Robert W. Sokoll Water Treatment Plant, City of Waxahachie, Texas

The Robert W. Sokoll Water Treatment Plant (WTP) is a 20-MGD microfiltration membrane plant designed to serve the City of Waxahachie and the Rockett Special Utility District, and treats water received from the Tarrant Regional Water District with a process that includes chlorine dioxide addition in the raw water pipeline for oxidizing iron and manganese; chemical induction rapid mixing and coagulation with alum or PACl; two 10-MGD, three-stage flocculation and sedimentation basins with tube settlers; five granular activated carbon (GAC) contactors for TOC removal and taste and odor control; immersed membrane filters; ultraviolet advanced oxidation with hydrogen peroxide for additional taste and odor control; and chlorine/chloramine disinfection. The project included a 4-MG precast concrete, baffled clearwell, and high service pump station.

Designed to initially provide 20 MGD of treated water, the major process components and the chemical storage tanks were designed conservatively so that the plant can easily be uprated to 25 MGD by adding membranes and UV equipment. To expand the plant to 40 MGD, the plant will need additional flocculation, sedimentation, GAC contactors, clearwells, high service piping, membranes, and UV equipment in the membrane building. The site is planned for an ultimate build-out capacity of 80 MGD.

During preliminary design, APAI conducted a disinfection evaluation to determine the appropriate technology for disinfection at the new plant. APAI evaluated systems for chlorine gas, bulk delivery of sodium hypochlorite, and on-site generation of sodium hypochlorite. The evaluation included a life-cycle cost comparison and an assessment of non-economic factors. The disinfection evaluation indicated that a chlorine gas system was less expensive than other alternatives on a capital-cost and present-worth cost basis; however, non-economic factors such as safety for the surrounding community and for the treatment plant operators favored the hypochlorite systems. On-site generation of sodium hypochlorite was selected for the design.

Based on the results of this analysis, APAI designed a 3,000-lb-per-day hypochlorite generation system, including solution water pretreatment, brinemaking, generation, storage, and solution pumping systems. As a backup to the generation facility, APAI designed the system with the capability to dilute and feed bulk hypochlorite.