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Miami-Dade Water and Sewer Department Disinfection Demonstration
Author: Peter E. Moffa; John W. Chorlog, Jr. (Miami-Dade Water and Sewer Dept.)
Date: 1/107
From WateReuse Conference, Sept. 2007

Miami-Dade Water and Sewer Department (MDWASD) conducted pilot testing of alternative disinfection technologies at the MDWASD South District Wastewater Treatment Facility (WWTF). Brown and Caldwell (BC) served as the consultant; Environmental Associates Laboratory (EAL) conducted select sampling and analyses. The purpose of this disinfection demonstration was to compare high-rate disinfection (HRD) using chlorine dioxide (ClO2) and ultraviolet light (UV) to high-level disinfection (HLD) with chlorine as defined in Chapter 62-600.440(5) of the Florida Administrative Code (FAC). HLD comprises sand filtration followed by conventional chlorination with 15 minute contact time at peak hourly flow which meets the administrative requirement of 5 mg/l TSS and the specified FC limits in Table 1. HRD-ClO2 and UV were piloted without preceding filtration using secondary effluent as influent; both were designed to treat 100 gpm. These pilots were operated in parallel to an existing 1-MGD HLD system previously installed at MDWASD. Equivalency of the HRD, ClO2 and UV to the HLD was based on meeting pathogen guidelines values stipulated by Florida Department of Protection (FDEP) as shown in Table 1. Table 1. Pathogen Guideline Values Recommended by FDEP Organism Units Average Maximum Giardia “viable” cysts/100l 1.4 5 Cryptosporidium “viable” oocysts/100l 5.8 22 Enterovirus PFU/100l 0.044 0.165 Note: 62-600.440(5), F.A.C requires that fecal coliform be below detection limit in 75% of the samples collected and not to exceed 25 CFU/100 ml It is important to note that the pathogen guidelines presented in Table 1 are in terms of “viable” protozoan counts per 100 liters and PFU/100L for enterovirus. Through discussions with the US EPA and negotiations with FDEP, consensus was reached on the interpretation and method of analyses of values specific to this demonstration. ”Viable” protozoan was interpreted by Miami-Dade as the true viability values resulting from the US EPA ATP method approved specifically for this demonstration. Three types of enumeration were considered; total 1623, DAPI+ and DAPI+ with internal structure (DAPI+IS). The DAPI+ was selected as the better of the three. Nevertheless, it is well documented that DAPI + results overestimate the number of “viable” protozoans. An Alternative testing procedure (ATP) was submitted and approved by EPA which served as the basis for the “viable” measurements. Percent reductions from the “viable” measurements were used to apply to influent quantities that were measured as DAPI+ to arrive at guideline values. This combination was necessary since analytical limitations required seeding of viable organisms at a lab scale on the one hand and seeding the influent at pilot scale in the field posed a health risk on the other. These results in addition to the DAPI+ results were used to compare the HRD/ClO2 and UV to HLD. Further instead of using Enterovirus method that results in PFU/100mL, a more sensitive method was used that resulted in MPN/100mL. Prior to the disinfection demonstration program, the existing HLD system was optimized to ensure it was working at its peak levels. Optimizing included increasing backwash rates for the sand filters and installing mixers for chlorine disinfection and increasing the chlorine dose.