Pilot Testing Indirect Potable Reuse Treatment in South Florida
- Jayson Page and Enrique Vadiveloo - Hazen and Sawyer
Many utilities are under tremendous regulatory pressure to produce reuse water to reduce impacts aquifers and recharge them. Often times, due to the dense urban development, the use of large area land application has limited potential and would be prohibitively expensive to implement. As such, the several utilities have explored numerous options to develop alternatives including large scale power plant cooling, coastal wetland rehydration, shallow aquifer and deep aquifer indirect potable recharge. The purpose of this paper is to present the results of indirect potable reuse pilot projects for the shallow aquifer and canal systems.
These facilities were intended to treat secondary effluent from the wastewater treatment plants by applying the best available treatment (BAT) technologies. These facilities each pose several unique challenges and opportunities. In some cases these challenges are source water related. For example, one is a pure oxygen wastewater plant that has limited hydraulic retention time and is difficult to convert to biological nutrient removal. Another source water issue is related to plants receive wastewater flow that is consistently high in colloidal fats, oils and grease FOG. FOG has been a nuisance at several facilities where advanced filtration was proposed. Colloidal FOG is more problematic because it tends to remain suspended throughout the process. This material can blind filters and cause high differential pressures across membranes they come in contact with, making the application of the BAT difficult.
The second type of problem, when considering wastewater reuse at a plant, is the location of the discharge. In many cases, surface water discharges have more restrictive water quality requirements. This study looked at the impacts of discharge requirements on reuse applications. In the case of surface water discharge, the location of the a canal may be convenient and lower conveyance costs but may require more treatment. As mentioned above, conventional aeration facilities are best suited to the nutrient conversion by biological processes. The conventional treatment plant in this study was located adjacent to a surface water drainage canal, providing the greatest potential for cost reduction.
The conditions at the first plant can complicate treatment operations and make it difficult to achieve typical nutrient limits. Initially, it was proposed that a subsurface discharge be used to eliminate the need for elevated nutrient removal. This created a need for injection wells, but was intended to be offset by the lower treatment cost. Whereas, the physical location of the second treatment plant in this study may reduce transmission costs, but be held to higher surface water limits, limiting its overall costs. Ultimately both plants were held to extremely low ammonia and phosphorous limits by the local regulatory agencies, as well as, monitoring and treatment of potentially regulated substances.
The pilot studies were conducted using the secondary effluent from the plants for the purpose of demonstrating the feasibility of the proposed treatment process, along with verifying water quality expectations and critical operational design parameters. The demonstration equipment included deep-bed sand filtration, membrane biological reactors (MBR), chlorination, membrane filtration (MF), reverse osmosis (RO), and advanced oxidation using ultraviolet light (UV-A) with hydrogen peroxide (H2O2). Based on initial results at the pure oxygen facility, ion exchange (IX) was added to the treatment process stream to reduce ammonia concentrations to below the local limit 0.5mg/l. At the time, this was the first IX system piloted for this application.
Water Reclamation Pilot Plant
The pilot tests took place over a 5 month and 12 month period based on regulatory requirements. Samples were taken daily to maintain for operational control. In the shorter, pure oxygen pilot, five MF vendors and two UV vendors were tested for future purchasing comparison and life-cyle cost evaluation. For both pilots, the key operational and analytical parameters believed to be governing for these facilities included: Total Organic Carbon (TOC), Total Suspended Solids (TSS), Total Nitrogen (TN), Total Nitrite plus Nitrate, Nitrate, Nitrite, Ammonia , Phosphorus. In addition to these substances the microconstituents 1, 4-Dioxane, and N-Nitrosodimethylamine (NDMA) were also monitored throughout the process as potentially regulated compounds. In addition the potentially regulated compounds an extensive series of testing for a large number of microconstituents was performed to determine the number of the compounds (i.e., plasticizers, pharmaceuticals, hormones, industrial chemicals) present. The pilot testing also considered ability of each unit process to remove them.
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