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Florida Lake Management Society Annual Conference, Naples, Florida, June 4 – 7, 2007

REDUCING PHOSPHORUS LOADING FROM A WETLAND RESTORATION PROJECT WITH A MOBILE ALUM INJECTION SYSTEM

V. R. Hoge, E. R. Marzolf, and R. Naleway St. Johns River Water Management District Palatka, FL T. DeBusk, T. Auter, and F. Dierberg DB Environmental Rockledge, FL

Introduction

Lake Apopka, the fourth largest lake in Florida, is one of the headwaters for the Ocklawaha Chain of Lakes. By the 1970s, Lake Apopka was considered the most severely polluted large Florida lake. To reduce nutrient loading to Lake Apopka, the St. Johns River Water Management District (SJRWMD) purchased 2,362 ha of farms west of the Apopka- Beauclair Canal between 1988 and 1992. Following the cessation of farming, the land was reflooded to encourage shallow marsh habitat. As a significant part of the restoration effort, the Marsh Flow-Way (MFW) was constructed to filter suspended solids and particulate phosphorus from Lake Apopka. The MFW consists of four independent cells capable of treating up to a total of 220 cfs.

The West Marsh properties comprise approximately 1,000 ha, located north of the MFW. Storm events during 2004, which included three major hurricanes, increased water levels in these parcels sufficiently to threaten adjacent properties and the integrity of project levees and pumps. The untreated discharge of six inches of water over the 1,000 ha would approximately equal the P loading SJRWMD has allocated to this property for a full year. Therefore, the objective of this project was to use liquid alum (aluminum sulfate) to treat discharge water at this site to significantly reduce phosphorus loading to Lake Apopka and the entire Ocklawaha system.

Methods

DB Environmental, Inc. in Rockledge, Florida was contracted to provide design and implementation assistance. Since the existing pump and retention pond layout built by the farmers was incompatible with an alum injection system, a temporary pump and the existing MFW system were reconfigured.

Due to the distance from existing electrical sources, a photovoltaic system was designed to power the alum-dosing pump. The initial pump used to move water over the existing levee was an axial flow diesel pump with a maximum capacity of 30 cfs. In order to mobilize the alum pump system at other sites rapidly on demand, the power and pump system was trailer mounted.

Discharge with alum dosing began on March 15, 2005 with the injection of approximately 18 mg Al/L (13 L/minute) into the intake side of the temporary pump. In response to lower total phosphorus within the West Marsh, dosing was reduced to 15 mg Al/L on March 30. Further reduction of dosing to 12 mg/L was implemented on November 1 to try to achieve higher flows and a shorter residence time. Following an initial drawdown, the system was shut down temporarily to install a culvert system to convert to a gravity flow system in June 2005.

Session 4 – Page 16

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