The water comes out of the bore clear. You can fill a glass with it, hold it to the light, and see nothing. Three weeks later the drippers at the end of the run are choking up with a reddish-brown sludge, the block's watering unevenly, and someone's blaming the filter. The filter probably isn't the problem. The water chemistry is — and it's a problem that was invisible in the sample.
This catches out more bore-fed growers than almost anything else, so it's worth understanding properly.
Why clear water turns to rust
Iron in groundwater is usually dissolved — in a form called ferrous iron. Dissolved, it's completely invisible. It doesn't clog anything and it won't show up when you eyeball a sample. The trouble starts the moment that water meets oxygen.
When you pump bore water to the surface and push it through a system, the temperature, pressure and pH all shift, and the water picks up oxygen. That oxygen converts the dissolved ferrous iron into ferric iron — the oxidised, insoluble form. Ferric iron is rust. It drops out of the water as reddish-brown particles, and those particles build up inside the dripline and physically block the emitters. The clear water that left the bore arrives at the dripper carrying solid rust it made on the way.
The concentrations involved are tiny. Iron levels as low as 0.1 ppm are enough to start clogging a drip system — a figure most people wouldn't think twice about on a water report. There's often a second front, too: certain bacteria feed on dissolved iron and secrete a slime called ochre, which combines with the rust and other particles to block emitters faster. So you can be fighting mechanical clogging and biological slime at the same time, both fed by iron you couldn't see.
Why a finer filter doesn't fix it
Here's the part that trips people up. The instinct, when emitters clog, is to fit a finer filter or blame the one you've got. But if the iron is still dissolved when it reaches the filter, there's nothing for the filter to catch — ferrous iron passes straight through any screen, disk or media bed, because it's in solution, not suspension. It only becomes a solid further downstream, after the filter, on its way to the emitters. A finer filter catches rust that's already formed; it does nothing about the dissolved iron that's going to turn to rust after it's passed through.
That's the whole trick of this problem. The filter can be perfect and the system still clogs, because the filtration is happening in the wrong place in the sequence.
What actually stops it
The fix is to force the iron to oxidise before it reaches your emitters, and filter out the rust it forms — so the precipitation happens on your terms, not inside the dripline. There are three ways to do it.
The first is aeration and settling: spray or cascade the water into a tank or pond so it takes on oxygen, give the iron time to oxidise and drop out, then re-pressurise and send the now-iron-free water to the system. It uses no chemicals — air is free — but it needs a holding tank and a second pump, so the capital cost is higher.
The second is chemical oxidation, usually chlorine. Injected upstream of the filter, chlorine oxidises the dissolved iron almost immediately, turning it to rust right there where a media filter can then catch it. The rule of thumb is roughly 1 ppm of chlorine for each 1 ppm of iron, aiming to hold a small free-chlorine residual at the far end of the line, which also deals with the iron bacteria. The non-negotiable detail is that the injection goes before the filter — inject after it and you've just made rust with nothing left to catch it.
There's a third route, and it's the one that matters most for anyone choosing a media filter: catalytic media. Instead of oxidising the water in a separate step and then filtering it, a catalytic media filter does both jobs inside the one vessel. The media — manganese-dioxide based, sometimes solid, sometimes a coated or infused grain — catalyses the reaction as the water passes through, flipping the dissolved ferrous iron to solid ferric iron on contact, then traps that precipitate in the same bed. A periodic backwash flushes the captured iron out, and the media itself isn't consumed in the reaction, so a good bed lasts years. One tank, oxidation and filtration in a single pass.
It isn't magic, and it's worth knowing the limits before you spec it. Catalytic media needs dissolved oxygen in the water to drive the reaction, and it's pH-sensitive — it works best in roughly neutral water and falls off if the supply is acidic, which may mean correcting pH upstream. And past higher iron loads, around 2 ppm and up, it usually still wants a pre-oxidant ahead of it to carry the load. Inside its range, though, it's the cleanest answer to the bore-water problem: the oxidation that was clogging your drippers now happens deliberately, inside the filter, on media built to catch what it creates.
Either way, the principle is the same and it's the thing worth remembering: with iron, the filter's job isn't to strain the water as it is. It's to remove the rust that gets created — whether you create it upstream with aeration or chlorine, or the media creates it inside the vessel. Get the sequence right — oxidise, then filter, or do both at once in the right bed — and the clear water that fooled you stays out of your emitters.
If you're putting a media filter on a bore, the question to ask isn't just how fine it filters. It's whether the system is making the iron drop out — before the dripline does it for you.