment facility, was ground and then sieved into 5 aggregate size ranges (<0.5, 0.5 to 1.0, 1.0 to 2.0, 2.0 to 4.0, and >4-mm). Phosphorus sorption isotherms for each aggregate size range were determined as a function of time (between 24 to 120 h). Reaction rate constants (k) were determined using a first-order reaction equation and Pmax values for each aggregate size range were calculated from the linear form of the Langmuir equation. The <0.5-mm WTRs aggregates had the highest k values and the reaction rates decreased with an increase in aggregate size. All isotherms showed that aggregate size ranges reached equilibrium between 72 and 96-h and there was a strong linear (r2 between 0.78 and 0.96) and significant (P between <0.05) relationship between the mean P equilibrium concentration and P sorbed. Coarser-sized WTR aggregates (between 1.0 to >4.0-mm) had Pmax values of <94 mg/g, while finer-sized (<1.0-mm) aggregates had values >98 mg/g. These results suggest that processing WTRs into finer particle size ranges will bind more P per weight unit of residual than coarser- size aggregates. Aggregate size has an important influence on WTRs P sorption characteristics; therefore, it is recommended that aggregate size should be strongly considered when employing residuals as a soil amendment to reduce non-point source P contamination of surface water bodies.

Last Modified: 4/16/2014