Phosphate

The association between algae and phosphate is fast becoming recognized as a major factor in the presence of algae in swimming pools-and its removal.

This summer, some of the highest recorded levels of phosphate have been reported In pools right across Australia, partially due to drought -related dust fallout. Apparently, the problem of phosphates in our oceans and groundwater has now Crossed over to our swimming pools as well. The biggest concerns for swimming pool owners from increased phosphate levels are excessive, stubborn algae blooms and rapid chlorine consumption.

For the swimming pool owner, removal of phosphate is imperative to maintain good water quality. Then, once the phosphate has been removed, it's important to maintain a low phosphate level. This will ensure clean water, better chlorine performance and no algae. All living things, including algae in swimming pools, need phosphate to survive and grow. It is common industry knowledge that removing the phosphate and increasing the chlorine concentration will result in less algae in the pool. But how are the two related and how can the phosphate concentration be reduced? Phosphate enters the pool water from a variety of sources including dust and rain, runoff from lawns and gardens fill water, some pool chemicals and leaves. Over time, this causes an increase in the phosphate concentration.

Small amount of algae attached to dust, leaves and the like are constantly being introduced into pools. They need only warmth, nutrients like phosphate, and sunlight to rapidly multiply and become a threat to pool sanitisation, through the more rapid destruction of chlorine. Chlorine has long been used not just to sanitise swimming pools but also to kill algae, as chlorine is a reasonably effective but short-lived algaecide. One traditional treatment to visible algae in pools has been to "shock” the pool with high concentrations of chlorine which kills off most of the algae. However, this does not reduce the amount of phosphate in the pool, so when the chlorine level in the pool drops back to normal, the conditions which allowed algae to grow before “shocking” are still there. And the algae will start to re grow. Preventing algae from regrouping requires that either the chlorine concentration is kept at high levels or that the conditions in the water are changed to be less favorable for algae growth.

In traditionally treated pools, phosphate concentrations tended to be limited to the removal of algae by filtration. This is not very effective because wet algae weigh 1000 times more that the phosphate needed to nourish it. Further, a portion of the phosphate contained in the algae is released back in the water as the algae die in the filter, and the algae are oxidized by chlorine in the water passing through the filter.

A more effective traditional treatment, especially when treating massive blooms of algae, is super-chlorination until the algae turn white, followed by flocking with aluminium compounds and lanthanum compounds. Iron compounds are used to effectively remove phosphate from sewage effluent and ponds but is undesirable in pools due to the stains that iron compounds create. Aluminium compounds are best suited for pools moderately polluted with phosphate, in the range of 1000 to 5000ppb phosphate and where the operator is familiar with vacuuming to waste. Aluminium compounds are effective for removing most of the phosphate, down to approximately 500ppb but, without a lot of bother, they cannot keep the phosphate below the 100ppb, which is required to be effective for algae control. The principle advantages of aluminium compounds are that they are cheap and therefore suitable for a one-off process of removing the bulk of phosphate accumulated in a pool. One of the principle advantages of lanthanum compounds is that when they come in contact with pool water containing phosphate, hydroxide and carbonate, they do not significantly interfere with the formation of lanthanum phosphate. This is due to the extremely low solubility of lanthanum phosphate relative to lanthanum hydroxide or lanthanum carbonate. Just a slight (20%) excess of a lanthanum compound will cause the phosphate to drop well below 100ppb. Concentrations lower than 10ppb are reasonably easily attained. Lanthanum compounds are a potent and specific phosphate remover. Another advantage of lanthanum compounds is that moderate amounts can be retained in pool filters without clouding the pool water. Crystals, largely undissolved, lodge in the filter then slowly dissolve to coat the filter media with lightly adherent, invisible-to-the-eye lanthanum carbonate (together with a little lanthanum phosphate). Here they retain their ability to specifically remove phosphate from water passing through the filter.

Lanthanum compounds are best suited for maintaining low levels of phosphate in pools where convenience and performance are important considerations. They are much easier to use and require much less operator time than aluminium compound phosphate removers. They are better suited to pool owners and busy pool service –people. Considering cost and convenience, they are best suited for pools with less than 2000ppb of phosphate.

To sum up, phosphate removal is the key to keeping pools algae-free, maintaining quality water and allowing other chemicals to work at their most effective level. As additional insurance against recurring algae, a “Long life” copper-based algaecide is recommended to be used in conjunction with the phosphate removal treatment. Chlorine, as mentioned earlier, can bee categorized as a short-term algaecide. A copper-based algaecide, in contrast, will maintain its efficiency over a long period of time and in doing so, assist the phosphate remover to do it"s job more resourcefully. Pool owners can avoid high levels of phosphate and keep their pools algae free in various ways.

1.    They should prohibit runoff from lawns, landscaping or washing decks entering the pool;

2.    They should remove leaves and other rubbish from the pool regularly;

3.    They should apply a lanthanum compound phosphate removal treatment;

4.    They should use a long-life algaecide all year round in conjunction with their phosphate removal treatment;

5.    They should get their pool professional to regularly test their pool water for phosphate.

Pool shops have at their disposal special test kits to check the level of phosphate in swimming pools. Should the result of the test exceed the recommended level then a product to remove the excess phosphate is now readily available through pool chemicals outlets right around Australia.

This product is called “Starver” and is now far and away the biggest selling item in the pool and spa specialty chemical range.

Extract from August/September 2006 edition of SPLASH pool & spa industry magazine