Shocking the pool with chlorine is the raising of free chlorine levels in the pool to such a high level that every living thing in the water is killed by cell disruption. I liken the process to a lightning bolt ripping through the water, and perhaps that's why they call it shocking the pool.
Oxidation is a term that's thrown around alot. Chlorine oxidizes the contaminants in the water, by attacking the cell walls of pathogens, algae and bacteria. Oxidation begins immediately when granular or liquid chlorine is added to water.
Add enough pool shock to the water, and you will reach what's known as "breakpoint chlorination", a threshold beyond which survival of any contaminant is unlikely.
There are basically three reasons to shock a pool - I call them the A-B-C's of Pool Shocking.
Algae: Green, Pink, Black - no matter what color, shocking with chlorine is one of the best treatments for algae. Small blooms in the corner, or a full-on assault by the single-celled enemy can both be treated effectively with granular or liquid chlorine. It's generally agreed that for algae removal, a 30ppm chlorine level will kill most active algae. This equates to about 4 lbs of 65% available, calcium hypochlorite pool shock, per 10,000 gallons. If you shock with liquid, about 3 gallons of 12% sodium hypochlorite per 10,000 gallons will do the job. Brushing, vacuuming and thorough filter cleaning is also required.
Bacteria: E. Coli, Legionella, Cryptosporidium, Pseudomonas, Shigella to name a few. Bacteria, viruses, and parasites can lead to Illness, hospital visits and in rare cases can be fatal. Water is a good breeding ground for bacteria and can support viral and parasitic outbreaks. They can enter the pool through many means, and infect users through absorption, or by breathing or drinking the pool or spa water. Bacteria is invisible in your pool or spa, but can be tested with bacteria test strips.
Chloramines: When a chlorine molecule is floating around in your pool water, looking for some cell to invade, it often is rendered useless when it combines with an ammonia or nitrogen molecule. These combined molecules, known as chloramines, are no longer effective sanitizers, and worse, are responsible for red eyes, chlorine odor, and itchy skin. A pool that has these symptoms is not over-chlorinated; and actually needs a whole lot more chlorine to correct the condition!
The level of chloramines in a pool can be measured with a DPD test kit. Subtracting Free chlorine from Total chlorine will give the level of Combined chlorine in the pool water. When combined chlorine (chloramines) are 0.3 ppm or greater, a good shocking of the pool is indicated.
To reach the breakpoint chlorination threshold mentioned above, you will need to raise the chlorine level 10 times the level of combined chlorine. If you don't quite reach this level, you will have wasted your pool shock, and only contributed to the problem - so be sure to "hit it hard"!
"C" can also stand for cloudy pool water, another condition that shocking the pool can help, except in cases where the turbidity is caused by suspended carbonates or calcium that have precipitated out of solution.
Cloudy, hazy, murky water can be caused by circulation, filtration or sanitation issues, and shocking the pool may be a temporary solution to a cause it cannot correct, such as an undersized and worn out filter or one that isn't filtering long enough each day. For spas or pools, shocking after heavy use is also advised to remove bather waste.
Most pool stores are fond of telling their customers to shock the pool monthly, whether it needs it or not. Some suggest that a weekly pool shocking should be performed. But such a general timetable would be hard to apply to all pools, in all situations.
Referring again to our A-B-C's of pool shocking, there can be only 3 reasons to shock the pool. You either have Algae, Bacteria or Chloramines (and sometimes cloudy water).
If you don't see any algae, and your tests for bacteria and chloramines are not concerning, then you can keep the pool shock on the shelf. No sense in wasting money, and treating the pool for conditions that don't exist. Those that shock the pool every week, whether it needs it or not, may be not only wasting money on pool chemicals, but could be harming their pool. Plastic, vinyl and steel can be weakened by high chlorine levels, so shocking the pool is best avoided until necessary.
Pool shock can also be used in smaller amounts, as a quick booster to your chlorine level. This practice I call Super-Chlorination. Usually just a pound of shock will bring chlorine levels up from zero to a range that will prevent problems. Waiting for tablets to dissolve could take too long, so if you find your chlorine level at zero, use a pound or two of pool shock to bring up the chlorine level quickly.
The moral of the story is that you have to test your pool to know when to shock. Shocking monthly, or even weekly is overkill in many cases.
Test visually for algae and cloudiness, and test your pool (and especially your spa) for bacteria with these inexpensive bacteria test strips. Chloramine tests can be performed with any good DPD test kit (not OTO, the yellow color test), or if you notice a strong chlorine smell and eyes are burning, you may assume that a good, strong pool shock is needed.
Cartoon is Copyright Dan Reynolds, Jantoo.com