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A part of this Technical Information Bulletin is taken from a document prepared by Technical Information Services, Bio-Lab, Inc., Decatur GA on 24 September 1999.

An Introduction to Phosphates

The element phosphorus (P) is one of the essential nutrients required for algal growth, along with carbon and nitrogen. Phosphorus is used by algae in many metabolic processes including photosynthesis. Sometimes referred to by its non-systematic name orthophosphate, phosphate (PO43-) is not only the most common source of phosphorus in natural waters, it is also the most easily assimilated by algae. Phosphates are measurable using selected pool test kits, but it is important to note that phosphonates and organophosphorus compounds can also be assimilated by algae, so these test kits may not indicate all of the phosphorus in the water that is available as an algal food source.

It is important to understand that the discussion of phosphates as a food source for algae is fuelled by information gathered in naturally occurring and unsanitised bodies of water such as lakes, streams, and ponds. Although dependent upon a range of factors, the impact of phosphates in natural bodies of water is generally well understood. In contrast, no literature is available describing the impact of phosphates on algal growth or sanitiser consumption in disinfected water, such as a pool or spa maintaining a chlorine or bromine residual. Many phosphate test kit manufacturers do not provide guidelines for what they consider to be a low, acceptable or high concentration of phosphate, simply because they cannot identify the impact of phosphates on sanitised bodies of water, where residuals of chlorine, bromine or other sanitisers are maintained, in part, to prevent algae growth.

Open bodies of water, such as swimming pools, will always have algal nutrients being present from a variety of sources. In the presence of all of the essential nutrients, algae will grow until one of the nutrients becomes limiting; that is, there is not a sufficient amount of that particular nutrient available to support growth. In many cases, the limiting concentration of phosphate is far lower than that which can be detected by the test kits. For example, if all other nutrients are available in excess and phosphate is added, algae growth will increase dramatically. On the other hand, if phosphate is present in excess but nitrate is absent, algae will not grow regardless of the phosphate concentration. Monitoring of phosphate concentrations alone is not a satisfactory indicator of algal growth.

Phosphate Removal Using Starver™

There are at least two products that are specifically targeted at removing phosphates from pool water. PhosFree™ from Natural Chemistry is based on enzyme technology and, at the time of writing, was not available in Australia. Starver™, manufactured by Vanson, Inc. and marketed in Australia by Lo-Chlor™, utilises a lanthanum compound to precipitate phosphate out of the water as insoluble LaPO4.(1) The product is compatible with all conventional sanitisers and oxidisers, including non-halogen compounds like potassium monopersulfate (eg. BioGuard OxySheen). In the supporting literature,(2) Vanson suggest that the phosphate residual in a swimming pool should not exceed 0.1 ppm.

When Starver was first released, the Technical and Development Department at Bio-Lab, Inc. in the US examined the product with the view to possibly incorporating it in the BioGuard range. After extensive laboratory and field trials, the decision was made not to pursue Starver for a number of reasons:(3)
1. It is a heavy slurry that proved difficult to apply.
2. Many applications of small doses are required in order to avoid high filter back-pressures whilst lowering phosphate levels.
3. The filter must remain on until phosphate levels fall to the recommended level.
4. Much of the product is lost during backwashing or cleaning.
5. In well-maintained pools, there is no need for the product, as algae is adequately controlled by maintaining sanitiser and algae preventative residuals.

In conclusion, phosphate-removing products will not remove all of the nutrient sources of phosphorus available for algae. The routine maintenance of an appropriate sanitiser residual and good maintenance practices (correct water balance, use of an algae inhibitor like BioGuard Back-Up, Back-Up II or Salt Pool Protector, etc.) is necessary to prevent algal growth in swimming pools.

References

1. Vanson, Inc., "Starver The Phosphate Remover", Technical Data Sheet, 6 September 1995.
2. Vanson, Inc., "Phosphate Testing and Removal", Technical Data Sheets, 26 July 1996.
3. M. Mueller, "Starver Versus Optimiser", Bio-Lab Australia Interoffice Memo, 6 April 1995.

The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.