The State of the Science for Automating Dewatering Processes

Dewatering processes represent both a significant capital and operating cost as well as an opportunity for savings at wastewater treatment facilities. Several studies have been completed and are underway both in the U.S. and abroad. These will help provide a better understanding of the mechanisms of solid-liquid separation, further refine various physical-chemical-mechanical thickening and dewatering processes to maximize the removal of water, while automating these processes to save money by reducing the amount of chemical conditioners and the level of operator attention. The paper also suggests savings by using automation to give operators real time information about the dewatering cost.This paper provides an overview of some of the relevant work on dewatering automation and describe several full-scale tests in the US and Europe. This includes the WERF report on project 98-REM-3 “Thickening & Dewatering Processes: How to Evaluate & Implement an Automation package”, published in December 2001, which investigated control systems and instruments for both feed back and feed forward applications. The WERF study demonstrated that the instrumentation is in place to reliably measure process variables, and established that a 15-20% reduction in dewatering costs is not unreasonable. It also demonstrated that automation packages do not consist simply of a single instrument and an instruction book. The wastewater research organization in the Netherlands, STOWA, also conducted a similar project on dewatering automation, which focused mostly on feed forward controls. Both WERF and STOWA projects have tested instruments and sampling techniques, and demonstrated that all of the elements of the centrifuge can be automated. The new control system, which is discussed in this paper, incorporates recommendations from both the projects.An integrated feed forward/feed back control combination already exists for belt presses; however, centrifuges presented special challenges. The authors address the question of overall centrifuge process control, This paper lays out a logical step-by-step path to complete the automation of the centrifuge. Automation of a centrifuge consists of feed forward control loops which prevent upsets and process errors, coupled with reactive feed back control loops that will correct those errors that do occur. Since the various loops interact with each other, each loop must be evaluated to determine the speed of response, and the order of precedence. Documentation of the control system will be most important so that persons other than the creators can modify and adjust it.