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Reinforced Concrete Cistern

by John Hammerstrom

This rainwater harvesting (RWH) system, with an above-ground, 7,500 gallon (usable) poured-concrete cistern, used for both potable and nonpotable purposes, was designed and constructed in 2001 as an integral part of a new single-family home in Key Largo, Florida. Rain is collected from a 1,700 square-foot white Galvalume roof and gathered in six-inch copper gutters with spash shields at roof valleys for occasional heavy downpours. Copper plumbing is used throughout the house as well. Although copper in rainwater harvesting systems has been subsequently discouraged, the levels of copper in the annual water tests of this system have never exceeded 20% (.26 mg/l) of the EPA Maximum Contamination Level (MCL) for copper, 1.3 mg/l. It has been reported elsewhere that because of current global levels of CO2 in the atmosphere, before accounting for any other airborne influences, the pH of rain would be 5.6. The reported pH of the annual test samples taken from an inside faucet has always been greater than 7, which could be attributed to the concrete tank. That does not explain why there seems to be no substantial copper leached from the gutters, since they are upstream of the tank. It remains for others to determine why the copper levels in the rainwater of this waterfront home are insubstantial, but it could be a function of omnipresent coral rock dust raising the pH. Salt air and deposits may have an impact. Another possible explanation is research describing the protective quality of different compounds that make up copper's patina.

PV panelsThe home was the first in the Florida Keys to have a photovoltaic (PV) system interconnected with the electric utility. Because of the relatively small capacity of the battery-backup PV system, the power consumption of the rainwater system was of concern. The most significant factor was the amperage limitation of the PV system's inverter, which meant that the RWH system pump had to be sized correctly. The construction plumber was unfamiliar with the integrated systems (photovoltaic and rainwater harvesting), and recommended a much larger pump that would have exceeded one of the inverter limitations. This would have meant that if the grid electric power was unavailable (after a hurricane, for example), the RWH system pump would have exceeded the inverter's capability and would have caused an electrical system shut-down. A relatively large 80-gallon pressure tank moved the water, which meant a smaller pump would suffice to repressurize the pressure tank when the lower pressure limit was reached, perhaps once or twice a day. Since sustainability was one of the goals of the home, a smaller pump than "normal" was used, in this case a 1 HP Goulds Jet Pump.

The first-divert system consists of two "Leaf Eater - Advanced", self-cleaning rain heads, and two "SafeRain" adjustable first-flush device. The two 4-inch downspouts are larger than necessary, considering the size of the roof (1,700 square feet), but there is no worry about a rainfall exceeding the downspout capacity!

Forms for concrete rainwater tankThe cistern itself was designed into the concrete structure of the house and includes rubber "dam" gaskets at the base and ceiling of the tank, at the junction of the separate concrete pours. The cistern has eight-inch reinforced concrete walls, with another partial wall in the center to divide the tank into two halves. The divider facilitates service on half of the tank when necessary, rather than neccessitating emptying an entire single tank.

The inside of the concrete tank was sealed with a "food grade" product advertised at the time for such—a cementitious product sold by STO. Since then, the National Sanitation Foundation has identified products that meet their standards and one can consult their current list by clicking on this link.

The water from each half is picked up by a "foot valve" placed approximately 8 inches above the bottom of the tank. In the picture, one will also see the weighted float that is the tank-side portion of the direct-read level indicator. No batteries required. The weighted float is connected through a tiny tube in the concrete wall to another weighted level indicator outside the tank and reads in reverse. An eyebolt with washers can been seen to the left of the filters. As the float in the tank rises, the level indicator descends. The washers were used to adjust the weight of the system such that it was heavy enough to overcome the friction of the small tube, descending to indicate greater quantity when the water level increased and yet be light enough to rise when the tank water level decreased. The "pass through" connecting both halves of the tank is visible at the top of the darker gray STO sealant. After twice removing with a shop vacuum the very small amount of sediment in the tank during annual cleaning, visual inspections without distrubing the biofilm at the bottom of the tank are now the practice. The construction plumber used 1-inch PVC pipes in the concrete wall to each half of the tank, which became a painful mistake. The intake for the pump is 1 1/4, which required hammering drill work to expand the hole through the concrete wall to make way for the larger pipe. Imagine the racket inside the tank while hammer-drilling two holes through reinforced concrete walls.

rainwater pumpThe 1 1/4 inch intakes and the Goulds Jet Pump are visible in the image. The pump has operated flawlessly for eleven years. Couplers are installed on each side of major components, to facilitate replacement should it become necessary. Although the foot valve has an integrated check valve in it, another check valve was installed prior to the pump to assure pump priming. A feature that should be added to this system is a low level cutoff, that would protect the pump from the possibility of the tank being pumped dry.


Downstream of the pump is the aforementioned pressure tank, whose functionrainwater filtration system is to reduce the total number of pump starts. Demand pumps with much smaller accumulators are now available that are designed to run whenever a spigot is opened, but this reporter is skeptical that they will last as long as pumps with a traditional pressure tank. After the pressure tank is a Pura Big Boy 3-canister filtration system that in this case houses a 1 micron filter followed by a carbon block and then ultraviolet.

There are more robust systems available, but this has worked well and the test results validate it. There are many filter options for sediment and carbon. Maintenance of this filtration system is routine except for the UV chamber on the right. The UV light is housed in a crystal tube/sleeve that is very vulnerable when removing the relatively heavy, water-filled blue canister for cleaning. It is advised that great care be taken when spinning off the blue housing, by using small increments of "unscrewing" and also by placing some weight-bearing structure (a crate, for example) approximately six inches underneath as a stop on which to rest the heavy load after the last thread is cleared and the water-filled canister is free. The UV light is changed through the housing box at the top, and while replacing the bulb annually, cleaning the inside of the crystal tube is advised as well. A long wire with a lint-free swab on the end moistened with alcohol has worked well. If the crystal sleeve breaks, it can be changed, but it is challenging not to break the new one in the replacement process. If you break one, order two. Consult the manufacturer for advice.

The home's water and electric systems are both "hybrids" in that they are both connected with the traditional utilities. The local water utility was very supportive of this private-use potable system and only required an RPZ (Reduced Pressure Zone) valve or a T-10 meter (with an integrated isolation valve to preclude backflow from the RWH system to the utility water system). Rainwater is the top priority potable source, so three subsystems were installed to further prioritize its availability: 1) One dedicated line to a faucet in the kitchen, 2) Showerheads and sinks and 3) Toilets and hosebibs. The Florida Keys has a modest dry season, and it is not uncommon to have a relatively long dry spell from January until May or June. When rainfall is plentiful and "refills" are expected, rain is the sole source of water for all purposes. In 2011, rain was used exclusively from August 1 to mid November. A bypass was recently installed that directs untreated rainwater to toilets and exterior hoses. There is controversy regarding this practice and it was installed with that knowledge.

When the dry spell is anticipated, perhaps in mid-November or December, the toilet and hose valve is switched to utility water, while all of the rest of the plumbing remains running rainwater. Near the end of the "dry" season, if the cistern level is under 1,000 gallons, treated rainwater use will be limited to the one faucet in the kitchen for drinking and cooking, while utility water will be used for all other external and internal uses. In the photo of valves above (which is not as complicated as it looks), the top three valves are on-off for water from the utility, while the three corresponding valves on the right are rainwater controlson-off for the three rainwater subsystems. The pictured configuration shows water "on" from the utility for toilets and hoses, while the other two valves from the utility are "off" because the sinks, showers and kitchen are supplied by the rainwater system. There is also an air-gapped fill line directly from the utility for the VERY unlikely event that it was desirable to fill the tanks with utility water. It has never been used.

One increasingly important characteristic of a rainwater capture system is its ability to reduce stormwater runoff. During the period between August 1 and mid November, 2011, 15,000 gallons of water were captured and used onsite that would otherwise have become stormwater, flushing contaminants and nutrients to nearby water bodies, in this case Florida Bay. During the wet season, when the tank is often full and more rain is expected, use of water in and around the home is accelerated, including car washing; long, hot showers (solar heating and free water that needs to be consumed to make room for the next rain event make long, hot showers guilt free); plant watering (the bananas love it), etc.

RPZ valveAs stated earlier, the local water utility required an anti-backflow device, and an RPZ valve was selected. It requires annual testing, which is a nuisance and an expense, but in the "greater good" interest of making it easier for regulators to allow rainwater harvesting systems to be used, it is a minor nuisance. The Florida Keys has a lengthy rainwater-harvesting history that parallels the Carribean islands.

Harvesting rainwater in the islands is routine. Indeed, in Bermuda (not in the Caribbean, but the Atlantic Ocean), it is a requirement. For those who have not seen an RPZ valve, an image is included here. Regarding conveyance components as stated earlier, there are two four-inch downspouts. Because the overflow capacity must exceed the intake capacity, the overflow is six inches in diameter. Two four-inch pipes have a combined area of 25.13 square inches. A six-inch pipe has an area of 28.27 square inches. In retrospect, since there are two downspouts serving the 1,700 square foot roof, three-inch downspouts would have easily accommodated the greatest expected rainfall per hour for south Florida of 4.5 inches per hour.

Probably the most controversial aspect of this system is that rainwater is the preferred source. Once aware of the relative primary and secondary water qualities of the two sources, treated rainwater is the clear winner (pun intended). Water samples from the rainwater system have been extensively tested for the past ten years, and compared to the annual reported results of the utility water, even though it is excellent relative to other municipal water sources, there is no doubt which water one would choose to drink and which would be relegated to flushing the toilets. These annual tests also evaluate organic and inorganic contaminants including solvents, pesticides, herbicides, trihalomethanes and metals. The list of regulated contaminants that are allowed in public drinking water is lengthy. The list of unregulated contamininants for which there are no EPA limits is frightening. The Four Star nonprofit charity Environmental Working Group has identified 201 unregulated contaminants in U.S. drinking water. These are contaminants for which there are no enforceable health standards - any level is legal. Among them are Vanadium - an industrial discharge; Bromide - a naturally occurring element that contributes to the formation of toxic byproducts; Radon - a radioactive breakdown product often found in groundwater; Perchlorate - a rocket fuel additive; MTBE - an octane-boosting fuel additive that is banned in most states, but present in groundwater; Acetone - a common solvent; Strontium - a metal used to produce TV set glass; Dacthal - an herbicide commonly used at golf courses; Methyl ethyl ketone (MEK) - a synthetic solvent; Alpha and Gamma Chlordane - isomers of banned pesticide Chlordane used for termites, plus 190 other unregulated contaminants.

This RWH system is located in Key Largo, Florida, is not open to the public for tours; however, the owner - John Hammerstrom, Past President ARCSA, is willing to answer questions, just post below.





Rain Harvesting

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Taking on Water

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Drinking Water

Tapped Out


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