Scaling On The Pool Surface – Is Calcium The Only Cause? What Is Really Causing Scaling?

When we talk about scaling in the pool industry, most of the time the offender that comes to mind is calcium. We all know that high calcium levels can lead to scale depositing on the pool surface, fixtures, in the plumbing, and inside equipment like heaters and salt chlorine generators.

What if calcium isn’t the only factor? What if the calcium hardness of fill water has always been high but scaling was never an issue until recently? Could something else be causing, or at least contributing to, the problem?

The answer is yes.

For decades it has been a well-known fact in the drinking water treatment industry that excessive phosphates will actually form phosphate scale. When phosphate levels go beyond the “critical concentration level”, scale forms. This “critical concentration” level is variable based on chemistry, but for most pools that maintain the chemistry within the NSPF guidelines, that critical load point is somewhere between 2000-4000ppb.

Most pools today have phosphates. In fact, very few do not. This is due to multiple factors, including residual phosphates from municipal water supplies, swimmers, and even some pool treatment chemicals that utilize orthophosphates as an ingredient. The ambient level varies from facility to facility, but with the advent of newer digital phosphate meters, we are finding many clients have phosphates beyond 2000 ppb. Some even have phosphates nearing the 10,000 ppb mark. Phosphate load is influenced by the type of swimmers in the pool, by source water, and by locally available pool treatment chemicals.

When the operators of these pools perform maintenance and attempt to clean the scale from heat exchangers, salt chlorination cells, or off of pool/deck surfaces and equipment, what they find is that the scale is incredibly difficult to remove. The reason is that the phosphates combined with calcium to form a stronger than average scale deposit. The scale that forms is not as responsive to typical acid washes, and hardens more than calcium scale alone will, typically requiring scraping off the pool/deck with a chisel or a similar tool and multiple diluted acid washes (do not use concentrated acid) for the chlorination cell, since scraping the plates will cause irreparable damage and should be avoided at all times. It also is not soluble in cold water like calcium scale is. If this sounds familiar at all, you may have a bigger problem on your hands than you thought.

High phosphates are not an insurmountable problem, but they do require attention. There are some simple steps that can be taken to prevent calcium phosphate scale from forming in the first place:

1.       Remove the phosphates before they become a problem – To do this you’ll need a high strength phosphate remover.  These chemicals should be used on a daily or weekly cycle to help maintain phosphates at as low a level as possible, even while more are introduced to the pool.

2.       Reduce the ability of the calcium and phosphates to “plate out” – To do this, you can utilize special chemicals with polymers that help to keep the phosphates and calcium in suspension. This will help reduce scaling but will also contribute to higher ambient phosphate levels as they are “trapped” in the water. This is really only recommended for facilities where phosphates are measured above 6000ppb and where treating for phosphates is difficult to schedule or maintain.

3.       Keep your water in balance, and customize your balance to your pool – When balancing your pool it is recommended to start with the NSPF guidelines and then incorporate the LSI (Langlier Saturation Index) to keep the water in line. For pools with high calcium, you’ll want to adjust your alkalinity and pH levels down a bit to keep your pool out of the “scaling” zone (higher than +0.3). If you have high calcium and high phosphates, pushing your LSI slightly towards the corrosive end (-0.3 being the limit) can help prevent scaling.

Calcium scale and phosphate scale are definitely situations that should concern you, but going into it with some knowledge of how to deal with the issue should allow you to keep your equipment in top shape and your pool crystal clear.