The Situation

Lake Waco is the only significant source of water for Waco, and in the future will be the primary source for much of McLennan County as ground water continues to be depleted. Unfortunately, the lake is experiencing a significant reduction in raw water quality due to significant algae growth. The primary reason for the increased algae growth is the major increase in dairy farms in the North Bosque watershed which feeds Lake Waco. During the latter part of the 20th century, the upper North Bosque River watershed experienced phenomenal growth in dairy cow population. Runoff from these agricultural lands has caused occasional organic loadings that have resulted in both taste and odor problems.

Current Treatment

The basic treatment process at both Mt. Carmel WTP and the Riverside WTP is the conventional coagulation, flocculation, sedimentation, and filtration process. The current instrumentation and chemicals are modern, but the process is relatively old. Blue-green algae are present in Lake Waco in large numbers. Addition of chlorination during the treatment process starts the algae death process, releasing the taste and odor causing compounds. A taste and odor absorbing material (powered activated carbon – PAC) is now mixed in the water. However, this very inefficient way to combine PAC with the algae is a short-term solution at best. To adequately handle the treatment of Lake Waco water, with the nutrient load and resulting algae load, and to minimize taste and odor problems, more advanced treatment processes are required.

In looking for a solution, The Wallace Group kept in mind the need for a plant enlargement in the next 5 – 10 years, in addition to more stringent standards for turbidity, total organic carbon, and disinfection by-products as early as 2002.

Proposed Studies

The Wallace Group reviewed several alternatives for advanced treatment techniques that could be considered to improve the quality of the finished drinking water at Waco. The alternatives for advanced treatment techniques were microfiltration and dissolved air flotation (DAF). Additionally, the use of granular activated carbon (GAC) filters were reviewed. The Wallace Group prepared budgets and cost estimates for conducting pilot plant studies on both DAF and Microfiltration, both followed by GAC Filtration.

Pilot Test – Summer and Winter 2000

The Wallace Group subcontracted with Leopold Water and Wastewater Products for a trailer mounted pilot plant with operators. The DAF and GAC pilot plant was operated at the Lake Waco raw water pump station for approximately 5 weeks in Summer 2000, and 3 weeks in Winter 2000.

The DAF technology proved very effective in the removal of algae from the Lake Waco water supply. While algae removal eliminates the generation of additional taste and odors in the water supply, it does not remove the odors previously imparted to the water. The GAC contactor filter was used to remove the odors previously imparted to the water supply. The use of both technologies was critical in fully addressing the taste and odor problems.

Treatment Options

To implement the DAF and GAC Technologies at the existing facilities will require a considerable change in the existing treatment processes. The Wallace Group provided several alternatives, including cost estimates, on implementing and staging the technologies for the City of Waco. Many items were considered including initial capital expenditures and disruption of the treatment process. Regardless of the options selected, the removal of both the source (algae) and the by-product (taste and odor) is a capital-intensive project. A long-term capital improvement program can be formulated with input from the City staff. The elimination of the nuisance problem created by the algae will provide an additional advantage of helping the City meet new stringent standards that will be in place in January 2002.

Author: Bob Wallace, P.E.