In accordance with the Office of the Chief of Naval Operations Instruction (OPNAVINST) 5090.1C, Navy vessels must limit oily waste discharges to less than 15 parts per million (ppm) under normal shipboard operating conditions. Newer bilgewater practices and ship design have led to higher concentrations of emulsified oils. The majority of existing Navy oily water separators (OWS) cannot effectively treat this emulsified bilgewater.
The Aircraft Appliances and Equipment Limited (AAE)/CINC Industries Combined System is an advanced two-stage treatment (OWS) system designed to treat bilgewater. The first stage is a centrifugal separator (CINC separator) designed to remove bulk oil and larger oil droplets. The second stage is a ceramic ultrafiltration (UF) membrane designed to treat emulsions and finer oil droplets. Oily wastewater processed by a ship’s OWS must contain less than 15 ppm oil in order to discharge overboard.
The performance objectives for the laboratory and shipboard evaluations were to demonstrate effluent quality, efficiency of the CINC separator, and water content of the waste oil. System reliability as well as operation and maintenance requirements were also determined.
The AAE/CINC Combined System provides continuous separation in two stages using the principles of centrifugation and UF to separate oil from bilgewater. A self-cleaning strainer prior to both stages removes finer solid particles. Bulk oil and larger oil droplets are removed in the first stage, while finer oil droplets and emulsified oil are removed in the second stage. The first stage is a centrifugal liquid-liquid separator. The centrifuge separates oil and water by amplifying the density differences between the two. The second stage utilizes a ceramic membrane module to remove emulsions and finer oil droplets. Separated oil and self-cleaning strainer flushes are discharged to a waste oil tank (WOT). An oil content monitor (OCM; provided by the user) measures the residual oil content before deciding if the system effluent is clean enough to discharge overboard.
The laboratory evaluation showed that the AAE/CINC Combined System was effective at producing an effluent with less than 15 ppm oil-in-water when processing non-emulsified and mechanically emulsified bilgewater with influent oil concentrations ranging from 0.02% to 100%. The AAE/CINC Combined System could not consistently produce a dischargeable effluent when processing oily water and aqueous film forming foam (AFFF). When exposed to oily water and liquid detergents, the system would go into recirculation minutes after starting a test. Bubbles in the effluent that formed from the detergent caused the Navy-approved ET-35N OCM to alarm. Oil break through was observed during the second mixed liquid detergent test. The system was unable to meet effluent criteria during the first oily water and chemicals (bleach, acetone, and paint thinner) test. However, the system was able to meet effluent criteria during the second mixed chemicals test. Oil was observed to break through the system when processing oily water and powdered detergent.
The CINC separator could not meet the test’s requirement of removing at least 80% of the influent oil during the laboratory evaluation. The CINC separator would typically remove 40%-46% of the injected oil in an oily water mix and near 0% of the injected oil in an oily water and detergent mix.
Over the course of the laboratory evaluation, waste oil generated from the AAE/CINC Combined System had an average water content of 90%, not meeting the test’s maximum allowable water content limit of 20%.
The laboratory evaluation results indicated that the AAE/CINC Combined System was an easily operated OWS. Overall, maintenance activities were easy to carry out and the system’s touch screen control panel enhanced the simplicity of some of the maintenance activities.
During the shipboard evaluation, water content of the waste oil could not be measured due to the system’s piping configuration. Program modifications were made to reduce the amount of water in the waste oil; however, it was still likely that water content of the waste oil was over 20%.
Four performance, one design, and three equipment failures occurred during the laboratory evaluation. During the shipboard evaluation, there were three performance, one design, and five equipment failures. The failures of the membrane feed pump were by far the most significant failures. After upgrading the membrane feed pump with more robust components, the pump functioned reliably.
During the shipboard evaluation, the AAE/CINC Combined System could not be operated without support from Naval Surface Warfare Center, Carderock Division (NSWCCD) prior to upgrading the membrane feed pump. After membrane feed pump modifications were carried out, ship’s force reported that the system was easy to use. Corrective maintenance actions were difficult to perform due to the compact design of the system.
The AAE/CINC Combined System may be a potential technology to treat bilgewater onboard Navy Combatants. This technology may be suitable as a new install or retrofit. The objective was to evaluate a centrifugal separator and a membrane polisher integrated into a single unit. Laboratory testing warranted further testing shipboard.
The goal was to identify a commercial OWS that could process the highly variable bilgewater shipboard and also require minimal operator involvement and footprint. The effective processing rate of the AAE/CINC Combined System was an issue for a ship installation. The system processes bilgewater at 3 gpm. Actuation and fluid dumping of the self-cleaning strainer was optimized to reduce water sent to the WOT. The system’s programming was modified to optimize actuation and fluid dumping of the self-cleaning strainer. This updated program was successfully demonstrated during the shipboard evaluation. Due to the system’s compact design, performing maintenance or repairs on any of the system’s pumps was very difficult.
The system demonstrated a low level of operator involvement during the laboratory evaluation. Two major equipment repair events during shipboard deployment required significant effort to diagnose and repair. Other than these two events, ship’s force did not report a high level of operator involvement. The system is complex; however, the touch screen control panel allows for easy training and operation.
Implemented equipment will likely be commercially available with custom system adjustments for each ship class, depending on space availability and ship specification. The system tested was full-scale, so there should be no scale-up issues. No proprietary or intellectual property rights issues are anticipated. The system can be purchased by any vessel with the need. The AAE/CINC Combined System purchased for this test was a standard size. Prior to shipboard installation, software modifications were made. During the shipboard evaluation, hardware modifications were made.
Numerous lessons were learned from a mechanical and system software standpoint. Several system modifications were made just prior to and during the shipboard evaluation based on system failures and to either improve robustness or performance. By far, the most important of these modifications was the replacement of the original membrane feed pump components with more robust ones. Prior to this modification, the AAE/CINC Combined System repeatedly broke down due to damage to the membrane feed pump. After this modification was performed, ship’s force was able to consistently successfully operate the AAE/CINC Combined System. Ship’s force reported that the system was never out of commission after the membrane feed pump modifications were made.
Program modifications were made to reduce the water content of the waste oil and shut off the potable water valve if the check valve failed to check during membrane backflushing. The shipboard demonstration showed that the potable water shut off program did, in fact, signal and prevent filling of the OWT with potable water. The water content of the waste oil was reduced by nearly half by reducing the flushing rate of the self-cleaning strainer. The shipboard demonstration showed that this reduced flush rate did not have any adverse effects on the strainer. Additional program modifications were made to increase the OCM flush time after shutdown and to add a low permeate flow warning to signal to the operator that the membrane needed to be hotflushed.
Further testing of the AAE/CINC Combined System was performed on DDG-57 for the shipboard evaluation. Shipboard testing demonstrated the system’s performance under real world conditions with actual bilgewater. Laboratory results were favorable for bulk and mechanically emulsified oil, but unfavorable for some chemically emulsified oil. For acceptable shipboard system performance, hardware and software modifications were required.