Desalination chemicals adjust pH, control scale, remove particulates, prevent biological fouling, clean and remineralise the highly purified product streams. Specialized chemistry is also required to synthesize the reverse osmosis (RO) membranes. Defining the consumption on a per-unit-water basis is difficult, though, owing to the many factors that determine which chemicals are used and at what levels, such as the type and quality of the water source, the desalination method used and the severity of the conditions operated in.
Thermal desalination, which essentially involves distillation of clean water from salts and other contaminants, is energy intensive, but relies on low-grade heat for much of the duty, reducing the energy cost. RO membrane processes consume less energy, but use electricity, which is more expensive. The membranes are also expensive, but they generally last at least seven years. RO membranes are semipermeable. They allow water molecules under high pressure to pass through but repel salts and other dissolved and suspended particles.
Trends in the desalination industry that directly impact chemical consumption include reducing energy consumption, increasing operating efficiencies and lowering costs, while also minimizing environmental impact. Specialty additives can be designed in response to the composition of different water feeds, perform at higher temperatures and lower pressures and under ever higher salt concentrations. The overall effect is to increase efficiencies, minimize waste and reduce cost.
Pretreatment chemicals used for brackish and seawater desalination include pH adjusters, coagulants and flocculants, deposit control agents (antiscalants, dispersants), biocides and reducing chemicals. In post-treatment, chemicals include chlorine, anti-corrosion additives and compounds for remineralisation.
Chemical treatments may be similar in thermal and membrane applications, but the technologies are adapted for each. For membrane processes in particular, chemical additives are designed to restore and prolong membrane life, maintain system performance, reduce downtime and protect the membrane from oxidizing agents.
In the case of some chemicals, changing technology has reduced demand. For example, to prevent scaling, the pH of feed water has traditionally been adjusted using acid. But it is becoming more common to replace acids with specialty antiscalants. The use of physico-chemical pretreatment with coagulants and flocculants (typically high-molecular-weight polyacrylamides) is also declining as pretreatment becomes more robust and the performance of equipment used to treat intake water improves, especially when combined with a set of organic coagulants.
Antifoaming agents (several classes of surfactants) are also necessary in thermal desalination processes.
Antiscalants for thermal processes need to perform at ever higher temperatures. Operating efficiencies in these plants improve at higher temperatures, and thus antiscalants need to continue to work effectively under such severe conditions. Biocides are also critical. They prevent biofouling, which is a major issue for membrane processes.
The water that leaves desalination plants is often of very high purity and thus may need to undergo remineralization to reduce its aggressiveness. This process,involves injection of carbon dioxide (CO2) and calcium hydroxide to form calcium carbonate. Chlorination and fluoridation typically follow as well.
Dew, a world leader in development of high performance specialty chemical solutions for desalination processes, we have introduced an extensive range of high performance solutions for smooth operation.
Dew Membrane Solutions include these three elements :
The DewtreatTM range of premium antiscalants has a solution for all of the fouling and scaling problems encountered in RO operation.
The DewtreatTM is a complete range of cleaners specially formulated to restore membrane performance through removal of inorganic, organic, silt, silica, and metal deposits.
Microbiological Control Agents
The DewtreatTM is a range of solutions that provide effective control of biofouling for both on-line and intermittent dosing operations.