After you have determined the size, location, and material of your water tank or cistern, ensuring that the tank has these essential components is your highest priority. If built and maintained properly, a water tank or cistern with these parts will be able to provide clean water efficiently and reliably for a long time with a minimized risk of complication or failure.
The Important Pieces
The Inlet is the pipe that delivers the water from the source(s) to the tank. This must be attached to the tank above the highest possible water level, which is determined by the overflow (explained next). This prevents the water in the tank from flowing back into the inlet and backing things up wherever the water is coming from. If possible, you should install a diversion or shutoff valve close to where the inlet arrives at and empties into the tank so that you can stop the flow when the time comes for cleaning and maintenance or just in case there is an emergency with the tank that must be addressed.
The Overflow is another pipe near the top of the tank that drains excess water when the tank gets full, preventing the water from splashing over the edge of the tank or flowing back into the Inlet. The height of the Overflow drain determines the water level, so placement is critical. Depending on the size of the tank, every vertical inch can equate hundreds of gallons of water storage, so you don’t want to aim too low. The Overflow also serves and extremely useful second function. When left in a large tank for a long time, most of the particles and debris that come in with the water will either float to the surface or sink to the bottom, leaving the water in the large middle part (where you pull your water from) nearly perfectly clean. If the mouth of the overflow is oriented horizontally so that it opens towards the tank’s ceiling and a funnel like extension is added to widen the circumference of the opening, then most of the water draining out will be this dirty surface layer, minimizing the loss of clean, usable water.
The Outlet is the pipe that will take water from the tank to deliver it to its end use such as irrigation and washing clothes or, if being used for drinking, cleaning, and other uses with high purity requirements, it will travel from the outlet through a proper filtration system before being delivered to its end use. To ensure you have access to as much of your clean stored water as possible, the outlet is usually located near the bottom of the tank. Due to the purifying settling effect mentioned earlier, there will be a layer of sediment that pools at the very bottom of the tank, with much cleaner water above. The Outlet should be installed as low as possible without allowing this sediment to be sucked out with the clean water. This can be as low as 6” above the floor on large (50,000 Gallon) tanks and perhaps lower on smaller cisterns.
The Drain is an important component when the time comes to empty the tank for cleaning or maintenance. The Drain is a pipe leading out of the tank, usually with a control valve or cap on the outlet, and can be located on the wall or the floor of your tank depending on what works best for your situation. However, to avoid leaving a reservoir of sediment, it should be installed at the tank floor’s lowest point to ensure the entire volume can be emptied without needing to bucket and sponge dirty water out of the tank by hand. If possible, designing a sump (a spot where the floor dips down below the level of the rest of the floor) into your tank with a drain located at the bottom of the sump can help ensure effective drainage. If the drain line is short and your tank drains directly to the ground around itself, a concrete “tongue” should be installed at the drain’s outlet to direct the water out and away, preventing the water from eroding the earth out from underneath your tank, which could put it at risk of tipping or collapsing.
The Access is the way people can enter and exit the tank when checking water levels or performing cleaning and maintenance. This is usually an opening with a lid installed on the tank’s roof above the water level, so that the tank can be accessed when it is full. There are two very important safety measures that must be taken with the Access. The first is to make sure that the access has a lid that can be locked shut to keep out critters and curious children for whom the tank can be a drowning hazard. As an alternative or additional measure for limiting unauthorized access, the ladder to the Access can be designed to be removed and stored elsewhere when not in use. The second important measure is making sure there is a way to get in AND out of the tank. A few ladder rungs or a climbing rope mounted to the interior walls of the tank can make falling in inconvenient rather than dangerous.
The Air Vent is the final of the essential components and is responsible for allowing air to escape and enter the tank as the water level rises and falls. It is usually located atop the tank and does not need to be very large, because the rate of air movement will generally be slow. It is important to ensure that the Vent (along with all other components) is built in such a way that prevents sunlight, insects, and small animals from entering the tank. This is important because sunlight and water together will inevitably become a home for life such as algae, which will attract more uninvited guests who will build a food chain in your potential drinking water. Making sure your tank is shaded from direct sun as much as possible will keep water temperature down, preventing algae growth and bringing cooler water to the tap. Critter proofing the tank with screens or valves at every point of access is also important because the tank is for water and water only, rats and squirrels need not apply.
While these are the most important components for a tank that can effectively and efficiently store clean water with minimal maintenance, there are many more ways you can improve the tank’s functionality and convenience. Much of the information in this blog post and more can be found in greater detail in Art Ludwig’s book “Water Storage”, one of a series of books that cover the many ways to harvest, store, use, and reuse the water falling or flowing on, in, or near your own home. If you are thinking of adding cisterns to your home’s existing water system or are building a new home and would like to include local water harvesting and storage features in the design, books like these can help you move forward with confidence in your own understanding of what you need to do and how it can best be accomplished, as well as advice from any neighbors or friends who have built similar systems for their own homes. Finally, if you are in doubt or your design requires expertise beyond what you can do alone, engineers and professional designers can help you make sure the system is safe, secure, and legal. Good luck!
Written by James Drumwright IV, Architectural Designer at H2D Architecture + Design