Getting a Grip on AGS Waste Solids: Settleability and Phosphorus Release

INTRODUCTION The Aerobic Granular Sludge (AGS) and other densified processes offer the promise of biomass intensification with the added benefit of nutrient removal. A key element needed to support AGS is the selective wasting of flocculant biomass and particles from the process. Wasting occurs from the top of the settled solids or by other physical methods (e.g. hydrocyclones). While this wasting approach is essential to the AGS process, it results in waste solids (WAS) streams that differ from conventional activated sludge (CAS) with higher flow rates, lower total suspended solids (TSS) concentrations, and biological nutrient removal (BNR) activity. Therefore, the AGS WAS must be managed in a completely different way from CAS WAS. Two key WAS management challenges need to be considered. The first challenge is managing intermittent AGS WAS flows, which are on the order of 5 to 15 times higher than CAS WAS flow rates on average and approaching 100 times higher for instantaneous flow rates. The second challenge for AGS WAS management is understanding the impact of the phosphorus accumulating organisms (PAOs) on the solids handling system and managing the potential for struvite formation. The objectives of the project work are the following: *Define AGS WAS settleability characteristics *Expand the understanding of phosphorus release potential (total OP release) and rates METHODS & APPROACH Solids Settling Characterization All WAS samples were taken directly from the pipe that transfers WAS from the AGS reactors to the solids buffer tank (Figure 1). Settlability was evaluated using a 5.5-inch diameter column settler (Figure 2). The settling column was filled with 10 L of WAS sample for each test. Sample ports, located every 2 L and at the bottom of the column, were used to collect supernatant and settled solids samples. Settled WAS and supernatant samples were taken and TSS was measured in both samples. Sludge volume index (SVI) values were also estimated. Phosphorus Release Rates and Potential Phosphorus release testing has been conducted using gang stirrer apparatuses with 1.5 L samples (Figure 3) and low-rate mixing (30-50 s-1). A carbon source was added to 10 of the samples giving F:M ratios ranging from 0.0 to 0.5 g-COD/g-VSS. Additionally, the initial TSS concentration was varied by providing some initial thickening in some samples resulting in a range from 1,500 to 5,000 mg/L. The initial total phosphorus and phosphate concentrations were measured in the raw WAS sample. Samples were subsequently taken from the release tests through a 34 h time period. Samples were filtered and phosphate measured. Results & Discussion AGS WAS Settling Rates The WAS volume was recorded as a function of time by measuring the depth of the sludge layer in two separate test batches. Initial WAS concentrations were 2,500-5,000 mg/L and thickened to 8,500-10,000 mg/L by the end of the test period. The final settled volume was reached within 4-12 minutes. The maximum settling velocity was 3.9-11.9 m/hr at time 2-4 minutes and decreasing over time (Figure 4). SVI5 values were estimated in the range 110-200 mL/g, and SVI30 values were calculated in the range 100-120 mL/g. The TSS concentration in the supernatant was 8-40 mg/L. Evaluation of modeled settling in a clarifier was conducted using a BioWin simulator based on the calculated Vesilend model clarifier coefficients. Model sludge blanket height and effluent suspended solids remained within the desired range, even with the additional solids. A state point analysis from BioWinTM indicated that the AGS WAS operating condition was within the solids-flux limitation of the clarifier (Figure 5). Phosphorus Release Rates and Potential Phosphorus release testing showed phosphorus release rates varied initially in response to carbon dosing with higher short-term release rates followed by reduced long-term release rates (Figure 7). Short-term (or initial) release rates varied from 0.4 mg-P/g-TSS/h for no carbon addition to 4.0 mg-P/g-TSS/h for carbon addition (acetic acid with F:M of ~ 0.4 g/g). The long-term release rates varied from 0.1 to 0.7 mg-P/g-TSS/h for initial TSS concentrations in the range 1,500-5,000 mg/L (Table 1 and Figure 8). The effect of starting TSS concentration was more important than carbon addition for this test batch with rates normalized to volume increasing exponentially as shown in Figure 8. The overall phosphorus released varied in the range 7-22% of the initial TP measured. A follow-up test was conducted showing a release of 25-30% of the initial TP over a 72-h release period. A phosphorus mass balance on the plant aligns with maximum releasable TP observed based on the phosphorus removed from the liquid stream. An evaluation of the settling characteristics of AGS WAS showed high settling velocities resulting in WAS TSS concentrations similar to CAS WAS TSS concentrations with low DSS concentrations. Based on the results of phosphorus release testing, struvite mitigation is recommended for anaerobic digestion of AGS WAS.