Treatment of Lake Inflows with Alum for Phosphorus Removal

Abstract

Alum is being used on an experimental basis to remove phosphorus (P) from the inflows of Fish and Tanners Lakes in the Minneapolis and St. Paul metropolitan area of Minnesota. Jar test studies and experiments with settling columns demonstrated the usefulness of these methods for proper design of inflow alum treatment systems. Mixing conditions in treatment facilities at both lakes promote good floc development. The degree of alum and water mixing, defined by the velocity gradient (G, s⁻¹) and mixing time (t, in s), or Gt, was 7,700–35,000 for Tanners Lake and the maximum potential Gt for Fish Lake was 42,000. The concentration of TP entering Fish Lake was greatly reduced at an alum dose of 8 mg Al L⁻¹ but was largely unaffected at 1 mg Al L⁻¹. Average annual TP removal from 1998 to 2003 with alum doses of 1.9 to 10.5 mg Al L⁻¹ ranged from 61 to 84% at Tanners Lake. Surface TP declined rapidly with alum treatment in both lakes. Average annual Secchi disk depth (SD) for the west bay of Fish Lake improved from 1.5 m in 1998 to 2.1 m in 2000 despite a 28 cm storm event in July 2000 that led to large inflows to the lake. Treatment at Tanners Lake, which occurred during the entire year, was effective in reducing the long-term (May through September) average TP concentration in the surface of Tanners Lake from approximately 50 to just under 30 μg L⁻¹. Alum floc, which was deposited in sediments of a settling pond prior to Fish Lake and in the lake sediments inhibited P release under anoxic conditions.

Key Words:

• alum treatment
• phosphorus removal
• design