The requirement for a ...

Published by

The requirement for a contact tank depends on the disinfection requirement of the water. UV is not very effective at killing viruses and to get a significant kill requires a much higher 'dose' than is generally applied (40 mJ/cm2) . Different virus species have different levels. However viruses are killed rapidly by a low dose of chlorine and so the residual chlorine dose will be effective for this. Generally the contact time will be much lower than required for a chlorine full disinfection where the CT is typically 15 and the contact time is 30 mins. I have used systems with a 10 min contact time after UV. 

Another point is that the DWI are now reinforcing the requirement for full disinfection within the site boundary. This means that the old contact main or 'it will be sorted out on the way to the res' won't wash and so if there is a virus potential then there needs to be a contact tank.

Also depending on your view of risk there is the issue of glass fragments and mercury from possible broken lamps. Personally I see this as a theoretical rather than an actual risk but it is often easier to compromise. A contact tank will help to capture fragments.

2 Comments

A further point is that there is not really a need for the contact tank to be a free surface atmospheric tank. For smaller volumes and where the system is pressurised pressure vessels can be used, thereby avoiding repumping.

Published by

Permalink

Thank you very much for your reply.

We run the CT tool and indeed the required contact time was below 5 minutes. The sources are quite ok, although there's a present risk for crypto, hence the UV. 

Good point about the DWI, we'll need to consider their requirements in a future.

Very interesting point about the glass fragments, I'll check how often this happens with the Ops teams.

 

Thanks very much again for your feedback. 

Gloria

Published by

Permalink