Closed Circuit DesalinationTM (CCDTM) technology is a continuous batch process that works by recirculating brine until any desired recovery level is reached. It uses standard RO membranes and equipment in a new configuration that provides substantial cost and performance advantages over conventional RO systems.
Conventional reverse osmosis (RO) techniques split a pressurized feed flow stream into a permeate stream and a pressurized brine stream.
Pressurized Feed flow = Permeate Flow + Pressurized Brine Flow
The ratio of the permeate flow rate to the feed flow rate, known as the recovery rate, is typically lower than 50% in high-salinity seawater RO systems. Therefore energy recovery devices, such as pressure exchangers, Pelton turbines or turbochargers, must be utilized to recover the energy contained in the rejected pressurized brine. In conventional RO systems fed with lower salinity water, higher recovery is achieved with multiple RO stages, often with additional pressurization between stages. The recovery in each stage is limited by the requirement to maintain sufficient brine flow across each membrane element. In high recovery systems, the energy in the pressurized brine is typically not recovered.
CCD technology is based on a consecutive sequential RO process in which feed is pressurized in a hydrostatic closed circuit from which water can leave only as permeate through the membranes. In this process, the pressurized feed flow rate is equal to the permeate flow rate.
Pressurized Feed Flow = Permeate Flow
In the CCD process, pressurized brine is recirculated until the desired recovery level is reached such that essentially none of its energy is wasted in the process. There is no investment of energy into pressurized brine, hence, energy recovery devices are not relevant in CCD systems. In addition, less pressurized feed is required, reducing the energy lost to feed pump inefficiencies. Thus, CCD technology greatly and directly reduces the energy consumption of the RO process.
The CCD process is typically performed under constant flow and variable pressure conditions. A CCD process sequence commences with an initial pressure that is a fraction of the constant operation pressure of conventional RO systems. As the salinity of the recycled concentrate increases, pressure gradually increases into the range in which conventional RO systems operate. Hence, the average operating pressure of a CCD process is significantly lower than the constant pressure of conventional RO systems, thereby reducing the basic energy required to create permeate.
Since the process recovery level is related only to the duration of internal recirculation sequence and not to the number of membrane elements per housing or per system, any CCD process is capable of reaching ultimate recovery in a single stage. Up to 65% recovery can be achieved in seawater RO (SWRO-CCD) systems and over 97% has been demonstrated for brackish RO (BWRO-CCD) and industrial water treatment systems, limited only by the scaling characteristics of the pretreated source water.
The hydrostatic CCD process is made continuous by replacing the brine with fresh feed at the end of each pressurization sequence without stopping the flow of pressurized feed or permeate. In SWRO-CCD systems, fresh feed is hydrostatically pre-pressurized in a side conduit that is engaged while the brine is displaced from the system. In BWRO-CCD and industrial water treatment systems, recirculation to the target recovery level is followed by a brief period of plug flow operation as brine is displaced from the system through a restricted outlet.
CCD systems are typically designed with three to four standard membrane elements per housing instead of six to eight in conventional RO systems. Cross flow is maintained by a circulation pump that operates at a constant flow and a constant pressure differential of below 1 bar (14.5 psi). High internal circulation combined with the fewer membranes per housing results in very balanced flux distribution along the membrane array. Hence the CCD process is typically operated at higher average flux rates, thereby requiring fewer membranes to achieve a required permeate production capacity. Higher average flux rates also provide superior permeate quality at any recovery level. Alternatively, the improved utilization of membrane surfaces may be leveraged for a further reduction in the process pressure requirement, thereby further reducing energy consumption.
Although a CCD process can be operated at higher average flux rates than conventional RO processes, the more balanced flux distribution limits the maximum flux and recovery experienced by each individual membrane element. In particular, high average flux in CCD processes is achieved without ever exceeding membrane manufacturer's operating recommendations for the first or head element in each membrane housing. This helps reduce head element fouling.
High cross flow supplied with the circulation pump reduces the deposition of particulate matter on membrane surfaces. All membrane elements are washed with both fresh feed and concentrate in the course of each sequence, disrupting and greatly reducing scaling, tail membrane element fouling and bio-fouling. These operational features, inherent to the CCD process, greatly reduce membrane erosion and cleaning requirements, thereby reducing maintenance expenses and enhancing system reliability.
In conventional RO plants, recovery, cross flow, pressure and the number of membranes per train are strongly coupled. CCD technology decouples the basic variables of the RO equation, enabling independent manipulation of these functions. The resulting extreme operational flexibility allows the RO process designer and operator to cope with variations in feedwater quality and customer requirements, optimize process performance and minimize capital and operating costs.
