Courtesy of Fran Wagner, here are the steps for conversion of an anaerobic septic tank (the most common kind) to a cistern.
Obtain a variance from the Department of Health (if needed). The fee will include an inspection by the Department of Health of the water in the new cistern to assure that it is not contaminated. The chlorine bleach described later will ensure that the water is properly disinfected.
Have the septic tank pumped out and pressure-cleaned by a certified septic pumping contractor. Most contractors have a pressure washer and fresh water supply on their truck. Ask them to thoroughly clean the inside of the tank and then pump out the wash water.
Connect the roof gutter downspouts to the tank using 3-inch PVC pipe. Usually it can be connected to a stub of the old fill pipe running from the house. It is a good idea to add a piece of 1⁄8-inch mesh screen between the gutter and the cistern to catch debris flushed from the roof during the beginning of a rainstorm.
Connect an overflow pipe to the cistern tank. This is usually a couple of 3-inch PVC 90-degree elbows, which allow water to flow out of the cistern if it overflows. Install a grate or screen to prevent access by rodents and mosquitoes. It is also a good idea to add a splash basin or gravel field to reduce erosion and guide the overflow where you want it to go.
Allow the cistern tank to fill. When it is about three-quarters full, add 3 to 4 gallons of unscented bleach. When full, add another gallon or two of bleach. This amount is more than sufficient for a very large tank (more than 1,500 gallons.) The chlorine will disinfect the tank by killing any remaining bacteria, including those in the pores of the concrete walls. Then it will slowly dissipate.
Install a pump with a pressure tank and connect it to a power source. The best kind is a shallow well pump. Most of these are supplied with a pressure tank that holds from one to five gallons of water. The pump draws water from the cistern and fills the pressure tank to 40 to 50 psi. When a hose is turned on, the pressure forces water out the hose. As the water flows out, a pressure-demand switch automatically turns on the pump to refill the pressure tank. Such pump/tank combinations can be purchased from Home Depot or on the Internet from discounters such as Harbor Freight Tools (www.harborfreight.com).
Connect an intake suction pipe from the cistern tank to the pump. The pump should be mounted off the ground to protect it from any future storm surges. Install an outdoor “GFI” power outlet in a weather-tight box near the pump. Include a switch to disable the pump if necessary.
The intake pipe should have a check valve installed near its end, about 6 to 8 inches above the bottom of the cistern. The remaining water will help weigh down the cistern tank and keep any groundwater pressure from “floating” it, or raising it partially out of the ground. Connect an output pipe from the pump to one or more garden faucets.
Within a few months after the bleach process, the Department of Health should be called to test the water, a requirement for the DOH certificate of approval. When the chlorine level has dissipated to the level allowed by the DOH (you can test it using a common swimming pool test kit), they should be called to perform their official test. This only has to be done once. In addition to chlorine, the DOH tests other measurements such as water clarity and bacterial content.
An optional float switch can be installed to prevent dry pump operation when the tank is near empty. This will shut down the pump if the water level falls below the float switch setting. This prevents the pump from running when the cistern is almost empty and the pump cannot pump water.