How to shock the pool (chlorinate to breakpoint)

Chloramines / Combined Chlorine

If you smell "chlorine", coming from your pool, what you really smell are combined forms of chlorine, also called chloramines. Chloramines are chemical compounds formed by chlorine combining with nitrogen containing contaminates in the pool water. These, are still disinfectants, but they are 40 to 60 times less effective than free available chlorine. Contaminates come from swimmer wastes such as sweat, urine, body oil, etc. This is why requiring all bathers to take a warm, soapy water shower is a good idea.

Three types of chloramines can be formed in water? monochloramine, dichloramine, and trichloramine. Monochloramine is formed from the reaction of hypochlorous acid with ammonia. Monochloramine may then react with more hypochlorous acid to form a dichloramine. Finally, the dichloramine may react with hypochlorous acid to form a trichloramine. Trichloramines cause the "chlorine" smell and hang in the air directly above the pool water level, often causing competitive or frequent swimmers to have asthma like symptoms. High levels of chloramines will also cause corrosion to surfaces and equipment in the pool area. The trichloramines are especially irritating to the eyes, nose and lungs.

Chloramines can usually be eliminated from the pool water by performing breakpoint chlorination with chlorine or super oxidation with a non chlorine oxidizer. Ultraviolet systems and ozone systems are effective at reducing chloramines in pools.

Breakpoint chlorination

Break point chlorination is adding enough chlorine to eliminate problems associated with combined chlorine. Specifically, breakpoint chlorination is the point at which enough free chlorine is added to break the molecular bonds; specifically the combined chlorine molecules, ammonia or nitrogen compounds. It takes a ratio of chlorine to ammonia atoms of 7.6 to 1 to reach breakpoint, other contaminants (i.e. bacteria, algae) are also present that must be oxidized, so 10 times the amount of combined chlorine must be added. When sufficient free chlorine (FC) is added to pool water, the inorganic chloramines are converted to dichloramine, then to nitrogen trichloride, and then to nitrogen gas. Any excess chlorine leftover will become the chlorine residual (FC).

The graph below shows what happens when chlorine (either chlorine gas or a hypochlorite) is added to water. First (between points 1 and 2), the water reacts with reducing compounds in the water, such as hydrogen sulfide. These compounds use up the chlorine, producing no chlorine residual.

The combined chlorine (CC) level is calculated by subtracting the free chlorine (FC) from the total chlorine (TC) in the pool/spa water. Rule 410 IAC 6?2.1?30(o) 2 requires testing of the pool/spa water for combined levels at least twice a week.

Rule 410 IAC 6?2.1?30(e) requires "The pool water shall be superchlorinated to breakpoint or superoxidized with a nonchlorine oxidizer, when the pool test kit reveals a combined chlorine (chloramine) concentration of five?tenths (0.5) parts per million (ppm) or greater." However, studies have shown that swimmers find pool water the most enjoyable if more than 85% of the total chlorine is free chlorine. Therefore, the Environmental Public Health Staff recommends superchlorination when the combined chlorine concentration is 0.2 ppm or greater (a total chlorine of 1.2 ppm and a free chlorine of 1.0 ppm provides 83% of total chlorine as free chlorine).

Note: Pools using bromine as a sanitizer must also perform breakpoint superchlorination using chlorine. Like chlorine, bromine combines with organic impurities to form combined bromine and bromamines.

Achieving Breakpoint Chlorination

To achieve the breakpoint, the free chlorine (FC) added to the water must be about ten times the amount of combined chlorine (CC). This is an "all or nothing" process. Not adding enough chlorine to reach breakpoint will make the problem even worse as the result is the formation of more chloramines and re?dissolving of chloramines back into the pool water. Continual "shocking" but not reaching breakpoint will result in the pool reaching a point of no return. Partial or complete draining of the pool water and refilling with fresh water may be the only remedy at this point. If an indoor pool facility has inadequate air exchange with outdoor fresh air, it will be necessary to add air circulation fans with doors and windows open to keep the air above the pool water level moving to prevent re?dissolving of nitrogen (by product of breakpoint chlorination) leading to more chloramine formation.

Please note as required in 410 IAC 6?2.1, Sec. 30 (g) "The pool shall be closed and remain closed during breakpoint chlorination" and adding too much chlorine, beyond breakpoint, will yield high chlorine residual that may require the pool to remain closed until the free chlorine residual drops to an acceptable level as required in 410 IAC 6?2.1, Sec. 30 (b).

Calculating Amount of Chemical to Achieve Breakpoint Chlorination

The DPD test does not measure combined chlorine (CC) directly, it measures free chlorine (FC) in Step 1 and total chlorine (TC) in Step 2. Total Chlorine is the sum of free chlorine and combined chlorine. Therefore combined chlorine is the difference between total chlorine and free chlorine. CC = TC – FC.

The first step in determining the necessity of a shock treatment is to determine the level of combined chlorine.

Using the D.P.D. testing kit, test for free chlorine (FC) and total chlorine (TC). After completing the water test, you subtract the free chlorine reading from the total available chlorine reading, the result indicates the combined chlorine (CC) or chloramine level in the pool water. Broward Beach County pool services

For example:

Combined Chlorine = Total Chlorine? Free Chlorine

2.3 ppm (TC) measured from test kit? 1.5 ppm (FC) measured from test kit = 0.8 ppm CC.

If the water has no chloramines, the answer to the subtraction will be zero (0) and a shock treatment is not needed. This is a desirable level. After determining the level of combined chlorine in the pool water, the pool operator must determine the breakpoint chlorination for that value.

The breakpoint chlorination value is 10 times the combined chlorine (CC) level.

For example: 0.8 ppm (CC) from the above example × 10 = 8 ppm of chlorine to achieve breakpoint. Taking into account the free chlorine all ready in the pool, chlorine will have to be added to the level of 8 ppm.

Determine the Amount of chemical to add*:

Example**: Calculate the chemical change to achieve Breakpoint Chlorination in 60,000 gallon pool with FC of 1.5 ppm and TC of 2.3 ppm. Using 67% Calcium Hypochlorite where the label states that 2 oz will produce a chemical change of 1ppm in 10,000 gallons of water:

STEP 1: Determine the amount of Combined Chlorine (CC)

Total Chlorine (TC) – Free Chlorine (FC) = Combined Chlorine (CC) 2.3ppm–1.5ppm= 0.8ppm

STEP 2: Calculate the breakpoint Chlorination (BPC) amount

Breakpoint (BPC) = CC × 10 0.8 × 10 = 8.0 ppm

STEP 3: Determine the desired change amount

Desired Change = BPC – FC

8.0 ppm – 1.5 ppm = 6.5 ppm

STEP 3: Determine the amount of chemical to add:

Amount of chemical from product label

Actual Pool Volume

Desired Chemical Change

Total

60,000

6.5

• 10,000 from product label
• 1.0 ppm from product label

2 oz.

• 6
• 6.5

78 oz

Convert answer to pounds: 78 ÷ 16 = 4.875 lbs; rounded to 5 pounds.

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