Algaecides are perhaps the easiest to apply of all the standard pool treatment products. In this newsletter we will try to explain how algaecides work, why they play a necessary part in good pool management, and how to select appropriately from the choice of algaecidesavailable.
Algae Algae are nothing more than plants which are adapted to live in water, so it’s no surprise that they thrive on the same nutrients as any other plant. They require nitrogen, phosphorus and potassium, just like the plants in your garden. They also need sunlight, as this is essential for the photosynthesis of which all plants are capable. This is a complex chemical process in which chlorophyll, which causes the green colour, utilises light energy and converts carbon dioxide into organic compounds essential for growth.
But how do these little plants, and the essential nutrients they need, get into pools and hot tubs in the first place ? They arrive by every means available – carried in on rain drops, blown in by the wind, walked in by bathers, or by hitch-hiking on leaves and twigs. Once established, they multiply very quickly, because few swimming pools contain the little grazers which would eat them up in the wild !
But what about their favourite nutrients ? Nitrogen is most likely to be introduced into the water by bather contamination. That’s why a pre-swim shower is so useful. Shock treatment (superchlorination), itself a good method of killing algae, ensures that nitrogen is released from pool debris as a gas rather than staying in the water as an assortment of nutritious (to algae at least !) nitrogen compounds. So shocking is a great way of starving them out AND oxidizing them away at the same time. Phosphorus is introduced into the water in a variety of ways: • It may be present in the water supply; small amounts are sometimes introduced into the water by the supply company to prevent corrosion to metal components • It may be introduced by using water softeners, which often use phosphate chemicals • In hot tubs, it is often found in scale control chemicals, which may contain phosphonates • It may build up if you use a liner cleaner containing phosphoric acid • Finally, it may arrive as a result of accidental introduction of field or garden fertilisers into the water from surface ‘run off’ or sprays. Unfortunately, this will also give your pool a dose of unwanted nitrogen and potassium too ! Just the ticket for an algal bloom to grow !
OK, my water has turned an unpleasant shade of green, and the kids are planning a pool party; what now ?
As always, prevention is better than cure. Regular pool maintenance and careful attention to water quality helps prevent algae from growing in the first place. This means keeping the pH in line (7.2 – 7.6) at all times and maintaining the recommended free chlorine ‘residual’ for your pool. The best algae prevention comes from a steady sanitiser level of 1-3 ppm for chlorine or 2-5 ppm for bromine for a pool, and 3-5 ppm chlorine or 3-6 ppm bromine in a spa or hot tub. Algae is less common in pools, spas and hot tubs which have a cover, as the cover reduces the amount of sunlight reaching the water; this makes it harder for the algae to get a foothold.
How to deal with an algal attack If all is green, use this checklist: 1. First and foremost, check the pool pH; more often than not, the pH will be too low (less than 7.2), which will cause your pool to consume chlorine rapidly, or too high, in which case you might encounter the dreaded sanitiser failure. At a pH above 7.6, chlorine becomes progressively less effective. At pH 8.5 it barely functions at all. If necessary, adjust the pH using an appropriate dose of pH Increaser or Reducer. 2. Now shock dose by raising the free chlorine level to above 10 parts per million if the water is a greenish colour and up to 25 parts per million if you really can't see the bottom at all ! Always keep bathers out of the pool until the chlorine level falls to less than 5ppm. 3. If you see colonies of algae (green threads or ropes) which have attached themselves to the pool walls, steps or other fittings, just brush ‘em off. The nastier types such as the black and mustard varieties are thankfully rare in the UK, but they can adhere so firmly that you may need to scrape them off – carefully, to avoid damage to your liner. 4. Now vacuum to remove the debris by filtration, giving the filter a regular backwash every couple of hours. 5. Now you can introduce an algaecide into the water. Yes, that’s right, finish with a ‘remedial’ dose of algaecide !
How should I choosewhich algaecide ? Algaecides are simply chemicals which are added to the pool water to control algae. While algaecides can kill algae at high dosages, most are used as algaestats, or agents which prevent algae from growing when chlorine is allowed to become depleted. However, the term ‘algaecide’ has stuck to the whole group.
In the past, the algaecide of choice would most likely have contained a copper salt, as copper is toxic to algae at low concentrations. In most situations now, the ‘polyquat’ algaecides have become the mainstay of modern pool management. Polyquat is (mercifully) short for ‘polyquaternary ammonium compounds’; agents of this type treat and prevent algae growth by disrupting the delicate cell membranes which protect the tiny plants which form an algal bloom. These agents are much safer to use than the older copper-based algaecides (though these still have a place in case of shutdowns during the season) because they will not stain you, your hair, or your swimming pool a rather attractive shade of blue ! If you have experienced any pool stains caused by metals in the past, you should use a polyquat algaecide to treat your swimming pool, and avoid copper-based products completely. Polyquat pool algaecides cannot cause staining or foaming.
Copper salts still have an important place as long-life algaecides. Copper disrupts the photosynthesis process on which all algae depend, but copper algaecides work quite slowly, whereas the polyquats effectively burst the little cells apart very quickly. Because they act slowly, the copper agents are very handy when you wish to shut down your pool for a long break during the season. Copper-based long-life algaecides usually containing copper chelate, a compound in which the copper is bound to a very stable chemical called a chelating agent. Copper algaecides will slowly release copper into the water over a period of 3 months or more. Don't be tempted to overdo the dosing of a long-life algaecide, otherwise copper may start to deposit on metal surfaces. Although modern polyquats are highly effective, we at Easychemicals sometimes encounter pools which suffer repeated algal blooms, in spite of shock treatments and heavy dosing with algaecide. The underlying reason is simple, but not immediately obvious.
Introducing a remedial dose of algaecide into the pool or hot tub will lead to the rapid destruction of algae. However, this rapid destruction also creates an increased amount of organic debris in the pool, consisting of the dead algae themselves. This can easily overwhelm the chlorine in the pool, because the chlorine will automatically try to oxidise the dead algae (just as it will oxidise bacteria in the pool). This means that the chlorine is essentially used up in carrying out a different task to the one intended for it, and there is now less left to deal with the onerous task of sanitising the pool. This is why many pool owners find that their chlorine consumption rockets after an algal bloom, and the pool quickly relapses into an unpleasant green state within days of an apparently successful algaecide clean-up !
OK, so how do I deal with this problem ? This is where we have to think laterally. We have just shocked our green pool and dosed with a ‘remedial’ dose of polyquat algaecide. The water contains an amazing amount (perhaps several pounds) of finely dispersed dead algae, and it looks hazy and dull. Our overworked chlorine is trying to ‘burn’ that haze away whilst at the same time dealing with the constant attack of airborne (and bather-borne) bacteria. It’s little wonder that a bit of help is needed. And here’s the trick. Flocculate it, then sweep it away !
Fans of our occasional e-mail newsletters will now be able to return to one of our previous issues, when we looked at the role of flocculants. This is where they really come into their own. New friends of Easychemicals, feel free to ask us for a copy of the guide to flocculants by clicking this link: firstname.lastname@example.org. Just type in the e-mail title, and hit . Flocculants work by helping small particles clump together to form larger ones, which are heavy, and therefore sink to the bottom of the pool. Once deposited, your trusty pool cleaner can vacuum this silt to your filter, or better still, to waste. Of course, the impact of several pounds of fine silt (ie. flocculated algae) on any filter is substantial, so it’s important to backwash every couple of hours during a post-flocc vac session. That’s tedious, but still much easier (and cheaper) than dosing with chlorine every 12 hours in a vain attempt to oxidise several pounds of dead, dispersed algae !
So, to wrap up: 1. Keep up with routine pool care, and algae should never darken your water again 2. A routine weekly dose of a plain polyquat algaecide will act as an excellent ‘algaestat’ 3. If the green devil appears, follow the checklist above – How to deal with an algal attack 4. Use a long-life algaecide to cover long periods when the pool is out of use 5. Dose with Winterclear every 3 months during winter shutdown; this product also includes a scale inhibitor and filter aid to help see your pool through the long dark nights.
Think of algae as weeds in your pool and you won’t go far wrong. Weeds in your garden can be prevented from gaining a foothold by regular maintenance and judicious use of weedkiller. Similarly, an occasional dose of algaecide as your pool ‘weedkiller’ always helps !
Remember - Always read instructions on packaging for proper handling, dosing and application of pool chemicals. Never mix chemicals. Re-test water daily and readjust if needed.
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