Long-Term Evaluation
of the Lake Ella
Alum Injection System


Abstract


For several decades, new land development projects constructed within the State of Florida have been required to utilize stormwater management practices such as retention or detention ponds, filtration systems or swales to minimize post-development discharges of pollutants to receiving water bodies. However, much of the existing land development in Florida occurred prior to implementation of the current stormwater regulations. Most of these areas contain no facilities for management of runoff, and therefore, discharge untreated stormwater runoff to lakes and other surface water bodies. Many of these areas are highly urbanized and fully developed with little available space for retrofitting activities using traditional stormwater management techniques. Therefore, interest has risen recently in developing a rapid and efficient method of reducing pollutant concentrations in nonpoint source discharges to surface waters which does not require a large commitment to often unavailable land area.

In 1986, a prototype system was introduced in a lake restoration project on Lake Ella in Tallahassee, Florida based on the flow-weighted injection of liquid sulfate, Al2(SO4)3.18 H2O, commonly called alum, into the runoff flow inside the storm sewer lines prior to discharge into the lake. Alum is an acid salt of aluminum which has been used for many years in treatment of drinking water, for phosphorus removal in the wastewater industry, and also in lake restoration projects for water column clarification and as a method of inactivating sediment release of phosphorus into the overlying water column during anaerobic conditions. However, prior to the Lake Ella project, alum had not been used for the treatment of stormwater inputs into a receiving water body.

Alum forms harmless nontoxic precipitates of Al(PO4) and Al(OH)3 which combines with phosphorus, suspended solids, and heavy metals, causing them to be deposited into the sediments in a stable inactive state. The soluble precipitates formed between alum and phosphorus in the sediments are exceptionally stable since they are immune to changes in sediment redox potential, and to a lesser degree, pH as well.

The alum stormwater treatment system resulted in immediate and substantial improvements into water quality in Lake Ella and appears to be promising for use in other locations. However, many questions need to be addressed concerning long-term pollutant removal efficiencies, floc accumulation and stability, ultimate floc disposal, and potential toxicity problems before these systems can be permitted for general use.

The research efforts described in this document are intended to generate further information on the performance and long-term impacts of the alum injection system constructed at Lake Ella in Tallahassee. The specific objectives of these research efforts were to:

  1. Evaluate the long term pollutant removal efficiencies of the alum treatment process and resulting water quality characteristics of Lake Ella;
  2. Examine the accumulation rate and migration of alum floc and heavy metals in the bottom sediments;
  3. Determine the stability of phosphorus and heavy metal sediment associations in Lake Ella before and after the initiation of the alum treatment system;
  4. Examine the effects of the change in pH and redox potential on the stability of phosphorus and heavy metals in alum treated sediments;
  5. Examine the toxicity and characteristics of floc accumulations with respect to ultimate disposal;
  6. Examine the chemical speciation of the aluminum ions formed during precipitation reactions and their relationship to potential toxicity; and
  7. Examine the effects of alum floc accumulations on the populations of benthic organisms.


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