Automated Primary and Secondary Solids Control Using Solids Density Meters

The Lincoln Wastewater System is currently using Toshiba Solids Density Meters for two applications within the Theresa Street Wastewater Treatment Facilities. The first application involves using the instrument to control the operation of pumps withdrawing solids from the primary clarifiers to provide a consistent, well thickened primary sludge. By controlling the density of the sludge, the amount of sludge pumped to the anaerobic digesters is reduced with a resulting increase in digester capacity. The main advantages of using this type of automated primary sludge control, to produce thicker and more consistent solids concentrations, are longer detention times in the digesters with increased digester capacity and reduced operator requirements.The second application includes using the solids density meter in conjunction with magnetic flow meters to automatically control the solids retention time of the mixed liquor in the activated sludge process. The implementation of the system has provided more uniform control of the solids inventory within the process, with fewer wide swings in sludge age. However, the biggest advantage of the control system has been the reduction in effort to take samples, conduct lab analysis, perform the necessary calculations, and set pump intervals and durations. The activated sludge process and wasting efforts have become essentially hands-free.