open valves immediately Applications Vacuum sewer systems may be the preferred system in the case of particular circumstances, such as: Transport Trains, aircraft, buses, and many ships with plumbing generally have vacuum systems with vacuum toilets. The lower water usage (less than a quarter of a liter per flush) saves weight, and avoids water slopping out of the toilet bowl in motion. Aircraft toilets may flush with blue disinfectant solution rather than water. A portable collection chamber is used; if it is filled from an intermediate vacuum chamber, it need not be kept under vacuum. Dry areas Lack of water in many countries and drastic water savings measures have led to difficulties with aging gravity networks with solids blocking in the pipes. Vacuum systems save water. Boggy, rocky, or permafrost terrain Flat terrain, unfavorable soil (rocky or swampy ground), or a high groundwater table (which requires dewatering trenches) can make gravity sewerage systems much more expensive. Vacuum sewers are small in diameter and leak inwards, and in frost-free areas, they can be laid close to the surface in small trenches. Water protection areas, environmental use Vacuum sewers can pass through water protection areas and areas with sensitive high ground water tables, because there is no danger of spoiling groundwater resources (vacuum sewers have a high leak tightness due to their material; and if they leak, they leak inwards). Vacuum systems are used in many environmentally sensitive areas such as the Couran Cove Eco Resort close to the Barrier Reef in Australia. They've also been used to replace septic tanks to reduce nitrogen levels in ground/surface water. Vacuum systems have also been applied to collect toxic wastewater from the environment. Seasonally sub-freezing climates If the temperatures in an area dip below freezing in winter, the vacuum line is buried below the frost line, in ground that stays unfrozen year-round (as are conventional gravity sewers). Valves, collection pits, intake vents, and control systems need to be designed to keep functioning despite cold, snow and ice. Temperature-monitoring sensors are also standard, so problems can be noticed early. In the case of Plum Island (Massachusetts), the island was prone to freezing temperature and excessive snowfall that initially made it difficult to locate a potential problem. Changes to their Pit setup and monitoring at the valve pit has helped with maintenance. Many Nordic Countries utilize vacuum sewers, it is helpful to have some type of