Pneumatics Help Cruise Ship Treat, Disinfect Waste Water

Hydraulics & Pneumatics - December 2000

Passengers on cruise ships rarely pause to dwell on how much happens behind the scenes on these huge floating resorts. But, as they consume copious amounts of food and drink, a lot of waste is generated. International regulations generally prohibit ships from dumping waste at sea, so some sort of disinfection system is required.

The answer is the Omnipure Marine Sewage Treatment system, manufactured by Exceltec International Corp. of Sugarland, Tx. The system uses electrochemistry to disinfect the water and directly break down the organic matter in the sewage stream in marine environments. The compact, lightweight units which incorporate lightweight pneumatic valving, can treat varying volumes of sewage without long retention periods or chemical additives - while meeting all environmental regulations governing the discharge of wastewater in territorial water. Omnipure is suitable for marine and offshore applications, including cruise ships such as those operated by Blue Lagoon Cruises, Princess Cruise Lines and Royal Caribbean Cruise Line.

The machine is a Coast Guard approved Type II Marine Sanitation Device (MSD). The Omnipure MSD passes the sewage, mixed with seawater, through a proprietary electrolysis cell and breaks down the organic matter by direct oxidation. This treatment method achieves 90-95% oxidation and a near total bacterial kill within minutes.

The disinfection process starts when sewage is collected in a surge tank. Once the tank level reaches the midpoint (system on) level switch, the system is activated, and a macerator (grinder) pump reduces the solid matter to particles that are 1/8 inch or smaller in diameter. This pump supplies the motive force for the treatment.

The macerated sewage is then pumped through the electrolysis cell for disinfection and oxidation. The discharge is then pumped into a retention tank. This tank fills, and overflow is allowed to spill over the top of the tank, leaving any solid waste to collect at the bottom of the vessel. The overflow is then discharged overboard.

The system will continue to operate until the level in the surge tank is pumped down to the system off level switch. The system will rest until enough sewage is collected to initiate the system to function.

The pneumatic system is used to control spring-to-close air operated valves to control cell flow direction. The inlet flow path is partitioned so fluid enters the top of the cell, travels down the first cell channel, through a crossover slot at the bottom of the cell, then back to the top of the cell through a second channel to the outlet. A pneumatic actuator shifts a three-way valve to alternate the inlet ports. At the end of each cycle, Entertron's Smart-PAK PLUS ePLC signals the actuator to shift the valve, causing the flow direction in the cell to reverse.

The flow direction is reversed for each cycle to prevent seawater hardness deposits from accumulating on the cell electrodes. In addition, the ePLC automatically performs a blowdown operation approximately once per day. The ePLC monitors the level in the surge tank and waits until it is at its lowest point. At this point, the ePLC closes a valve located at the inlet of the macerator pump. Next, a valve opens at the bottom of the retention tank. The macerator will then pull any solids that have settled on the bottom of the retention tank and discharge them back into the surge tank through the normally open return line. This operation lasts approximately one minute. The valves are then returned to their normal positions, and the unit will return to normal operation. This blowdown cycle allows solids to be reprocessed.

Exceltec chose industry-proven microprocessor controllers manufactured by Entertron Industries Inc., Gasport, NY, to power the Omnipure control panels because (like Exceltec), Entertron offers non-standard product designs, which has drastically reduced the company's R&D costs. One of the major advantages Exceltec has found in this unit is the field - changeable EPROM. When customers require a change in the control of their Omnipure units, a new EPROM can be programmed at the Exceltec plant in Texas and shipped to the customer anywhere in the world within 1-2 days.

Exceltec changed over to ePLC control vs. hard-wired logic, because they needed to be able to expand the control scheme to cover all of these automatic functions. The ePLC reduced cost due to less wiring and fewer components. Troubleshooting was also greatly improved due to the alarm and function monitoring capabilities of the ePLC. The ability to go on-line and see the ePLC in operation was a great help in troubleshooting. The use of pneumatic valve actuators afforded greater reliability at less cost than electromechanical actuated valves.

Minor programming errors popped up when testing the first all automatic Omnipure system. However, because Exceltec engineers were able to monitor the program on-line, it was relatively easy to fix the errors and finish testing. All in all, though the pneumatics functioned very reliably.

In addition to two standard configurations and a range of treatment capacities, Exceltec can build custom-designed units to the requirements of any ship. To enable this degree of flexibility, a sophisticated, microprocessor-based control system with a liquid crystal is being integrated into most units. Through this panel, engineers and technicians can control valve switching, reduntant pumps, and perform control and alarm handling for the units.

For more information on the Smart-PAK PLUS, any other ePLC or how Entertron can best address your control application needs, as was done for Exceltec, contact us at 716-772-7216.