On-Line Cake & Liquid Solids Measurement For Bioenergy Process Optimization, and Green Energy Efficiency

Between 1997 and 2002, there were two significant dewatering automation development projects; one by WERF, and the other by STOWA in the Netherlands (STOWA is the equivalent of a combined WERF and AwwaRF). The purpose of these projects was to advance the state of technology with respect to dewatering automation and optimization control. During project meetings several bioenergy systems operations staff identified a need for cake percent solids sensors. They advised the WERF project team that a combination of automation software, and flow and solids sensors, designed around the combined dewatering and bioenergy unit process boundaries would provide meaningful Mass Flow Control.This matched a 1997 EPA finding regarding the importance of Mass Flow Control for better performance of biosolids thermal oxidizers (incinerators) and for bioenergy technologies in general.STOWA Report 99-26, a module of the above mentioned automation project, identified and tested promising solids sensors in liquid sludge. Two companies, Alfa Laval and Alpine Technology, who were both involved in the WERF and STOWA projects, used these results as the basis for their sensor selection for their respective dewatering and thickening automation control systems.STOWA Report 2002-43 documents the performance of an Automation Control System over a twelve month period and confirmed that technology was available for routine and reliable dewatering automation. The WERF 2001 Report 98-REM-3 detailed the advantages of such control technology.In 2008, during an update project for the EPA 1987 Process Design Manual for Sludge Treatment and Disposal, the EPA project manager highlighted the importance of Mass Flow Control, combined with cake percent solid sensors, for enhanced process performance. He made specific reference to the need for modern process control tools for thermal oxidation (incineration), thermal drying, and other bioenergy technologies.During 2009, for a bioenergy upgrade project, MSD Cincinnati selected fluid bed incinerator technology, and the district's optimization team defined their required operational philosophy as follows:Sludge feed to dewatering: Install density meter on TPS/TWAS (blended thickened primary sludge and thickened WAS) inlet header to dewatering sludge feed wells. Continuously monitor density of incoming sludge (3 - 5% TS) to calculate polymer consumption in active lbs of polymer per dry ton of solids processed.Dewatering centrifuge feed: Operate one progressing cavity pump per centrifuge and set flows to match the sum of the flow readings on the TWAS and TPS transfer pumps' discharge headers. Acceptable flow range to each centrifuge should be established based on accomplishing 26-27% TS @ 95%+ capture efficiency.Dewatering centrifuges: Install density meter on centrate outlet to continuously monitor centrate quality (capture efficiency). Capture efficiency should be maintained at 95% or greater.Dewatered solids cake pumps: Install sludge density meter on each discharge header to fluid bed incinerator (6 headers total - 2 per incinerator). Continuously monitor cake density to maintain 26 - 27%TS feed to incinerator.The above process control requires that the following variables be monitored: TPS/TWAS suspended solids, centrate quality (solids capture efficiency), cake percent solids, and polymer consumption.In addition to improved control for proven bioenergy technologies, these new generation control and sensor tools enable the process boundary to be expanded to include additional green energy fuel streams for bioenergy reactors. For example, Mass Flow (ratio) Control enables importation of fuels from other wastewater facilities, use of FOG, industrial green waste, and woodchips. The solids sensors enable accurate blending of multiple streams, thus providing optimum feed to the bioenergy reactor. These tools enable existing technology to be upgraded to process super autogenous green fuel feed. Energy utilization equipment, such as steam and gas turbines, can then function at much higher efficiency.