5.  Conclusions and Recommendations

5.1  Summary

     Desalination technology offers the capacity to produce high quality potable water from saline waters.  Using this technology, water supplies have been provided for areas where potable water is scarce.  Furthermore, membrane technology is one of the best available technologies (BAT) for drinking water production.
    The use of desalination technology continues to be  developed worldwide.  Countries from the Middle East or the Caribbean favor SWRO to provide potable water to their population.  The United States has been using both SWRO and BRO extensively for the past 20 years.  In Florida which represents more than 60% of the membrane potable water of the US, 200 millions of gallons of potable water are produced every day thanks to membrane technology.  However, for every 100 gallons of potable water produced, around 20 gallons of desalination potable water by-product or concentrate has to be disposed which represents 50 MGD for Florida alone.  The concentrate has a composition similar to the source water used by the water treatment plant but with concentration of contaminants 3 to 4 times higher than the source water. Concentrates from different plants have differing characteristics depending on the source water, the pre- and post-treatments and the desalination technology chosen by plant operators.
    Several options for concentrate disposal include land applications, sewer, deep well injection, or surface discharge and numerous regulations exists to control the disposal of desalination by-product.  An NPDES or UIC permits must be obtained in order to discharge concentrate in surface water or to use a deep well, respectively.  The control of permits is authorized to state governments by USEPA.  Disposal of the concentrates remains as a major concern.  Concentrates from various locations continue to fail bioassays.  Cause of toxicity have been ascribed to seawater ion imbalance, or simply because a specific toxicant (or group of toxicants) is present at an elevated level.  However, very little is known on the environmental impact of the concentrate and each disposal method.

5.2  Recommendations
 
 

Table 5-1. Future task force for a better understanding of environmental effect of desalination potable water by-product.  These suggestions should help to clarify the concerns about the disposal of the concentrate and determine the future of membrane technology as a potable water production technique. 
- Investigate and study the possible environmental impact of concentrate discharge on receiving waters.
- Improve communication among the desalination world.
- Favor reused water policies and encourage potable water conservation.
- Take advantage of the NPDES monitoring program and use the data already being generated to perform plant 
  to plant comparisons.
- Evaluate the application of the mixing zone concept for desalination discharges and determine if dilution is an 
  acceptable solution to concentrate disposal.
- If dilution appears as an acceptable solution, continue to develop engineering work on diffusers and mixing 
  apparatus at the point of discharge of the concentrate. 
- Review the classification of desalination potable water by-product as an industrial waste.
- Research deep well injection techniques.
- Use hybrid solutions to dispose of the concentrate from membrane facilities.
- Determine mechanisms of concentrate toxicity.
- Do not rule out seawater desalination and ocean disposal of concentrate.
 
5.2.1  Environmental impacts.
    Environmental assessments of the impacts of surface water discharge of RO concentrate should be performed. Very little information is available on the environmental impacts to surface water disposal of concentrate.  The only references that can be gathered are cases in which standards have been violated or bioassays have failed, but the effects need further evaluations.  Important questions to ask include:  Are there any impacts of concentrate discharges on surface water quality and aquatic life, and what is the magnitude of those effects?  Are there any impacts on population and diversity of aquatic organisms in receiving water? Does the quality of receiving waters show significant changes as the result of concentrate disposal?
5.2.2 Communication.
     Better communication among the desalination world should be encouraged.  Dialogues between the desalination plants operators and the legislators, among States, among countries, between the professionals and the public should be induced in order to favor feedback and improve technology and predicting protocols through annual workshops and conferences.
5.2.3 Reuse  water policies.
    Policies designed to save water by reusing the produced water from wastewater treatment plant or by informing the general public of the importance of saving in their every day life could help to decrease the anticipated potable water demand of the next twenty years. Reducing potable water demand of the community would lessen the need to increase desalination potable water treatment plants capacity in the future.
 
5.2.4 NPDES Monitoring.
    The data gathered by the NPDES required monitoring should be a starting point for these environmental impact investigations. An intensive review and analysis of this data should be initiated to determine if there are similar trends among all the membrane plants.  Ratios of groundwater to concentrate composition, correlations between toxicity and ion imbalance, influences of pre- and post-treatments on concentrate can be examined by reviewing the data required by the discharge permits.  These data should be used to document the environmental impacts of concentrate disposal to surface water.
5.2.5  Dilution
    Elements present in groundwater are concentrated by roughly a factor of four in the concentrate.  Moreover, a 25% blend of concentrate and receiving water passes the toxicity tests.  Therefore, a fourfold dilution of concentrate should be adequate to return it to the initial composition of the groundwater. A mixing zone at the point of discharge should be sufficient to achieve such dilution. However, the concept of a mixing zone requires additional studies. How large a mixing zone can or should be allowed?  What dimensions should the mixing zone be? Is there any risk of sediment accumulation of radioactivity, metals, or other pollutants? Is a minimum circulation or flow necessary in order to get an efficient mixing zone?  What concentrations can the mixing zone handle?
    For receiving waters with poor flushing rates, like the Indian River Lagoon, additional study of groundwater seepage rate and influence on surface water should be quantified to put the concentrate studies into context.
5.2.6 Engineering Solutions to  Dilution.
    Dilution may be a good solution to address ion imbalance toxicity.  The use of diffusers at the end of the discharge pipes should allow a small and efficient mixing zone. More work should be done in this direction if dilution appears as an acceptable solution to concentrate disposal.
5.2.7 Legal Approach.
    An new classification of concentrate has to be reconsidered.  The industrial waste label does not take into account the site-specific aspect of concentrates and the seawater ion imbalance issue. An another approach could be to allow deep well injection of the concentrate into Class I municipal well by changing the UIC classification of deep wells.
 5.2.8  Research Deep Well Injection.
    Deep well injection may appear to be the favored alternative to surface water discharges.  This method has been used successfully all over the country for many applications including concentrate disposal. However, the reliability of deep well technology and its long-term effects on the environment still have to be determined and investigated.
5.2.9  Use of hybrid-solutions.
    The use of hybrid solutions (blending with wastewater and deep well injection, surface discharge and wastewater) would probably resolve problems that each method presents individually. Mixing an industrial wastewater with a domestic wastewater changes the blend classification and can be disposed of under conditions less stringent than in the case of an industrial wastewater alone, and Class I municipal deep well could be used without upgrades.
5.2.10 Develop bioassays and research mechanisms of toxicity.
    Toxicity tests that can determine the cause of chronic effects the concentrate can present are needed in order to differentiate between toxicity due to ion imbalance and/or to the presence of toxic concentration of a specific compound.  Moreover, further studies have to be done for a better understanding of the mechanisms of ion imbalance toxicity in order to prove or refute its role in concentrate toxicity.  If toxicity is confirmed, is it the result of an excess or the lack of one or several of the major ions? Do all of the major seawater ions have an influence or only several of them?
5.2.11  Seawater Desalination and Ocean Discharge.
    Seawater desalination and ocean discharges should not be completely ruled out.  This process is successfully used in the Middle East and in the Caribbean. Seawater desalination may be  more expensive and require more maintenance but its concentrate is less sensitive to source water characteristics, more uniform from one plant to the other and appears less toxic to marine organisms.
 

back                        next
table of content
home
Desalination Links

about the author
take a look at my resume.

drop me a lign or two with
your comments and advices.
[email protected]
 
 

Hosted by www.Geocities.ws

1