Cruise ships go for vacuum toilet systems

Whenever a new cruise ship is ordered or delivered, there is a tendency to concentrate on the glamorous aspects while ignoring much of the hidden equipment that contributes to the comfort of crew and passengers. One such area is the toilet system, an essential component on any ship but especially true for passenger vessels which produce large quantities of grey and black water.

All toilet systems for cruise ships nowadays are based on the vacuum principle which has been steadily developed and improved since the 1970?s. The main reason for this is that the vacuum system is the most cost-effective solution due to the high-level of installation flexibility, small pipe diameter, simple installation, low water consumption and vertical lift capability.

The world?s largest and leading supplier of vacuum toilet systems to the cruise ship market is the Finnish company Evac with approximately 80% of the market, while the other leading supplier is the Norwegian company Jets Vacuum.

The Swede Joel Liljendahl developed the first vacuum toilet system in the 1950?s by

creating a vacuum using conventional vacuum pumps installed on a vacuum tank. While this system has good vacuum generating capacities, it also has some drawbacks including the fact that a vacuum tank is needed.

This is particularly a problem with large systems requiring capacious and bulky tanks and it is also difficult to install, is space hungry, expensive and complicated. Liljendahl sold his concept to Electrolux, who saw a great potential in the maritime ship market and gradually it became quite common on passenger ships and large

ferries.

Market comeback

Although this system was overtaken by more recent types, new pump technology vacuum tanks are making a strong comeback in the market. Advantages include extremely high efficiency, no foam, small footprint, volume and weight, longest overhaul intervals and silent operation. A disadvantage is that there is no aeration effect of sewage although this can be accomplished in the main holding tank.

In 1975 Evac introduced a simpler and more economical system as a result of a new technological solution that is based on a sewage ejector powered by a centrifugal pump. The waste water is pumped through an ejector nozzle and back to an atmospheric collecting tank until a discharge valve is opened and the water discharged for further treatment. This system with ejectors is now the most common solution for newbuildings.

Advantages include ease of operation, low maintenance costs, aeration of the waste, quick reaction to low vacuum levels and the collecting tank always has holding capacity. Disadvantages of the system are a large footprint, volume and weight, foaming at high water temperatures, inefficient holding tank volume and the large number of pumps.

The vacuum macerator (vacuumarator) type screw pump solution is the latest innovation of the vacuum toilet system and is produced by Jets Vacuum, a company established in 1986. Vacuum is created in the piping by water sealed screw pumps. Heavy particles are first separated from the waste to a container located before the pump, after which all waste is macerated by an integrated macerator before entering the pump.

The vacuumarator, which is at the heart of the Jets system, is a simple solution for operating a vacuum toilet system and is internationally patented. The compact and light weight vacuumarator device generates a vacuum, macerates sewage and pumps the effluent to either a treatment plant or to a collecting tank for short holding, air separation and efficient pumping. The system allows for an in-line installation of a toilet systems vacuum generator and simplifies the total installation while eliminating the requirement for a tank in order to generate vacuum. The first cruise ship to be fitted with the Jet system was Cunard?s Seabourn Pride back in 1988.

Advantages include the flexible installation of vacuum generating equipment, no tank required for vacuum creation, small footprint, weight and volume. Disadvantages are the high-energy consumption, high consumption of fresh water, short maintenance intervals and no aeration effect on sewage which has to be arranged in the main holding tank.

All modern vacuum systems are ideal for retrofitting on ships with old gravity

systems.

The high degree of installation flexibility combined with a minimal space requirement allows for installation of a vacuum unit almost anywhere in a ship in order to improve capacity, reliability, the function of the sewage treatment plant, reduce operating cost and eliminate foaming problems.

Environmental

considerations

Recent research shows that very few shipboard biological treatment plants produce effluent discharges that are within the US Coast Guard and IMO regulatory limits.

Some vacuum systems can create conditions that can seriously hamper the biological treatment process because of the long retention time in the holding tanks without aeration. A common practice is however to collect black water in intermediate holding tanks which is then transferred in batches to the treatment plant. On cruise ships such tanks are usually about 10m3 and the retention times can reach 12 hours. In such circumstances anaerobic conditions are likely to occur with growth of undesirable micro-organisms and formation of sludge although this is not the case with ejector

systems.

Another problem can be shock loads when too large a batch of black water is transferred to the treatment plant after long retention time in holding tank. This gives frequent and instantaneous upsets to the treatment process because of the shock loads that can carry sludge with a very high anaerobic content. However, a modern automation system, which is often installed onboard cruise ships, is an efficient way to control the distribution of sewage in small batches to the onboard treatment plant. Evac has supplied this concept for all Voyager class vessels.

A few countries have national standards for vacuum sewage systems and these are guidelines for design, capacity and installation. Classification societies have no regulations or formal approvals for vacuum systems and therefore there are no certificate requirements. These systems are, in principle, a transport conduit for black water and, as such, not covered by IMO, US Coast guard, or DOT/MSA regulations. These regulations however apply for all overboard discharge.

The last ten years has seen a dramatic increase in the number of vacuum toilet systems installed in all types of ships, particularly for passenger ships, and the majority of Far Eastern shipyards are now using vacuum toilets as standard.

Analysis of operating expenses for the three types of marine vacuum sewage systems shows that the basic vacuum pump system has the lowest running costs while the ejector and vacuumarator are equally both higher.