A method for comparative evaluation of whole-lake and inflow alum treatment

A method is described to evaluate two methods of phosphorus (P) management in lakes using aluminum sulfate (alum)—in-lake and tributary (inflow) treatment—and compare the resulting in-lake P levels. For in-lake treatment, a technique is described to calculate the optimum alum dose based on measurement of “mobile P” in lake sediments. Mobile P is defined as loosely sorbed and Fe–P, the fraction of sediment P subject to release under anoxic conditions. A linear relationship (r2=0.90) was found between P-release rate and the mobile-P content in sediment cores. Addition of alum to aliquots of sediment showed predictable relationships between (i) alum dose and aluminum-bound P (Al–P) formed and (ii) mobile-P loss and Al–P formation. The decrease in sediment P release that would result from in-lake alum treatment was estimated from the residual mobile P after treatment. A method also is presented to estimate the amount of alum needed to bind potentially mineralizable sediment organic P. For inflow treatment, jar tests with urban runoff in metropolitan St. Paul and Minneapolis, Minnesota (USA) were used to study effects of alum dose on P removal from water. With sufficient mixing, a dose of reduced total P (TP) and soluble reactive P to low levels regardless of pH, TSS, and TOC, but doses did not significantly reduce TP compared with settling alone. A modeling approach is described to compare the effects of inflow and in-lake treatment, and a case study is developed to illustrate the approach for typical eutrophic lakes.