AMI SAC254 Online Continuous UV Absorption Monitor - Swan Analytical USA
These online devices are a fairly new product in our neck of the woods, so we don’t have a lot of feedback on optimization of the reagents, especially given that comparisons from one year to another are hard to obtain in water purification. The idea of determining the dose of coagulant or granular activated carbon (in the case of a TOC abatement) based on the amount of organic matter estimated via the UV absorption measured at 254 nm seems to me to be a move in the right direction in terms of manual dosing (not very reactive) or according to the turbidity (non-linear dose).
I use the word "estimated" because the correlation between TOC and 254-wavelength absorption is difficult to establish. However, this will always be preferable to oxidizable matter that incorporates certain minerals.
I'm not familiar with this device but have used HACH and E&H. These analyzers are very useful for treatment plants susceptible to TOC, but they have their limitations.
First, it's an optical measurement. Anything that gets in the way of the sensor will create interference, i.e. turbidity and fouling of the measuring chamber. Therefore, if possible, install these devices on clear water and remember to choose models equipped with wipers, otherwise you will have to clean them every week.
In practical terms this means that raw water measurements are prone to error, which will be significant in the case of high turbidity. Similarly, if placed at sedimentation tank outlet, TOC peaks will often be linked to turbidity peaks. For measurements taken under these conditions, it is imperative to use the analyzers with turbidity meters.
At filter outlet or with treated water, the issue does not arise. Nevertheless, not everything is as simple because, as has already been said, the UV/TOC correlation is not constant. In the case of treated water, for example, it varies from 0.5 to 1.5. Therefore, precautions should be taken when a plant is near the 2-mg/l TOC limit in order to ensure it is not exceeded with the risk of over-consumption of reagents.
Despite these negative points, I strongly recommend the use of analyzers. Well placed and in the right location, they are very reliable and provide an immediate picture of the evolution of organic matter throughout the water treatment process. And beyond the value itself, any drift in the measurement should be tracked to ensure the entire system is working properly.
Finally, be careful to calibrate the optical length (1, 5, 10 cm) according to the desired TOC measurement. The longest optical lengths are for the lowest TOC values.