Phosphates, algae and your swimming pool

The association between algae and phosphates is recognized as a major factor in the presence of algae in water/swimming pools-and its removal.

The biggest concern for swimming pool owners is that increased phosphate levels can result in excessive and stubborn algae blooms.

For the swimming pool owner, removal of phosphate is imperative to maintain good water quality. Once the phosphates have been removed, it’s important to maintain a low phosphate level. This will ensure clean water and no algae.

All living things, including algae, need phosphates to survive and grow. It is common industry knowledge that removing the phosphate below 150ppb will result in less algae by method of starvation.

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 amounts 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 your beautiful pool.

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 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.

An effective traditional treatment, especially when treating massive blooms of algae, is super-chlorination until the algae turn white, followed by flocking with aluminium sulphate. Aluminium sulphate is fairly effective for removing phosphates, but without regular use, it cannot keep the phosphates below what is required to be effective for algae control. It is therefore crucial that small quantities of aluminium sulphate are applied regularly to the pool. (example – 1 tablespoon weekly per 10kl of water or when dust is found to be settling on steps or pool bottom. This should be sprinkled lightly on the pool surface and then the pool brushed well. Water may turn slightly milky but will clear up quickly. Normally on the following day, you will find that the aluminium sulphate has trapped phosphates + other particles and dropped to the pool floor. Vacuum this drop-out to waste or backwash immediately after vacuuming if not to waste for smaller pools.) ALUM is cheap and easily available but there is a more effective product as per below…

To sum up, phosphate removal is the key to keeping water algae-free, maintaining quality water and allowing other chemicals/O3 to work at their most effective level. Chlorine, as mentioned earlier, can be categorized as a short-term algaecide and works very well in conjunction with O3.

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 control phosphate levels by using a product that targets phosphate removal regularly;

4. 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. Algae cannot grow if the phosphates are kept below 150ppb. If your pool shop does not know about phosphates then best you find a new pool shop that understands water chemistry.

Water Rhapsody’s poolside tank system allows you to re-use your backwash water. This eliminates the need to top up after each backwash with municipal water. In just 24 hours, the once dirty water is clean and can be returned to the swimming pool. This not only saves you money on water but also on chemicals as the Ph balance of the water returned is the same as the water in the pool whereas municipal water dilutes the chemicals.

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