Is your purifier cartridge working?

[Manky Sanke]

Testing the effectiveness of dechlorinators
Water purifiers are frequently sold as "able to dechlorinate X number of gallons". The first thing to note is that these are sometimes US gallons so you only get about 0.8 of that figure in UK gallons. The next thing to note is that the number of gallons it will dechlorinate during its life depends on the chlorine content of the water. Manufacturers assume a typical tap-water chlorine value and calculate the life of their cartridges or activated carbon from that.

Frequently, water supply authorities add extra chlorine to sterilise the pipes. Chlorine purifiers will remove the extra chlorine but since they can only remove so much chlorine before they are “used up” it shortens their life in doing so.

Also, your supply may contain a higher level of chlorine than the manufacturer assumed. If, (for easy numbers), it was double, the cartridge would only last half as long before it needed replacing, and its life would be further shortened by chlorine spikes.

Testing whether the chlorine cartridge or the activated carbon in a purifier is still working isn't hard to do. For koi, the chlorine or chloramine level should ideally be zero but certainly should not exceed 0.02 mg/L. Manual chlorine test kits for swimming pools are cheap but they will not read down to such a low level. Cheap chlorine meters sold on ebay for less than twenty pounds are not reliable at very low levels. But when you top up your pond, you don't need to know the actual chlorine level in the water coming out of the purifier, you just need to be sure that there isn't any, and this is cheap, easy, and very quick to do.

Obtain some DPD 4 tablets on line or from your local swimming pool supplier. They should only cost around £10 to £15 a hundred [2014 prices]. Fill a standard 10 ml sample tube with water from your purifier or pond and drop in a DPD 4 tablet. Don't look through the side in the usual fashion, place it on a white surface and look down through it. This will make it easier to see a faint colour change. If there is no chlorine or chloramine in the sample it will stay clear. The tablets begin to show a noticeable pink colour change in the range 0.01 to 0.02 mg/L so, although this is too indistinct to be able to say what the actual level is, it is valid to say "no colour means a total chlorine level of less than 0.02 mg/L". If you can see the faintest pink colour in the sample, it means there is a trace of chlorine in it and there shouldn't be.

Strictly, a DPD 4 tablet doesn’t just test for chlorine, it is also looking for any other oxidising reaction, so, although it provides an instant colour change when any form of chlorine is present, it will then very slowly produce a pink colour change due to dissolved oxygen. This is especially noticeable if the sample is left exposed to the air so that more and more oxygen can enter the solution. Look for the colour change immediately because even if there is no immediate pink colour, showing that there is no chlorine in the sample, after several minutes dissolved oxygen will begin to turn it pink anyway.



Additional information about types of DPD 4 tablets
There are two types of DPD 4 tablets. The cheapest ones are called "Rapid Dissolving" and they do exactly that but the ingredient in the tablets that causes them to disintegrate and dissolve rapidly results in them tinting the water white. They still turn the sample anywhere from pale pink if there is a trace of chlorine or chloramine present to deep pink if there is a lot of it present but a few people have said that they find the white background colour confusing.

There are also "Photometer Grade" DPD 4 tablets which are a little more expensive. These are difficult to dissolve (which is why the rapid dissolving ones were produced) but they leave the sample totally clear unless there is any form of chlorine present. Don't shake the sample tube vigorously in an attempt to dissolve these types of tablets, that will dissolve oxygen into the sample and may cause a false positive reading. The tablets should be crushed in the bottom of the sample tube with a specially made tablet crusher or with something that resembles the blunt end of a plastic knitting needle. After that, a couple of inversions of the tube will be all that is necessary to ensure good mixing.

Both types of DPD 4 tablets react identically to chlorine/chloramine and the only difference is that the rapid dissolving filler in the rapid dissolving types adds the white colour as well as any pink colour change due to chlorine.

I use both grades according to whether I need the accuracy of a photometer or a quick estimate of chlorine by eye and find no problem seeing pink in the sample, either in a clear sample or one with the white tint as well as any pink.

Try the cheaper rapid dissolving ones and, if you don't like the white tint, get the photometer grade tablets instead.

[ginboomerang]

Thanks,

A useful and informative post. Our water has had a 'chlorine smell' to it recently and I know that Severn Trent are laying new water main pipes in our area. I changed the carbon filter and added another carbon pod 'just in case'

I'll be buying them there DPD 4 tablets.

[Manky Sanke]

Mark,

I came up with the idea of using DPD 4 tablets that way when I was testing some water samples for very low levels of chlorine a few years ago and began thinking that the average koi keeper who doesn't have electronic meters needed a simple way to test for such low levels instead of taking the chance that their purifiers were working properly.

I've had that piece on my website ever since but, although the test is very simple, it's important that it's done exactly as described or the very low level of pink at extremely low chlorine levels won't be noticeable. If you, or anyone else, likes the simplicity of the idea and passes it on could you please also point to the correct method, either on here or on my website.

[Drakoi]

Well there are test for Cl so those will work as well and you can proly find them at your local Pet shop.

[Manky Sanke]

I'm well aware that there are chlorine testers on the market. I spent 30+ years running a swimming pool company and, apart from selling chlorine test kits of all types to my customers, I must also have used every form of chlorine test kit or meters that is available. As it says in the post, the reason why I came up with the idea of using DPD4 tablets and a sheet of white paper as a cheap and simple way for koi keepers to test for very low levels of chlorine was that electronic test kits are expensive and that there isn't a manual kit on the market that can detect chlorine at very low levels.

For koi, the chlorine or chloramine level should ideally be zero but certainly should not exceed 0.02 mg/L. Manual chlorine test kits for swimming pools and similar applications are cheap but they will not read down to such a low level. Cheap chlorine meters sold on ebay for less than twenty pounds are not reliable at very low levels.

The test method I described above gives a simple "go-no go" test that clearly indicates whether there is no chlorine present or whether chlorine is present at a level that is a threat to koi health.

[Pondwithnoname3]

I appear to live in an area with relatively low chlorine in the supply but I don't have much faith in the three pod unit I have, I've refilled two of the cartridges in recent months with activated carbon and renewed the first pre filter cartridge.
I add sodium thiosulphate on top ups and partial water changes, my question is, is the three pod redundant or is it removing contaminates such as heavy metals or should I bin it's?

[Manky Sanke]

The three pod system isn't redundant if you have the correct cartridges. The mechanical filter filters out small particles such as rust and other solid contaminants. A metals cartridge will remove any trace of dissolved heavy metals such as lead or copper. A carbon filter will remove chlorine or chloramine.

[Pondwithnoname3]

The three pod system isn't redundant if you have the correct cartridges. The mechanical filter filters out small particles such as rust and other solid contaminants. A metals cartridge will remove any trace of dissolved heavy metals such as lead or copper. A carbon filter will remove chlorine or chloramine.

Thanks Syd, best I get get the metals cartridge back in, out of interest what's the material in a metals cartridge and how's it work at removing heavy metals?

[Manky Sanke]

Manufacturers regard information like that as commercially sensitive and are reluctant to give away any information other than sales hype but a common industrial way to remove heavy metals is called KDF (kinetic degradation fluxion). It is relatively cheap and easy to manufacture into suitable cartridges/pads that can be fitted into water feeds or water flows where heavy metals are likely to be present or accumulate and need to be removed.

Essentially KDF filters consist of pads impregnated with shards of copper and zinc which act like small batteries when they get wet. The electric field causes them to attract a whole range of metals and adsorb (similar to absorb) them onto their surfaces.

[Pondwithnoname3]

Manufacturers regard information like that as commercially sensitive and are reluctant to give away any information other than sales hype but a common industrial way to remove heavy metals is called KDF (kinetic degradation fluxion). It is relatively cheap and easy to manufacture into suitable cartridges/pads that can be fitted into water feeds or water flows where heavy metals are likely to be present or accumulate and need to be removed.

Essentially KDF filters consist of pads impregnated with shards of copper and zinc which act like small batteries when they get wet. The electric field causes them to attract a whole range of metals and adsorb (similar to absorb) them onto their surfaces.

The stuff I removed and replaced with carbon resembled loose chips of copper/brass I dumped it because there was only a handful of the stuff in the canister and I probably wrongly thought carbon was a better option.
The last few refill canisters I bought weren't pond specific but produced for domestic supply and or pond, could this be a problem do you think?

[Manky Sanke]

No, the cartridges in water purifiers are usually made to standard specifications that perform best at whatever they are supposed to do. There will be variations, of course, but the principles will be the same.