Ballasted Biological Treatment Process Removes Nutrients and Doubles Plant Capacity

The Town of Sturbridge, MA WWTF has experienced steadily increasing flows over the last 20 years due to significant population growth in the community. Plant loading rates are now above design levels, and NPDES permit requirements are becoming more stringent for both nitrogen and phosphorous. Furthermore, a limited footprint is available for expansion. The Town and its consulting engineer, Tighe & Bond, considered many potential solutions to address these issues, including a system upgrade using Membrane Bioreactor (MBR) technology. During an evaluation of treatment alternatives Tighe & Bond recognized that while MBRs can provide high level treatment in a small footprint they can also be energy intensive, relatively expensive to own and operate, and subject to membrane fouling, cleaning and replacement issues. Based on these issues the Town decided to explore an emerging ballasted biological treatment technology called BioMag.BioMag is a ballasted flocculation treatment technology that shares the benefits of MBRs, but has a substantially lower lifecycle cost and has no membranes to clean or replace. BioMag uses finely divided magnetite to increase the specific gravity of biological floc. Magnetite is Fe3O4, a readily-available, inexpensive, inert iron ore that is not magnetic itself, but is strongly drawn to a magnet. Magnetite has a specific gravity of 5.2, approximately twice that of sand. Increasing the specific gravity of the floc increases settling rates and allows operators to appreciably increase the mixed liquor concentration, while still maintaining adequate settling and thickening in the secondary clarifiers. The process works by impregnating biological floc with magnetite. The dark magnetite particles can be easily seen with a microscope as black specs within the floc. The magnetite is recovered from the waste activated sludge using high shear mixing and magnetic drum separation, and is returned to the aeration tank.Tighe & Bond worked with Cambridge Water Technology (CWT) and the Town of Sturbridge to conduct a full scale BioMag trial on one of three activated sludge trains at the Sturbridge POTW. An identical, parallel train was used as the control. The purpose of the trial was to compare the effectiveness and economics of CWT's BioMag technology to that of a Membrane Bioreactor (MBR) solution, to meet the Town's upgrade objectives. BioMag is an emerging activated sludge upgrade alternative that can more than double a facility's biological treatment capacity and free up tankage for biological nutrient removal, while utilizing only existing aeration tanks and clarifiers.During the Sturbridge pilot program it was demonstrated that the BioMag could provide nearly complete Biochemical Oxygen Demand (BOD) and Ammonia removal. Testing also demonstrated that total nitrogen concentrations of less than 10 mg/l, and total phosphorous concentrations of less than 0.2 mg/l could be achieved while operating the process in a Modified Ludzack-Ettinger (MLE) mode of operation. Magnetite recovery rates observed during the pilot consistently exceeded 95%.