Water. Desalination + reuse
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TECHNOLOGY DispOsaL mETHODs bECOmE LimiTiNG faCTOr Economic considerations can become the limiting factor for water resource managers' and policy-makers' decisions to implement water reuse and desalting technologies. Membrane technologies continue to decrease in cost, as recovery efficiencies continue to increase. Currently recovery efficiencies are found in the 85-95% range with proper blending, and process configuration. Disposal Option Consideration Discharge to oceans or bays No adjacent oceans or bays, or conveyance means are not economical Discharge to surface water Adjacent natural channels/washes have intermittent flows Deep well injection Required geological composition not present or conveyance means are not economical Discharge to sewers High total dissolved solids (TDS) may interfere with biological wastewater treatment processes Evaporation ponds Regional climate experiences freezing temperatures so additional storage volume is required Table 1. Disposal methods and considerations. We find that there appears to be less room to improve the recovery efficiency, and hence find significant cost savings. There is more room to decrease the costs of the concentrate disposal through the introduction low-to-no-energy technologies. Considering the three primary challenges listed above, the discharge disposal methods (sewer, surface waters, oceans or bays) can become quickly excluded, as regulation criteria are typically stringent in reducing the introduction of large volumes of such concentrate into existing systems that are unable to quickly or easily equalize concentrations of high TDS, low dissolved oxygen or those with higher specific weights. This leads to the implementation of evaporation ponds becoming increasingly feasible when climate conditions are favorable, such as in the southwest US. brackish water supplies and treating wastewater for water reuse is to become a sustainable and cost-effective option in a community's water supply portfolio. These considerations provide the opportunity to delve deeper in two primary areas; 1. Developing innovative methods to reduce the concentrate stream volume from membrane processes 2. Developing innovations in the processes and materials associated with enhancing evaporation. Both can reduce the costs and footprint of implementing evaporation ponds for concentrate disposal and help support the feasibility of increasing the reliance on brackish water sources at inland locations. When considering the implementation of evaporative ponds for the disposal of concentrate from membrane treatment, the primary factors are 1. The cost of the land and liner 2. The efficiency of the desalination processes. EvapOraTiON Or sOLar pONDs Specifically looking at selecting evaporation ponds or solar ponds as a concentrate disposal method, we have found this can be a cost-competitive solution in semi-arid In this research, an average land cost of and arid climates with relatively low land US$ 23,721 per acre, was developed for costs and relatively level terrain. However, available undeveloped, or underdeveloped it can also become financially exclusive land adjacent to existing urban or suburban due to extensive surface area requirements municipal water and wastewater treatment and the associated impermeable liner costs. When concentrate discharge to sewers or surface waters, injection to deep wells, or land application are not a viable disposal method at inland locations, the development of innovations in evaporative methods acres becomes fundamental, if implementing membrane Figure 2. Average Land versus Liner Costs technologies for desalting Cost, $ desalination include target control items such as inorganic scale, organics, and biofilm, where pretreatment, antiscalant and pH adjustment are typically used to increase recovery. Additional considerations include the use of chemicals in membrane maintenance including: citric, hydrochloric and sulfuric acid; sodium hydrochlorite; chlorine gas; and hydrogen peroxide. Pretreatment is considered to prevent surface scale from occurring on the membranes, which is a product of crystallization of soluble salts that occur as concentrations exceed solubility limits. Pretreatment chemicals can be used to manage scale-forming compounds by interfering with the nucleation process and inhibiting the formation of crystals. These chemicals typically end up in the concentrate stream and are part of the disposal method selection decision. The use of antiscalants can improve recovery rates by lowering the scaling potential of soluble salts beyond their solubility limits or saturation levels. These chemicals also end up in the concentrate stream and need to be considered. The reduction of the feed pH is completed to reduce the potential of calcium carbonate scaling on membrane surfaces. Hydrochloric acid (HCl) or sulfuric acid (H2SO4) are typically used to lower the pH and typically end up in the concentrate stream and are part of the disposal method selection decision. All of those considerations factor into the concentrate management design and process selection and may add to one or more of the three primary challenges of concentrate management, which are: 1) High levels of dissolved salts in the solution 2) Low levels of dissolved oxygen in the solution 3) A higher specific weight of the solution. Each disposal method has its own selection considerations. These are listed in Table 1. May-June 2013 | Desalination & Water Reuse | 33 |