Sodium hypochlorite & deep bed filtration technologies help WWTP meet effluent TSS limits
The Miami-Dade Water and Sewer Department (WASD) is the largest water and sewer utility in the southeastern U.S., serving nearly 2.3 million residents and thousands of visitors each day. With five water treatment plants and three regional wastewater treatment plants, the department treats 300 million gal of water per day and disposes of 300 million gal of wastewater per day.
In April 2004, the WASD entered into a consent order with the Florida Department of Environmental Protection (FDEP) when nitrite was detected in the well monitoring system at the South District Wastewater Treatment Plant (WWTP). The South District plant discharges its effluent via a deep injection well system, and the FDEP was concerned about potential leakage of treated effluent into an underground secondary source of public drinking water. With that in mind, the consent order required that the WASD construct a high-level disinfection (HLD) system to produce effluent that met FDEP HLD reuse standards as well as primary drinking water standards prior to disposal.
The project’s result was the construction of one of Florida’s largest wastewater facilities, which is now able to treat to stringent reclaimed water standards. Two of the key technologies used in the upgraded plant were the ClorTec on-site sodium hypochlorite generation system and the TETRA DeepBed filtration system—both from De Nora.
As part of the upgrade, the WASD decided to replace its gaseous chlorine systems with on-site sodium hypochlorite generation. After comparing systems from two manufacturers, the utility selected the ClorTec high-output CT series on-site generating systems. The 21,000-lb-per-day treatment capacity of the ClorTec systems makes the South District plant the largest ClorTec installation in the world. The systems generate 0.8% sodium hypochlorite by combining salt, water and electricity and provide a powerful disinfection method without the need to store gaseous chlorine on site. The CT series systems meet requirements for 21,000-lb-per-day chlorine equivalent.
Testing Two Technologies
Because the South District plant had no alternate method of effluent disposal when its effluent was out of compliance, the FDEP consent order required that it meet the total suspended solids (TSS) requirement of less than 5 mg/L on a continuous basis under all conditions. The plant had typical secondary clarifier effluent TSS levels of 20 mg/L or less. Its new filtration system would need to be capable of handling influent TSS of 45 mg/L, which is the weekly average TSS permit limit for the plant. The secondary effluent TSS typically averaged around 10 ppm.
Hazen and Sawyer, an environmental engineering firm in Hollywood, Fla., which served as the improvement project’s general contractor, determined that the requirement for continuous TSS compliance was unique and that there was no existing design data that complied with the reuse standards. Nevertheless, the firm concluded that two technologies—membrane disk filters and deep bed filters—were the most promising options for the plant upgrades, and pilot tests of the technologies were conducted. Hazen and Sawyer, assisted by Miami-Dade WASD staff, performed the pilot tests on a side-by-side basis in order to assure identical influent and relative loading conditions.
The results of the pilot tests indicated that the membrane disk filtration technology did not successfully meet the required effluent TSS limits except at relatively low influent TSS levels and relatively low hydraulic loading rates. The deep bed filter selected by Hazen and Sawyer and Miami-Dade WASD, the TETRA DeepBed filtration system, successfully met the effluent TSS limits under a variety of testing periods, influent levels and hydraulic loading rates.
Upgrades to the South District plant began in the spring of 2005 and were completed in 2012. The project increased the plant’s flow capacity by 27% – from 225 million gal per day (mgd) to 285 mgd—making it one of the largest wastewater facilities in Florida.
“We have been very happy with the ClorTec and TETRA systems,” said Steve Kronheim, chief plant operator at the South District plant. “They are both working well.”