Today there are two popular secondary swimming pool sanitation technologies available to help eliminate algae, bacteria, viruses, and protozoa in your pool water. The first is Ultraviolet (UV) pool water sanitizer which is a non-chemical process that uses UV light to eliminate organics in the water. Water is pumped from the pool into the filtration system then passed through the UV housing chamber. In the UV chamber is a powerful UV bulb that eliminates pathogens and chloramines and does not allow organic replication. Although this is a highly effective pool sanitizer, it is considered a secondary sanitation system because the system will only disinfect the water that is being pumped and passed through the system. Water in the pool that has not been pumped will still need to be sanitized for swimmer protection. The UV system combined with traditional chlorine or salt water systems is highly effective in eliminating pathogens, and the UV system will significantly reduce the need for chlorine or salt needed (30% or more) to sanitize properly. Also, UV sanitation systems are much more efficient at killing protozoa such as giardia and cryptosporidium (crypto) which is are highly chlorine resistant viruses. With the UV system eliminating chlorine by-products chloramines and the reduced need for chlorine, the UV system helps reduce chlorine irritants such as strong asthma induced chlorine smells and red itchy eyes and skin. For the UV system to work most efficiently the water being treated must not be cloudy which can absorb the UV light making it difficult to sanitize.
The second secondary pool sanitation system is called ozone sanitation. Ozone sanitation is set up similarly to the pump and filtration system but also utilizes a venturi system. Two different ozone process systems can be used for swimming pools. They are corona discharge and UV light. Corona discharge and UV light both create ozone (O3) through splitting oxygen (O2) molecules in the surrounding air. The single oxygen molecule attaches itself to unaffected oxygen creating ozone (O3) which is a volatile gas that is then pushed and mixed with water through the venturi system which will then kill and eliminate unwanted organics from the stream. Ozone systems are very effective, and some units can reduce the need for chlorine up to 90% or more. Ozone systems also kill protozoa up 99.9%. Like UV any water that has not been oxidized may be contaminated and will need to be eliminated through residual means such as chlorine.
Better sanitation and technologies are always being tested and created, but the proven sanitation system that has killed more algae and bacteria than any other systems combined is chlorination. Although there has been a push for complete elimination of chlorine, it is arguably the simplest, most common and recognized system. Personally, I would opt for a combination of UV or ozone with a chlorine/salt water system just to be safe and to have a piece of mind.