Rainwater Harvest and Storage

Chapter 14

Drawing of rainwater tank or components

The roof overhang and steel diagram are examples only. If the roof of a reservoir of 75 m3 (20,000 gallons) extends 75 centimeters outward (30"), rainwater collection increases by fifty percent.

An overhang of 1.4 m (55") doubles the roof area, this size is a construction project that will require consideration of the bracing methods during construction. An overhang of ninety centimeters (35") has proven strong enough in the past to support 15 cm (6") of soil and sod. The steel diagram omits small pieces used to reinforce abobe the wall, where the roof curve turns upward.

The next drawing has been altered slightly to better illustrate the wall thickness in proportion to the height. A more accurate thickness depiction is necessary to render the minimum fifteen centimeter air gap between the lowest point of the inlet and highest point of water surface when the tank is full. This distance was originally maintained to prohibit possible storage water contamination from flowing into a public water supply or water well. In this case the gap provides a separation which makes a barrier to contamination of the stored water through direct and prolonged contact with outside debris which may have collected on the roof.

The simple inlet shown requires further consideration for shading sunlight that promotes moss growth in the reservoir. While these green plants are not harmful, they are a life cycle of growth and decay which might eventually host something undesirable. Locate the inlet on the north side of the tank. Construct a small ferrocement umbrella to shade most of the light. This design will also help divert wind that blows water away from the inlet before it enters the reservoir.

Drawing of rainwater tank inlet screen

Although sanitation is primary, one should not be afraid of water which falls as rain. Bacteria grows everywhere, even inside our healthy bodies. It is true that bacteria spore are very durable; space scientists, for example, work in clean rooms to keep from contaminating their experiments with earthly life forms that may ride with their experiments and perhaps contaminate other planets. Astrophysicists have determined that precursors of life are floating on interstellar and galactic winds, and that they probably were collected by our planet and deposited on the surface by rain and snow. Today there are millions of people using anti-bacterial soaps and living with worry that their bodies might be invaded. Relax. Most problems will be avoided by realizing that the roof which collects the water should be treated like a clean dish rather than the kitchen floor. Those who are so terrified of germs they use antibacterial soaps should be more concerned about the environmental consequences of the soap passing through sewer systems into rivers, and evolution of super-germs from the offspring of the surviving germs.

Wash your hands after using the bathroom. Wash your shoes before walking on the roof. Most germs do not want to live on a hot roof, still, a quick rinse of your shoes in soapy water will wash away the dog poop you may not have noticed in the trail leading to clean the rainwater collection system. If hands and shoes are not clean, its better to stay off the roof.

I have friends who built a modest new home, in Hawaii. They collected rainwater from the roof and saved it in a plastic swimming pool under the house. What is shared here is for thought and should not be taken as the ultimate words on rainwater harvest and storage. There are codes in some areas which position the rainwater cistern underground, near the foundation. This is something I do not recommend. I agree with the friends who replaced the plastic swimming pool under the house with an above-ground ferrocement cistern. Why? Contaminates from any source do not seep into a reservoir placed on top of the ground. The above-ground reservoir is bathed in cleansing sunlight and all water borne contaminants drain downward and away.

A small rainwater wall encircling the roof is very low cost. Make the wall 10 - 15 centimeters high (6"). Make two “L” shaped strips from welded wire. Attach them to the roof. Cover with fine mesh and then plaster.

The position of the overflow pipe determines the minimum air gap. The lowest point of the overflow pipe is the maximum height of the stored water. Notice that the water containment wall is not centered on the tank. The far side from the inlet needs to be slightly higher so water flows with the circular arrows to the inlet.

Draeing of rainwater capture on a cistern roof

The simplest version of a reservoir roof water collection system includes a rinse plumbing design which is a good idea but presents a problem. If it is not maintained, it soon fills with damp material that acts as a growing medium.

The idea is that a preliminary portion of a rainfall is used to rinse the roof. Then the rain is contained in the reservoir itself. This is accomplished by connecting the horizontal rinse pipe to a vertical pipe with a 90º ell fitting or a clean-out tee. A slightly open faucet at the bottom of the vertical pipe continuously drips. The first portion of rain will rinse the roof and flow down the stand pipe, since that water merely drips from the faucet, the vertical pipe fills. When the rinse pipe is full, rainwater flows into the reservoir via the rainwater inlet. When the rain stops, the dripping faucet allows the vertical pipe to drain and be ready to accept rinse water from the next rain.

Rainwater falls on the roof and flows toward the inlet. The rinse pipe is lower than the inlet so the water enters the rinse pipe first. Rainwater rinses dust and other contaminants into the vertical pipe until it fills. A valve at the bottom restricts the outflow. The amount of rainfall for rinse is controlled by the size of the vertical pipe; large roofs will thus be better cleaned by larger pipe. This system works well for areas which have heavy rain. Those who live in arid climates will probably not use this design and simply use a broom once or twice a year. Some regions have abundant rain and little debris while others are semi-arid with much debris. The range of conditions varies greatly and the technique used to collect and store rainwater will vary as well.

Once I helped repair a reservoir for a rainwater collection system that serves an orphanage in the Bahamas; the roof had never been cleaned and the children were bright-eyed and healthy. The down spouts from rain gutters lead to intermediate open junctions which utilize window screen to catch any leaves or other material. The junctions are where rain gutter down pipe makes a transition to 10 cm water pipe (4"), they are located where excessive deposits are quickly noticed and can be easily cleaned with a tool located nearby, for the rare occasions when the screen has caught enough leaves and twigs to block and divert the water. I have also seen rainwater systems in Hawaii that were never cleaned.

drawing of rinse pipe

A forest environment drops a large amount of vegetation which can turn into soil and support vegetative growth in rain gutters within one year. Large size down pipes and more specialized material separation equipment is required in forested areas where periodic rainfall may be brief and heavy. In such an environment, the simple tank roof rinse pipe previously illustrated will be 10 to 15 centimeters in diameter (6"), the lower valve fitting will have a reducer bushing or perhaps a less expensive cap or plug with a small hole drilled through it. The fittings at the bottom of the rinse pipe should only be hand tightened for easy removal of collected material.

Although it is usually possible to locate and purchase every part described, there will be projects and locations which require fabrication on site. This chapter is intended to give a brief introduction to a plumber’s parts inventory that may be of use to those interested in ferrocement reservoirs. To be fair, these parts are not completely confined to a single trade. Carpenters, for example, often install the rain gutters which bring rainwater to the reservoir. Carpenters or subcontractors specializing in rain gutters may also be the ones who install vertical pipes which accept the first rinse from a roof as described previously.

A related subject is ultraviolet light used to remove bacteria. Installation and maintenance of the UV light canister is similar to changing a neon light bulb. Size is determined by flow rate and volume, this is similar to selecting a pump from a pump size chart.

More than one roof may be used for collection of rainwater that is stored in a single reservoir. Such an arrangement might be the only source of fresh water for an entire neighborhood after of a large storm. This situation directs attention to sanitation requirements not yet discussed and the reader is urged to acquire professional training where public health is involved. With that said, we can now proceed to the fundamentals of collection from several roofs into one tank. Keep in mind that if many people will be bringing water containers to fill after a large storm, it will be necessary to assume that at least one of those persons has a dirty container, or dirty hands, or both. This is not a negative judgement upon the quality of people in general, it is simply the way one must treat a central public water source so that it does not become a vector for sudden and rapid spread of epidemic disease.

There should be a method to place the container so there is a standard air gap between the plumbing and the container, especially when those who need the water fill the water containers. Do not use a water hose which has been lying on the ground and handled by many people. Do not insert a hose in the container opening. This precaution may sound elementary but after a large storm people are not necessarily thinking as clearly as they usually do. One person should be chosen to supervise the dispensing of water after a storm. Older women have proven to be good managers of water after a storm.

The water dispensing area must have an additional faucet nearby for public hand washing. The entire container filling and washing facility must be located downhill from stored water. Be sure that all the water is usable without power so people do not put buckets into the reservoir to lift out remaining water. If water from the hand washing area simply spills on the ground, place that faucet approximately three meters from the container filling station. Be sure that the faucet heights are lower than the floor of the reservoir and disease will not become a problem.

drawing of water bottle filling area

Whether rainwater collection serves a surfing resort, a rural village, or a series of urban apartment buildings, consideration of the delivery of water from multiple roofs to a single reservoir is primarily a plumbing design task.

The final illustration contains several necessities for a sustainable economy; roofs to keep us warm and dry, clean water to drink, solar power to pressurize the water, and fun without pollution. Adequate diet, health, education, clean air and freedom to enjoy the Earth complete this list, which is for discussion. Do you suggest other ideas also?

The smaller cistern collects material washed from the roofs. Rainwater water flows to the main cistern when the small cistern is full. The cistern for material collection drains after the rain has passed and should be cleaned out regularly. A cross tee with larger drop pipe helps material flow into the small cistern and allows easy cleaning. Place clean-out tee along delivery lines, never use corrugated thin wall pipe. Screen the inlet vent. Slope the floor inside the debris cistern to a “V” shape, any tool can be used to clean the bottom, provide access from both sides and a similar channel to the drainage valve.

multiple roof rainwater harvest

Continue to Glossary and Business Model