AWWA ACE61614

The low-pressure membrane market has grown with the applications of these systemsexpanding significantly. Early low-pressure membrane applications were historically forturbidity removal of relatively high quality source waters, but now they are used toprovide complete treatment on difficult-to-treat source waters in basically one step.As such, the solids characteristics of the reject wastewater from these systems havechanged significantly also. Based on some actual testing of membrane reject waters, thesettling and thickening characteristics of direct feed membrane reject waters aresignificantly different than conventional water treatment sludges. Conventional gravitythickening or flotation thickening appear to realize sludge concentrations of no greaterthan 1.5% (and polymer addition does not help significantly), while mechanical (i.e. beltsor centrifuges) or enhanced thickening (i.e. DensaDeg) will be required to reliably achieve a thickness of 2 to 3 percent. A case study is presented of the City of Lancaster, Pennsylvania installing low-pressure membranes (Zenon 500) totreat two different river waters (two different water plants) directly. Both of thesesources require a coagulant for pretreatment and the feedwater quality can be highlyvariable. In addition to the varying solids loadings that are significantly impacted by influentturbidity, the varying hydraulic loadings from the Zenon-500 (as well as other submergedmembrane systems) may be an even greater impact to the solids handling facilities, evenwith a direct discharge to a sewer system. For example, the Zenon-500 submergedsystems regularly use a partial or full tank drain. In some cases, this may mean asmuch as 27,000 gallons must be discharged from a treatment train in a minute. This typeof rapid discharge can significantly impact the performance of a gravity (or flotation)thickening process. Additionally, it can adversely impact the flow to sewer lines that canbe overwhelmed with these short-term tank drains. As such, some form ofequalization is required in Lancaster as well as with most other submerged membranesystems that employ some form of a tank drain.Therefore, to determine the effectiveness of its existing thickeners and their ability tomanage the Zenon-500 reject water, the City conducted a large number of settleabilitytests with the pilot plant membrane wastewater to determine if the existing thickenerscould handle the new membrane system wastewater flows. Includes table.

Product Details

Edition:

Vol. - No.

Published:

06/17/2005

Number of Pages:

5

File Size:

1 file , 240 KB