Innovative Use of Membrane Technology to Thicken Waste Activated Sludge

All biological wastewater treatment processes generate sludge that represents a collateral problem for their handling and disposition. The overall cost for sludge treatment can constitute a large percentage of the total life cycle cost of the plant.Membrane bioreactor (MBR) is a biological process, which has grown very rapidly over the past decade because of excellent effluent quality and compact footprint. Many MBR plants operate at sludge retention times (SRT) ranging from 10 to 30 days. However, even with extended SRT, the mixed liquor typically does not meet Class B requirements for disposal, without additional treatment. Therefore a majority of the MBR plants are designed with aerobic digestion. To minimize capital (i.e., reduce aerobic tank size) and chemical costs, ZENON has designed several plants with membrane sludge thickeners.The ZeeWeed® membrane sludge thickener is designed to thicken waste activated sludge (WAS) from the membrane bioreactor without the use of polymer. The thickener increases the waste sludge solids concentration from approximately 1% dried solids (DS) to a thickened sludge concentration of 3.0 – 4.5% DS. The thickener is operated in a modified Batch mode or stand-by train mode. These operating strategies ensure the membranes are not continuously exposed to the high solids concentrations all the time, which optimizes membrane performance.Cauley Creek Water Reclamation Facility in Georgia, with a design capacity of 5 mgd, is equipped with a membrane sludge thickener to minimize the aerobic digester tank volume. The ZeeWeed® membrane thickener is designed to handle a WAS flow of 100,000 gpd at 0.7 – 0.9% DS to produce thickened sludge of 25,000 gpd at 3 – 4.5% DS. The thickened sludge is aerobically digested and dewatered with a centrifuge decanter.The membrane sludge thickener is a reliable process that can be utilized for thickening WAS from MBRs and also other activated sludge processes. This technology eliminates the use of polymers, which constitute a significant operational cost and it reduces the capacity of tanks and equipment required. The performance of ZeeWeed® membrane thickener is comparable to conventional sludge thickening processes such as dissolved-air flotation and rotary drum thickeners. In a membrane bioreactor application, the membrane thickener can be easily integrated into the overall membrane filtration system rather than being considered a separate unit process.