Densified Activated Sludge: Successful Analytical Methods for Process Performance Monitoring

Introduction The densified activated sludge (DAS) process is a relatively new procedure that Water Resource Recovery Facilities (WRRFs) are adding to improve treatment efficiency. Certain inherent properties of DAS, however, can cause interferences with conventional laboratory methods. Overlooking these inherent granule properties can result in compromised laboratory results and misinformation for DAS process monitoring and operation. Metro Water Recovery's (Metro's) process engineers, operations, and laboratory worked together throughout a 2-year DAS demonstration project to identify, refine, and modify appropriate analytical methods for DAS. This presentation will summarize four of Metro's analytical methods developed for DAS as described below. Methods Metro initially assessed the applicability of standardized and newer wastewater methods for settling velocity1, TSS2, microscopy3, and PSD4 to DAS. Over the duration of the DAS pilot operation, Metro adjusted these conventional methods in response to analytical problems and errors noted in the datasets as discussed in the following. Particle Settling Characteristics Conventional Method Issues (CMI) Sludge volume index (SVI) is a routine method that has been used to assess the settleability of sludge for over 90 years5. Settling velocity is a variation of the SVI method which provides valuable process information for DAS, specifically in the first 6 minutes when much of the DAS settling occurs1. Despite the value in settling velocity, there is some inherent variability from the need for nearly immediate readings before a clear defined front has formed making it difficult to accurately measure settling. Modified Procedures (MP) Metro defined three method adjustments to improve the reproducibility of the method: 1) using flashlights to aid in the readings, 2) standardizing the method for readings between laboratory staff to ensure consistency, and 3) optimizing sample homogenization time to clarify front formation. Metro will present images and videos to illustrate these findings and share recommendations to standardize this method in other laboratories. Total suspended solids (TSS) CMI TSS is a commonly used laboratory method to assess solids inventory, solids residence time, wasting rates, and granulation efficiency. Due to the rapid settling of DAS, measuring the exact sludge volume for TSS using graduated cylinders and standard pipettes can underestimate TSS. Metro developed modified analytical protocols for accurate TSS measurements. DAS can also cause online and handheld TSS measurements to systematically underestimate true concentrations due to particle settling and stratification in the basins. MP Metro's laboratory modified their standard operating procedures (SOPs) for DAS to overcome interference due to DAS settling. Using wide-bore pipettes to measure and transfer sample to filter from the sample container (rather than graduated cylinders), better maintains granules in suspension through the sample volume measurement process. The fact that TSS online and handheld probes read lower values compared to laboratory TSS controls was overcome by determining instrument calibration reference factors (ratio of laboratory values to probe readings) and correction factors used to adjust online readings. These adjustment methods resulted in more consistent readings closer to the true TSS (lab) values. Microscopy CMI Microscopy is used to assess sludge properties such as microbial community and filament abundance6. It is an equally important tool for DAS providing details on the size and morphology of the granules, density, and filaments. Appropriate techniques for wet mount slide preparation ensures the structure of granules is not compromised and quality images are produced. MP Metro developed a modified wet mount procedure for DAS analysis, emphasizing how to place the cover slip to avoid DAS compression. Metro will demonstrate this method in the presentation. Figure 1 shows the granules in-tact which prevents errors and Figure 2 shows flattened granules. Particle Size Distribution (PSD) CMI Particle size distribution (PSD) is not a common wastewater analysis method but can be extremely important when working with granular sludge. It is beneficial to quickly quantify the size distribution of the granules being produced so operational changes can be made to maintain the appropriately sized granules and sufficient nutrient removal. Sieving and PSD via instrumentation were assessed by Metro. MP Sieving is a common method for PSD analysis, but it is slow, labor intensive, prone to bias, and only provides information on the discrete sieve sizes used. While PSD measured with instrumentation using light diffraction methods is more of a qualitative method, Metro demonstrated that PSD analysis by instrumentation provides reliable, reproducible PSD results (Figure 3). This is done in less than ten minutes per sample allowing for timely data for operational decisions, whereas the sieving procedure can take between 30 minutes and two hours. Conclusions Metro's team has identified important method modifications that laboratories should consider when analyzing the characteristics of DAS. By validating and adopting these recommendations in other laboratories, operators and process engineers will increase confidence in the data being used