Naval Surface Warfare Center Carderock Division (NSWCCD), Environmental Engineering, Science and Technology Branch (Code 633) performed a full-scale performance evaluation of an advanced high shear rotary membrane system (HSRMS) technology through laboratory performance verification and shipboard demonstration. Previous research for the development of the HSRMS disk technology was executed by NSWCCD through SERDP/Office of Naval Research (SERDP WP-1671). Results showed the effective treatment of chemically emulsified and solids laden simulated bilgewater, which is in some instances the most challenging waste stream to treat.

Through ESTCP Project WP-201520, NSWCCD Code 633 investigated currently available commercial HSRMSs that could be recommended for shipboard applications. The Novoflow NDCF-10 HSRMS was identified as a potential alternative to existing shipboard oil/water separators (OWS) based on its compact configuration and ability to accept the KERAFOL ceramic disks. With endorsements from Naval Sea Systems Command Wet Environmental Systems and Discharges Technical Warrant Holder and the Future Fleet Project Office for Product Director Army Watercraft Systems, ESTCP awarded the NSWCCD Environmental Engineering, Science, and Technology Branch (Code 633) proposal to evaluate the Novoflow HSRMS as project WP-201708.

The laboratory evaluation was completed between March and October 2018. The Novoflow HSRMS was able to process oil, detergent, solids, and salt water mixtures and produce high quality effluent under all test conditions. Oil concentration, measured by Environmental Protection Agency 1664, was below 15 ppm for all samples. However, membrane life did not meet the criteria of greater than 300 hours and was found to be between 147-415 hours. During the laboratory evaluation, the HSRMS operated for a total of 959 hours with zero “relevant critical failures” and zero “relevant major failures”, thereby satisfying the mean time between failures acceptance criteria. The failures that did occur include the following: 1) at 50 hours into testing, the upper bearing assembly of the membrane stack leaked grease due to scoring on the silicon carbide seal and 2) at 850 hours into testing, the silicon carbide seal was found to be chipped after disassembling the membrane stack for inspection. The manufacturer identified the cause of the first failure as a torsion spring with insufficient clearance pressing the seal faces too tightly. Novoflow provided a redesigned bearing assembly as a replacement. The second failure was due to operator error.

The shipboard demonstration was completed between July 2021 and April 2022. The system treated a total of 7,963 gallons while underway with an average effluent flow rate of 4.0 gpm. The effluent oil concentration was below regulatory limits (15 ppm) for the full shipboard evaluation. The Marine Environmental Protection Committee.107(49) approved commercial-off-the-shelf oil content monitor, Turner TD-107, did not trigger any high oil content alarms and made the decision to discharge 100% of the time. Overall, the system was able to treat bilgewater under shipboard conditions and produce high quality effluent. The HSRMS did not meet the performance criteria of exceeding 300 hours of operation before requiring a membrane regeneration. After 28 hours the membrane resistance exceeded the maximum threshold (1.49 psi/gfd @ 68°F), indicating the need for membrane regeneration or replacement. However, it was discovered that the system was unintentionally operated as a dead end filtration after 26 hours, which expedited the membrane fouling. In addition, the Novoflow HSRMS, in its current configuration, did not meet the reliability criteria for maintenance ratio. This was due to corrective maintenance needed to clean a level sensor and correct tolerances with the feed pump variable frequency drive (VFD).

Reliability criteria could be met with the following considerations, 1) an automatic rinse function during shut down to prevent oil fouling sensors, 2) relocating sensors to avoid fouling from oil, 3) redesign to avoid the use of VFDs, and 4) redesign processing capacity using full scale evaluation results.

Overall, the results indicate HSRMS as a promising technology for bilgewater treatment; however, its current design would need to be repackaged for Department of Defense (DoD) shipboard applications.

Further considerations would be needed for militarized ships, including redesign for shock, vibration, electromagnetic interference and other requirements (e.g. cyber security). A reliability study of the redesigned system needs to be performed to ensure design changes meet the OWS requirements for DoD ship platforms. Based on the results of the laboratory study and shipboard evaluation, a redesigned HSRMS with disk membranes as the core treatment technology could be a candidate for new Army, Military Sealift Command, Navy, and United States Coast Guard ship acquisition programs that are seeking a small, compact and complete OWS system.