The Process and Water Quality Specialists

Desalination
  • desal 10
Desalination Membrane Process

High Pressure Membrane System: In a typical RO facility with cylindrical pressure vessels each houses several membrane elements.  A group of cylindrical pressure vessels are operated in parallel is called a stage.  To increase system recovery, the concentrate from one stage can be fed to a subsequent stage (sometimes called as brine-staged or multistaged system); and to improve solute removal, permeate from one pass can be fed to a second pass (a two-pass system or permeate staged system).  In multistage systems, the feedwater flow rate reduces in each succeeding stages due to permeate extraction from the feed water.  To maintain sufficient crossflow velocity and to avoid solute accumulation on the membrane surface, multistage systems are designed with decreasing number of pressure vessels in each succeeding stages.  Various configurations for multistage and two-pass systems exist and the specific design depends upon the feedwater and product water quality and goals. Array Configurations of RO facilities can be seen here:

ED/EDR: The basic ED/EDR unit consists of several hundred cell-pairs bound together with electrodes on the outside and is referred to as a membrane stack. These stacks are then placed in series and a three-stage EDR process is shown here. Unlike RO systems, concentrate and permeate both are fed to the subsequent stages and permeate water is taken only from the final stage (upon obtaining desired TDS concentration). Importantly, RO/NF energy cost is based on the volume of water treated; whereas for ED/EDR processes, it is proportional to the salts removed. Therefore, these processes are usually only suitable for brackish feedwaters with a salinity of up to 12,000 mg/L TDS.  With higher salinities the ED/EDR process becomes more costly than other desalination processes. As a rule of thumb, approximately 1 kWh is required to extract 1kg additional salt using ED/EDR. Bacteria, non-ionic substances and residual turbidity are not affected by this process and can therefore remain in the product water and require further treatment before certain water quality standards are met.

Post treatment:  Permeate water is typically low pH, low hardness and low alkalinity and process requires post treatment. Post treatment typically consists of pH and alkalinity adjustment that can be accomplished by a variety of treatment processes.  Addition of disinfectant in product water is must to ensure microbial safety in the distribution system. If the desalinated water is being combined with other sources of water supply, it is very important to ensure similar water quality characteristics for both water sources. If the desalinated water will serve as the sole water source for the distribution system, it is important that additional impacts on horticulture and taste and odor concerns are adequately addressed.