Water and wastewater treatment is a profound technical challenge because of the diversity and variability of processing wastes and the multiple water quality goals geared to the end uses of the treated water. It can encompass chemical, physical and biological processes and requires good chemical and microbiological control, good engineering design and careful management to assure reliability and continued successful operation.
The arsenal of available technologies is large and growing as scientists and engineers create processes to solve new and existing treatment challenges and to achieve greater efficiencies and higher water quality targets. Some of these are driven by regulatory requirements and some by end use process needs, such as wastewater reuse. As always, efficiency and economics are important drivers.
Treatment applications include the need to meet pretreatment and environmental discharge requirements mostly driven by federal and state Clean Water Act National Pollutant Discharge Elimination System discharge and effluent guidelines. Additional motivations include water recovery for producing internal process water to meet quality requirements that are often more stringent than drinking water standards, food and beverage process applications for product and non-product uses, and ultimately producing drinking water as that becomes more widely acceptable.
One process growing in use is desalination. The term desalination specifically refers to desalting, but the same technologies also remove most organic chemicals and most microbial contaminants, whether during the desalination process or in other applications. Membrane processes are experiencing expanded applications and research is continually developing improved membranes with better efficiencies, lower operating pressures and greater resistance to damage. Thermal and membrane processes produce aggressive water that must be stabilized often by the addition of limestone or calcium carbonate to prevent excessive corrosion of pipes, valves and other contact surfaces.
The International Desalination Association reports that as of mid-2015, there were at least 18,426 desalination plants in 150 countries with total daily production capacity of 86.8 million cubic meters of water. That is equivalent to 22.9 billion U.S. gallons per day. The desalination inroad into the U.S. is more limited. Perhaps the most notable examples are Tampa, Florida which has been operating since 2007, and Carlsbad, California. which has recently begun operations. After numerous problems, Tampa now produces up to 25 million gallons per day. which supplies about 10 percent of the region’s needs. Many other projects are planned and underway in water short states.
Source waters range from sea water to brackish groundwaters, process wastes and brines, domestic wastewaters with high total dissolved solids, and the on-site treatment of water used in hydraulic fracturing and produced water, to wastewater recycling for direct or indirect potable reuse. The composition of an industrial wastewater will significantly affect whether membranes can be used.
The U.S. Geological Survey estimates that saline groundwater represents about 1 percent of the total world water volume, and the available freshwater is only about 0.76 percent. Desalting brackish groundwater represents a virtually untapped opportunity to increase freshwater availability. The cost of desalting brackish water is considerably less than desalting seawater because the salts concentration is much less and lower pressures may be used. Both multimillion gallon-per-day and home-sized desalination processes are in use. Volumes have been written on desalination technologies and the details of applicability, design and operation, so this will be a conceptual overview.
Salty source water considerations include:
- Total dissolved solids range from about 1,000 to 3,000 milligrams per liter (mg/l) in some brackish groundwaters, to about 35,000 mg/l in open ocean seawater, to 45,000 mg/l in the Arabian Gulf.
- Ionic content broadly includes sodium, calcium, and magnesium cations; chloride, bromide, sulfate and carbonate anions; and other ions.
- Lower and higher molecular weight natural and synthetic types of total organic carbon may be present.
- Microbes and other organisms may be included.
Thermal processes
The thermal processes are multistep distillation methods that range from simple, small-scale distillation to commercial sea water desalination. Thermal seawater desalination has been practiced on a large scale for more than 60 years initially in the Middle East, where it is still the dominant desalting technology. Although its energy consumption and costs are generally higher than membrane processes, it currently provides most of the processed drinking, irrigation and process water in those countries. Principal thermal processes include:
- Multiple flash distillation (MSF) — Heated water boils during a rapid pressure drop and the vapor is condensed. The condensing vapor occurs on surfaces that are in contact with feedwater, so that the heat of vaporization is mostly recovered and the feedwater is preheated. This occurs sequentially in several stages. Freshwater recovery is in the 25 to 50 percent range. MSF plant characteristics include high volume, corrosion, scaling and high use of treatment chemicals.
- Multi-effect distillation (MED) — Plants may have vertical or horizontal tubes in which steam is condensed on one side and heat transfer occurs to the salt water on the other side. Pressure is reduced in each stage (effect) as the temperature declines and additional heat is provided.
- Vapor compression distillation (VCD) — These plants function by compressing water vapor on the heat transfer surface which has salt water on the other side that recovers the heat of vaporization. The salt water is heated causing vaporization. The compressor increases pressure on the vapor and lowers the pressure on the brine to decrease its boiling point.
Thermal desalination also involves removing some chemical contaminants that might be in the sea water. If petroleum contamination is present, all the volatiles may not be removed because frequently only one theoretical plate is used in each volatization stage. Also, the potential for entrainment and steam distillation of some higher molecular weight contaminants, for example, some algal toxins, may exist. The concentrates are usually returned to the sea. Although efficiency improvements are being made, thermal processes are usually energy intensive, which drives the greater use of membrane processes when designing new facilities.
Membrane processes
Membrane processes involve the passage of water through a semipermeable membrane under pressure. The pressurization reverses the spontaneous transport of water that would occur from the dilute to the more concentrated side to equalize the free energy of the fluids. Several types of membranes are used in wastewater treatment, e.g., membrane bioreactors, but desalination processes require reverse osmosis (RO) membranes; nanofiltration can have some applications. The pore sizes of the membranes range from 0.1 to 1 micrometer (µm) for microfiltration (MF), 0.001 to 0.1 µm for ultrafiltration (UF), +/- 0.001 µm for nanofiltration (NF), and 0.0001 to 0.001 µm for RO. Membranes are also used in electrodialysis processes.
The membranes were originally cellulose acetate, but now are usually layered polyamides and polysulfones with a porous support material. Ceramic membranes are also becoming available that can provide greater ruggedness and resistance to substances that can damage polymers.
MF and UF membranes have numerous applications including pretreatment to reduce particulate loading and fouling on RO and NF membranes. NF membranes can reduce multivalent ions, but they are not effective for monovalent sodium and chloride ions.
RO membranes are operated at high pressure in the range of 1,700 to 6,900 kPa (265 to 1,000 psi) to improve their efficiencies. RO membranes can remove up to 99 percent of ions, and organic chemicals with molecular weights of about 200 daltons and larger. Solvent chemicals can be a problem because they may dissolve in the polymeric membrane and be transported to the processed water side. Water recoveries can reach about 85 percent, more or less depending upon the composition of the feedwater and design. Recoveries can be increased with subsequent brine retreatment. Periodic treatments are required to remove scaling .
Granular activated carbon (GAC) is frequently used to dechlorinate the water before it reaches the RO membrane because free chlorine can damage most membranes. That can allow bacterial regrowth in the GAC column. Carbon fines may also be released, which can contribute to membrane fouling, so additional prefiltration may be necessary. Non free chlorine pretreatments to control microbial growth involve pH lowering and other disinfectants, such as chloramines or peroxyacetic acid, that do not damage the RO membranes.
One area of research and development is in forward osmosis technology. Forward osmosis involves the use of a prepared "draw" solution with higher ionic strength than the contaminated saline water to be treated, so it functions more like natural osmosis allowing water to be transferred across the membrane. The draw solution usually contains ammonium carbonate (ammonia plus carbon dioxide). The draw solution becomes more diluted as the water passes into it through the membrane. It is then thermally regenerated because the ammonium carbonate will revert to ammonia and carbon dioxide gases, which are then recycled to regenerate the new draw solution. The process is attracting attention because of the potential lower energy costs.
Managing desalination concentrates
All desalination processes produce a concentrated brine (potentially as high as 8 or 9 to 1) and a concentrate of other chemicals that were present in the contaminated water being treåated. Some concentrates might contain recoverable products. Managing the concentrates is an important challenge. Shoreline sea water desalination facilities generally have the opportunity to return the concentrate to the sea via a long pipe, but usually after pre-dilution and thermal equilibration. Inland brine facilities could be limited to lined evaporation ponds, additional treatment to concentrate the salts perhaps to dryness or possible deep well injection. Chemical concentrates will be subjected to additional requirements, except for deep well injection, before disposal because of their probable toxicities as well as salinity.
References
- Desalination Technology: Health and Environmental Impacts. Cotruvo, Voutchkov, Fawell, Payment, Cunliffe and Lattemann, eds. CRC Press. ISBN 1843393476.
- Puretech Industrial Water. http://puretecwater.com/what-is-reverse-osmosis.html
Dr. Joe Cotruvo is president of Joseph Cotruvo and Associates LLC, water, environment and public health consultants and technical editor of Water Technology. He is a former director of the EPA Drinking Water Standards Division.
