Disinfection 2005 Manuscript Presentation Number: 1 ALT B: Evaluation of Submerged Induction Mixers to Improve Coliform Disinfection by Chlorine at the Sacramento Regional Wastewater Treatment Plant

Abstract

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