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MWMC FACILITIES PLAN Design criteria used for sizing and siting the facility include the following: · Average filter .flow = 2.5 gpm/sf · Peak filter flow = 5 gpm/sf · 75% TSS removal · Total filtered flow = 30 mgd · Total filter area = 7200 sf · 8 modular filter cells each at 26 ft x 35 ft. For granular media filtration a large supply of chlorinated water is needed to provide backwash flows to the filters. It was assumed that a new filter backwash pump station would be constructed in the existing chlorine contact basins utilizing vertical turbine pumps for this purpose. It was also assumed that in the initial stages one cell of the existing aeration basins could be used for backwash equalization flow, as this volume is currently unused. Other filtration technologies listed above are well suited for smaller filtration projects and should be considered to provide filtration for level IV reuse water. 6.3 Disinfection Alternatives The WPCF currently uses a liquid/gaseous chlorine disinfection system with sulfur dioxide for dechlorination. The primary use of chlorine at the WPCF is effluent disinfection. The current disinfection design capacity is approximately 60 mgd at 60 minutes hydraulic detention time and 175 mgd at peak wet weather flows limited by chlorinator capacity. Flow in excess of 200 mgd during peak wet weather flows have been adequately disinfected in the facility only because of the reduced chlorine dosage as a result of the diluted wastewater flows. Chlorine is also used to control filamentous bulking organisms through addition to the return activated sludge (RAS) line. Table 3.2.2-4 outlines the existing WPCF disinfection unit process and equipment. Up to the secondary treatment capacity of 103 - 111 mgd, all of the PE is routed through the secondary system and disinfection facilities. During PWWFs, PE in excess of 111 mgd is diverted around the secondary treatment facilities and blended with the SE upstream of the disinfection facility. This is accomplished through a dedicated diversion channel. Projected peak instantaneous flows to the facility are estimated at 277 mgd. The modified secondary treatment system will handle a peak flow of 165 mgd. Therefore, flows to the facility in excess of the secondary treatment system capacity will be diverted around secondary treatment and will need to be either disinfected separately or combined with the SE prior to disinfection. Because the effluent pipe from the current disinfection system is limited in its capacity to 165 mgd (once the secondary system is modified), it will be necessary to keep the diverted PE and SE flow separate for disinfection. Due to growing concerns about operator and public safety, MWMC initiated an investigation to evaluate disinfection altematives other than the use of chlorine and sulfur dioxide gas. Alternatives would be required to disinfect a projected peak flow of 277 mgd. The criteria for which chlorine alternatives were evaluated were taken from the guidelines published in the Washington Department of Ecology Orange Book, which are summarized 6-22 MWMC_6.0_REV11.DOC