The health department requires 0.4 mg/L free residual at the outlet. We however note that if the flowrate is reduced, the outlet residual is reduced, for a significant period (due to det time increase and resulting HOCl decay). We are trying to model the situation, to see the extent of the problem.

Has anyone experience in this situation which would resolve the problem without increasing our inlet residual?

Thanks

Grant H 

Just to start the ball rolling if your tank is baffled you could try a baffle redesign  so that at low flow a greater percentage can short circuit but at high flow more is held back or has to take the long way. My original thought was if you had a maze type arrangement of baffles you put some small holes through the baffles to allow some flow to pass through relatively quickly. This would impact on your mean detention time obviously and the hole sizing will be "interesting".

Regards

TerryF

I absolutely agree with your comment  I would be an interesting design problem and it may require the whole tank configuration to be reviewed. The intent of my suggestion was to have a CCC that performs very differently  at different flow rates offerring a shorter length path at low flow and a longer for some of the flow at high flow.

Regards

TerryF

We maintain a constant reuse system flow and water not sent to customers overflows a weir and spills into the normal effluent channel to mix with the rest of the plant effluent for de-chlorination and discharge.  That way the deterntion time in the reuse contact chamber is constant and much easier to operate.

Where are you from?

I thought the 23 total coliform requirement for reuse was bad enough, but a free chlorine requirement. Isn't that going to kill the grass? Are they going to make you check for THM's and HAA5's.

Mark

E.coli < 10/100mL

Cryptsporidium & Giardia <6 log reduction from raw sewage conc'n

Virus <7 log reduction from raw sewage

Helminths approx 4 log reduction

These standards are unmeasureable and so the plant rely on a HACCP approach of monitoring treatment parameters and residuals, to guarantee that each process is working. It is a very big effort, but the effluent can be guaranteed as to be fit for purpose.

As for the chlorine residual, we are dechlorinating with bisulfite. I have read a paper on IWA's Water Practice that 1 mg/L will just kill the grass.

Regards

Grant H, Australia

Is it possible to put a second chlorine feed point half way to the effluent weir?  Perhaps activated by a (low) flow signal?

David

We are thinking about a spray header and perhaps aeration mixing to rechlorinate the first pass(es) of the tank.

Alternatively we are looking at a recirc system to recirc the outlet back to the inlet and rechlorinate the flow, until the residual rises and we can discharge