A Comparison of Municipal Activated Sludge Treatment with Membranes

The use of membranes is growing in popularity as a means of treating municipal wastewater. Typically, membranes are added-on to an existing wastewater treatment process or may be considered in the design of a greenfield plant. In either case, many times the unit processes are designed independent of one another, which can lead to higher costs of ownership.The most recent improvements to the Fountain Hills Sanitary District WWTP in Fountain Hills, Arizona included, among others, upgrades to the existing activated sludge process, the addition of cloth media filters and a microfiltration system. Each unit process was provided by an independent supplier. Currently, a Multiple Barrier Treatment Process (MBTP) is available that integrates biological treatment with cloth media filtration and membrane separation. This MBTP takes advantage of the intimate knowledge of the interdependence of the unit processes to provide effective treatment at the lowest cost of ownership.The biological process design of the MBTP can be customized to meet the treatment objectives. The separation of aeration and mixing results in process optimization. Another key aspect of the MBTP is the marriage of cloth media filtration and microfiltration. The pile cloth media has unique characteristics that allow high solids loading while maintaining a consistent effluent quality that is low in TSS and turbidity. Because the effluent from the filter is consistent regardless of fluctuations in influent solids loading, it has proven beneficial to the operation of the hollow fiber microfiltration process as membranes perform optimally on feed streams containing low solids and minimal loading variations. A resultant savings in membrane surface area of 50% to 80% can be realized.The consistent, high quality feed to the membrane process allows the membranes to operate at flux rates that are much higher than those typically realized on secondary clarified effluent or direct mixed liquor filtration. In addition, because the cloth media filters are able to meet strict effluent reuse objectives as a stand-alone process, they offer additional flexibility to the plant operator in terms of redundancy and point of discharge.Capital costs are reduced, mainly because of the significant reduction in membrane surface area. The trade-off between cloth media and membrane surface area also serves to dramatically reduce operating costs. Further reductions in operating costs result from the separation of aeration and mixing in the biological reactor portion of the process.