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Wastewater to 21 mgd Water Supply Augmentation Pilot Testing at Miami Dade South District

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The Miami Dade Water and Sewer Department is implementing a major upgrade program for its existing South District WWTP in response to capacity upgrade requirements, regulatory mandates and commitments to optimize water resources. The existing facility provides secondary treatment using high purity oxygen activated sludge. In 2005, MDWASD initiated implementation of one of the largest High Level Disinfection (HLD) Programs in the country - a 285 mgd peak hourly flowrate capacity at South District WWTP. More recently, due to water supply consumptive use limitations and related regulatory mandates, and concurrent with the ongoing HLD Upgrade, MDWASD also initiated implementation of one of the country's largest water reclamation facilities −26.2 mgd of treatment to deliver 21 mgd of finished product for water supply augmentation. Treatment processes planned for the SDWRP apply best available technologies; however, pilot testing was required to fill in some of the experience gaps with very low nutrient limitations and for site specific process performance verification, to assist with defining final permit limitations for some specific parameters, design criteria, process optimization, and qualification of microfiltration and ultraviolet disinfection - advanced oxidation equipment manufacturers.

A five month pilot test program was conducted at the South District WWTP. Unit processes tested included deep bed mono-media filters, microfiltration, reverse osmosis, ammonia removal(via ion exchange or breakpoint chlorination) and ultraviolet disinfection/advanced oxidation. Product stabilization was also reviewed using the RTW model for Corrosion Control and Process Chemistry 4.0. Key parameters of concern monitored throughout the pilot program included Total Suspended Solids (TSS), Fats Oils and Greases (FOG), Turbidity, Silt Density Index (SDI), Total Dissolved Solids (TDS), Ammonia, Total Phosphorous (TP), and NDMA. Treatment limits/goals were established as part of the study based upon primary and secondary drinking water standards, the Florida Administrative Code (62-610.564) for groundwater recharge and indirect potable reuse, a Memorandum of Regulatory Understanding with Florida Department of Environmental Protection regarding future potential permit limits, and local Miami Dade (MD) Code requirements and an understanding of these requirements with the MD Department of Environmental Regulation (DERM).

For key parameters of concern (TSS, Turbidity, FOG, SDI, TDS, and ammonia), the pilot program successfully verified the ability of the proposed unit processes to achieve the expected goals/limits for treatment for the reclamation facility. However, these concerns were identified:

Total Phosphorous limits proposed as a result of the pilot testing are 0.02 mg/l, as compared with a narrative limit of “non-degradation” stated in the MD Code; it is anticipated that regulatory agencies will accept the 0.02 mg/l limit as sufficient to meet the non-degradation intent.

Pilot test NDMA average concentrations of 0.00088 ug/l were slightly above the goal/limit of 0.0007 ug/l, but the full-scale facility is anticipated to be able to comply with the limit.

Five RO system vendors were tested for performance verification. Rejection, permeability, and flux decline were compared for all systems tested. Pilot data were also used to confirm cleanability / recovery of the RO units.

A very low limit of 0.5 mg/l of ammonia was established by the regulatory agencies. Pilot testing confirmed that this limit could not be achieved by RO. A review of breakpoint chlorination as compared with ion exchange treatment for ammonia removal downstream of the RO process was performed which supported the use of ion exchange for this application. Breakpoint chlorination testing revealed that very high doses of chlorine (on the order of 25 mg/l) were needed to achieve the ammonia limits, and that these levels resulted in a corresponding increase in NDMA levels and exceedance of those limits, so ion exchange was selected.

Concerns with potential membrane fouling by FOG in the secondary effluent were addressed during the pilot using permeability, membrane pressure, flux and other parameters since direct FOG measurements are difficult and not useful for assessing FOG impacts. Results showed no detectable impact to membrane performance via these parameters and therefore, no expected fouling concerns.
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Keywords: NDMA; advanced wastewater treatment; breakpoint chlorination; deep bed filtration; ion exchange; microfiltration (MF); reverse osmosis (RO); ultraviolet disinfection — advanced oxidation (UVA); water supply augmentation

Document Type: Research Article

Publication date: 01 January 2011

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