Innovative Approaches to RO Concentrate Management: Beneficial Reuse and Concentrate Minimization
Desalination of brackish waters and reclaimed waters using reverse osmosis (RO) is often limited by the options available for concentrate disposal, particularly in inland areas. Dewatering the concentrate is alternative approach to disposal but the available thermal and membrane technologies are still cost prohibitive for most applications. Beneficial reuse of the RO concentrate represents a sustainable alternative to more traditional concentrate disposal and treatment options because the concentrate becomes a resource rather than a pollutant. This paper presents results from a series of innovative tests utilizing ion exchange (IX), bipolar membrane Electrodialysis (BMED) and Electrochlorination (EC) technologies to recover useful products from RO concentrate that can be utilized at the treatment facility. Two of these technologies including the BMED and the EC are currently in the patent process for this and other applications. Experiments were conducted on RO concentrate obtained from a pilot-scale integrated membrane system (IMS) treating wastewater. The IX experiments focussed on recovering phosphate from RO concentrate using a chelating ion exchange resin and converting the phosphate rich regenerant into struvite, a commercially viable fertilizer. Reasonably long run lengths of up to 700 BVs were obtained with actual RO concentrate using one of the tested resins. BMED was used for generating mixed acids and bases from the RO concentrate solution after suitable pretreatment. Reasonably high concentrations of acids and bases (0.2- 0.5 M) were produced and were shown to be dependent on the concentration and the volume of the used salt solution. It is expected that upto 1 M acid and base concentrations can be produced from this process. Additionally, the RO concentrate was desalted as well. Electrochlorination using RO concentrate was utilized to convert this waste stream into hypochlorite disinfectant of 0.6% similar to that currently being utilized at water treatment plants.
The above processes present the potentially viable alternative of utilizing the RO concentrate for the production of useful products instead of having to dispose of the waste brine into the environment. Costs are currently being developed for implementation of these three strategies and will be presented at the conference.
Membrane treatment is a critical component of water recycling programs being implemented globally to increase limited water supplies. RO treatment is employed when removal of specific dissolved contaminants and total dissolved solids (TDS) is needed to achieve the target finished water quality. Installation of RO facilities has increased dramatically as the cost of membrane modules continues to decline (Adham et al, 2005). But there is a significant cost associated with the technology when utilized in non-coastal communities. RO treatment produces waste brine that can range from five to 25 percent of the influent flow. While coastal communities can utilize ocean discharge, inland facilities must rely upon conventional alternatives that are not considered environmentally sustainable and are becoming increasingly difficult to permit (Mickley, 2001).
The industry is seeking cost-effective concentrate brine handling alternatives that are more environmentally sensitive. Beneficial reuse of RO represents a promising and sustainable alternative to Zero Liquid Discharge (ZLD) methods that have been applied on a limited scale because of the large energy needs associated with the process. Recovery and beneficial reuse of phosphate and acids and bases from RO concentrate was selected for evaluation after extensive screening of the alternatives currently available.
Bipolar Membrane Electrodialysis is a membrane based electrochemical process, which uses bipolar membranes for separation of ionic species from a salt to produce the respective acid and base. This process has been used in the food industry to produce organic acids like lactic acid, ascorbic acid and salicylic acid with a reasonable amount of success (Wilhelm, 2001; Bazinet et al, 1998). It has also been applied to pure NaCl solutions to produce HCl and NaOH Mazrou et al, 1998). RO concentrate is mostly made of Na+ and Cl- ions with substantial amounts of divalent ions. However proper pretreatment of the RO concentrate stream is required for beneficial reuse of this stream for production of mixed acids and bases. Phosphorus removal from wastewater has been studied in detail as wastewater containing phosphorus that is discharged to surface streams can cause major environmental impacts (WERF, 1994). The use of the ion exchange process for this application has also been studied over the last decade (Zhao and Sengupta, 1998). Further, the ion exchange process for phosphate recovery has been enhanced by using the phosphorus recovered in the regenerant stream to produce an excellent nutrient for plants called struvite (Johnston and Richards, 2003). Struvite belongs to a class of chemicals called metal ammonium phosphates, which are insoluble in water and have an excellent biological based slow release mechanism. Struvite is magnesium ammonium phosphate (MgNH4PO4) and can be formed by addition of ammonia and magnesium to phosphate. RO concentrate from the IMS utilized in this study was expected to have between 10 and 20 mg/L P making struvite recovery from this waste stream a possibility.
Several water treatment plants have been switching to onsite chlorine generation as a disinfectant source due to operational safety concerns and ease of implementation. Conceptually, commercial hypochlorite generators split NaCl using an electrolytic cell to produce hypochlorite. Again, since RO concentrate contains significant amounts of Na+ and Cl- ions, this stream could be beneficially reused through production of hypochlorite
MWH and Sandia National Laboratories recently completed a proof of concept study looking at the above innovative beneficial reuse alternatives for the RO concentrate using bench scale testing. The beneficial reuse alternatives investigated included: production of the fertilizer struvite from the phosphate-rich RO concentrate utilizing ion-exchange and chemical addition; production of mixed acids and bases from the salts present in the RO concentrate using bipolar membrane electrodialysis; and production of usable hypochlorite utilizing RO concentrate RO as a feed stream by electrolytic reduction. This paper presents the results from this study.