The purpose of the chlorine is to prevent the growth of microorganisms and oxidize organic wastes. There is a great disparity between the amount of chlorine available, when comparing a spa to a pool. It is the presence of chlorine or other sanitizers, that helps maintain sanitary conditions. Once the sanitizer level is depleted, microorganisms will start to flourish and organic wastes will start to accumulate.
Assume that the spa and pool are being used, while the water temperature is the same. If the spa and pool are used by two adults, it would be expected that the amounts of microorganisms and organic waste added, to the water, would be about the same, over similar periods of time. If that is the case, the spa, with only a reservoir of 3 grams of chlorine, would quickly be impacted and the free chlorine would start dropping towards zero.
The pool, on the other hand, has 114 grams of chlorine available to help sanitize and destroy organic wastes. Over similar time spans, the pool would appear relatively unaffected. The conclusion to draw is the sanitizer levels, in a spa, drop rapidly, as compared to a pool, once the units are in use.
