marker or monitoring to locate valves when they are buried under the snow for extended periods. Low or seasonal population density With lower population densities, the costs for the collection chambers and vacuum stations are less important than the costs of installing pipe and, for gravity sewers, pumping stations, etc.. Pneumatic pipes are generally smaller than gravity-drained hydraulic ones In frost-free climates, the pipes for a vacuum system can also be buried more shallowly than a gravity system. High specific conduit lengths, where the required pipe length is longer than ~4 metres per inhabitant, will tend to make a vacuum system cheaper. In seasonal settlements (recreation areas, camping sites etc.) with conventional gravity sewer systems, sedimentation problems can easily occur as automatic flushing by daily waste water does not take place. High flow velocities within vacuum sewers prevent such sedimentation problems. The Formula 1 race tracks in Shanghai and Abu Dhabi are using a vacuum sewer system for that reason. Historic sites Historic sites may have old buildings, narrow streets, and steep terrain. Tourism may also cause strong seasonal fluctuations in population density. Vacuum sewer systems may be selected for their fast (avoiding conflicts with traffic and tourism), cost-effective and flexible installation. Examples include Flavigny-sur-Ozerain, France, and Khasab and Al Seeb in Oman. Treatment Vacuum sewer systems can be set up so that they collect concentrated blackwater (toilet wastewater) only, with the greywater from sinks and baths being collected separately (it is much easier to treat greywater than blackwater). The biosolids from a vacuum system need not be diluted with flushing water. Sewage systems usually thermophilically compost biosolids which have been separated and dewatered from a standard gravity sewer. This process is simpler if the biosolids are never watered. Composting at high temperatures kills pathogens and seeds. Biosolids compost is required to be composted at high temperatures. Sewage can also be treated in an anaerobic process with the production of biogas. This design has the potential to increase sustainability of water infrastructures. Examples Arctic In the Bering Strait region of Western Alaska both the Native Village of Savoonga, located on St. Lawrence Island, population 600, and the Native Village of Saint Michael located on Norton Sound, have been using vacuum sewer systems for years. Selawik, renovated its vacuum sewer system in 2014, saving over $217 000 a year on energy costs. Arid regions Especially in the Middle East (United Arab