Application of Measured Rheological Data for Improved Sludge Process Design

1.0 Purpose Wastewater sludges that contain more than 1.5 to 2 percent solids have fluid characteristics that are different than clean water. The complex non-Newtonian behavior of sludges requires more in-depth hydraulic analyses, especially when there is variability in solids concentrations. These complexities can be quantified using rheology, the study of the deformation and flow of matter. Rheology is required to appropriately design systems for non-Newtonian fluids such as primary sludge, waste-activated sludge, combined undigested sludge, thickened sludge, digester recirculation, and dewatering feed sludge. The primary goal of measuring and representing these rheological characteristics is to achieve a higher precision in hydraulic design. Rheological data was gathered for a solids improvement project at Kitsap County's Central Treatment Plant to develop hydraulic model inputs in AFT Fathom„¢ to then determine thickened sludge process design criteria. Measured rheological characteristics and updated design method findings were compared to typical rheological values and historical design methods to inform final design criteria for the thickened sludge process improvements. The measured rheological data and alternative design approach can be applied to future projects by leveraging a growing data base of rheological characteristics and alternative design approach performance data. 2.0 Rheological History and Historical Design Approaches There are typically two approaches to sizing sludge pumps. When plant data cannot be collected, it is common to use standard rheological values. A downside to this approach is that it is typically only recommended for pumping lengths of less than 100 feet. Alternatively, pump testing can be used to determine rheological characteristics but it requires a pump to already be installed and, in many situations, hydraulic analysis and design must be conducted prior to testing. It can also be difficult to estimate the pressure spike associated with pump startup to initiate sludge movement. 3.0 Rheological Experimentation An alternative approach to estimate sludge pumping hydraulics is to test plant sludge with a rotational rheometer and use that data as an input to a non-Newtonian hydraulic model. Through the use of a Brookfield DVNext rotational rheometer, rheological sampling and analysis for pumped sludges can be conducted (Ametek Brookfield Inc., 2019). This piece of equipment applies a shear force and torque to a fluid using a rotating spindle. It records the shear rate, which is a function of the spindle speed, the shear force, torque, and viscosity. Testing these values by varying the spindle speed creates a viscosity and shear stress profile for each sample. This profile characterizes the rheological properties of the fluid. The samples are also tested to determine the solids concentration so a relationship between the rheological profile and percent solids can be defined. 4.0 Application of Rheology to Modified Design Approach The purpose of these plant-specific studies is to improve hydraulic calculations using the observed rheological properties of the sludge to then establish design criteria for sludge pumping applications. AFT Fathom„¢ is a hydraulic modeling software that has capabilities to input observed rheological data and compare it with various non-Newtonian fit models to estimate the hydraulic impact of a given sludge. Such models include the standard Power Law, Bingham Plastic, and the Herschel-Buckley fits (Water Environment Federation and the American Society of Civil Engineers, 2010). AFT Fathom can then use these characteristics and fit models, along with other information such as preliminary pipe lengths and fittings, to determine the total dynamic head and other hydraulic design criteria (AFT Fathom 2023). To further improve the analysis, AFT Impulse„¢ can be used to estimate transient pressure spikes that may occur during pump startup The Fundamentals of Unfamiliar Fluids (Clark, 2021), (AFT Impulse 2023). 5.0 Case Study HDR is designing improvements for various solids treatment processes at Kitsap County's Central Treatment Plant. Amongst other miscellaneous solids improvements, new primary sludge, septage, and WAS thickening equipment and two new digesters will be designed. Equipment to support these process improvements such as thickened sludge pumps, process piping, meters, etc. are included in the design. Rheological data was collected from the plant to design new sludge pumps and ensure adequacy of existing sludge pumps for process modifications. 5.1 Experimental Approach Data was collected over four sampling events, two different seasons, and varied operational scenarios to determine a modeling and design basis for different sludge streams. Co-thickened primary sludge and septage, digested sludge, and thickened WAS samples were collected and analyzed with the rotational rheometer on site. Triplicate runs were conducted for a wide range of shear rates to collect each stream's rheological properties per testing methods recommended by the rheometer manufacturer (Ametek-Brookfield, Inc., 2017). Once trials were complete, onsite laboratory staff used standard methods to determine each sample's percent total solids and total volatile solids. 5.2 Experiment Results and Data Evaluation Test results revealed that shear stress values are, generally, positively correlated with the total solids of the samples. As anticipated, seasonal temperature impacted viscosity measurements with the colder conditions prompting more conservative design values. For each stream, shear stress and shear rate data were evaluated using the Bingham Plastic, Hershel-Buckley, and Power Law rheological fit models. Tested samples had a lower percent solids (4-6%) than the anticipated design condition after the thickener (5-8%). To address this discrepancy, measured rheological fit curves were extrapolated to estimate the properties of thickened sludge with a higher percent solids concentration than what was tested. This approach required multiple hydraulic model runs using each rheological fit model to determine the most reasonable design conditions and to avoid oversizing of the pumps. In summary, measured data for each sludge was input into the hydraulic model for each rheological fit and compared to typical design approaches to estimate the total dynamic head (TDH) that the thickened sludge pumps would experience. In some cases, typical design approaches provided more conservative TDH values as compared to the alternative design approach using measured rheological characteristics. Figure 1 depicts one example of testing results for thickened waste activated sludge (TWAS), thickened primary sludge and septage (TPSS), and digester sludge. Figure 2 depicts the rheometer on-site during a sampling event. Comparison of TDH for typical design approaches and alternative modeled design approaches based on measured data are compared in Table 1. Another rheological sampling event will be conducted in January 2024 to measure additional characteristics and seasonal variability for the sludges in question. These additional results will be compared to summer sampling data to finalize elected design criteria for the thickened sludge process improvements. 6.0 Future Development of Rheology In preparation of the aforementioned rheological testing, an internal best practice document was developed in collaboration with Matt Higgins at Bucknell University to define appropriate rheological testing criteria, data analysis methods, and design approaches using measured data. It is expected that this document will continue to be refined as more sampling events are conducted. Upon construction of each project, performance data will be evaluated to calibrate and improve the experimental criteria and alternative design approach. Rheological data sets from sludges tested at various plants can be compiled into a database to evaluate trends and consistencies in different sludges. Depending on the depth and breadth of data available and resulting findings, this comprehensive dataset may ultimately be used as a standard reference for design.