The Impact of Magnetite Ballast on Oxygen Transfer and Alpha

A new process, known as mixed liquor ballasting, increases the capacity and treatment efficiency of the activated sludge process by greatly enhancing the mixed liquor settling rate. The BioMag™ system is a ballasted biological treatment system that uses magnetite, a high density, inert form of iron oxide which can be recovered magnetically from the waste activated sludge and reused in the process to maintain an extremely dense, fast settling biomass. As a result, very high biological solids concentrations can be maintained, and the plant footprint can be extremely small. Many studies on other high biomass activated sludge systems have found that the oxygen transfer efficiency of fine bubble diffusers is negatively impacted by high biomass concentrations. While the BioMag system normally operates with very high mixed liquor suspended solids, the magnetite, which typically makes up about half of the suspended solids, is inert and has properties very different than conventional activated sludge biomass. The oxygen transfer efficiency of fine bubble diffusers has not yet been tested in a rigorous manner in a high magnetite, ballasted activated sludge system. The impact of high concentrations of magnetite on fine bubble aeration is vital to know in order to properly size a diffused aeration system in the BioMag® system. To obtain pertinent design information, the field oxygen transfer efficiency of a commercial fine bubble disc diffuser was examined using off-gas measurement in a 3.0 meter deep test column. Various air flow rates and magnetite concentrations were tested, and it was found that magnetite has no negative impact on oxygen transfer efficiency in a fine bubble aeration system.