A cistern is a watertight receptacle, typically larger than a rain barrel, designed to collect and store water, most often rainwater, for household use. Modern systems are an increasingly popular way to conserve water and reduce reliance on municipal supplies. While rainwater is initially pure, collecting and storing it introduces numerous risks of contamination. Cistern water can be made safe to drink, but only through diligent management, regular testing, and a multi-step treatment process.
Common Sources of Contamination
The journey from the sky to the storage tank subjects the water to three main categories of contaminants: biological, chemical, and physical. Biological contamination poses the most significant health risk, primarily stemming from animal waste deposited on the catchment surface. Bird droppings, rodent feces, and other organic matter on the roof contain harmful microorganisms, including bacteria like E. coli and pathogens such as Salmonella, Cryptosporidium, and Giardia.
Chemical contaminants are introduced through the atmosphere and the materials of the collection system itself. Airborne pollutants, such as industrial fallout or vehicle exhaust, can be washed into the system during rainfall. Furthermore, roofing materials can leach substances like copper, lead, or zinc, especially if the roof is old, painted, or made of certain metals. Pesticides or herbicides used nearby can also be picked up by the runoff and concentrated in the stored water.
Physical contaminants, including debris like leaves, dirt, silt, and insects, create conditions where biological hazards thrive. Accumulated sediment and organic debris settle at the bottom of the cistern, forming a sludge that harbors bacteria and creates a breeding ground for microbial growth, known as biofilm. This sludge also increases the water’s turbidity, making the subsequent disinfection process less effective.
Testing and Professional Monitoring Requirements
The only reliable method to confirm water quality is through professional laboratory testing, which should be done at least semi-annually. The most important test is for coliform bacteria, particularly E. coli, as their presence is a clear indicator of fecal contamination and potential waterborne pathogens.
Beyond bacterial testing, cistern water should also be analyzed for its basic chemical properties and potential heavy metals. Parameters such as pH and total dissolved solids (TDS) provide insight into the water’s corrosivity and overall quality. Testing for nitrates is also recommended, especially if there are septic systems or agricultural operations nearby, as high levels can be particularly risky for infants.
If the catchment surface is metal or the plumbing system is older, a heavy metal test for lead and copper is prudent. Only certified laboratories should be used for drinking water analysis to ensure the methods and reporting meet established quality standards.
Effective Water Treatment Methods
Once testing has established the water’s baseline quality, a multi-barrier treatment system is necessary to ensure potability. The first step in this process is filtration, which removes physical contaminants that interfere with disinfection. Initial sediment filters trap particles like dirt and rust, protecting the finer treatment components downstream.
Following sediment removal, activated carbon filters are installed to improve the water’s taste and odor. Carbon filtration works by adsorption, effectively removing organic compounds, chlorine, and certain chemical contaminants that may have leached into the water.
The final and most effective step for eliminating biological hazards is disinfection, with ultraviolet (UV) light being the most common household method. A UV system neutralizes bacteria, viruses, and protozoan cysts by disrupting their DNA, preventing them from reproducing. This process is chemical-free and does not alter the water’s taste, but it requires the water to be clear, underscoring the importance of preceding filtration. Chemical disinfection, such as adding unscented liquid household bleach, can also be used for emergency situations or shock chlorination, but maintaining a consistent, safe residual level requires careful monitoring.
Cistern System Maintenance for Ongoing Safety
Safe cistern water requires an ongoing commitment to system maintenance, not a one-time achievement. A routine, professional cleaning is recommended at least once a year to remove accumulated sediment and biofilm from the tank floor and walls. Sludge removal is important because it harbors bacteria and reduces the effectiveness of disinfection chemicals.
Preventative measures begin on the roof itself, using a “first flush” diverter, which redirects the initial flow of rainwater away from the cistern. This initial runoff contains the highest concentration of contaminants, including dust, leaves, and animal droppings, which settled on the roof between rain events. These diverters significantly reduce the biological and physical load entering the storage tank.
The cistern structure must also be regularly inspected for integrity and security. The tank’s access lid should fit tightly to prevent insects, rodents, and light from entering, which can cause contamination and algae growth. All vents and overflow pipes need to be properly screened and downturned to prevent debris and pests from entering the system.