In a developing world, potable water is a limiting factor for development. Providing drinking water to an increasing population is more complex every year due to the reduction of quality and quantity of surface or ground water or simple geological conditions (such as salt-water intrusion of the water table or desert, dry surface conditions).  Many communities are reaching the limit of what their fresh water resources have to offer. In the Arabian Gulf, water is more valuable than oil and its control often leads to war.  In states like Florida or California, which both have experienced water shortages, water management is an essential issue for growth management.  Alternative sources are necessary to meet the demand for potable water for the future generations.
     Desalination techniques, such as membrane technology, offer one solution to this difficult problem.  By removing contaminants such as salts, heavy metals, radionuclides and pathogens from water of lesser quality, brackish or even saline water can be used to produce potable water.  For the past two decades, the use of a drinking water production technology called reverse osmosis to provide potable water from brackish and saline water sources has increased significantly.

1.1 What is Reverse Osmosis?

     Osmosis is the diffusion flow between two solutions (solvent plus dissolved species or solute) separated by a semi-permeable membrane that only allows the solvent to pass through and acts as a barrier for the solute.  The solvent flows from the medium of lower concentration toward the one of higher concentration (Raven and Johnson, 1991) (Figure 1.1 (a)).  The pressure gradient between the two solutions is called osmotic pressure difference and varies according the concentrations of the solute and the type of solvent present in the two solutions (Figure 1.1 (b)).  Reverse osmosis (RO) is effected when a pressure gradient greater than the osmotic pressure difference is applied on the medium of high concentration.  In this case, the flow is reversed and the solvent travels from high to low concentration areas and the separation of water from solute becomes possible (Bhattacharyya and Williams, 1992) (Figure 1.1 (c)).
 
 

     Applying pressure equal or greater than osmotic pressure to a saline solution will reverse the osmotic flow, and potable water will be produced on one side of the membrane while ions and dissolved gas will be concentrated on the other side. Seawater, brackish water or groundwater are treated through RO by using appropriate membranes and can be separated into potable water and concentrate.

1.2  Application of Membrane Technology in Drinking Water Treatment

     Among all the desalination techniques, the RO process allows the production of cleaner drinking water (Pontius, 1996).  Other desalination techniques include NanoFiltration or Membrane Softening (NF or MS), MicroFiltration (MF) and UltraFiltration (UF) and Electrodialysis/Electrodialysis Reversal (ED-EDR).  However, RO remains the Best Available Technology (BAT) according to USEPA (Pontius, 1996) to remove pathogens and most inorganic and organic materials from water (Table 1-1) and is the only desalination process with a filtration size smaller than 1 nm (Table 1-2).
 
 

1.2.1 Descriptions of Desalination Processes (Pontius, 1996)

1.2.1.1 Pressure Driven.

Saltwater Reverse Osmosis (SWRO) (high pressure RO) and
Brackish water Reverse Osmosis (BRO) (low and medium pressure RO):
Reverse Osmosis uses a very small pore size membrane and can operate at different pressures according the salinity of the source water.  Brackish surface and groundwater sources are preferred to ocean water due to higher costs for operation and maintenance of SWRO (energy costs for high pressure pumps, maintenance of membranes and pumps degraded by scaling and corrosion).

Membrane Softening (MS) (also known as NanoFiltration) (ultra-low pressure):
Membrane softening operates at much lower pressure due to a membrane more pressure sensitive than the one used in RO plants, and is very effective with groundwater with low Total Dissolved Solids (TDS).

MicroFiltration (MF) and UltraFiltration (UF):
Micro and ultrafiltration use membranes with pore sizes larger than the membranes used by RO and MS.  Consequently, they are not efficient filters for most of the ionic compounds. Micro and ultrafiltration cannot be considered as full desalination techniques, and are only used to clarify the source water.  The water produced by those methods can not be considered potable unless additional post-treatment is applied. MS and UF are not used in the US but represent an interesting research subject.  Most MS and UF plants are found in Europe.  The largest plant is located near Paris, France and has a capacity of 15MGD [57,000 m3/d] (Adham et al, 1996).
 

1.2.1.2 Non-Pressure Driven:

1.2.2 Pre- and Post-Treatment to the Desalination Processes

1.3  Concentrate Disposal

    Worldwide, desalination plants use RO more than any other desalination process to treat seawater as well as brackish and fresh water (AWWA-MTRC, 1998).  In Florida, the production of potable water from brackish groundwater by RO has increased exponentially for the past three decades (Figure 1.2). If the impacts on surface and groundwater supplies due to the pumping of increased amounts of water to be treated can be minimized, the major concern of the application of RO is for concentrate disposal.
 
 

1.3.1 What is the problem?

1.3.1.1 Why is concentrate classified as an industrial waste? Of what consequence is this classification for the disposal of concentrate?

1.3.1.2 Ion imbalance and toxicity.

1.3.2 What to do with the concentrate?

1.3.2.1 Dilution in surface water.

1.3.2.2  Deep Well Injection (DWI)

     Concentrate is an industrial waste but has not been included in the list of hazardous waste that can be injected in Class I wells.  Desalination by-product requires a Class V well for deep well disposal.  The costs associated with upgrading a Class I municipal well to a Class V well may limit the use of DWI for disposal of concentrate.  In addition, the environmental consequences of deep well injection are not completely understood and require further investigations.

1.3.2.3 Sewage treatment or Publicly Owned Treatment Work (POTW).

1.3.2.4 Land application (Evaporation ponds and irrigation).

1.3.2.5  Water minimization

     According to Mickley (1993), the solution to concentrate disposal other than surface discharge that answers the best any financial and environmental concerns lies in deep well injection (when the hydrogeology allows it) for large capacity plants (>0.5MGD [>1,900 m3]) and in land application (spraying) for smaller units (Table 1-6).
 
 

    Recently, the city of Melbourne has reviewed the costs associated with three different alternatives for disposal of concentrate from their RO facility which presently  discharges to the Eau Gallie River.  The cost of a deep well injection solution were ranging from 3.5 to 10 millions dollars while displacing the point of discharge from the creek to the Indian River Lagoon was estimated to cost $1 million (A final alternative considered by the city involved an ocean outfall with an estimated cost of $4 millions).
    Membrane technology is a relatively new technology developed industrially for only two decades.  Much information about the possible toxicity remains to be thoroughly investigated.  As an example, seawater ion imbalance as a cause of toxicity for aquatic organisms has only recently been discovered.  Similarly, little is known about the effects of disposal methods.
 

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