How Does it Work?
Given our City’s history with taste & odor issues and the well-chronicled watershed challenges, simply adding capacity without enhancing our treatment processes is not an option.
Federal and State regulatory changes beginning in 1998 increased the number of potential contaminants to be addressed through the treatment process over the next five years (60 in the EPA Contaminant Candidate Lists 1 and 2), and lowered several limits on compounds that affected the city’s treatment processes.
The challenge was to increase capacity while enhancing quality. An advanced treatment evaluation conducted by the Wallace Group in 2001 revealed that a process widely used throughout the world for algae and organic removal would produce the best results. This process, called Dissolved Air Flotation, has been used successfully in Greenville, South Carolina as well as several other locations in the U.S.
The five-year Lake Waco Comprehensive Study preliminary results identified a significant Cyanobacteria (Blue-green algae) population existing in Lake Waco and dominating the lake system, even in cold weather. This is due to the cyclic nutrient loading from the watershed. The Cyanobacteria produce the compounds that result in taste and odor problems in the drinking water system
The second phase of the Water Quality and Quantity project is designed to dramatically improve the taste of the finished water. In order to complete this task the City of Waco is the constructing a new clarification facility, featuring Dissolved Air Flotation (DAF) for the removal of algae and other suspended particles from the raw water supply. DAF is particularly effective on waters with significant amounts of lightweight particles such as algae.
Algae are the primary source of taste and odor causing compounds in Texas waters. DAF works by attaching air bubbles to particles suspended in the raw water and floating them to the surface of a tank for removal. The DAF treatment process includes ‘flocculation’ to bind particles suspended in the raw water into larger particles that can more easily be removed; a saturator that entrains air into a side stream for injection into the process; an air nozzle header that releases extremely fine air bubbles that attach to the flocculated particles; a skimmer which removes the suspended particles after they float to the surface of the basin; and effluent laterals which collect the clarified water off the bottom of the basin.
The odor causing compounds MIB and Geosmin are found inside the algae cells and get released to the water when the algae is killed or damaged. By removing the algae at the lake site before it has a chance to be killed or damaged during piping to the treatment plant or the water treatment process itself, water quality will be significantly improved. The reduced levels of MIB and Geosmin in the raw water once the algae is removed will allow any residual taste and odor compounds to be more easily removed in later stages of the treatment process. After DAF clarification, the water will be treated with chlorine dioxide for primary disinfection and then sent to the two treatment plants where it will be filtered through sand and granular activated carbon for removal of residual taste and odor causing compounds and any remaining suspended materials.
The taste and odor associated with Waco’s water is a serious problem, that unfortunately, requires an expensive solution. Coupled with the tightening regulatory standards and future water demand projections, the City staff and technical experts were challenged to develop a plan that would meet these requirements. The critical component of the plan is the DAF facility and its effectiveness.