Algae may be one of the things feared most by rainwater harvesting practitioners, but by following the recommendations below, they can be prevented from growing, multiplying, and eventually ruining the water.

  • The most effective and most important tip is purchasing a poly storage tank made from food grade polyethylene or another approved material like good quality concrete, Aquaplate or stainless steel. Water tanks can be smooth or corrugated, and generally smooth tanks have thicker walls. Corrugations give greater strength, so the walls can be made a bit less thick.
  • Studies have shown that smooth walled tanks can help prevent the growth of algae more than corrugated tanks. However, you can help offset this effect by following proper tank maintenance procedures.
  • If you’re not a huge fan of poly tanks, you can get a stainless steel tank made from 304 or 316 grade stainless steel. Residential stainless steel tanks are usually corrugated.
  • Since both heat and light help algae grow, it can help if the tank is positioned in the shade. However, it’s still possible to have a well-functioning water tank even it’s not possible to position the tank so it’s directly under an awning or something similar. Tanks made from approved materials like food grade, UV treated polyethylene can be placed in full sun.
  • Ensure that the catchment, gutters, and downspouts are free of algae growth. They should, more importantly, be made from materials that do not facilitate algae growth. Regular cleaning and inspection will help ensure the tank water is clean and free from algae.
  • A multi stage filtration system, especially with a UV filter, is one of the best ways to keep collected rainwater clean. As a rule, the more filters you have set up in your rainwater collection system, the cleaner the stored water will be.
  • A properly meshed strainer as well as properly installed pipework are vital in preventing algae growth in the tank, since they stop most light from getting into the tank.
  • Certain chemicals that can kill and inhibit algae growth can be added to the contents of the water tank, however this is one of the riskier methods of household water disinfection as adding too much or the wrong chemicals can be dangerous. It’s not recommended to do this yourself.
  • Cleaning the tank interior and exterior on a regular basis, about one to three times a year, can help prevent algae from growing and thriving in the tank. Only qualified contractors should remove thick sediment buildup in the tank.
  • Drain the water before you start cleaning the tank interior. To do this, open the outlet valve that is located at the tank base. A hose can be connected to the outlet valve so that the water can be redirected to a different container. You can also try to clean the tank when there’s not much water inside, but this may not always be possible.

Once all the water is drained, mix mild dishwashing detergent with hot water to create a liquid cleaner. With a long-handled hard bristle brush or sponge, scrub the tank interior until all dirt has been completely removed. You’ll need quite a bit of strength to do this, so if you’re qualified to use one, a high-pressure hose can make the job a lot easier. However, it is best to call in a professional tank cleaner to do this type of cleaning once a year or so as part of your normal water tank maintenance schedule.

After scrubbing, it’s important to rinse the water tank thoroughly. A water tank that is completely dry is ready to be used again for rainwater harvesting.