The primary objective of this effort was to perform system selection for a High Shear Rotary Membrane System (HSRMS) to ensure the highest level of success in potential laboratory and shipboard evaluations. This effort selected a system which integrated advanced high shear rotary membrane technology and performance based operating configurations for the effective treatment of shipboard bilgewater. The technical objectives of this project were divided into three primary segments:
Conventional cross-flow membrane systems pump the influent at high flow rates to produce large cross-flow velocities near the membrane surface. The surface is scoured, reducing the thickness and concentration of the solute boundary layer, which increases the effluent flux. The increased cross-flow rate is maintained by a high flow pump in a recirculation loop. Pumping produces maximum cross-flow velocities as well as the transmembrane pressure (TMP) needed to drive influent through the membrane. As feed concentration increases, with time or stage number, it becomes increasingly difficult to maintain high velocities because of the increased feed viscosity. Lower cross-flow velocity, higher solute concentration or concentration polarization, and membrane face plugging lead to a reduction in effluent flux from membrane fouling or pore plugging.
Concentration polarization and cake formation can significantly reduce membrane performance. While conventional membrane systems apply high cross-flow velocities, the HSRMS generates shear forces by rotating the membrane within the process fluid. This approach enables a straightforward configuration, trading pumps for mechanical drives to generate conditions for cross-flow. Methods to further enhance turbulence and shear at the membrane surface include incorporating stationary shear elements between each membrane disk or interleaved or nested multiple disk stacks. Also, because the feed delivery and pressurization are decoupled from turbulence or shear promotion, the HSRMS can be operated at lower transmembrane pressures, which decreases solute boundary layer compaction and pore plugging in comparison to conventional cross-flow membrane systems.
The previous Strategic Environmental Research and Development Program (SERDP) (WP-1671) and Office of Naval Research (ONR) research investigated a HSRMS consisting of vertically stacked membrane disks that rotate about a hollow shaft inside a cylindrical housing. The feed stream enters the membrane chamber under pressure and is distributed across the membrane surface. Effluent is forced through the membrane and discharged through the hollow center shaft. This project investigated the correlation between membrane disk diameter, material, pore size, rotation rate, and the effects of back pulsing and continuous cleaning to performance requirements relevant to Navy shipboard waste streams. Only ceramic disk membranes manufactured by KERAFOL® proved capable of being configured and used to treat simulated bilgewater containing chemical emulsions and solids to an oil concentration of 15 ppm or less. Based on cumulative findings, recommendations were to pursue further scale-up, configuration and demonstration of larger (e.g., 312 millimeter [mm] or 374 mm) diameter, 60 nanometer pore size, ceramic disks rotating at 500 revolutions per minute for bilgewater treatment to demonstrate and validate the technology for dynamic shipboard use.
Through market research and site visits, metrics were developed which will govern the design of a HSRMS suitable for shipboard applications and potential future procurement. From the developed metrics, a contracting package was drafted and is undergoing review by the Acquisition Program Manager (APM). Once feedback is received from the APM, the package will be submitted to contracts for potential award and procurement in the event laboratory and shipboard demonstrations are pursued.
A successful shipboard demonstration of an integrated HSRMS will provide sufficient evidence of the technology’s ability to meet specific military shipboard requirements for bilgewater treatment. With successful completion of the shipboard evaluation, HSRMS can be a candidate for new ship acquisition programs that are seeking a small, compact and complete oily water separator system. Because of the compact nature and processing rate of the HSRMS, this system can also be transitioned for potential retrofit of Navy vessels through the respective Program Executive Offices and Army vessels through Product Directorate Army Watercraft Systems.
System procurement, initially seen as an area of potential risk, has been greatly mitigated through the efforts pursued in this project. With close discussions executed between the technical and administrative departments, procurement is on schedule in the event a laboratory demonstration and potential shipboard validation are pursued. The data provided from these evaluations will inform stakeholders of HSRMS potential to provide improved bilgewater treatment with lower lifecycle cost within a smaller footprint.
Because system function must be thoroughly validated, significant design changes are not anticipated. Scale up and implementation for land-based testing is currently not considered high risk. From laboratory tests, it will be determined if the HSRMS is truly suitable for shipboard applications via a go/no go decision point planned after the laboratory evaluation.