How to Manage Sludge Build-Up in Rainwater Tanks

To ensure rainwater collected in your tank remains clean and fresh, it is important to properly maintain your tank. Rainwater tank maintenance recommended by the Australian Health Department involves inspecting inside your tank and cleaning out any sediment and organic matters that has accumulated at the base of your tank.

What is Tank Sludge?

Even with strainers, plant material, waste and sediment will still find their way to the bottom of your tank. Over time this turns into a layer of “sludge” which some say can be a breeding ground for harmful bacteria and dangerous to your family’s health.

Many assume this layer to be detrimental to tank water quality, yet there is growing dissent as new research reveals the biofilms in rainwater tanks appear to be highly effective at removing lead and other contaminants from the water column.

It is said that te microbial communities found in biofilms are predominantly harmless soil or environmental bacteria that consume other bacteria, nutrients and chemicals. Since rainwater tanks are a low nutrient environment, the biofilm is hungry which actually means it is good for preserving water quality.

Nonetheless, the Australian Department of Health states is the published, Guidance on Use of Rainwater Tanks:

“In addition to six-monthly inspections, tanks should be inspected every 2-3 years for the presence of accumulated sediments. If the bottom of the tank is covered with sediment the tank should be cleaned.” (Chapter 6. Monitoring and Maintenance)

Preventing Sludge Build-Up

The best way to manage sludge is to prevent it from forming in the first place. Here are some steps you can take:

• Keep your roof and gutters clean: Regularly clean your roof to remove leaves, bird droppings, and other debris. Ensure your gutters are clear and consider installing leaf diverters (rain heads) or leaf guards (gutter mesh) to prevent sediment from entering the downpipes.
• Use a first flush diverter: A first flush diverter allows the first flow of water from the roof to be diverted, taking any debris and contaminants with it, thus preventing them from entering the tank. This significantly reduces the amount of sludge that forms at the bottom.
• Calm your water in-flow: Calming the water flowing into your tank helps to reduce the velocity of water entering, preventing sediment from stirring and spreading throughout the water. Tools like a calmed inlet and solar shields work by softening the in-flow of water.

Avoiding the Sludge Layer

• Install a higher outlet valve: almost all water tanks comes with a ball valve installed at the very base of the water tank, normally used as a drainage valve, but often also as the valve connected to an external pump. However, this position means the valve will likely draw from water from the sludge layer. If you have the space an budget, you could install an additional ball valve approximately 100mm above the base of the tank to avoid drawing straight from the sludge layer.
• Use a floating off take valve: Installing a floating off take valve ensures that your pump pump draws water from your cleanest water, which is just below the water surface and well away from the sludge that settles at the bottom.

Cleaning Out the Sludge

If sludge has already built up, here are some methods to clean it out:

• Automatically sludge removal: if you have a smooth-wall poly tank or a concrete tank, you could use a auto-tank vacuum system to automatically remove the sediment around your outlet valve when your tank overflows. This vacuum system siphons sediment and small debris from the base of the tank when the tank is full and water overflows.
• Manual cleaning: To clean your tank, you can either drain the tank fully, hose and sweep it out. You could also create a siphon hose or use a 12v pump to suck out the sediment from the base of the tank. However, this method is time-consuming and can be risky if your tank is very large and needs to be entered, so it’s often easier and safer to hire a professional water tank cleaning company.