The limitations are due to the chemistry.
The Cyanuric Acid (CYA) in Trichlor (i.e. stabilized chlorine pucks/tabs) is not a separately added binding agent. The chlorine is attached to the CYA as a unique chemical compound -- one that is a solid that when formed with high pressure tends to stick together and slowly dissolve in water.
Dichlor (i.e. stabilized chlorinating granules though sometimes that is crushed Trichlor) is also a solid, but is powdered or granular and dissolves much more rapidly in water. Both Trichlor and Dichlor increase the CYA level.
Calcium Hypochlorite (i.e. Cal-Hypo) is also a solid and can be compressed into pucks, but doesn't stay together well in water so extra binders are added. Even so, it still tends to fall apart as it gets smaller and the binders can leave a bit of a mess. Special chlorinators are needed for Cal-Hypo pucks that are different than Trichlor feeders. Generally, Cal-Hypo is added as a powder since the pucks are so messy.
Lithium hypochlorite is also a solid, but it dissolves too quickly in water to form pucks/tabs and it's very expensive.
Sodium hypochlorite (i.e. chlorinating liquid or bleach) is put in water so is a liquid and at very high concentrations becomes unstable which is why you don't generally see chlorinating liquid above 15% nor see any "solid" sodium hypochlorite.
Chlorine gas is obviously a gas and is toxic and dangerous to handle unless you are a professional so this is not used in residential pools except by a few pool services.
So it just works out from the chemistry that there is nothing chlorine can be attached to chemically that would 1) be in solid form and 2) slowly dissolve in water and 3) not add anything you don't want. Since Trichlor and Dichlor increase CYA and Cal-Hypo increases Calcium Hardness (CH), the choices with the least side effects are lithium hypochlorite, sodium hypochlorite and chlorine gas. However, lithium hypochlorite is very expensive and chlorine gas is too dangerous so that just leaves sodium hypochlorite. This is a good choice as it is also close to pH neutral over time (i.e. when accounting for the acidity of chlorine consumption/usage) and its only side effect is to increase salt levels more than some of the other chlorine choices, but salt is far more innocuous than high levels of CYA or CH.
There are options for automation of adding chlorinating liquid or bleach including The Liquidator and peristaltic pumps. Of course, there is also on-site chlorine generation using a saltwater chlorine generator (SWG).
Richard
