Swimming Pool Blog

Phosphates & Nitrates in Pools


by Rob CoxJuly 30, 2010

Phosphates & Nitrates in Pools

nitrates in pools and spas
Phosphates were pretty big news in the seventies, when environmental groups began battling big industry against the practice of dumping waste water high in phosphates and nitrates into local watersheds. This was creating something called eutrophication. This is a process where bodies of water receive excessive nutrients that stimulate plant growth. Lakes and rivers were becoming overgrown with algae and plants, which dramatically affects the water chemistry, and ability for fish and other aquatic life to survive.

In swimming pools, our main goal is to keep eutrophication to a minimum. We don't want to see any plant life growing - unless you are operating a "natural pool", so it makes sense to keep levels of Phosphates and Nitrates to a minimum in swimming pools.

Phosphates and Nitrogen (the source of Nitrates) occur naturally, and they are all around us. Phosphorous occurs as a mineral and Nitrogen is a gas. Outside of the pool, they pose no problem, but once in the pool water, these two elements create conditions that increase chlorine demand to fight against microscopic plant life that uses both elements as a food source.


What Are Phosphates and How did they get into my Pool?

Phosphates (PO4) are generally referred to as pollutants, although many sources of Phosphate occur naturally. We most often think of phosphates as what is used to give our laundry detergent its sudsiness. Phosphates are in cleaning supplies, fertilizers, some beverages, and even in some pool chemicals. You can't see, smell or taste phosphates in your pool water, but it can be tested for, with a Phosphate test kit.

Phosphates commonly enter pools through wind blown fertilizer. If you have a lawn care service treating your beautiful yard, or if you fertilize azaleas surrounding the pool, it can be difficult to keep fertilizer from reaching the pool, via wind or water. Phosphates can also be left behind in our swim trunks, after removing them from the washing machine. Soaps, Shampoo and cosmetics all contain phosphates.

Phosphates usually enter a pool in the form of Tri-Sodium Phosphate, or TSP. During shocking of the pool, or adding of enzymes, TSP is broken down into the tastiest algae food known - Ortho-Phosphate. In this free state, ortho-phosphate provides all types of algae with a good food source.


What Are Nitrates and How did they get into my Pool?

Nitrates and Nitrites are products of Nitrogen gas. Nitrate, which consists of a single nitrogen atom connected to three oxygen atoms (NO3), is extremely stable or hard to remove. Nitrogen (NO) can enter your pool water from a variety of sources. It combines with oxygen to form a Nitrite (NO2). This extra oxygen atom is not just free floating around the pool (unless you are using an ozonator), so the agressive Nitrogen Oxide will steal another O atom from our chlorine molecule, HOCl. This makes our chlorine less effective and increases your chlorine demand. This continues as Nitrites take on another oxygen atom to become Nitrates, (NO3). 

Nitrates have many of the same sources as Phosphates. Lawn care products, Acid Rain, Perspiration and Urine or other types of Ammonia (NH4). One study done in Miami found a higher incidence of Nitrates in community pools and pools which allow children, and lower levels in adult only pools. Nitrates cannot be removed from a pool except by dilution. AskAlan suggests draining 20% and refilling, repeatedly, until Nitrate has been lowered to manageable levels. You can test your pool for Nitrate levels, using a test strip for Nitrates.


How do I know if I have a Phosphate or Nitrate Problem in my Pool?

The first clue to you, may be that persistant algae that keeps coming back. If you have "tried everything" and the algae keeps returning, you probably have some amount of Phosphate or Nitrate contamination. If suspected, you can use the test strips to determine the level present in the water. Your local pool store may be able to test for this, or alternatively, perform a Chlorine Demand test, which can be an indication of a Phosphate or Nitrate presence in high concentrations.


How do I remove Phosphates or Nitrates from my Pool?

As mentioned above, the treatment currently used for nitrate removal in pools is "by dilution". Replace the water. Draining a pool completely can cause problems, so it's best to drain partially, refill, drain partially again, refill, etc. Phosphates, however, can be removed partially or controlled with good sound water balance and frequent shocking of the pool water. For those extreme cases, however, it has become quite convenient to use a phosphate remover, such as Phos-Free by Natural Chemistry, or the Orb-3 phosphate removal product. 

shower before swimming

Keeping an eye on the use of fertilizers, water run-off and introducing only clean swimmers into the pool will also help to keep the algae food of Phosphates and Nitrates down to a minimum.