Multi-Point Polymer Addition to Reduce Overall Polymer Demand

INTRODUCTION Mechanical dewatering of solids before or after digestion or stabilization processes is typically employed by utilities to reduce the water content of their solids in order to decrease transport costs. Prior to the mechanical dewatering process (such as centrifuges or belt filter presses) the solids need to be flocculated to increase the particle size and strength to make the solids amenable to mechanical dewatering such that the water drains easily through the particles and the particles can withstand the shear during dewatering. The process of preparing the solids for dewatering is typically referred to as 'conditioning'. The vast majority of WRRFs use high molecular weight, cationic polymers to condition their solids. Unfortunately, polymer costs have risen dramatically in recent years and these costs have become significant. In some cases, the availability of polymer has also become limited due to supply chain issues. As a result, WRRFs need approaches to reduce polymer demand for their conditioning process while not sacrificing performance in terms of cake solids and capture of solids. One approach to reducing polymer demand is through a multi-point polymer addition scheme in which a smaller dose is added at one point in the solids pipeline, this dose is allowed to mix, and then additional dosages are added downstream to fully condition the solids. This approach has been shown to reduce overall polymer demand, although the effects on cake solids and capture have not been well studied. The objectives of this research are to evaluate the impact of multi-point polymer conditioning on the overall polymer demand as well as cake solids and capture. METHODS AND MATERIALS The solids used for this testing were from a conventional, mesophilic anaerobic digester that was a mix of primary and waste activated sludges. To evaluate multi-point addition of polymer, a laboratory-based method of dewatering was employed. The details of this method have been described by Ngwenya et al. (2017). Figure 1 shows an overview of this method. Briefly, the method utilized controlled mixing to impart the same velocity gradient, G, for the same amount of time during the conditioning step since the shear during mixing has been shown to impact polymer demand. The extent of conditioning or flocculation is assessed using the capillary suction time (CST) and the optimum polymer dose is taken as the polymer dose with the lowest CST. After finding the optimum polymer dose, the solids are then dewatering using the centrifuge cup method, again, all parameters are held constant such that the only variable would be the characteristics of the solids. To evaluate the multi-point polymer addition versus traditional approach with all the polymer addition at one location, the solids were conditioned using a constant mixing time and all the polymer dose added at one time and the optimum polymer dose was determined using the CST versus polymer dose curve. To compare this conventional approach with multi-point addition, the dosage was broken up, typically applying half the original dosage at time equals zero, and then the additional dosages were added at different time intervals. In all cases, the total time of mixing remained constant such that the shear did not vary during the experiments. An example timeline for the multi-point addition experiments is shown in Figure 2. At the optimum dosage, the solids were then dewatered to determine the impact on cake solids and the filtrate solids were also measured to assess capture. The full details of the method will be described in the paper. RESULTS AND DISCUSSION The CST versus polymer dose curves for single, two- and three-point addition are shown in Figure 3. The curves show the typical U-shape in that the CST decreases as the polymer dose increase, reaches a minimum, and then begins to increase due to polymer overdosing. Using the minimum CST as a the optimum dosage, two point addition reduced the polymer demand by about 20% relative to the control, while three point addition reduced the polymer demand by about 30% which would result in substantial cost savings. The box plot of cake solids for these three polymer addition schemes is shown Figure 5. The one- and two-point addition had the same cake solids, around 19%, while the three-point addition had a slightly lower cake solids, around 18%. Interestingly, the cake solids were generally not impacted much using multi-point addition as long as good mixing intensity was used. In cases where less intense mixing was used, the multipoint addition decreased the polymer dose, but also resulted in a greater reduction in the cake solids. These various results will be discussed in more detail in the full paper, along with an overall economic analysis to show the tradeoffs between reducing polymer demand versus the impacts in cake solids. SUMMARY The results showed that significant polymer reduction can be achieved with multi-point addition schemes without sacrificing cake solids as long as mixing was sufficient. This approach could be a simple approach that WRRFs could implement to reduce costs by reducing their polymer demand while not sacrificing cake solids.