Environmental Considerations for The Disposal of Desalination Concentrates
Dr. Mike Mickley, President,
Where demands for water exist beyond the amount freshwater sources can meet, desalination of lower-quality waters provides the only new water source. As a result, there has been a dramatic worldwide increase in the number of and capacity of desalination plants.
Although the technologies of thermal and membrane desalination have a substantial history and the industry is well established, the industry-regulatory interface is relatively new and is in an early stage of development. The degree of regulation of desalination byproducts has varied greatly from country to country and has been perhaps most intense in the United States, where the degree of concentrate regulation has evolved rapidly since 1986.
Desalting of brackish water and seawater to provide needed drinking water fulfills a basic social need. And, in general, it does this with less environmental impact than other water processing options. There are no fatal flaws in desalination technologies and there is a substantial history of environmentally safe operation of desalination plants. There is a need, however, to identify, understand, and address environmental concerns that are increasingly being raised. Society, through research and eventually the legislative and regulatory process, must wrestle with the or cost/risk issues to define when and how environmental effects warrant preventative and corrective action. The trend has been and will likely continue to be of both increasing concern for and increasing regulation of environmental impacts. For negative environmental impacts, the industry needs to recognize this trend and determine ways of mitigating the problems. More generally, the industry needs to be proactive in addressing these issues that may, if not addressed, slow or limit realization of the tremendous potential for desalination to meet the growing needs for alternative water sources in an environmentally safe and cost-effective way.
The paper reviews the characteristics of membrane (reverse osmosis) and thermal concentrates and the environmental concerns associated with each method of concentrate disposal. Particular attention is paid to the surface discharge of concentrates since it is the most frequently used method of concentrate disposal, both for seawater and brackish processes.
Characterization of Desalination Concentrates
Factors affecting concentrate characteristics include:
Concentrate characteristics are summarized in Table 1 for the various desalting processes considered. Unlike most industrial processes, the major waste stream produced by the desalting processes - the concentrate - is not characterized by intentionally added process chemicals. Rather, the concentrate reflects the raw water characteristics, the raw water composition at a more concentrated level. Raw source water is pretreated with additives to control scaling, fouling, and corrosion of process components, all of which compromise system performance. The level of these added chemicals is relatively low in most cases (typically less than 10 ppm), so that concentrate is overwhelmingly defined by the constituents present in the raw water. An exception to the low level of added chemicals is the use of acid to reduce scaling potential from carbonate species. Acid may be added in amounts up to a few hundred mg/L. Besides lowering the pH, the addition of acid will increase the concentration of the acid anion species such as sulfate in the case of sulfuric acid addition. Such anions, however, are not of toxic concern.
Thus the feed water to the processes is slightly modified raw water. In the process itself most or all of the raw water constituents get concentrated depending on the particular process. Typical concentration factors, different for the various processes, are given in Table 1.
The temperature of reverse osmosis (RO) concentrate is ambient, while the thermal process concentrate is typically 10 to 15¡F above ambient temperature. Due to the much higher processing temperatures in thermal processes, the potential for corrosion is significant. The industry has shifted towards the use of more corrosion resistant materials as well as away from the continuous use of acid. Consequently, corrosion potential has decreased. Where acid and chlorine are used, good control of chemical levels is important. Post-treatment of concentrate is usually minimal. Brackish reverse osmosis (BRO) concentrates that originate from groundwater sources may be low in dissolved oxygen and high in dissolved CO2 and even H2S. Post-treatment may consist of aeration for oxygen, degasification to get rid of CO2 and H2S, and pH adjustment to bring the pH back up to neutral levels that will minimize corrosion potential and be compatible with receiving water lifeforms. Seawater reverse osmosis (SWRO) processes may have only pH control as post-treatment.
Concentrate from thermal processes, such as multistage flash evaporation (MSF) and multi effect distillation (MED), is typically mixed with once-through cooling water prior to discharge. The cooling water usually contains a certain amount of free chlorine which is dependent on the sophistication and effectiveness of chemical control. The dilution of concentrate results in a final discharged effluent that is rarely more than 15% higher in salinity than the receiving water. In larger thermal desalination plants that are sited along with steam turbine power plants, the concentrate may be further diluted with condensate water. SWRO concentrate, on the other hand, can be 100% higher in salinity than the receiving water.
Other wastes produced by the desalination plant, such as cleaning wastes, may be mixed with the concentrate and discharged together. The intermittently produced cleaning wastes may be stored and continuously bled into the concentrate or discharged to the sewer. Membrane chemical cleaning agents include acids, bases, complexing agents, enzymes, detergents, and disinfectants. The wastes also contain the scaling and fouling materials that are cleaned from the membrane system. In thermal processes, the fairly standard acid cleaning produces a waste that may also contain elevated levels of metals due to tube corrosion/etching.
In summary, whiles desalting concentrate depends on the particular process involved, there are several important shared characteristics. Desalting concentrate may be described as having low levels of process-added chemicals so that raw water characteristics determine final concentrate characteristics. The concentrates are characterized by few parameters other than high total dissolved solids (TDS) relative to the raw water and higher temperature in the case of thermal process concentrates. Membrane concentrates may be further characterized by low pH and thermal process concentrates (frequently) by the presence of trace metals and residual chlorine. It is important to realize that, in the absence of corrosion products and with good chemical control and use of non-toxic additives, desalting processes do not produce more pollutant material or mass; they redistribute (concentrate) that which is present in the raw water.
Concentrate Disposal and Environmental Concerns
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