Process Capability Assessment of Centrifugal Sludge Thickening Under Various Operating Conditions

APPLICABILITY Solids thickening is an indispensable unit process for process intensification of biosolids treatment. The fluctuations of feed sludge characteristics often impact the stability of the thickening process, reduce process capability, and increase operation and maintenance costs. Often, the optimization of sludge thickening is focused on meeting performance requirements as detailed in specifications. This study expands the assessment of sludge thickening by including both thickening equipment and its up- and downstream infrastructure from a process capability perspective. The presentation will benefit engineers and utility operators in better design and troubleshooting of sludge thickening processes with designed thickened sludge solids total equal to or higher than 6%. INTRODUCTION The practice of wastewater treatment is evolving to transform wastewater treatment plants into WRRFs with the adoption of advanced wastewater treatment and solids handling equipment, processes, and systems. Enhanced sludge thickening plays an important role in achieving footprint reduction, chemical and energy efficiency, and process control of solids handling [1]. Several studies have suggested that enhancing sludge thickening plays an important role in achieving energy self-sufficiency [2]. Studies showed that improvements in primary sludge (PS) and waste activated sludge (WAS) thickening significantly increased biogas production [3]. Thickened sludge with higher than 4% total solids could be sufficient for achieving a positive energy balance in two-step AD processes. Sludge thickening is taking an increasingly important role in process intensification in biosolids treatment, either through pass-through thickening prior to sludge digestion or recuperative thickening integrated with digestion processes. Centrifugal thickening is one approach to achieving high-rate sludge thickening. The control of the thickened sludge total solids (TS) usually involves the control of Relative Centrifugal Force (G-force), solids retention time, differential rotation speed between centrifuge bowel and screw, and polymer dosage. The application of an in-line density meter or pressure-indicating transmitter could provide control signals for optimizing the performance of centrifugal thickening through mechanical adjustment of the size of thickened sludge outlet [4] or adjustment of the specific gravity of thickened sludge through air injection [5]. Feed sludge characteristics have a significant impact on the performance of thickening centrifuges. The variables of the sludge characteristics include hydraulic loadings, solids loadings, and the mixing ratio between PS and WAS. The fluctuation of feed sludge characteristics could negatively affect the performance of thickening centrifuges and interrupt subsequent operations of biosolids processes. At the same time, fluctuations in feed sludge characteristics are expected and should be accommodated in the design of sludge thickening. The objective of this study is to investigate two cases of centrifugal thickening installations, respectively equipped with GEA and Centrisys thickening centrifuges. This study will provide practical recommendations for mitigating potential bottlenecks in achieving 6% TS of thickened sludge. This study will introduce the use of process capability assessment as a tool for O&M. METHODS Collection and organizing of site information. One challenge for thickening improvement is to systemically document and organize the site information associated with sludge thickening. This study investigates the design and operation of thickening centrifuges and the immediate upstream and downstream unit processes of the thickening centrifuges at two water resource recovery facilities (WRRFs). The design conditions of the two thickening centrifuges systems are summarized in Table 1. The process capability of sludge thickening is affected by both performance of the thickening centrifuges and the performance of up and downstream infrastructure, including wet wells, sludge transfer pumps, the design of process piping, and flow measurement devices. In this study, we developed a check list for a detailed documentation of the thickening unit process as well the immediate upstream and downstream processes, as shown in Table 2. Process capability assessment. Process capability assessment is a manufacturing tool to gain process performance knowledge by analyzing a measurable property of a process to the specification [6]. In this study, an easy-to-use process capability assessment tool is developed for operators to document the thickening performance as a record for understanding process performance and preventative & predictive maintenance of the thickening equipment on a monthly basis. RESULTS A scoring matrix is developed based on the assessment checklist (Table 2) for the evaluation of the positive and negative factors impacting efficient thickening for the two WRRFs. The scoring matrix is used as a survey and communication method to collect opinions from operating crew, maintenance crew, and engineers. Figure 1 shows an example of the scoring matrix. The scoring matrix will be further developed to document the progressive process improvement of sludge thickening. As it is not practical to adjust feed sludge TS in a controlled approach at WRRFs, the impact of solids loading on the performance of sludge thickening is evaluated through time series analysis and histogram analysis of feed and thickened sludges. Figure 2 shows the time series analysis of the feed sludge and thickened sludge TS. As illustrated in Figure 3, histogram analysis of the same set of data reveals the process operation in respective to specified performance (Table 1). Hydraulic loading of thickening centrifuges could be readily adjusted by throttling feed pump. The impact of hydraulic loading is evaluated in terms of solids recovery ratio under different thickened solids TS. As shown in Figure 4, a trade-off exists between hydraulic loading and the solids recovery under the same thickened sludge TS. The results of Figure 4 is based the WRRF 2. A similar curve will be developed for WRRF 1 for this study. Figure 5 shows the change of apparent viscosity as sludge is thickened to TS of 2.5%, 4.5%, 6.0%, and 8.5% respectively. The significant increase in thickened sludge viscosity leads to a higher transfer pump discharge pressure. Depending on different process piping design, the thickened sludge transfer pump's discharge pressure varies 90-110 PSI for WRRF 1 and 70-100 PSI for WRRF 2. It is necessary to size the thickened sludge transfer pump appropriately to minimize process interruption due to insufficient pumping capacity. Figure 6 provides an example of the developed process capability assessment method for monthly evaluation of the performance of thickening centrifuges. The assessment serves as a standard approach to understand the process stability and capacity in related to specified performance requirements. CONCLUSIONS This study documented the impact of sludge characteristics on the performance of thickening and subsequence sludge transfer. The significant increase of sludge viscosity when thickened from 4% to 6% TS requires a design of higher discharge pressure of thickened sludge transfer well. The findings of this study suggested that special attention is needed in the design of the process piping, pumping, and process controls to realize the capability of thickening equipment. The process capability assessment tool applied in this study provides plant operators with means to document and understand the performance of thickening centrifuges. The method developed in this study could be applied to the process capability assessment of other types of thickening technologies beyond thickening centrifuges in WRRFs. This method can serve as a communication tool between the operation and maintenance crews/departments, as the results addressed both process performance and equipment conditions. With the adoption of more advanced and sophisticated solids handling equipment, developing comprehensive process capability assessment tools for respective unit processes may lead to improved performance and reduced life cycle costs of solids handling at WRRFs. This study adopts process capability assessment in evaluating the performance of sludge thickening based on two types of centrifugal thickening machines with different control mechanisms. Attendance at this presentation will benefit design engineers interested in designing centrifugal thickening as part of the process intensification of solids handling in WRRF. The presentation will provide practical suggestions for understanding the process capability of installed thickening equipment for performance improvement.