The Sacramento Regional Wastewater Treatment Plant (SRWTP) began implementation of a submerged induction mixers (SIMs) pilot study in late 2003 to evaluate mixing and disinfection efficiencies. A relatively high chlorine dosage had been found necessary to achieve effluent requirements for total coliform from a pure oxygen activated sludge treatment process using the current turbine pump, chlorine injectors, and flash mixers (flash mixer system, or FMS). Preliminary economic analyses indicated that there was a possibility of chemicals cost savings of chlorine and sulfur dioxide and electric energy that would recover the SIM costs in less than a year.The pilot study included installation of six SIMs -four 20 horsepower units and two 25 horsepower units in two channels at the effluent observation structure (EOS channels). One 25-hp SIM, and two 20-hp SIMs were installed in each EOS channel, directed against flow, with an average wetted cross-sectional area of 96 ft 2 (8 ft by 12 ft) and an average velocity of 1.2 to 1.8 ft/sec. Water quality and other operational data were collected prior to and post installation of the SIMs and would be used to compare with the current FMS that had a relatively low energy coefficient "G". Initial chlorine capacity tests indicated that chemical feed capacity of each SIM was far less than projected by the manufacturer. Additional observations included inadequate mixing, decrease of pH, and higher chlorine dosages. To address these issues, the SIM manufacturer recommended replacing the smaller vacuum enhancer with larger ones to increase chemical feed capacity and to place additional units in each EOS channel to provide better mixing. Additional tests of the SIMs with large vacuum enhancer rings, the placement of six SIM units in one EOS channel were run, and results were found to be similar to previous tests. In addition, there was no improvement of disinfection efficiency as compared with the flash mixers system even though chlorine dosage almost doubled. The SIM pilot study was therefore terminated.It is apparent from these results that the hydraulic mixing regimes of the SIM in channel configurations, similar to SRWTP, are not well known, and care must be exercised in applications for large channels. KEYWORDSChlorine, submerged induction mixers, disinfection feed capacity reduction, pilot plant testing, channel velocity and configuration issues, flow bypass and unsatisfactory performance. BACKGROUND
Methylmercury (MeHg) is a highly toxic and bioaccumulative contaminant, which can be produced via methylation by sulfate reducing bacteria in natural waters. The fate and transport of MeHg at wastewater treatment plants is largely unknown. The Sacramento Regional County Sanitation District performed a study to investigate the fate and transport of MeHg throughout its treatment process and recycle streams. This study was conducted from September 2004 through May 2005 at the Sacramento Regional Wastewater Treatment Plant (SRWTP). The SRWTP uses conventional primary treatment followed by high purity oxygen secondary treatment, chlorine disinfection, and discharges treated effluent to the Sacramento River in the town of Freeport, south of the city of Sacramento. The study utilized a two-phased approach; Phase 1A and Phase 1B. Phase 1A focused on the principal liquid train locations and return flows. Phase 1B sampling plan was developed from the results of Phase 1A and focused on data needed to investigate MeHg production and removal mechanisms in more detail. The study's main objectives were to evaluate the fate and transport throughout the plant, and to identify key factors that affect MeHg. Results of this study concluded that approximately 75% of MeHg in the influent was removed by the treatment process, with the majority or the removal occurring in the secondary treatment process. Approximately 98% of total mercury (THg) was removed by the treatment process. The key difference between the two mercury forms is their particle affinity. The partition coefficient for MeHg (log K p ≈ 4.0) is approximately ten-fold lower than for THg (Log K p ≈ 5.0). Mass transport models show that this attribute is the dominant factor controlling the difference in removal efficiencies between MeHg and THg at the SRWTP. Methylation may be occurring in the anaerobic digesters, as is evident by higher concentrations of MeHg in particles in the digesters and solids storage basins. Return flows from the on site Biosolids Recycling Facility and Solids Storage Basins increase the influent MeHg loading by approximately 13%.
The Advanced Treatment Technology Pilot (ATTP) project provides a valuable example for successfully implementing a fast-paced project to demonstrate compliance with stringent nutrient removal and tertiary treatment permit requirements. The ATTP project was designed, built and commissioned in ten months on schedule, on budget, with no injuries, and at a construction cost of $12 million. This paper contains a description of how the selection of a design, build, own, and operate (DBOO) delivery implementation benefitted the project. Lessons learned regarding pilot startup and operations and maintenance of the analytical instrumentation are also included. The DBOO implementation allowed and fostered a successful ATTP project which achieved:1. Adherence to an extremely tight schedule 2. Managed risk to the District 3. Appropriate equipment and materials selection 4. Seamless testing, startup and commissioning
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