Enhancing polymer efficiency through use of diluted polymer for final dewatering

Introduction
The rising costs of polymers significantly contribute to the operational expenses of treatment plants, particularly in the dewatering of thermal hydrolysis-based digestate, where polymer demand tends to increase due to the increased soluble and colloidal organic concentrations. At the Blue Plains Advanced Wastewater Treatment Plant, final dewatering is achieved using belt filter presses, which typically require an average polymer demand of 20 ± 5 lb per ton of total solids (TS) (2021 to 2024 average). This demand is influenced by the characteristics of the sludge, including seasonal variations in the ratios of primary to waste activated sludge (WAS) and changes in solids retention time.

Operational conditions, such as the effectiveness of polymer mixing with solids, also play a crucial role in determining polymer demand. The Blue Plains facility is designed to utilize a 0.25% polymer solution, and includes dilution of the digestate solids from an initial concentration of 5% to 3.5%, thereby enhancing polymer efficiency.

Recent optimization efforts at DC Water have successfully reduced polymer demand by approximately 10%, primarily through improved operator awareness and responsiveness. Building on these findings, our study investigates the potential for further enhancing polymer efficiency through the use of diluted polymer solutions. We hypothesize that a more diluted polymer will improve the mixing process, thereby increasing the polymer's effectiveness and reducing the overall dosage required for successful dewatering. Both bench-scale and full-scale tests are being conducted to evaluate this approach.

Methods
Bench-scale dewatering testing (Higgins et al. 2017) were different polymer solutions (0.2%, 0.15% and 0.1%). The capillary suction time (CST) at different polymer dose scenarios was used to determine the optimal polymer dose (OPD) as the measure of the extent of conditioning and floc formation. In addition, filtrate quality (expressed as TSS) and cake solids were evaluated in relation to the CST curves. Tests were performed over 5 consecutive weeks in which all three polymer solutions were tested over a span of 2 days (minimum solids characteristics changes) and with freshly prepared and ages polymer.

Three full-scale testing campaigns were also performed. Each campaign focused on three belts isolated and the polymer tank used to store emulsion polymer was used to dilute down the polymer batched in the plant to add to the three belts, while the rest of the facility used the non-diluted polymer (0.25%). Charge titration was used to confirm the polymer concentrations prepared.

Results and discussion
1. Bench-scale testing
The CST curves, filtrate quality and cake solids in function of polymer dose for the three polymer concentrations tested are shown in Figure 1. A small shift to the left was observed with decreased polymer concentrations (Figure 1A). A lower CST at lower polymer dose was observed at decreased polymer concentrations. However, results were variable, and polymer savings were inconsistent when examining the optimal polymer dose determined by the inflection point in the CST curve. In contrast, the use of 0.1% polymer solutions consistently improved filtrate quality, demonstrating that fines were more effectively captured with diluted polymers. Cake solids remained stable across all conditions and were less affected by variations in polymer concentration. Given that bench-scale tests indicated potential reductions in polymer demand and enhancements in filtrate quality, full-scale testing was conducted to gain further insights.

2. Full-scale testing
Standard operations involved using a 0.25% polymer solution for belt filter press dewatering. Before commencing the experiment, the polymer dose was optimized on three belts to establish a baseline and determine the daily polymer demand. Additionally, 3-5 filtrate and cake samples were collected for the initial baseline as well as for each new condition thereafter. Polymer samples were also taken to confirm the polymer concentration using charge titration (see Figure 2A).

Once the baseline was established, polymer solutions were diluted to 0.18% and 0.11%. The optimized polymer doses for each concentration are shown in Figure 2. A decrease in polymer demand was observed across all tested belts when the concentration was reduced from 0.25% to 0.18%. While there is no significant improvement from 0.18 to 0.11% (p>0.05). Additionally, the use of diluted polymer doses showed no significant impact on filtrate quality or cake total solids (TS) (Figure 3). We hypothesized that a lower polymer concentration could enhance the interaction between sludge and polymer, reducing the likelihood of polymer binding to itself. Another factor that might have played a role in the inconsistency of results between belts was the belt age that was different amongst belts. Overall, we estimate a potential polymer savings of about 10% by diluting the polymer to 0.18%.

Conclusions
In summary, reducing polymer demand by using a more dilute polymer solution, appears feasible, achieving approximately 10% savings. Full-scale testing showed no significant differences in cake TS% or filtrate TSS. Additionally, we will further investigate the optimal combinations of TS concentrations and polymer concentrations to enhance polymer efficiency. Diluted polymer at 0.18% will be implemented full-scale at Blue Plains in early 2025.