Pigeons, polluted rain and reverse osmosis filters

Inevitably as the weather gets drier those not lucky enough to have a good store of rainwater are wondering how to get hold of clean water.

One possibility we’ve recently been asked about is whether you can use reverse osmosis filters to clean up tapwater.

Reverse osmosis filters – which force tapwater under pressure through a semi-permeable membrane – seem to be sold as domestic appliances for people not happy with their tap water. It looks like they would cost several hundred pounds to get installed, and then have some regular running costs for filters and maintenance.

I haven’t investigated them carefully – just scanned one companies website – but they appear to have a serious flaw as far as ponds are concerned. Although the filters had a very high removal rate for most chemicals – usually in excess of 90% – for nitrate, which will usually be the biggest problem, the removal rate is only 50 – 90%.

So you could fit the filter and then remove only half the nitrate. If your tapwater comes with 25 milligrammes of nitrate in each litre (which it often does) halving this amount would still leave you well above natural levels.

Which brings us to Robert in Lewisham who has a pigeon infested roof with passing traffic depositing all sorts of dirt on the roof as well. And to compound the problem, he has no downpipes to get the water from anyway.

The only practical suggestion I have is to see whether there are any friendly neighbours who might be prepared to share water from their roof. You collect the water from their place and bring it back to your garden pond.


3 Responses to “Pigeons, polluted rain and reverse osmosis filters”

  1. K Skiper Says:

    I can’t see what the problem with tap water is. It’s highly purified, and safe. In some areas there is a higher mineral content, but that’s no real problem when compared to natural mineral addition to garden ponds. Any chlorine (which is only present in low ppm quantities anyway) will disappear after half an hours standing. Fill a water but with tap water, cover it, and leave for an hour or so – perfect for topping up ponds.

  2. Jeremy Biggs Says:

    Hi K Skiper – You’re right that the tapwater is safe to drink and that minerals in water, like the calcium that forms limescale, are completely harmless. Indeed for lots of plants and animals, calcium and other minerals are essential.

    But tapwater often contains unnaturally high levels of nitrogen, and sometimes also high levels of phosphorus. This makes it far from perfect for topping-up ponds – and is the cause of many of the complaints people have with algae, excess duckweed and a general failure to grow underwater plants. So tapwater is often, as far as ponds are concerned, polluted. You can check how bad your own water is (look for nitrate) by looking at the drinking water quality reports published on your water company’s websites (most of the bigger companies have a postcode searchable database of this information). Data on phosphorus are not easily available – water companies may add it to stop lead leaching out of old pipes into the drinking water but we don’t know how widespread the practice is. One water company we have spoken to is adding phosphorus at a concentration 10 times the level that’s natural in freshwater. That is serious pollution, and is pretty ironic as water companies have spent hundreds of million of pounds (funded through water bills!) upgrading sewage works to reduce the amount of phosphorus that is discharged into rivers, lakes and the sea.

    Nitrogen and phosphorus are both, of course, fertilisers but in high concentrations in water they are also pollutants causing excessive growths of algae at the expense of submerged water plants. Human activity has caused nutrient pollution on a vast scale – hardly any ponds, rivers or lakes in lowland Britain, for example, remain unpolluted by nutrients.

    As you say chlorine isn’t really a problem – if it hasn’t already been converted to chloramines by the addition of ammonia in the water treatment process (the water companies do this because chlorine gas can form cancer causing compounds when they react with organic matter in the water) it will quite quickly disappear from the standing water.

    Getting nutrients out of water once they’re in there isn’t anything like so easy. To remove nitrogen it may be possible to run the water though a biological filter where denitrifying bacteria can sometimes turn enough of the dissolved nitrogen (mainly nitrate) into nitrogen gas which simply joins all the other – more or less harmless- nitrogen in the atmosphere. To solve the problem of phosphorus you need to remove plants, mud or animals once they have taken it up, and then stop adding more. Phosphorus can’t be turned into a gas – so you need to physically remove it one way or another.

    The point about rainwater-fed garden ponds is that they could become a host of little islands of clean water in a land where unpolluted water is almost – but not quite – a thing of the past.

  3. jonspond Says:

    Using Reverse Osmosis filters is also incredibly environmentally unfriendly as it takes 4 litres of waste to produce 1 litre of RO water with a TDS reading of 0ppm – I used to have a marine fish tank and I was really concerned over the amount of water wasted for the RO water used to mix artificial seawater for water changes.

    Every 25 litres means that around 100 litres goes down the drain. I have read on marine fish keeping forums that the waste water has been placed into garden ponds – hyper polluted water!


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