Water Environment Federation (WEF) Discussion ForumsWater Environment Federation (WEF) Discussion Forumshttp://www.wef.org/TechnicalDiscussions/jfuller@wef.orgSat, 22 Nov 2008 11:10:19 GMT20http://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxDavid</P><P>We are thinking about a spray header and perhaps aeration mixing to rechlorinate the first pass(es) of the tank.</P><P>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 dischargeThu, 24 Jan 2008 07:25:53 GMT01780898http://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxGrant</P><P>Is it possible to put a second chlorine feed point half way to the effluent weir? Perhaps activated by a (low) flow signal?Wed, 23 Jan 2008 14:23:19 GMTD Shulmisterhttp://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxThe plant and reuse scheme is in Victoria Australia. Australia is taking a QMRA approach to setting limits for reuse water based on DALYs. In the case of Class A (unrestricted reuse, but not cooking and bathing) water legislation requires:</P><P>E.coli &lt; 10/100mL</P><P>Cryptsporidium &amp; Giardia &lt;6 log reduction from raw sewage conc'n</P><P>Virus &lt;7 log reduction from raw sewage</P><P>Helminths approx 4 log reduction</P><P>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.</P><P>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.Wed, 23 Jan 2008 02:21:46 GMTGrant Hhttp://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxA free chlorine residual is a little much I agree, but potable water does not kill the grass so 0.4 in reuse water shouldn't either. I guess this is what you get if the health dept is controlling the plant. I too wonder which state this is in.Tue, 11 Dec 2007 15:29:22 GMTD Shulmisterhttp://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxGrant,</P><P>Where are you from? </P><P>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. </P><P>MarkTue, 11 Dec 2007 12:36:02 GMTDvanthttp://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxCan the solution be as simple as not flow pacing the feed? Set the constant dose for peak flow conditions and as flow drops off the mg/l dose increases due to the constant dose rate. </P><P>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.Mon, 10 Dec 2007 13:58:40 GMTRob Baurhttp://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxDavid,</P><P>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.</P><P>Regards</P><P>TerryFWed, 05 Sep 2007 17:19:56 GMTTerryFhttp://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxI would think that allowing short-circuiting is defeating the purpose of baffle walls in the CCC. The goal of chlorination is disinfection; short-circuiting theoretically will allow passage of pathogens to the effluent. Even though you might get a higher residual due to less detention time, the effluent may not be acceptable for reuse.Wed, 05 Sep 2007 13:13:27 GMTD Shulmisterhttp://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxGrant,</P><P>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".</P><P>Regards</P><P>TerryFWed, 05 Sep 2007 00:13:52 GMTTerryFhttp://www.wef.org/TechnicalDiscussions/Topic6552-12-1.aspxWe have designed a chlorine contact tank for a reuse application. The plant is a BNR plant with low ammonia and P. Due to the low ammonia we designed to chlorinate to produce a free residual, reducing tank size over chloramination. The tank presently has a Tmean 71 min and t10 estimated at 47 min.</P><P>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.</P><P>Has anyone experience in this situation which would resolve the problem without increasing our inlet residual?</P><P>Thanks</P><P>Grant H Tue, 04 Sep 2007 23:49:18 GMT01780898