On a long enough flight, every passenger will need to conduct a restroom "layover." It is an essential system for passenger service that could have disastrous results if not properly engineered.

Once upon a time, yes, aircraft did release the biological waste of passengers over oceans and countryside. Shamefully it sometimes still happens, typically due to a valve or drain malfunction, and sometimes over populated towns. But surprisingly, a good amount of greywater still gets released in flight.

Thankfully the bathroom tech of airliners has trounced on. And today passengers get to experience the delight of vacuum toilets - and on some premium airlines, they even offer showers for long-haul flights.

The water and waste utilities on aircraft are two distinct systems. So let's start at the beginning - getting water onto the aircraft.

Water system

Figure 1. Potable water supply chain for aircraft. Figure 1. Potable water supply chain for aircraft.

Potable water is typically supplied to the aircraft through the help of a water truck that is specifically designed for aircraft ground service. Some terminals may offer a water cabinet, which is a hosed connection directly to the airport utilities.

These trucks often have a low profile that helps them fit beneath aircraft wings. This truck connects its supply hose to a check valve by the use of a coupling, and once fully secured the valve is opened to permit water flow from the truck. Water is pumped from the trucks to the potable water tank located within the aircraft fuselage.

Within the tank, sensors give both the flight and cabin crew an indication of the remaining amount of potable water in the tank. An electric pump pressurizes the potable water to ensure a positive rate of flow toward the galley, washbasin and toilet. To ensure potable water delivery to various fixtures, pipes are carefully laid out from the tank to the galley and lavatory. To prevent uninsulated plumbing lines from freezing in cold temperature, heat trace wiring may be taped to the pipes.

Figure 2. Water tank capacity of common passenger jets. Figure 2. Water tank capacity of common passenger jets. The potable water will typically arrive at sinks and basins in the aircraft galley and steward stations, as well as in restroom sinks. Most modern aircraft toilets are of the vacuum variety, which typically use little or no water per flush. However any of that water will ultimately reside in the sewage holding tank, which will be processed on the ground.

One notable trend is the Implementation of showers on long-haul flights lasting 20 hours or more. This is typically a luxury option for travelers who need to freshen up after long day of air travel. The water is warm, but showers are timed to conserve water - and weight, and therefore fuel.

While the definition of "potable" is safe to drink, the water coming from the aircraft faucets is not advisable to consume as there are many possible points of bacterial contamination, such as the inside of the water tank, ground servicing and faucet. This is the reason why most airlines offer bottled water to avoid poisoning their passengers. Water used for cooking or beverages is typically distilled or filtered. Random testing of water on aircraft by Health Canada in June 2006 found that 15.1% of the aircraft tested positive for total coliform bacteria and 1.2% tested positive for E. coli.

Wastewater systems

Greywater is typically ejected from the plane while in flight via drain mast. This is an external vertical winglet on the airplane that is heated to prevent the water from freezing. The ejected water typically evaporates within the atmosphere. If the sinks or basins are used while the aircraft is on the ground, the water ultimately lands on the runway.Figure 3. Aircraft drain mast locations. Source: ThermocoaxFigure 3. Aircraft drain mast locations. Source: Thermocoax

Notably, this subsystem requires the use of an air stop valve which was designed specifically for this application. An open drain to the plane's external atmosphere would depressurize the cabin. Instead, this mechanism safely permits the one-way flow of fluids through the drainage system. Flow through the valve is managed by a paddle and two seals. Negative pressure keeps the valve closed until the weight of the water column overcomes the negative pressure. This device also prevents large backflows that could result from clogs, which could be more dire when the plane is airborne.

Lavatory waste

Human waste collection today is entirely different from passenger airline heydays. Honeypots and the like are no longer the norm.

Most modern aircraft feature vacuum toilets, which collect waste and then transfer it to a holding tank. The toilet is powered by the pressure differential of the cabin versus the atmosphere. A little bit of water may be added to help clean the toilet bowl, which is typically a corrosion resistant and low friction materials such as stainless steel or PTFE.

The recirculating toilet has fallen out of fashion. These are chemical-type toilets which use a sanitizing liquid to dilute and deodorize waste. These models feature the famous "blue dye" which is still common in portapotties and marine toilets. Waste passed through a recirculating toilet may pass through a macerator.

Ultimately recirculating toilets have become less common due to the added systems needed to operate, and additional weight created by the blue dye liquid.Figure 4. Lavatory service truck. Source: Raimond Spekking/Creative CommonsFigure 4. Lavatory service truck. Source: Raimond Spekking/Creative Commons

Once the plane has parked, sewage service is provided by a dedicated lavatory service truck, which has low profile to ease maneuverability around the airplane. A hose connects to a outlet under the fuselage of the aircraft, and will later transfer the waste in the dedicated depository for processing.

Improvements on the horizon

Water and waste systems of an aircraft cannot be overappreciated, and the quality of these facilities is often a defining factor in the passenger experience. Aerospace suppliers are developing new solutions that will continue to push passenger satisfaction, such as lavatories that self-clean with the power of UV light, lavatories with modular walls so they can be made larger during flight, doors, faucets and toilets that self activate, and quieter vacuum pumps.

To contact the author of this article, email GlobalSpeceditors@globalspec.com