Rainwater harvesting is the collection of rainwater directly from the surface(s) it falls on. This water would otherwise have gone directly into the drainage system or been lost through evaporation and transpiration. Once collected and stored it can be used for non-potable purposes; in other words, this water is NOT safe to drink.
However, it CAN be used for toilet flushing, garden watering and clothes washing using a washing machine. You should note that where used for washing machines, if the quality of the collected water is poor, there can be issues with both colour and odour.
Despite the common perception that it rains a lot in the UK, our water resources are under pressure.
A high volume of water is taken from the environment for human use. Demand for water is rising because the population is increasing, lifestyles are changing and the impacts of a changing climate are becoming clearer. In the South East of England, where large numbers of people live and work, water is scarcer than anywhere else in the UK. In fact, there is less water available per person in this region than in many Mediterranean countries.
Based on our extensive experience in the treatment of water in swimming pools, spas and hot-tubs, we at EasyChemicals believe that we are ideally placed to advise owners of rainwater harvesting systems in the chemical maintenance of their installations. We can also supply the simple water treatment chemicals required to make sure that harvested rainwater is safe to use.
Although it is relatively easy to treat rainwater in such a way that it becomes fully ‘sanitised’ and therefore suitable for drinking and cooking, the Environment Agency strongly recommends that harvested water should NEVER be used for human consumption.
Untreated harvested water will almost always be contaminated with bacteria and algae, and it is important to treat the water to ensure it is safe.
Chemical treatment of a harvesting system is very simple.
The Environment Agency recommends that harvested water should contain a chlorine level of no more than 2 mg/L, or 2ppm (parts per million). This is sufficient to kill all bacteria and algae which can easily colonise tanked water. Chlorine is an effective sanitiser down to just 0.5 mg/L, so that should be viewed as the minimum figure. In other words, your aim is to keep the chlorine level between 0.5 mg/L and 2 mg/L at all times.
This level of chlorine is best achieved by dosing small amounts of sodium hypochlorite solution at a concentration of 14/15%. The dose required will clearly depend on the tank capacity. As a general guide, we recommend that Liquid Chlorine 14/15% should be dosed every week at a rate of 7 ml per 1,000 litres of tank capacity for each 1mg/L of chlorine.
Prior to testing, it is important to check the ‘free chlorine’ level, to avoid over- or under-dosing. This is done with a 3-in-1 paper test strip, which is simply dipped in the harvested water at a convenient location – perhaps a toilet cistern or garden tap.
The test strip consists of 3 colour blocks which give clear indications of the levels of various chemical parameters. Most of time, we are only interested in the ‘free chlorine’ and pH.
pH is a simple measure of acidity or alkalinity, with a pH of 7.0 being neutral, a pH of 0 being fully acidic, and a pH of 14 being fully alkaline. Chlorine works best at a pH between 7.2 and 7.6.
It is a simple task to adjust the pH using two chemicals – pH Increaser and pH Reducer.
To summarise, when the system is first brought into use, check the pH, and adjust if necessary.
Then dose with sodium hypochlorite to reach the desired chlorine level of 2 mg/L. Never allow the free chlorine to drop below 0.5 mg/L.
Chlorine is consumed slowly as it does its job – this is why weekly checks and top ups are required.
As you get used to your system, you will gradually learn the optimal top-up frequency and dosages.
As a general guideline, a storage tank with a capacity of 1,000 litres would require the following doses:
For each 1 mg/L increase in free chlorine level: add 7ml directly to your tank; so, to establish an initial level of 2 mg/L, you will need to add 14ml. For each upward shift in pH of 0.1 unit: add 5g of granules, dissolved in hot water before adding to your tank For each downward shift in pH of 0.1 unit: add 5g of granules, dissolved in hot water before adding to your tank
Liquid Chlorine should be transferred from the 5L container by pouring the required dose into a 100ml plastic measuring jug. The Liquid Chlorine is simply poured in the harvesting tank.
Carefully wash and rinse the jug after use.
pH Increaser and Reducer MUST be pre-dissolved in warm tap water before pouring into your harvest collection tank. If you dose them as dry granules, they will not disperse efficiently in cold water.
Just weigh out the required dose of granules using reasonably accurate scales, then transfer the granules into the (washed) 100ml jug which you used to add the Liquid Chlorine.
In summary, the starter pack of products required to run a harvesting system is as follows:
1 x 5kg pH Increaser
1 x 5kg pH Reducer
1 x 5L Liquid Chlorine 10/11%
1 pack 3-in-1 test strips
DO NOT DRINK THIS WATER