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How to Disinfect Your Rainwater Tanks if Something Falls In

Angelo (admin)

Rainwater tanks are a vital source of water for many households, especially in rural and remote areas. However, these tanks are susceptible to contamination from various sources, such as animal droppings, dead animals, organic matter, and other foreign objects. Such contamination can pose serious health risks, making the disinfection of rainwater tanks a critical task.

Here’s a detailed guide on how to safely and effectively disinfect your rainwater tank using pool chlorine if you suspect contamination.

Using Chlorine to Disinfect Water

Water in rainwater tanks isn’t normally chlorinated on a regular basis like a swimming pool, it’s only treated with chlorine as a once-off remedial action to deal with an issue such as contamination, as the effectiveness of chlorine is short-lived. It only acts on the water in the tank at the time it is applied, and diminishes in concentration over time. As such, any fresh rainwater gathered by the tank after chlorination will probably not be disinfected, as most of the chlorine would have dissipated by then, and the remaining concentration will be too low to be effective.

Chlorination is an effective method for eliminating harmful bacteria, many viruses, and Giardia from rainwater. By disrupting the cell walls and internal structures of these microorganisms, chlorine ensures that the water becomes safe for consumption. However, chlorination has limited effectiveness against Cryptosporidium, a resilient pathogen with a tough outer shell that makes it more resistant to chlorine treatment.

In addition to disinfecting the water, chlorination can also help remove unpleasant odors. This is achieved by oxidizing the chemicals responsible for these smells, rendering them odorless. When chlorine is added to water, it reacts with organic matter and other impurities present. The amount of chlorine required for effective disinfection depends on the concentration of these impurities. Higher levels of organic matter and impurities necessitate a greater amount of chlorine to ensure all pathogens are effectively neutralised.

To effectively disinfect water, you need to add enough chlorine to ensure that there is a free chlorine residual of at least 0.5 mg/L (milligrams per liter) remaining in the water after it has had a contact time of 30 minutes. This means that, after the chlorine has reacted with any impurities and organic matter in the water, there should still be at least 0.5 mg/L of free chlorine available to continue disinfecting the water. You can measure this residual chlorine level using a chlorine test kit, such as those commonly used for swimming pools. Ensuring this residual level is crucial for maintaining the water’s safety for drinking.

As a general guide, since the level of contaminants may vary, to ensure effective disinfection of water, chlorine doses of approximately 5 mg/L (5 ppm) are required. This is achieved by the addition of 40 mL of liquid sodium hypochlorite (12.5% available chlorine) per 1000 L of water or 7 g of granular calcium hypochlorite (75% available chlorine) per 1000 L of water. The procedure for accomplishing this is explained in the following section.

Step 1. Assessing the Contamination Situation

Before starting the disinfection process, it’s important to first remove any visible contamination, such as dead animals or organic matter such as leaves, from inside the tank if they are present. If the contamination is severe or if you suspect the water is unsafe, it’s best to empty the tank completely and clean it before refilling.

Contamination is more likely to occur upstream of a sealed or screened rainwater tank, particularly on roofs or in gutters where rainwater is collected. Animal and bird droppings on roofs used for rainwater harvesting can contaminate the collected rainwater. Additionally, small animals may fall from trees or power lines onto roofs, and dead birds can end up in gutters, often remaining hidden until their smell becomes noticeable in hot weather.

In such cases, it is necessary to divert the water away from the tank when cleaning roof and gutter areas to avoid further tank contamination.

You never know what kind of contamination you might find on your roof when collecting rainwater from it!

Step 2. Preparing for Disinfection of the Water Tank

To disinfect the water in your tank, you’ll need either powdered swimming pool chlorine (calcium hypochlorite, 65% available chlorine) or liquid chlorine (sodium hypochlorite, 12.5% available chlorine). These pool chlorine products can be purchased from large supermarkets, hardware stores or swimming pool stockists.

NOTE: Do not use stabilised chlorine (chlorine cyanurates) as it’s not effective in enclosed tanks, and is not suitable for disinfecting drinking water either.

NOTE: When handling and storing chemical compounds, it’s important to carefully read and follow the safety instructions on the package label. Pay special attention to the recommendations regarding personal protective equipment, such as gloves and goggles.

Step 3. Calculating the Required Amount of Chlorine

For effective disinfection of rainwater in a tank for drinking, the recommended chlorine concentration is typically 3-5 milligrams per liter (mg/L), which is equivalent to 3-5 parts per million (ppm).

Incidentally, this is close to the safe level of chlorine in drinking water set by the World Health Organization (WHO) and the United States Environmental Protection Agency (EPA), who both set the maximum safe level of free chlorine in drinking water at 4 mg/L (4 ppm). Since chlorine dissipates fairly quickly, water treated to higher levels soon drops to within this level within a day or so.

In these instructions, to ensure effective disinfection, we should aim for a chlorine concentration of around 5 ppm (parts per million), which is equivalent to 5 milligrams (mg) of chlorine per liter (L) of water in your tank. This can be achieved by adding either 7 grams (g) of calcium hypochlorite or 40 milliliters (mL) of sodium hypochlorite per 1000 liters (L) of water.

Tip: A level dessert spoon can serve as an approximate measure for 7 grams of powdered calcium hypochlorite.

Depending on the size of the water tank, a lot of the chlorine may be required for disinfection.

For those interested in how we can calculate the chlorine concentration in the water from the amount used, the calculations are explained below:

Step 4. Adding Chlorine to the Tank

Carefully add the calculated amount of chlorine to the tank. It’s advisable to wear protective gloves and goggles during this process to avoid any skin or eye irritation from the chlorine.

The granular calcium hypochlorite should be dissolved in rainwater in a clean plastic bucket, preferably in an open, well-ventilated area, before adding it to the tank. Always add the disinfectant to the water, not the other way around.

When adding the concentrated chemical mixture to the tank, spread it as widely across the surface as possible to promote thorough mixing (though this may be limited by restricted access).

Step 5. Testing Free Chlorine Levels to Ensure Proper Disinfection

After adding the chlorine, wait for at least 30 minutes and then test the water for free chlorine levels. There should be at least 0.5 mg per litre (mg/L) [or parts per million (ppm)] of free chlorine present. This indicates that the chlorine has effectively disinfected the water. You can use a pool chlorine test kit for this purpose.

Note that we need to be testing for free chlorine levels and not total chlorine levels, these are very different measurements. The total chlorine level is a sum of free and bound chlorine, and therefore measures much higher.

Step 6. Verifying the Water Disinfection

Approximately one hour after treating the water, a faint chlorine odor should be detectable. If the chlorine smell is not present, it might be necessary to repeat the addition of a similar quantity of chlorine to guarantee proper water disinfection.

Step 7. Waiting Period Before Use

Allow the water stand for a minimum of one hour before using it. For best results, refrain from using the tank for at least 24 hours after treatment. This waiting period allows the chlorine taste and smell to dissipate and ensures that harmful microorganisms are destroyed.

Step 8. Maintaining Safe Water With Chlorination

To maintain a safe water supply after the initial disinfection, a chlorine concentration of around 0.5 mg/L (0.5 ppm) is required. This step is generally not necessary for rainwater tanks, as they are typically not kept chlorinated. However, it is included here for completeness in case there is a need to maintain chlorination.

To attain a chlorine concentration of 0.5 ppm, add 1 g (gram) of powdered calcium hypochlorite or 4 mL (millilitres) of liquid sodium hypochlorite per 1000 L (litres) of water in the tank.

Tip: The volume of a metric teaspoon is equal to 5 mL.

Allow the water to stand for a minimum of two hours before use. The water will be safe to drink as long as the chlorine smell is not too strong.

The explanations of the calculations are as follows:

Determining the Volume of Installed Water Tanks

To determine the necessary amount of chlorine for the appropriate disinfection concentration, it’s essential to know the volume of water being treated. Tank sizes can vary widely, from small modular tanks holding approximately 750 liters (around 165 gallons) to very large tanks exceeding 50,000 liters (over 11,000 gallons).

Tip: To convert a volume in gallons to a volume in litres, multiply the number of gallons by 4.5.

Many commercial tanks come with their capacity labeled, but if not, the capacity can be easily calculated using mathematical formulas.

Method to Calculate the Volume of Rectangular Tanks

To calculate the volume of a Rectangular Tank, use the formula:

For example, the volume of a 1.6m (160cm) by 0.5m (50cm) by 2.4m (240cm) tank in litres would be:
Volume = 160 x 50 x 240 ÷ 1000 = 1920 L

Method to Calculate the Volume of Cylindrical Tanks

To calculate the volume of a Cylindrical Tank use the formula:

(where π = 3.142, and radius is equal to half of the diameter)

For example, the volume of a tank with a diameter of 2m (200cm) [with a radius of half of that which is 1m (100cm)] and a depth of 2.6m (260cm) would be:
Volume = 3.142 x (100 x 100) x 260 ÷ 1000 = 8169 L

Additional Tips

By following these detailed steps, you can effectively disinfect your rainwater tank, ensuring a safe and clean water supply for your household. Remember, the safety of your water is paramount, so it’s crucial to act swiftly and efficiently at the first sign of contamination.

References

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