The low-pressure membrane market has grown with the applications of these systems expanding significantly. Early low-pressure membrane applications were historically for turbidity removal of relatively high quality source waters, but now they are used to provide complete treatment on difficult-to-treat source waters in basically one step. As such, the solids characteristics of the reject wastewater from these systems have changed significantly also. Based on some actual testing of membrane reject waters, the settling and thickening characteristics of direct feed membrane reject waters are significantly different than conventional water treatment sludges. Conventional gravity thickening or flotation thickening appear to realize sludge concentrations of no greater than 1.5% (and polymer addition does not help significantly), while mechanical (i.e. belts or 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) to treat two different river waters (two different water plants) directly. Both of these sources require a coagulant for pretreatment and the feedwater quality can be highly variable. In addition to the varying solids loadings that are significantly impacted by influent turbidity, the varying hydraulic loadings from the Zenon-500 (as well as other submerged membrane systems) may be an even greater impact to the solids handling facilities, even with a direct discharge to a sewer system. For example, the Zenon-500 submerged systems regularly use a partial or full tank drain. In some cases, this may mean as much as 27,000 gallons must be discharged from a treatment train in a minute. This type of 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 can be overwhelmed with these short-term tank drains. As such, some form of equalization is required in Lancaster as well as with most other submerged membrane systems that employ some form of a tank drain. Therefore, to determine the effectiveness of its existing thickeners and their ability to manage the Zenon-500 reject water, the City conducted a large number of settleability tests with the pilot plant membrane wastewater to determine if the existing thickeners could handle the new membrane system wastewater flows. Includes table.