How to budge the sludge in your water tank 

Published: September 2019 I By: Justin Law

Sediment in the bottom of your water tank isn’t always a bad thing, but there are times when you need to make a clean start.

Harvested tank water provides a quality water source option for farmers. Source: iStock/Getty Images.  
Farmers who’ve lived on harvested water all their lives have a wisdom when it comes to water tanks: if it ain’t broke, don’t fix it. By that they mean if your rainwater at the kitchen tap is clear and odourless, there’s nothing wrong with it.

Even so, a routine inspection of a water tank can be confronting. A thick layer of sludge might be in the bottom of a water tank that has been used for some time, and a film of material might also be at the water surface.

To the untrained eye, this can look like a dirty water tank, but the experts say it’s all quite normal and is the sign of a healthy water treatment process.
“That layer of sludge, or biofilm, actually improves the water quality,” says Dr Peter Coombes, a Dubbo farmer and managing director of Urban Water Cycle Solutions, who has researched rainwater harvesting for 20 years.

“It’s pretty hungry and strips metals, bacteria and chemicals from the water as a natural filtration system so it’s an important part of the purification process. Debris and bacteria can also be trapped at the rainwater surface layer – the water surface microlayer – due to water skin friction, and proximity to oxygen and warmth. However, these materials do not enter the water column deeper in the tank.

“So, evidence of some material on the surface of stored rainwater does not indicate poor quality water,” Dr Coombes says. “Clear and odour-free rainwater at the tap remains a key indicator.”
Quality control in maintaining your water tank 
However, there are times when water quality can suffer, particularly following a rain event after a long period without rain. Properties on dry land will see dust, leaves and bird faeces accumulate on the roof and can end up in a tank, which may not be optimally set up to treat the water.

“When there are no leaf diverters, first flush devices and mesh screens on inlets, this can allow an inflow of debris, sediments and other materials,”

Dr Coombes says. “Decomposing organic material [such as leaves] can cause discolouration of rainwater water and odour.

Leaf diverters, first flush devices and mesh screens on inlets help to keep out debris, sediments and other materials. Source: iStock/Getty Images. 

“Even with best practice with the water treatment train, there can be an increase in bacteria and sediment entering tanks during rainfall. However, our research shows that these particles settle quickly and rainwater quality improves. The faecal indicator bacteria E. coli was not frequently found in tanks and was found to be unsustainable in the rainwater column.”

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This result is achieved by the biofilm at the bottom of the tank drawing in bacteria. But when the water at the tap is consistently discoloured and smells like rotten eggs or body odour, it’s time to budge the sludge.

Dr Coombes says there needs to be a very good reason to clean a water tank because a lot of water can be wasted and you can be without potable water for some days.
“Cleaning will disrupt the equilibrium inside the tank and there may be a need to wait for rainwater quality to improve,” he says. 

“Chemical additives may also not solve this type of problem so be judicious if you choose to incorporate them into the cleaning process.

“But most importantly, work out how the water became contaminated in the first place – it may be easy to solve with the proper water treatment train.”

Michael Smit, the technical and sustainability manager at Kingspan Water & Energy, agrees that a properly installed water treatment train will reduce the need for regular cleaning.

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“Material clogging pipelines means there’s a problem in the setup so getting that right is an important step,” he says. “A first flush diverter, proper mesh filters over roof gutters and a leaf diverter rain head are essential front-line defences against rainwater contamination. They will greatly reduce the risk of leaf matter and other contaminants entering the system.”

A first flush diverter is particularly useful in dry and dusty rural areas as it captures the first run of water from the roof during a rain event after a dry spell. It’s essentially a downpipe that captures around 20 litres of water before closing and allowing the rest of the water to flow into the tank.

A charged downpipe is also recommended for tanks not adjacent to a building or where a graded downpipe from the roof to the tank is not possible. It is basically a flat U shape with water from the roof descending vertically to a buried horizontal section which is then connected to an up-pipe next to the tank.
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Water pressure pushes the water into the tank, but when it’s not raining, water in the pipe can become contaminated if debris has been allowed in. This is where a rain head leaf diverter comes into play. It can either be a sloping mesh screen covering the entire length of the gutter or a mesh filter on the downpipe usually just below the gutter. Either way, it should ensure leaves don’t get into the downpipe.

How to check if your tank rainwater is safe to drink 
  1. Check the rainwater supply at the tap to see if it is discoloured and if it smells.
  2. If so, inspect the rainwater stored in the tank. Is the water discoloured below the surface or contaminated with suspended fine debris and organic material?
  3. If it is, wait for 6-24 hours after a rainfall event, and check to see if the rainwater quality improved.
  4. If there’s no improvement, reserve the water for garden use only, or drain stored rainwater from the tank and inspect the sludge layer.
  5. Also check to ensure that debris cannot enter the tank by installing mesh screens, leaf diverters, and a first flush system.
  6. If you have a wet system or a charged line (underground pipes that divert collected rainfall into the tank) ensure that leaves and debris cannot enter the underground pipes. You may need to flush this line to remove decomposing debris.
  7. If the sludge layer is near or above the rainwater supply outlet within the tank, an adequate cleaning technique can be chosen.
  8. Following tank cleaning or tank refilling, wait until rainwater supply at the tap is clear and free of odour before using as drinking water.

The best way to clean your rainwater tank

If it’s absolutely necessary to attack the sludge, there are a few effective methods: 
  1. Get in there with a broom: After draining the tank via the bottom sludge gate, get in the tank and sweep out the sludge. While the temptation is to make it spotless, it’s not recommended to clean out all the biofilm material on the floor and walls as this acts as a natural filtration system.

  2. Try Vacuuming: A suction hose is inserted into the tank and draws up the sludge layer in a process that can take a few hours. This is a good way to avoid losing all your water, but it will also stir up contaminants, which will take a day or two to resettle. There are maintenance businesses that offer this service.

  3. Install a self-cleaning system: A simple overflow pipe setup can create an automatic cleaning system that draws sediment from the bottom of the tank during a rain event. As the water level rises, it pushes water up a riser tube to the overflow near the top of the tank and creates a vacuum. Water from the bottom of the tank is sucked out until the water level settles below the outlet level.

  4. Sanitise the water: While not recommended in most situations, the World Health Organization has recognised that chemical treatment can be used to sanitise water. Chlorine dioxide, chlorite and chlorate are chemicals used to combat harmful bacteria and are available in Australia through some water treatment companies. However, good bacteria will also be affected in this process.

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