Rainwater pre-filtration: where the filter goes and which type to choose
Anyone planning a rainwater harvesting scheme, or trying to fix one that silts up, reaches the same question sooner or later: where in the system does the filter belong?
There are four positions, and each does something the others cannot.
Key points at a glance
A textile filter bag is the option that suits a tank already in the ground , because it needs no pipework change. On a new scheme, coarse separation…
Rainwater filtration in this sense is particle separation. It removes solids and protects storage, pumps and irrigation downstream.
Three questions, in this order.
What rainwater tank filtration is and is not
Rainwater filtration in this sense is particle separation. It removes solids and protects storage, pumps and irrigation downstream.
It is not water treatment. Dissolved substances, bacteria and colour pass through any of the filters above. A rainwater harvesting scheme intended for anything beyond irrigation, washing or process use needs treatment designed for that purpose, and a filter bag is not it.
Choosing between water filters for rainwater tanks
Three questions, in this order.
Is the tank already installed? If yes, the in-tank filter bag is usually the only retrofit that does not involve excavation.
What arrives on the roof? Trees change everything. A roof under deciduous cover needs coarse separation upstream, or the fine filter will load in weeks.
Who services it, and how often? This decides the rating more than the water quality does. A scheme checked twice a year needs a different setup from one checked monthly.
The retrofit guide works through the measurements and the practicalities. Where the scheme includes infiltration, the soakaway side is covered separately.
Frequently asked questions about rainwater pre-filtration
These are the questions we most often receive from private customers and specialist contractors
Do I need a filter in a rainwater tank at all?
Without pre-filtration, everything that lands on the roof can enter the storage tank and settle on the bottom. That sediment is disturbed whenever the pump runs and adds load to pumps and irrigation equipment. A filter significantly reduces this input.
Which filter fits my rainwater tank?
That depends on whether you are retrofitting or planning a new system. For retrofitting without structural alterations, the textile fine filter bag is the obvious solution and the inlet opening diameter is the key dimension. Other filter types can be considered in new designs with a dedicated filter chamber.
Does the filter make the water potable?
No. Solids filtration does not remove dissolved substances or microorganisms. Drinking-water use requires suitable treatment and the relevant legal conditions.
How often does a pre-filter need maintenance?
That depends on roof area, surrounding vegetation and weather. There is no fixed interval. A practical routine is to check in spring and autumn and after a long dry period followed by heavy rain.
Can I combine more than one filter stage?
Yes, and that is often useful. A leaf catcher in the downpipe noticeably reduces the load on the fine filter at the tank inlet because coarse material never reaches it. Combining stages extends the service interval of the finer filter.
What is the difference between a pre-filter and a fine filter?
The terms are not used consistently. In practice, installation position and the material to be retained are more useful distinctions: remove coarse debris as early as possible and finer suspended solids as close to the storage tank as practical.
Still unsure which filter or next step fits?
Send us the medium, dimensions and application. We will help you decide whether a standard product is suitable or whether the case needs a custom solution.
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