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Dewatering Reverse Osmosis Concentrate from Water Reuse Applications Using Forward Osmosis

Project: 05-09
Type: Report
Year Released: 2007

Program: Principal
Funding Partners: Bureau of Reclamation, California State Water Resources Control Board
Total Investment: $203,596.35 (Cash: $79,999.99, In-Kind: $123,596.36)

Principal Investigator: Samer Adham, Ph.D., MWH

Background

Reverse osmosis (RO) is an effective barrier in water production systems when removal of dissolved contaminants or salts is needed to achieve high finished water quality. RO concentrate usually comprises 10–30% of the influent for surface water and 50–75% of the influent for seawater. While coastal communities can utilize the ocean to discharge the RO concentrate, inland facilities must rely upon more problematic conventional alternatives, such as surface water or sanitary sewer discharge, evaporation ponds, deep well injection, and land applications. These options are costly, not environmentally sustainable, and increasingly difficult to permit. Thus, proper handling and disposal of the RO concentrate have become a critical environmental issue, particularly for an inland community.

A novel process of dewatering RO concentrate is forward osmosis (FO). FO is defined as the net movement of water across a selectively permeable membrane driven by a difference in osmotic pressure across the membrane. FO has been studied for a variety of applications such as volume minimization of sanitary landfill leachate, concentration of fruit juices, and desalting emergency water supplies for homeland security operations. The main advantage of using FO in water and wastewater treatment is the very low energy consumption rate, since no external pressure is required while rejecting a wide range of contaminants with possibly a lower membrane-fouling propensity than pressure-driven membrane processes have. The main challenges, however, exist in the manufacture of high-performance FO membranes, the selection of easily separable draw solutions with a high osmotic pressure, and the optimization of process configurations to minimize concentration polarization (CP).

Goals and Objectives

The project investigates:

  • the viability of utilizing FO for dewatering RO concentrate,
  • alternative membrane configurations for FO applications,
  • innovative draw solutions and compare them with baseline draw solutions, and
  • the economic feasibility of dewatering RO concentrate by using FO

Research Approach

Task 1: Membrane Optimization: Conduct a series of experiments on various membranes to determine the most optimum membrane type for DO applications.

Task 2: Draw Solution Optimization: Employ the most optimum membrane determined from Task 1 to evaluate the relative performance of several draw solutions.

Task 3: Real World Application: Use the best performing membrane and draw solution identified from previous tasks to dewater IMS RO concentrate.

Task 4: Beneficial Reuse Optimization: Determine the improvement in beneficial reuse of the dewatered concentrate.

Task 5: Economic Viability Assessment: Evaluate the economic viability of the DO concept for RO concentrate treatment.

Findings and Conclusions:

  • The FO process is shown to be economically feasible for RO concentrate minimization.
  • Dendrimers are a promising osmotic media as these macromolecules provide high osmotic pressure.
  • Ultrafiltration (UF) has the potential to reconcentrate the dendrimer along with its surface ions, as preliminary UF experiments achieved 87.3% rejection of the surface sodium ions.
  • Bench-scale experiments have shown that the RO concentrate was further concentrated in FO using salt (NaCl) as the draw solution.
  • Magnetic nanoparticles were evaluated in this study as a candidate draw solution. However, the magnetic fluid could not provide the high osmotic driving force for dewatering RO concentrate.
  • Albumin—a protein that has a special role in regulating the osmotic pressure balance within blood vessels— was also not able to provide the high osmotic pressure required for dewatering RO concentrate.
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