Making Good Better: Optimizing Salinity Management Using Time-Step Water and Salt Balance

Good, Better, Best. Never let it rest. To proactively manage their limited water resources, many inland utilities and private sector water consumers have embarked and maintained certain level of salinity management activities. This includes developing regional salinity monitoring and control program as part of relevant master planning documents, establishing plans for developing brackish groundwater, surface water, and reclaimed water resources, banning the use of timer-based softeners, setting up local limits on salinity as part of the industrial pretreatment program, etc. Some utilities established water and salt mass balance calculations using excel spreadsheets for their systems or subsystems. These water balance may be sufficient to accomplish their specific and limited objectives in the past. But to better plan for and effectively design, implement, operate, and most importantly, optimize the desalination and concentrate management facilities, accurate tracking of flow and salt concentration (both TDS and individual ions) are crucial. A complete case study is used to demonstrate how more accurately tracking flow and salt concentrations can result in significant saving and performance improvements for salinity management. A sanitary district in Arizona provides tertiary treated effluent to three local golf courses and three parks for irrigation water. The District currently operates a 3.0-mgd Advanced Water Treatment Facility (AWTF), which further treats the effluent by ultra-filtration (UF), for storage and recovery in the aquifer. The quality of the treated effluent, while consistently meeting all permitted water quality standards, has evaluated TDS and sodium, not ideal for turf irrigation. The District evaluated the feasibility of implementing desalination and zero liquid discharge (ZLD) or near ZLD concentrate management alternatives, including unit processes such as primary RO, inorganic pretreatment (Lime Softening plus Granular Media Filtration versus ion exchange), Secondary RO or Electrodialysis Reversal (EDR), Brine Concentrator, Crystallizer, and / or Evaporation Ponds. One unique challenge about the project was about evaporation pond sizing. Considering that the Town is nearly built-out, the maximum possible size of evaporation ponds was limited to 4 to 5 acres. The pond level and operation over time must be modeled closely. Because the actual flow to the AWT and the brine stream generated from the future RO system could change day by day, the conventional way of pond sizing based on monthly average flow and annual average evaporation rates are not sufficient and would result in oversized ponds less than 5 acres. A dynamic time-step based mass balance model was utilized to optimize the blending operation so that just enough water is treated by RO to meet the blended water sodium goal (110 mg/L). This approach minimizes the brine stream going to the pond. The daily pond level is animated in the model, allowing the user to track the pond operation and sizing the pond. Other features of this tool include user-friendly graphical interface, treatment alternative analyzer, what-if and sensitivity analyzer were also leveraged to project treated water quality under various conditions, develop a reliable and reasonable basis of design, set forth the footprint requirements, establish an evaporation pond operation plan, and
generate capital and O&M costs. The project considered several innovative salinity and concentrate management technologies, including monovalent selective ion exchange membrane, which can selectively remove sodium. Another technology investigated during this study was mineral polarization technologies, which enhance plant growth and remediate soil salinity by acting on charged minerals and polar non-minerals present in water. When salts are hit by a pulsed electrical field, they can break down into an uncoupled charged form, which passes through the root zone easier and faster, facilitating better drainage and turf growing conditions. Due to the proprietary nature of the technology and the difficulty in quantifying any changes in measured sodium concentrations, phone interviews of selected golf courses that are currently using these technologies were conducted. The feedbacks were fairly interesting. The study also touched on establishing comprehensive salinity management strategies for the District regarding the local salinity imbalance. Several supplemental practices that could potentially improve the turf growth on the golf courses were discussed. These items may not be sufficient as stand-alone solutions to the District’s salinity issues. However, when used to supplement the proposed engineered alternatives, they could reduce the need of treatment costs.

This presentation is available to AMTA Members only.