Retrofit a rainwater tank filter ➔ what fits a tank already in the ground and which measurements you need. Read the guide now!
Key points at a glance
Most rainwater tanks are filtered as an afterthought, once the sediment on the floor has become a problem. At that point the options narrow sharply, because anything requiring…
One measurement decides the order: the clear internal diameter of the tank inlet opening, taken at the widest point. That gives the DN size, from DN325 to DN800.
The bag hangs at the inlet so that incoming water runs into it, held by the straps at the existing opening. Three points decide whether it works in…
What can be retrofitted to a tank that is already in the ground
Most rainwater tanks are filtered as an afterthought, once the sediment on the floor has become a problem. At that point the options narrow sharply, because anything requiring pipework means excavation.
What can be retrofitted without digging is a filter that works inside the tank, hung from the existing inlet opening. That is the practical case for the textile filter bag. What cannot be retrofitted easily is anything upstream: a vortex filter needs a housing and a drain connection, and a downpipe separator needs the downpipe to be accessible.
The measurements you need
One measurement decides the order: the clear internal diameter of the tank inlet opening, taken at the widest point. That gives the DN size, from DN325 to DN800.
Two more are worth taking before you order. The depth available below the inlet, so the filled bag hangs clear rather than resting on the floor. And the access opening, because a bag that fits the inlet is no use if it cannot be brought through the manhole.
For openings between sizes, oval sections or unusual constructions we make to measure.
Fitting
The bag hangs at the inlet so that incoming water runs into it, held by the straps at the existing opening. Three points decide whether it works in service.
The bag must hang freely, not against the tank wall, or water short-circuits down the side. The top edge must sit above the highest expected water level at the inlet. And whoever services it must be able to reach it from the access opening without entering the tank.
Cleaning, and when to replace instead
The bag is washable. Rinse from the clean side outwards so material is pushed out of the fabric rather than further into it, and let it dry before refitting where practical.
Replace rather than clean when a seam has opened, a strap has torn or the fabric has thinned to the point of tearing under load. A bag that fails in the tank drops its contents exactly where you were trying to keep them out.
Blockages
A blocked inlet filter backs water up rather than letting it through. If that happens repeatedly, the answer is usually not a coarser bag but coarse separation upstream: a leaf separator in the downpipe takes the load that is blinding the bag, and the bag then handles the fine fraction it was designed for. The position comparison is on the application page.
Frequently asked questions about retrofitting a rainwater tank filter
These are questions we regularly receive from operators of existing systems
Do I have to pump the rainwater tank empty before retrofitting the filter?
Usually not. The bag is inserted from above through the access opening and fixed at the inlet. There must be enough clearance between the bottom of the bag and the water level for the loaded bag to hang freely.
What is the best filter for a rainwater tank?
There is no single best filter for every system. For retrofitting without structural alterations, the textile fine filter bag is usually the most practical solution. Other filter types can be considered for new installations with a dedicated filter chamber.
How often should I clean the rainwater tank filter?
There is no fixed interval. A check in spring and autumn, and after a long dry period followed by heavy rain, is a practical starting point because that is when a large amount of sediment can arrive at once.
What is a sediment trap, and do I need one?
A sediment trap is an upstream chamber in which heavier particles settle. It is a different component and not a filter in the narrow sense. In an existing system, retrofitting one is often structurally disproportionate; a textile filter stage can achieve a similar objective for suspended solids with much less work.
Can I leave the filter bag in the tank over winter?
Yes, provided the rainwater tank is installed frost-free. For above-ground tanks or systems exposed to frost, follow the system manufacturer’s winter instructions.
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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