Selection of Treatment Technology for Removal of Fluoride, Arsenic and Nitrates from Groundwater
- Silvana M. Ghiu, PhD, PE, Kevin Alexander, PE - Hazen and Sawyer
The Estrella Mountain Ranch-Southern Solutions Water Supply Project (Project) will introduce a new potable water source to the Estrella Mountain Ranch community located in the City of Goodyear, Arizona. This project provides the Community with a local source of water to augment the other available sources including the imported water from the north as well as recycled water. The project is located in the southern end of the City of Goodyear, Arizona and includes two wells, a water treatment plant and a concentrate disposal facility. The project is necessarily due to the source water (groundwater) having inorganic constituents at levels that exceed State and Federal limits for drinking water. These constituents are Nitrate, Arsenic, Fluoride and Total Dissolved Solids (TDS), which are common in groundwater. Nitrate is attributed to agriculture, whereas Arsenic, Fluoride and TDS are common in the local geology and are dissolved into the water in the ground over time.
The final product water quality from water treatment plant will be required to meet both the Federal and State regulatory limits including less than 10 mg/L Nitrate Nitrogen, less than 10 ug/L Arsenic, and less than 2.0 mg/L of Fluoride. Such statutory limits drive the treatment process selection. The statutory limit for TDS at 500 mg/L is considered a secondary Maximum Contaminant Level and can be exceeded since it is an aesthetic standard and it was agreed to match the City of Goodyear typical water quality. An analysis was conducted on two technologies and treatment strategies that would meet the required water quality. The two technologies are Activated Alumina (AA) and Reverse Osmosis (RO). The first technology, AA, is efficient for removal of Fluoride and Arsenic. It uses a media that is engineered to attract and remove Fluoride at low pH while also removes Arsenic in the process. The media can reduce Fluoride levels from 5 ppm to less than 0.1 ppm by adsorption. The amount of Fluoride removed from the water depends on the contact time and it is known that removal is more effective at lower temperature and lower pH water. Ideal pH for treatment is 5.5, allowing for 95% or greater removal rat. However, this technology does not remove Nitrates and therefore, the treated water would require a blend with other water source that is treated and imported from the northern part of Goodyear.
The second technology that was evaluated was reverse osmosis (RO). RO process will remove the majority of the TDS, Nitrate, Arsenic and Fluoride as well as other inorganic and organic constituents including hardness. RO removes a significant portion of the salts, therefore, the system will be designed with a bypass stream around the RO to allow blending in order to achieve the target product water quality levels.
The Activated Alumina generates a waste stream (7% of feedwater) that is acidic and which would need collection, equalization, neutralization and pumping up to a semi-enhanced evaporation pond system prior to disposal. The RO process generates a concentrate stream containing most of the dissolved salts present in the feed. The concentrate stream would be 10% of the feedwater based on projections from the water quality. Based on the detailed analysis on capital costs, ease of adding future capacity, the ability to treat the water completely to the desired water quality and for the ability to adjust to future water quality changes, Reverse Osmosis was selected as the recommended technology for water treatment. This paper presents details of analysis for the proposed technologies and elaborates on the process of selection.
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