I’ll confine my response to the production of drinking water. Ultraviolet rays are used to disinfect by emitting a controlled dose that inactivates microorganisms, preventing them from reproducing. It is essentially a chemical-free process that produces only trace oxidation. Chlorine and ozone, on the other hand, are strong oxidizing agents that kill microorganisms, remove organic matter, and precipitate various minerals (iron, manganese, sulfides, nitrites, etc.) Ozone is generally used after filtration and is more effective than chlorine on pathogens, but it has limited residual time. The lasting effects of chlorine, on the other hand, make it is a better choice for water storage and maintenance throughout the distribution network. I am not familiar with any major WWTP that use UV treatments. The literature indicates that UV reactors must be specifically calibrated and optimized, but in this context the technology has proven to be effective on a host of microorganisms.
Calgary Alberta Canada has a great deal of success with UV in Wastewater, also EPCOR uses it in many of their Wastewater systems, Check with the Edmonton Waste Management Centre of Excellence, (plug plug lol) they have a great course that I have had the honour of delivering a few times.
Chlorination has no effect on parasites like cryptosporidium or giardia, while UV is effective on both types of protozoa.
Ozone works on Giardia, not on cryptosporidium. The drawback to UV is persistence: because it only neutralizes microorganisms and does not remain in the water, it is mandatory to add chlorine afterwards to protect the distribution network.
Increasingly treatment plants are turning to UV disinfection prior to distribution.
To answer this question in terms of wastewater, for tertiary disinfection, UV is the most common in Europe upstream of silica or rotating-disk filters.
Ozone is used primarily to remove particle pollutants; chlorine is used less frequently because of the byproducts it produces.
The big difference is the residual nature of chlorine (an advantage) in the drinking-water network and the byproducts (disadvantage) in regard to wastewater treatment. In term of drinking water, ozone is usually used in conjunction with chlorine. I didn’t think UV disinfection was allowed for drinking water systems.
1 important thing: only chlorine ensures residual disinfection; UV has no residual effect and ozone has very little. UV is a very good disinfectant (not banned because we use it), but be careful because its effectiveness is seriously compromised by water turbidity (suspended solids create shadows that prevent light from hitting microorganisms). You have to provide for pre-filtration or make sure your raw water is very clear. To ensure optimum treatment, you can add chlorine downstream for residual protection. As Breizhau says, ozone is a very powerful oxidant. It is added after filtration because it reacts with everything that's in the water. There are several drawbacks to using ozone:
- It oxidizes everything in its path, so unless all your pipes and fittings are stainless steel, you might encounter problems. (We ran into trouble with the ladders in our treated water tanks.)
- Ozone is degassed in the storage tanks to remove bubbles, so make sure you have good ventilation because it will burn through your lungs.
- By oxidizing any matter present in water, ozone breaks down chain molecules, some of which can present risks. Hence the use of activated carbon filters to remove them.
You can't use chlorination immediately after ozonation, because the remaining ozone will destroy the chlorine too. So, chlorine is usually added after the storage tank phase or further downstream on the network.
Chlorination can be used on its own only with very good quality water. Since all water carries organic matter, the chlorine rapidly gets used up and creates chloramines (hence the familiar bleach smell). If the water is turbid, you're better off using filtration upstream.
To recap, for me there is no best treatment—you have to adapt it to the type of raw water you process and the results you want to obtain.
Hoping I shed a bit of light on the situation…
For drinking water, the combination of UV + chlorination at output is the most effective approach. UV neutralizes bacteria + viruses and protozoa and then allows you to greatly reduce the amount of chlorine required to ensure bacteria doesn't develop again. Careful what UV system you choose and check with local authorities to ensure you are buying products approved for your particular application.
One thing not mentioned is the quality of water prior to disinfection. Low turbidity the comments provided would be correct. As turbidity increases the effectiveness of UV will decrease and byproducts increase with chlorine usage during these events.
These are not the only alternatives. We recently treated Primary effluent in Vancouver with a CFU count of 8,000,000. In 15 minutes it was reduced to 15 and in four hours<1. There are no residuals and all single cell microorganisms can be destroyed. The system is proven in the field and is very scalable as well as capex friendly and low operating and maintenance costs.
We are working to establish new regulations for re-use and recycling of domestic waste water. A guaranteed. low cost method of killing bacteria is essential to allowing the re-use and recycling of water which will take the demand off expensive potable water input . In drought stricken areas this is essential.
It really depends on what you are trying to achieve. With regard to parasites such as crypto and giardia chlorination is very ineffective. Whereas UV is very effective given the proper treatment conditions. Water transmissivity must be high - generally greater than 90%. Effectiveness increases as transmissivity approaches 100%.
It is not uncommon to treat with UV - followed by chlorination for secondary disinfection in the distribution system.