Newman Lake restoration: A case study. Part II. Microfloc alum injection

Abstract

Microfloc alum injection, in contrast to whole-lake alum treatments, is a relatively new technique for alum delivery that has been implemented in a relatively small number of lakes and reservoirs. Microfloc alum injection is primarily designed to precipitate phosphorus in the water column using low doses applied on a continuous or intermittent basis. The technique employs diffusers to create ultra-small aluminum hydroxide particles with lower settling velocities and longer residence times, compared to whole lake treatments, to strip dissolved phosphorus from the water column. Because relatively few microfloc systems have been deployed, a brief review of their characteristics and performance is provided. In Newman Lake, Washington, a microfloc alum injection system has been in use since spring 1997. We tested microfloc residence times using in situ enclosures; residence time was determined to be at least 10 days, supporting the underlying concept. In the lake itself, turnover events prior to alum injection consistently produced higher algae growth with accompanying lower Secchi transparency. Since alum injection, post-turnover Secchi depths have all improved. In 18 years with 32 monitored turnover events, this relatively robust dataset consistently indicates the short-term ability of microfloc to improve transparency through algae reduction. Volume-weighted total phosphorus concentrations also have generally declined following alum injection. Adverse biological impacts were not observed in either enclosures or the lake. In the enclosure studies, total zooplankton and cladoceran densities and biomass in treatments were not significantly different from controls. Further applications and study of this promising technology are warranted.